- Email: [email protected]
The geological history of Northeast Syrtis Major, Mars
Accepted Manuscript
The Geological History of Northeast Syrtis Major, Mars Michael S. Bramble , John F. Mustard , Mark R. Salvatore PII: DOI: Reference:
S0019-1035(16)30349-9 10.1016/j.icarus.2017.03.030 YICAR 12424
To appear in:
Icarus
Received date: Revised date: Accepted date:
2 July 2016 28 March 2017 30 March 2017
Please cite this article as: Michael S. Bramble , John F. Mustard , Mark R. Salvatore , The Geological History of Northeast Syrtis Major, Mars, Icarus (2017), doi: 10.1016/j.icarus.2017.03.030
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ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 1 Highlights
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We use HiRISE observations to characterize and map geomorphic units at Northeast Syrtis Major, Mars. Mineralogy and physical properties of the geomorphic units are characterized with CRISM and THEMIS. Stratigraphy unifiable with 5 geomorphic units despite heterogeneity at decameter scale. We identify intriguing large linear features and circular landforms of the olivine-rich unit. Synthesizing mapping results with the literature, a detailed geological history is constructed.
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ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 2 History of Northeast Syrtis Major, Mars Michael S. Bramble a,*, John F. Mustard a, and Mark R. Salvatore b a
Department of Earth, Environmental, and Planetary Sciences, Brown University,
b
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Providence, RI Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI
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*Corresponding Author Contact Information: Michael S. Bramble
Department of Earth, Environmental, and Planetary Sciences
324 Brook Street, Box 1846
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Providence, RI 02912, USA
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Brown University
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Email: [email protected]
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Third revision date: 16 March 2017 Second revision date: 16 January 2017
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First revision date: 16 November 2016 Original submission date: 1 July 2016 Pages: 82 – Figures: 21 – Tables: 2 (4 supplementary) – References: 101 Keywords: Mars, geomorphology, mineralogy, spectroscopy
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 3 Abstract As inferred from orbital spectroscopic data, Northeast Syrtis Major bears considerable mineral diversity that spans the Noachian-Hesperian boundary despite its
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small geographic area. In this study we use observations from the High Resolution Imaging Science Experiment camera, supplemented with Context Camera imagery, to characterize and map the lateral extent of geomorphic units in Northeast Syrtis Major, and constrain the geomorphic context of the orbital-identified mineral signatures. Using
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recent observations, we confirm previous mineralogy identified with the Compact
Reconnaissance Imaging Spectrometer for Mars, and greatly extend the lateral extent of visible to near-infrared investigation utilizing the greater coverage. Analysis of Thermal Emission Imaging System observations reveals further physical properties and
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distribution of the geomorphic units. The stratigraphy, which spans the NoachianHesperian boundary, displays significant morphological heterogeneity at the decameter
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scale, but it is unifiable under five distinct geomorphic units. Our paired morphological
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and mineralogical analysis allows us to construct a detailed geological history of Northeast Syrtis Major. Several geological events that occurred in Northeast Syrtis
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Major—including the formation of the post-Isidis crust, the emplacement of an olivinerich unit, the formation of sulfate minerals, and the emplacement of the Syrtis Major
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Volcanics—can be related to regional and global processes constraining the local chronology. Other mineralogical indicators, particularly the formation of Alphyllosilicates, are difficult to place in the temporal sequence. They are observed in isolated patches on the post-Isidis crust, not as a distinct stratigraphic unit as observed elsewhere in Nili Fossae, suggesting their formation via isolated leaching or through
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 4 alteration of initial compositional heterogeneities within the crust. Exposures of an olivine-rich unit are intermittently observed to form quasi-circular landforms, suggestive of emplacement in circular depressions, which may indicate a period of cratering between the formation of the Isidis basin and the deposition of the olivine-rich unit. We
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identify and discuss intriguing large linear features of the olivine-rich unit, reminiscent of dyke-fed volcanism, that have raised bounding ridges suggestive of contact
metamorphism with the crust. We compile, review, and discuss many of the outstanding questions and running hypotheses relevant to our mapping area. A synthesis of our
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geomorphic mapping with recent literature reveals a well-defined geological history with extensive aqueous activity at Northeast Syrtis Major that is amassed in a stratigraphic sequence spanning a time likely greater than 250 million years of geological history. Our
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geomorphic and spectral analyses confirm that Northeast Syrtis Major exhibits considerable geomorphic and mineralogic diversity within a relatively small geographic
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area that is representative of the geologic processes occurring throughout the broader Nili Fossae region during the Noachian and Hesperian. Northeast Syrtis Major adds to
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this sequence by exposing the diverse environmental history of this region as observed
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through the presence of alteration minerals not present in this fidelity or proximity
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elsewhere in Nili Fossae.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 5 1. Introduction At the nexus of the northwestern border of the Isidis basin and the northeastern extent of the Syrtis Major volcanic province lies the region informally known as
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Northeast Syrtis Major (Figure 1; ~ 17–18 °N and 76–77 °E), a diverse martian landscape with varied geology and mineralogy that spans the Late Noachian and Early Hesperian boundary (Ehlmann and Mustard, 2012). At Northeast Syrtis Major
(henceforth NE Syrtis), four distinct aqueous environments were inferred via the
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presence of orbital-identified minerals: shallow crustal Fe/Mg-phyllosilicate-bearing terrains, carbonate-bearing olivine-rich terrains, Al-phyllosilicate-bearing terrains, and layered sulfate-bearing terrains (Mustard et al., 2007, 2009; Ehlmann et al., 2009; Ehlmann and Mustard 2012). The stratigraphy of key mineral-bearing strata are
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interpreted to show a trend from early neutral-pH water environments in the Late Noachian to acidic conditions in the Early Hesperian (Ehlmann and Mustard, 2012),
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though the paradigm of alkaline mineralogy inferred from Fe/Mg-smectites has recently
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started to be questioned as these minerals have been shown to form at mildly acidic pH conditions (e.g., Peretyazhko et al., 2016). Previous investigators examined the
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mineralogy and stratigraphy at a regional scale (e.g., Ehlmann et al., 2009; 2011; Ehlmann and Mustard, 2012), but this left unexamined important geomorphic
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characteristics and stratigraphic relationships that can only be revealed through detailed orbital analyses at the meter to decameter scale. Here, using the highest spatial resolution data available, we investigate the geomorphic context of the previously identified orbital mineral signatures to further our understanding of the relative age relationships and geomorphic textures. Our objective
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 6 is to investigate the lateral extent of the mineral stratigraphy first identified in the area by Ehlmann and Mustard (2012) over the broader NE Syrtis region, and observe how the morphology and composition of units identified in NE Syrtis varies spatially and through time by correlating and characterizing multiple exposures throughout the region at a
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high level of detail. NE Syrtis has a complicated but rich geological history that is
expressed over a large area (~2500 km2) with a unique sequence of units. We aim to correlate stratigraphic contacts and mineralogy mapped in NE Syrtis to the surrounding Nili Fossae, including the Jezero crater watershed, as the spatial proximity of these
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analyses to our study region provide valuable points of comparison. Lastly, we aim to construct a detailed geologic history of NE Syrtis using the most comprehensive mapping of the region to date. Unraveling this history through the detailed, high-
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resolution analysis of the morphology and mineralogy presented here will further our understanding of the Late Noachian to Early Hesperian transition, and the context of NE
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Syrtis in the geological history of Mars.
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2. Background
The age of the key geomorphic units around NE Syrtis are reasonably well
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constrained due to two stratigraphic markers: the Isidis basin and the Syrtis Major
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volcanic province. The region lies near the 1500 km ring of the Isidis impact basin (Frey et al., 2000), halfway between the 1100 and 1900 km diameters rings (Schultz and Frey, 1990), and the crater size-frequency distribution of impacts on the basin and ejecta generates a formation age of 3.96 Ga (Werner, 2005). An olivine-rich unit lies directly on the terrain excavated by the Isidis basin-forming event, establishing its age as contemporaneous with or immediately post-Isidis (Mangold et al., 2007; Mustard et
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 7 al., 2007). The younger stratigraphic marker is the volcanics from Syrtis Major capping layered rocks that are sulfate-bearing. Crater retention ages date the lavas to the Early Hesperian, with an age range between ~3.5–3.8 Ga (Hiesinger and Head, 2004; Ivanov et al., 2012). Additionally, the formation of the Nili Fossae fractures occurred after both
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the formation of the Isidis basin and the emplacement of the regionally-extensive
olivine-rich unit, and before the emplacement of the Syrtis Major volcanic province
(Mustard et al., 2009). This is well constrained as the fractures cut the olivine-rich unit but are embayed by the Syrtis Major volcanic province (Mustard et al., 2009). Therefore,
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approximately 250 Myr of geological history is recorded between these two stratigraphic markers.
The regionally-extensive olivine-rich unit, which is well exposed in NE Syrtis, has
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been studied with multiple orbital spectroscopic instruments including the Thermal Emission Spectrometer (TES) (Hamilton et al., 2003; Hoefen et al., 2003; Tornabene et
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al., 2008), the Thermal Emission Imaging System (THEMIS) (Hamilton and Christensen,
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2005; Tornabene et al., 2008), l’Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) (Mustard et al., 2005; Mustard et al., 2007; Ody et al., 2013), and
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the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) (Mustard et al., 2009). Two hypotheses have been proposed for the emplacement mechanism of the
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Nili Fossae olivine-rich unit. The first hypothesis is that the olivine-rich unit was emplaced as either basaltic volcanic flows in-place when Isidis formed (Hamilton and Christensen, 2005) or as picritic lava flows emplaced subsequent to the Isidis basin formation (Tornabene et al., 2008). The clear stratigraphic context for the olivine-rich unit emplaced post-Isidis precludes a pre-Isidis intrusive igneous origin (Hoefen et al.,
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 8 2003). The second hypothesis is that the unit comprises impact melt produced during the formation of the Isidis basin (Mustard et al., 2007, 2009). Supporting evidence includes the observation that the spatially-extensive unit ranges across ~3 km of elevation (Hamilton and Christensen, 2005), drapes the post-Isidis topography, and is
which are morphologically consistent with impact melt.
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distributed across the ring structures of the Isidis Basin (Mustard et al., 2007), all of
The olivine-rich unit is of particular significance because of its high olivine content
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(up to ~40%, Poulet et al., 2009), large areal extent (>113,000 km2, Hamilton and Christensen, 2005; Mustard et al., 2009), and intimate association with Mg-rich
carbonate (Ehlmann et al., 2008; Ehlmann and Mustard, 2012; Edwards and Ehlmann, 2015). The variable alteration to Mg-rich carbonate and isolated observations of
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serpentine are particularly intriguing aspects of the olivine-rich unit (Ehlmann et al., 2008, 2010; Murchie et al., 2009; Mustard et al., 2009; Ehlmann and Mustard, 2012;
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Mustard et al., 2014). Proposed carbonate formation mechanisms include hydrothermal
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alteration of the ultramafic host rock at slightly elevated temperatures, contact metamorphism, precipitation from transitory shallow lakes, or weathering of the olivine-
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rich rocks (Ehlmann et al., 2008; Brown et al., 2010; Viviano et al., 2013; Bramble and Mustard, 2016). The olivine-rich unit is observed throughout the western Isidis basin
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(Mustard et al., 2007; 2009) including on the floor of Jezero crater (Goudge et al., 2015), in the vicinity of the large Nili Fossae graben (Ehlmann et al., 2008; Mustard et al., 2009), to the north in a region informally called the Nili Fossae carbonate plains (Ehlmann et al., 2008; Edwards and Ehlmann, 2015), and in the southern Isidis basin at Libya Montes (Mustard et al., 2009; Bishop et al., 2013a). Furthermore, the carbonate-
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 9 bearing unit is significant as it may be a potential reservoir of atmospheric carbon, though the degree to which carbonate-bearing rocks may have acted as a carbon sink is difficult to constrain from orbit (Wray et al., 2016), however recent efforts have begun addressing this question (Edwards and Ehlmann, 2015). The Nili Fossae olivine-rich
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unit has been hypothesized as a source region for the plumes of methane observed in the martian atmosphere (Mumma et al., 2009; Wray and Ehlmann, 2011) and the
fracturing of the olivine-rich unit may allow methane, perhaps diffusely produced in the
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subsurface from serpentinization of the olivine-rich unit, to reach the surface.
Orbital visible to near-infrared spectroscopic investigations have identified varied aqueous alteration and igneous mineralogies in the Nili Fossae and NE Syrtis. Based on regional-scale stratigraphic relationships, previous studies have deduced a
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sequence of aqueous mineral formation, suggesting it progresses from phyllosilicate, to carbonate, to kaolinite, and terminating with sulfate formation, as distinct layers rich in
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these minerals are isolated in well-defined ascending stratigraphic positions (Ehlmann
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and Mustard, 2012). Many phyllosilicate minerals have been identified (e.g., Mangold et al., 2007; Mustard et al., 2008; Ehlmann et al., 2009), with the dominant clay mineral in
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the region identified as Fe-rich smectites and observed to correlate with exposures of the Noachian crust (Mangold et al., 2007; Mustard et al., 2008; Ehlmann et al., 2009).
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Al-bearing clays were identified in NE Syrtis and were suggested as an alternative alteration pathway of the Noachian low-Ca pyroxene crustal materials (Ehlmann and Mustard, 2012). Serpentine was identified in conjunction with the Noachian plains olivine-rich unit that has been variably altered to Mg-rich carbonate (Ehlmann et al., 2010; Ehlmann and Mustard, 2012), and was suggested to be a product of
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 10 serpentinization of the olivine-rich unit in a hydrothermal aqueous environment (Ehlmann et al., 2010), perhaps set up by the emplacement of hot olivine-rich material (Viviano et al., 2013), or by deep burial by Syrtis Major lavas (Michalski and Niles, 2010). Mechanisms are under consideration for the formation of the crustal
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phyllosilicates for Mars in general and in our mapping region include formation as
surface weathering products incorporated into the megaregolith by impact processes, hydrothermal alteration in the shallow crust (Ehlmann et al., 2011), magmatic
precipitation (Meunier et al., 2012), impact origin (Tornabene et al., 2013), and
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alteration in the presence of supercritical fluids during the cooling of the martian magma ocean (Elkins-Tanton, 2008; Cannon et al., 2016). Kaolin-group minerals may suggest that pedogenic-type leaching or hydrothermal alteration occurred (Gaudin et al., 2011),
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and these outcrops may be the result of formation under circum-neutral pH waters, or localized acidic leaching (Ehlmann et al., 2009).
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Sulfates are observed at the boundary of NE Syrtis and the Syrtis Major volcanic
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province (Ehlmann and Mustard, 2012; Quinn and Ehlmann, 2014a, 2014b). They are hypothesized to record a transition in the aqueous environments towards bearing acidic
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waters in both a regional and global context (Bibring et al., 2006; Ehlmann and Mustard, 2012). Spectral analysis resolved diagnostic absorptions in multiple locations that were
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consistent with the jarosite, polyhydrated sulfates, and/or a mixture of jarosite with other hydrated phases (Ehlmann and Mustard, 2012). The sulfate-bearing unit is ~500 m thick and displays meter-scale layering (Quinn and Ehlmann, 2014a, 2014b). The morphology and structure of raised boxwork ridges, erosion-resistant ridges in a polygonal network confined to the sulfate-bearing unit, suggests formation via filled
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 11 volume-loss fractures in a sedimentary setting (Quinn and Ehlmann, 2014b). Ehlmann and Mustard (2012) offered two formation hypotheses for the sulfates at NE Syrtis. First was a volcanic-hydrothermal hypothesis where acidic fluids circulate through layered volcanic flows altering the rock and precipitating sulfate. A variation on this hypothesis
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would be the deposition of the layers as ash flows or falls (Quinn and Ehlmann, 2014b). Second was a lacustrine-groundwater hypothesis where the layers are deposited as evaporite sediments in a geologic settings similar to those observed at Meridiani Planum (McLennan et al., 2005) or Terra Sirenum (Wray et al., 2011), and the
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circulation of fluids may be related to extensive groundwater aquifers (Andrews-Hanna and Lewis, 2011) or a hot volcanic layer leading to contact metamorphism of underlying sediments. The abundant carbonate and sulfate minerals in Nili Fossae and NE Syrtis
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corroborate the presence of diverse aqueous environments, as carbonates require near-neutral conditions and some sulfates, such as jarosite, require acidic conditions.
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The assemblages bearing phyllosilicate, carbonate, and sulfate minerals present in NE Syrtis illustrate that spatially and temporally distinct environments of aqueous alteration
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were active in the region (e.g., Poulet et al., 2005). This mineralogy is hypothesized to
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indicate a global trend in the planet’s chemical history suggestive of the transition of waters from circum-neutral pH towards acidic conditions (Bibring et al., 2006; Ehlmann
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et al., 2011; Ehlmann and Mustard, 2012), and also shows that the stratigraphy of NE Syrtis spans the Phyllosian-Theiikian boundary (Bibring et al., 2006). Additional intriguing geological observations in Nili Fossae and NE Syrtis
generate further outstanding questions about the evolution of Mars. Unaltered crystalline igneous units in Noachian-aged crust, enriched in low-Ca pyroxene, would
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 12 provide testable constraints on geochemical and petrological formation models for the early crust of Mars (Elkins‐Tanton et al., 2005; Skok et al., 2012; Grott et al., 2013). Raised ridges in Nili Fossae and NE Syrtis are an outstanding question as running hypotheses for their formation includes mineralized fracture zones (Saper and Mustard,
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2013), as well as breccia dykes and crater-related faults (Head and Mustard, 2006). Megabreccia bearing internal fabric, including layering and possible phyllosilicates, demonstrate active geological processes in the region prior to the basin-forming
impacts, and may bear geochemical remnants of the primitive and hydrothermally-
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altered crust of Mars and provide access to the subsurface (Mustard et al., 2009;
Tornabene et al., 2013). The question remains whether the altered megabreccia blocks originated as primary ancient crust, impact processes, or were produced by later
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igneous intrusions.
The greater Nili Fossae region has recently been under intense analysis,
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indicative of the exceptional exposure of geologic units in this region, the
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unprecedented wealth of orbital data, and its importance in understanding the evolution of Mars. Immediately north of our study area is the watershed for the deltas observed in
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Jezero crater to the northeast (Goudge et al., 2015), further north are the carbonate plains with the planet’s largest exposure of carbonate-bearing rocks (Edwards and
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Ehlmann, 2015), and to the northwest is the Nili Fossae trough (Ryan et al., 2016). The spatial proximity of these analyses to our study provides valuable points of comparison for the discussion of NE Syrtis. In their geomorphic mapping of the Jezero crater and watershed, Goudge et al. (2015) demonstrated that the mineralogy of the fan deposits is detrital in nature and reflects the composition of their associated watersheds. They
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 13 mapped 28 geomorphic units and observed a mottled olivine- and carbonate-bearing unit overlying basement units bearing Fe/Mg-rich smectite, similarly reflecting broader mineralogical and morphological trends in Nili Fossae (Mangold et al., 2007). Edwards and Ehlmann (2015) performed a joint thermal infrared and visible to near-infrared
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analysis of the Nili Fossae carbonate plains where several geomorphic units were
identified and their mineralogy and physical properties constrained. They demonstrated that carbonation processes at low temperatures likely altered olivine-enriched basalts to compositions bearing at most ~20% Fe/Mg carbonate, which suggests carbonate-
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bearing rocks in Nili Fossae only sequestered <12 mbar of CO2.
The diverse geomorphic units in a stratigraphic sequence at NE Syrtis demonstrate the variety of testable hypotheses addressable via future exploration or
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return of samples to the Earth. These hypotheses are compiled in Table 1. It is beyond the scope of a single paper to address each of these hypotheses, but our geomorphic
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map and combined geomorphic and spectroscopic analyses presented here enable
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findings that constrain some of the above hypotheses. Additionally, our detailed mapping, which pushes the highest resolution orbital data available to its limits, helps to
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clarify what can and cannot be resolved with orbital analysis alone. Certain unavoidable limitations—image resolution or non-uniqueness of geomorphic indicators as observed
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in orbital data, for example—similarly limit the ability to address outstanding hypotheses with the investigation herein. In this contribution, we investigate the morphological characteristics of NE Syrtis and the stratigraphic setting of the multiple mineralogical signatures identified by orbital spectroscopy using the available high-resolution orbital
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 14 imagery, in an aim to constrain the spatial, stratigraphic, and temporal relationships in NE Syrtis. 3. Data and methods
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To investigate the characteristics and lateral extent of identified geomorphic units and the geomorphic context of the orbital-identified mineral signatures, a geomorphic map was created utilizing high-resolution image data. Tandem analyses in the visible to near-infrared (VNIR) and thermal infrared (TIR) further constrained the properties of the
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geomorphic units. 3.1 Morphological mapping and analysis
The foundation of the mapping of NE Syrtis was two basemaps, created using
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the high-resolution imagery of the region acquired by the Mars Reconnaissance Orbiter (MRO) spacecraft (Zurek and Smrekar, 2007). The first basemap was a mosaic of
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images from the Context Camera (CTX) (Malin et al., 2007) produced using the USGS Integrated Software for Imagers and Spectrometers (ISIS) and rendered at a resolution
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of ~7 m/pixel (Anderson et al., 2004). Capitalizing on the excellent coverage of the
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region with images from the High Resolution Imaging Science Experiment (HiRISE) (McEwen et al., 2007), a second basemap was created utilizing HiRISE images and
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Environmental Systems Research Institute’s (ESRI) ArcMap geographic information system (GIS) software. HiRISE images were spatially referenced to the CTX mosaic prior to mosaicking. Digital Elevation Models (DEMs) were produced using CTX and HiRISE stereo image pairs with the NASA Ames Stereo Pipeline (Broxton and Edwards, 2008; Moratto et al., 2010). The DEMs were map-projected and referenced to the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 15 martian areoid. The DEMs allowed for constraints on both regional and localized topographic and stratigraphic relationships, and for morphometric analysis of geologic units. The CTX and HiRISE images utilized in this analysis are listed in the
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supplementary Table S1 and Table S2, respectively. The geomorphic mapping area was bounded to the north by the Jezero
watershed and the region investigated by Goudge et al. (2015), to the east by the
unnamed 7 km crater south of Jezero crater, and to the south and west by the first
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contact with the Syrtis Major volcanic flows (Figure 1). The region investigated has
near-contiguous HiRISE coverage allowing for the meter-scale geomorphology to be studied. In regions with HiRISE coverage, the surface was investigated at a mapping scale of 1:1,000, and unit contacts were drawn at this scale. The mapped geomorphic
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units were initially characterized at HiRISE resolution, and the transition to regions without HiRISE coverage results in the loss of the fidelity required to accurately map
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these units. The contacts between these units occur below the resolution of CTX. In
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regions without HiRISE coverage, CTX images were investigated and unit contacts were drawn where geomorphic units were discernable at a mapping scale of 1:5,000.
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Regions where the resolution was too low to identify the HiRISE-characterized units as the boundaries were below the resolution of CTX, or regions where multiple geomorphic
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units were in close spatial proximity hindering mapping at CTX resolution, were incorporated into the Undifferentiated Terrain (UNT) unit. Figure 2a identifies the area in NE Syrtis where HiRISE imagery was used in the geomorphic unit mapping. The geomorphic mapping was performed without regard to the mineralogical analysis. An exception was made for regions without HiRISE coverage where CRISM spectral
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 16 summary parameters for the mineral olivine were utilized to confirm suspected geomorphic units matching other spectrally-confirmed olivine-rich exposures. In general, geomorphic units were defined by their characteristics identifiable in the HiRISE images and derived DEMs. These characteristics include, but were not limited to, the qualitative
unit, DEM topography, and stratigraphic position. 3.2 CRISM spectral analysis
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albedo, overall two-dimensional geomorphic shape, texture and patterns internal to a
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The VNIR spectral characteristics of the identified geomorphic units were
investigated using hyperspectral images from CRISM aboard MRO (Murchie et al., 2007). Full-resolution (~18 m/pixel) and half-resolution (~36 m/pixel) targeted observations made by the CRISM infrared detector (1.00 to 3.92 μm) were processed
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by dividing the I/F image by the cosine of the incidence angle, where I/F is the radiance
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at the sensor divided by the solar irradiance divided by π (Murchie et al., 2007). The image was divided by a scaled volcano scan observation empirically optimized for each
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observation as a first-order correction for atmospheric gases. Spectral summary parameters were calculated to guide the spatial identification of mineral signatures
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(Pelkey et al., 2007; Salvatore et al., 2010; Viviano-Beck et al., 2014). Images were
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map-projected and georeferenced to a Mars Orbiter Laser Altimeter (MOLA; Smith et al., 2001) digital terrain model to correlate with the geomorphic mapping. A vector image of the geomorphic map was overlain on the projected CRISM images to spatially compare the spectral features identified with the mapped units. To identify mineralogies of the mapped units, CRISM image pixels that displayed a strong signal in the parameter bands were collected from the map-projected images, spatially averaged
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 17 within the mapped unit, and then the spectra were divided with in-scene, spectrallybland spectra to diminish residual atmospheric and instrumental artifacts. For comparison with laboratory spectra of mineral samples, the map-projected pixels with the strongest spectral features were located in the non-projected camera-space images
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and the collected spectral pixels were divided by a spectrally-bland region spanning the same along-track detector width. CRISM observations utilized in this analysis are listed in supplementary Table S3 and their coverage of the mapping area is shown in Figure 2b. The spatial locations of CRISM spectra presented in the figures below are listed in
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supplementary Table S4.
Absorption features observed in the CRISM spectra were characterized by their shape and wavelength position and were compared to a suite of laboratory-measured
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mineral spectra. Laboratory VNIR spectra of pyroxenes display broad absorptions centered near ~1 and ~2 µm, and the centers of these features vary primarily as a
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function of the mineral’s Ca content, with increasing Ca content shifting these
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absorptions towards longer wavelengths (Adams, 1974; Burns, 1993; Klima et al., 2007, 2011). Olivine has a prominent ~1 µm feature composed of three absorptions produced
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from octahedrally coordinated Fe2+ that varies its band center with changes in olivine
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Fe2+ content (King and Ridley, 1987; Burns, 1993). Fe/Mg-smectites, including Fe-rich nontronite or Mg-rich saponite, bear
prominent vibrational absorptions at ~1.4 (first OH stretching overtone and H2O structural combination tone), 1.9 (structural H2O combination tone of OH bend and H-OH stretch), and 2.29-2.31 µm (combination tone of OH stretch and Fe/Mg-OH bend; Clark et al., 1990; Bishop et al., 1999, 2002; Frost et al., 2002). A suite of absorptions at
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 18 ~1.4, 1.9, and 2.2 µm suggest the presence of a kaolin-group mineral, with the first two absorptions matching the sources in Fe/Mg-smectites and the 2.2 µm doublet absorption resulting from a OH stretch and Al-OH bend combination tone (Clark et al., 1990). The mineral talc has a sharp and deep 1.39 µm absorption from OH vibration
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and a broader 1.44 µm absorption (Clark et al., 2007). Narrow talc absorptions at 2.32 and 2.39 µm are of comparable width and the latter is ~50% the depth of the former (Clark et al., 2007). Similarly, an absorption at 2.47 µm is ~50% the band depth of the 2.39 µm absorption. Talc does not have H2O in its crystal structure and therefore is
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lacking an absorption at ~1.9 µm.
Narrow absorptions observed at ~2.3 and 2.5 µm can match fundamental vibrational mode overtones of the CO3 in the carbonate mineral structure, and paired
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band centers observed at ~2.31 and 2.51 µm or 2.32 and 2.52 µm are indicative of Mgrich carbonate (Hunt and Salisbury, 1971; Gaffey, 1987). Orbital carbonate
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identifications on Mars also bear an absorption at ~1.9 µm (e.g., Ehlmann et al., 2008;
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Ehlmann and Mustard, 2012), which may indicate the Mg-rich phase is hydrated (Calvin et al., 1994) or is mixed with another hydrated phase, such as a smectite (Bishop et al.,
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2013b).
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Sulfate minerals have been identified in NE Syrtis (Ehlmann and Mustard, 2012) and the particular phases identified were jarosite, based on the identification of a Fe-OH asymmetric doublet centered at 2.21 and 2.26 µm (Swayze et al., 2008), and polyhydrated sulfates, identified by structural H2O absorptions at 1.4 and 1.9 µm and a diagnostic inflection at 2.4 µm. 3.3 Thermal infrared analysis
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 19 Thermal infrared (TIR) data from the THEMIS aboard the Mars Odyssey spacecraft (Christensen et al., 2004) were investigated in the mapping of NE Syrtis to analyze variations in the thermal inertia (TI) as well as broad trends in the dominant surface mineralogy, which influences the shape of the primary Reststrahlen features in
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the TIR wavelength region (Christensen et al., 2004). THEMIS images I27043003,
I39820004, and I44288005 were the focus of our analyses and together these images cover the entire mapping area. For compositional analyses, THEMIS TIR data were processed using the techniques described by Salvatore et al. (2016). Summarized here,
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the data were collected by THEMIS with 10 spectral bands located between 6.8 μm and 14.9 μm and a surface spatial resolution of 100 m/pixel. These data were calibrated to top-of-atmosphere effective emissivity (Christensen, 1998; Rogers et al., 2005) and
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atmospherically corrected (Bandfield et al., 2004) to produce surface emissivity data. Windowed decorrelation stretch (DCS) images were produced using THEMIS radiance
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data (Edwards et al., 2011), and THEMIS DCS images with red-green-blue combinations of band 8 (11.79 μm), band 7 (11.04 μm), and band 5 (9.35 μm),
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respectively, were utilized in the mapping and the corroboration of geomorphic units
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identified using HiRISE and CTX images. Decorrelation stretch images using this particular band combination have been shown to be effective in differentiating spectrally
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distinct surface units, especially variations in mafic compositions (e.g., Hamilton and Christensen, 2005; Rogers et al., 2005; Edwards et al., 2008, 2011). Quantitative TI measurements, which indicate surface properties including the
effective particle size and induration of the geomorphic units, were collected from the mapped geomorphic units using a global TI map (Fergason et al., 2006; Christensen
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 20 and Fergason, 2013). Modeled effective particle sizes for each unit were generated as relationships between TI and the effective particle size of materials have been shown to be applicable on Mars (e.g., Kieffer et al., 1973). TI is related to the degree of induration and the grain size properties of the surface; for example, competent rock outcrops have
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a high TI while surfaces covered in fine-grained dust have low TI. As a means of
minimizing dust cover and quantifying only the best exposures, measurements of the TI were made within each of the mapped units at locations exhibiting the highest TI values
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and in an area large enough to be confirmed within the bounds of the mapped unit. 4. Results
Despite NE Syrtis bearing great morphologic heterogeneity at meter to decameter scales, the geology of the area is unifiable under five distinct units of paired
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morphology and mineralogy (Figure 3, Table 2). Ascending the stratigraphic column
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(Figure 3) they are: (1) a Basement Unit (BAU) that exposes areas of low-Ca pyroxene and Fe/Mg-smectite and is observed in a range of morphologies including crustal
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mounds, smooth and knobby plains, and raised linear ridges; (2) an extensive Fractured Unit (FRU) that bears olivine-rich VNIR and TIR spectra, is variably altered to
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carbonate, and tapers off into Large Linear Features (LLF) that appear to descend
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downsection; (3) a Capping Unit (CAU) with an unremarkable VNIR spectrum that preserves craters and sheds boulders; (4) a 100s of meters thick slope unit (FSU) exposing layered sulfates and raised boxwork ridges; and (5) the dated Syrtis Major Volcanics unit (SMV) bearing a mineralogy of high-Ca pyroxene. Isolated and distributed outcrops in the Basement Unit display spectral signatures of a kaolin-group
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 21 mineral, and while these locations are at the surface of the Basement Unit they are not a significant stratigraphic unit as seen elsewhere in Nili Fossae (Ehlmann et al., 2009). Raised linear features that are meters in width are observed in the mapping area
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and in two stratigraphic positions. Some of these ridges have been previously mapped by Saper and Mustard (2013) as raised ridges. Ridges meters in width are observed at the edges of the olivine-rich Fractured Unit descending into the downsection Basement Unit. Raised Linear Ridges (RLR) are observed in coherent outcrops in the Depression
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region (see below), are mapped as a subunit of the Basement Unit, and are most
comparable to those discussed by Saper and Mustard (2013). Raised Boxwork Ridges (RBR) are stratigraphically equivalent to the ~500 m thick Feature-bearing Slope Unit (FSU), and are high-standing above light-toned outcrops (Figure 3).
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From the geomorphic mapping and spectral analysis of NE Syrtis (Figure 4) we
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identify three regions (Figure 1c): (1) the Plains region, consisting of the relatively flatlying region from 17.6–18.0 °N and 76.8–77.3 °E, (2) the Depression region to the south
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and west of the Plains region that transitions from the Plains region towards the Syrtis Major Volcanics, and (3) the Syrtis Major Volcanics region. Our mapped geomorphic
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units are discussed in detail below and summarized in Table 2.
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4.1 The Plains of Northeast Syrtis Major The topographically subdued region outside the southwest rim of Jezero crater is
termed here as the Plains region of NE Syrtis (Figure 5). The Plains region has a dynamic topographical range of <100 m, and a shallow regional slope towards the south and east, away from a topographically-high large crustal mound. The Plains region is
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 22 additionally differentiated via the large contiguous area of HiRISE coverage, where ~30×30 km of HiRISE stereo images have been acquired and used to generate highresolution topography.
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4.1.1 Basement Unit The stratigraphically lowest geomorphic unit identified in the mapping area is the Basement Unit (BAU), which is differentiated into five geomorphic subunits (Table 2). No unit is identified underlying the Basement Unit, and therefore it is of undetermined
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thickness. The Crustal Mounds (CMD) subunit of the Basement Unit is observed as mounds exhibiting significant vertical relief with a peak, plateau, or elevated ridge from which lineations on the slopes fan away (Figure 6a). The slopes are slightly darker in tone and do not exhibit boulder shedding. A subunit differentiation is made for Large
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Crustal Mounds (LCM), which share many morphological similarities with the Crustal
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Mounds, with the exception of their heights reaching of 100s of meters of elevation. These mounds are light-toned with lineations fanning away from the topographic high.
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The Crustal Mounds are more sparsely cratered than the Fractured Unit or Capping Unit (see below), likely due in part to a combination of its physical properties and amount of
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debris cover.
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Two plains units are mapped in NE Syrtis: Smooth Plains and Knobby Plains, both subunits of the Basement Unit. Smooth Plains are characterized by an expansive, relatively dark-toned smooth surface. Isolated ridges or spatially-insignificant mounds akin to the Crustal Mounds or the other mapped units are observed in the Smooth Plains but are relatively uncommon. Knobby Plains are extensive smooth regions interspersed with frequent ridges or mounds akin to the Crustal Mounds or other
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 23 mapped units imparting a knobby texture in the HiRISE and CTX data. The Crustal Mounds, Knobby Plains, and Smooth Plains may be indicative of an erosional progression, with the Crustal Mounds being the most intact and the Smooth Plains representing the greatest amount of degradation. An intermediary step would be the
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variably dissected exposures of the Crustal Mounds that are spatially related in
seemingly coherent structures with light-toned surfaces and dark-toned smooth gradual slopes.
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Dispersed exposures of massive blocks or megabreccia are observed in the subunits of the Basement Unit. The megabreccia or massive blocks exhibit a diversity of morphologies including both angular and rounded textures, internal structure including possible layering, and disaggregation into seemingly drawn-out filaments. The
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megabreccia are up to 10s of meters in diameter, and are of similar characteristics to those observed elsewhere in Nili Fossae (e.g., Mustard et al., 2009; Goudge et al.,
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2015). The megabreccia are spatially distributed throughout the Basement Unit as
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small, isolated occurrences of light-toned blocks that are meters in size with no topographic signature. Multiple surface textures are observed within the Smooth Plains,
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where the lowermost is a darker-toned plain bearing abundant megabreccia, and meters above is a smoother, lighter-toned plain (Figures 6b and 6c). These textures and
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the megabreccia are not mapped as separate units as they are not spatially significant at the mapping scale. CRISM pixels displaying strong signals in the parameter images were analyzed from the Basement Unit. Narrow absorptions at approximately 1.4, 1.9, and 2.3 μm are observed in isolated patches of the Basement Unit (Figure 6e) where the dust cover is
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 24 low. These absorptions are consistent with the prominent vibrational absorptions of Fe/Mg-smectite, including Fe-rich nontronite or Mg-rich saponite, and are interpreted as such. Broad absorptions are observed at ~2 μm consistent with the presence of low-Ca pyroxene (Figure 6g). Low-Ca pyroxene is common and pervasive in the Basement Unit
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and subunits via this feature identified in several exposures.
Distinct absorptions at ~1.4, 1.9, and 2.2 µm correlated with isolated light-toned features 10s of meters in size are diagnostic of the presence of a kaolin-group mineral
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(Figure 7). Outcrops exhibiting these absorptions are only observed in small and
isolated patches in the two plains subunits, and are associated with light-toned features on the surface that are qualitatively among the lightest-toned features in a given HiRISE scene. The light-toned surfaces display a massive texture with no internal fabric at the
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highest resolution apart from dark-toned fractures and cover (Figures 7b and 7c). No positive or negative topographic irregularities are observed correlating with these light-
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toned features using a HiRISE DEM, and the features appear part of the basement and
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not a separate stratigraphic unit eroded to its current exposure. As the geomorphic units were identified independently of CRISM data, outcrops bearing a kaolin-group mineral
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could not be identified on morphology alone and therefore were not mapped as separate subunits. While some of the circular light-toned massive blocks could be
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interpreted as rounded megabreccia, they may also be interpreted as circular depressions formed by small craters filled with light-toned materials that were later eroded to form a level surface (Figure 7b). TIR investigation of the Basement Unit produced a TI value of 255.11 ± 18.05 J m–2 K–1 s–0.5, which generated a modeled effective particle size of ~260 µm. These
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 25 values likely correlate more closely to the regolith and debris observed covering the Basement Unit, rather than inherent thermophysical properties of the basement itself, consistent with the patchy distribution of strong CRISM signatures of low-Ca pyroxene, Fe/Mg-smectite, and a kaolin-group mineral. The Basement Unit is predominately green
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in the THEMIS DCS mosaic (Figure 8a). The green color is indicative of generally higher values in band 7 than the surrounding materials due to more silica-rich
compositions shifting the main Reststrahlen features towards shorter wavelengths. Green units in this THEMIS DCS band combination are generally consistent with
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basaltic compositions with alteration phases present (Salvatore et al., 2016), and this observation is corroborated here by the identification of Fe/Mg-smectites and Alphyllosilicates in CRISM pixels correlated with the Basement Unit.
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4.1.2 The Fractured Unit
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The Fractured Unit (FRU) is a spatially extensive unit with a hummocky, corrugated surface of light-toned blocks on the scale of 10s of meters seen closely
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packed in a darker supporting material stratigraphically above the Basement Unit (Figure 9). The dark-toned material forms polygonal shapes around the light-toned
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blocks giving this unit a fractured appearance, and a fractal pattern of smaller fractured
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blocks is observed within the larger blocks (Figure 9). A sharp contact is seen between this unit and the Basement Unit, and the Fractured Unit is observed to embay the Basement Unit (Figure 6a). Fields of light- and dark-toned boulders are observed, and interspersed are small crater-preserving mesa mounds and small mounds resembling the other units identified but not spatially significant. The Fractured Unit is commonly covered with patches of aeolian cover forming dune fields with an approximate north-
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 26 south dune orientation. The dune fields are commonly located in the topographic lows and craters of the Fractured Unit. Parallel lineations are occasionally observed in the Fractured Unit. The lineations
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most frequently observed are a variable banding that is concentric and parallel to the upsection Capping Unit (Figure 10a). This banding appears to occur on a relatively flat surface. Other less-frequently observed lineations are suggestive of layering. These lineations are most commonly found in the Depression region (see below) and display
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alternating light- and dark-toned bands (Figure 10c). The bands are not of uniform
thickness and display a wavy appearance. The bands can span 20 to 30 m of elevation when outcropping on slopes.
The Fractured Unit conforms to the underlying topography on both local and
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regional scales. The unit follows the topographic gradient and, over 100s of meters,
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remnants are observed in linear topographic lows and form circular features (Figure 11). The Fractured Unit embays the slopes of large mounds with the underlying unit often
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exposed at the crest. The unit is also observed to conform to, or drape, the larger-scale regional topography, being observed at elevations from –2000 m (relative to the MOLA
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datum) in the north of the mapping area and descending to –2450 m in the Depression
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region (see below) and disappearing under the Syrtis Major Volcanics to the south. This draping characteristic has also been noted by Mustard et al. (2009) at elevations as high as –500 m in their investigation ~250 km north of our mapping area, and suggests that the Fractured Unit is the local expression of the Nili Fossae olivine-rich unit (e.g., Hamilton and Christensen, 2005; Mustard et al., 2009).
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 27 The thickness of the Fractured Unit is variable within the Plains region. At the contact between the Fractured Unit and the Basement Unit, the Fractured Unit is commonly ~5–10 m thick. This thickness can range from a few meters to a few tens of meters, and may be governed in part by variable thin regolith cover or preferential
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erosion of the Basement Unit.
CRISM spectra collected of the Fractured Unit bear a broad absorption at ~1 μm and narrower, less prominent absorptions at 1.9, 2.3, and 2.5 μm (Figure 12). We
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interpret this spectrum to indicate a mixture of olivine and carbonate minerals, and
possibly an additional mixture phase of smectite, consistent with the interpretations of Ehlmann et al. (2009) and Ehlmann and Mustard (2012). In NE Syrtis, the carbonate spectral signatures appear more spatially pervasive and less variable in the olivine-rich
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unit than observed in other locations in Nili Fossae (e.g., Ehlmann et al., 2009). This may suggest that aqueous alteration was more extensive and complete in this region
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than elsewhere in Nili Fossae. Possible unaltered olivine-rich outcrops lacking 1.9, 2.3,
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and 2.5 µm absorptions are observed in a limited number of exposures relatively free of aeolian cover, not only in dunes as previously reported (Ehlmann et al., 2010; Ehlmann
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and Mustard, 2012).
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Several exposures of the olivine-carbonate-bearing Fractured Unit appear to bear a weak absorption at ~2.39 µm (Figure 12). This absorption is often approaching the level of the spectrum noise, and the approximate CRISM band minima varies between the 2.3840, 2.3906, 2.3972, and 2.4104 µm channels. While it has been suggested that talc, with an absorption at 2.386 µm (Clark et al., 2007), would be an expected mineral phase from contact metamorphism of the olivine-rich unit (Brown et
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 28 al., 2010; Viviano et al., 2013), it is not clear that talc is supported by the available data, as no 1.39 µm absorptions are observed, and the 2.39 µm absorption, consistent with talc (Clark et al., 1990; Brown et al., 2010; Viviano et al., 2013), is also present in Fe/Mg-phyllosilicates. We interpret spectra bearing a broad ~1 µm absorption, narrow
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1.9, 2.3, and 2.5 absorptions, and a weak 2.39 µm absorption as a mixture of olivine and Mg-rich carbonate with lesser amounts of a phyllosilicate, possibly saponite (Figure 12). The previous identification of serpentine in NE Syrtis (Ehlmann et al., 2010;
Ehlmann and Mustard, 2012) is in an outcrop located at the southernmost extent of the
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Fractured Unit in our mapping area, and is not a widespread spectral component.
Regions mapped as Fractured Unit display a predominately purple color in the THEMIS DCS mosaic (Figure 8b). This observation corroborates the enrichment in
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olivine observed by CRISM, as purple tones are indicative of olivine in this band combination (e.g., Edwards et al., 2008). The Fractured Unit displays the highest
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thermal inertia of the mapping area at 420.29 ± 20.74 J m–2 K–1 s–0.5, and a modeled
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effective particle size of > 1 mm. This value correlates well with values produced by a similar scale joint TIR–VNIR analysis by Edwards and Ehlmann (2015), where thermal
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inertia values of ~400–500 J m–2 K–1 s–0.5 were reported from the modeling of olivine-
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and carbonate-bearing materials on the more-northern Nili Fossae carbonate plains. 4.1.2.1 Large Linear Features There are multiple linear features observed in NE Syrtis ranging from linear
raised ridges (e.g., Saper and Mustard, 2013) to previously undocumented Large Linear Features (LLF) of distinct morphologic character and size. These Large Linear Features typically: (1) are 10s to 100s of meters wide and 1000s of meters long (Figure 13a); (2)
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 29 join seamlessly with edges of the Fractured Unit (Figure 13b) and share the same olivine-carbonate-bearing VNIR spectral signature; (3) the material comprising the Large Linear Features is light-toned and blocky in a dark-toned supporting material, and raised rims commonly bound the edges of the Large Linear Features (Figure 13c); (4)
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they appear to follow the trend of the local topography and are sometimes exposed as raised ridges or recessive troughs (Figure 13d). These linear features are distinct from the raised ridges mapped in the Nili Fossae region discussed below (Saper and
Mustard, 2013; Ebinger and Mustard, 2015). A select number of Large Linear Features
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are mapped as Capping Unit (see below) as a material consisting of rough surfaces with dark-toned boulder-shedding slopes is supported on, and conforms to, the Large Linear Feature shape (Figure 5).
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A cross-cutting relationship between the Large Linear Features and the Basement Unit is observed in the Plains region. The Large Linear Features are
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commonly observed sharply bifurcating at angles into multiple Large Linear Features,
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including orthogonal intersections and triple junctions (Figure 13a). Large Linear Features are observed in present-day troughs. These troughs are a few meters in depth
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with the Large Linear Features in the topographic low at the end of smooth gradual slopes of Basement Unit material on the sides of the trough, as measured by a HiRISE
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DEM. Additionally, smaller raised ridges meters in width and 10s to 100s of meters in length are observed extending off the edges of Fractured Unit and Large Linear Features and tapering into the Basement Unit. Some of these ridges tapering off of the Large Linear Features were mapped by Saper and Mustard (2013), and, while they appear innately related to the Large Linear Features, for select ridges it is difficult to
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 30 constrain whether the ridges are a seamless part of the Large Linear Features, or downsection ridges rising and halting at the Fractured Unit or Large Linear Feature contact.
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Raised ridges or linear mounds are commonly observed along the contact of the Large Linear Features and the Basement Unit. Topographic profiles across the Large Linear Features measured from the HiRISE DEMs (Figure 14) show the bounding
ridges to be meters in height when the Large Linear Feature is present on the two plains
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subunits and the center remains a few meters above the level of the host unit. The
bounded Large Linear Features bear a center comprised of light-toned blocky material with a darker supporting material. The light-toned bounding rims are interpreted as erosion-resistant rims of the Large Linear Feature or possible aeolian cover lapping
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onto the linear feature.
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Fractures that follow the track of the linear feature are observed at the center of select Large Linear Features. These fractures are ~1 m wide and 10s to 100s of meters
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in length and have dark, shadowed crevices suggesting that they are relatively free of dust cover. Fractures of a similar scale are also observed in the mapping area in the
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other mapped units, including the units identified by CRISM to be olivine-rich. The dark
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shadows formed by these small fractures suggest they may be meters in depth. 4.1.3 The Capping Unit The Capping Unit (CAU) is stratigraphically above the Fractured Unit (Figure 15)
and is characterized by: (1) an uppermost rough, crater-preserving surface. On finer scales, this surface bears abundant meter-scale light- and dark-toned boulders
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 31 surrounded by a dark-toned material (Figure 16). (2) Lower dark-toned slopes shedding both light- and dark-toned boulders variably exposing internal light-toned fractured blocks. In select expressions where the crater-retaining cap appears to have been significantly eroded, light-toned and angular blocks are observed within a fractured and
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polygonal dark supporting material (Figure 15c). Some expressions of the Capping Unit display coarse banding in this lower slope and/or a lack of boulder shedding. The
uppermost surface and underlying boulder-shedding slope were mapped together as the Capping Unit (Figure 16), as they exist in various states of preservation and
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exposure. An erosional progression is observed for the Capping Unit where the unit is removed in successive steps and the end stage is a cap of light-toned angular blocks with no crater-retaining cap or boulders (Bramble and Mustard, 2015). Smooth-Sloped
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Mounds (SSM), a subunit of the Capping Unit, have equal stratigraphic position and were mapped where the capping material displays smoother slopes, subdued boulder
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shedding, darker-toned slopes, and no evidence of crater preservation (Figure 9). The Capping Unit commonly forms mesa-like landforms in conjunction with the downsection
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Fractured Unit (Figure 16). The Capping Unit bears a greater crater density relative to
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the Fractured Unit.
The thickness of the Capping Unit is also variable within the Plains region. The
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Capping Unit is commonly 10 m in thickness as measured from HiRISE DEMs, and the thickness is observed to vary within a range of ~5–15 m. An approximately meter-scale benching is observed (commonly in the absence of discernable layering) across the Capping and Fractured Units. Select mesa-forming expressions of the Capping and Fractured Units have vertical expressions approaching the ~1 m resolution of the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 32 HiRISE DEMs, in particular where the contacts occur on a gradual slope. The SmoothSloped Mounds subunit is notable due to its significant topographic relief, with some mounds reaching 60 m in height.
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An additional observed characteristic of select mesa-forming expressions of the Capping and Fractured Units is a quasi-circular nature of these two units. These
Capping Unit mesas are observed with a quasi-circular rim of Fractured Unit and
variably isolated from the larger expanses of the Fractured Unit (Figure 17). The quasi-
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circular mesas are of a relatively uniform size, all being a few 100 m in diameter, though even much larger mesa-forming expressions of these two units, on the order of several kilometers in diameter, appear to have approximately equal perpendicular diameters (Figure 17e).
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CRISM spectra collected of Capping Unit surfaces are unremarkable in the
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spectral parameter images and spectra collected from the Capping Unit display a flat, featureless spectrum. Select slopes of Smooth-Sloped Mounds bear a broad absorption
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centered at ~2.2 μm indicative of high-Ca pyroxene materials in these deposits. These spectral signatures appear correlated to the mantling material and are not an inherent
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property of the Smooth-Sloped Mound subunit. Analyzing spectra collected of the
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Capping Unit or Smooth-Sloped Mounds subunit do not reveal absorptions indicative of high-Ca pyroxene, corroborating the association of the ~2.2 µm absorption with the dark-toned mantling material. The Capping Unit displays a green-yellow color with isolated spots of purple (Figure 8c), indicative of higher values in band 7 likely due to the main Reststrahlen features being centered towards shorter wavelengths, showing slightly more silica-rich
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 33 compositions. Our observation of a TI of 276.9 ± 18.5 J m –2 K–1 s–0.5 for the Capping Unit similarly correlates with the TI range of 250–300 J m–2 K–1 s–0.5 reported for the lowolivine capping unit by Edwards and Ehlmann (2015). Our modeled particle size of ~375 µm for the selected Capping Unit outcrops is at the lower end of the ~300–700 μm
4.2 The Depression southwest of the Plains
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range reported for their capping unit (Edwards and Ehlmann, 2015).
To the south and west of the Plains region is a topographic depression
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containing variants of the geomorphic units described above (Figure 1c). CTX DEM topography shows the Depression region’s lowest elevations lie ~500 m below the average elevation of the Plains region. From the Depression region northwards to the 6 km crater is a rise in elevation towards a Large Crustal Mound outside the mapping
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area to the north of this crater. The Depression region lacks extensive HiRISE
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coverage. The isolated HiRISE imagery in this region provides insight to the units present, but the ability to map geomorphic units characterized at HiRISE resolution is
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hindered.
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4.2.1 Depression variations on Capping Unit and Fractured Unit Large, rough, crater-preserving surfaces matching morphological characteristics
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of the Capping Unit are observed in the Depression region, but here they are commonly larger in area (few kilometers across versus few 100 m) and bearing an extensive, smooth, dark-toned cover (Figure 18a). Similarly, in the Depression region there is a variant of the Fractured Unit termed the Dark-Toned Fractured Unit (FRU–DT) that is of matching morphology but the light-toned, polygonally-fractured surface bears a patchy
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 34 but smooth, dark-toned cover. This unit is commonly identified by exposures near the contacts with neighboring units, where the surface of light-toned blocks closely packed in a darker matrix is observed (Figure 18b). Beginning within a few 100s of meters from the contact and increasing in apparent abundance towards the center of these units is a
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dark-toned cover. The same dark-toned cover appears to be partially obfuscating both the Fractured Unit and Capping Unit. The lateral extent of the dark-toned cover matches the orange color in the THEMIS DCS image (Figure 8d). Orange in this band
combination suggests the presence of exceptionally silica-rich compositions or sulfates,
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though little can be definitively said about this orange color as the color may be
explained by coatings, alteration rinds, or sulfates that are generally invisible to VNIR detectors.
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CRISM analysis of the FRU–DT corroborates its relationship to the Fractured Unit as a strong and broad absorption is observed at ~1 µm and narrower and weaker
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absorptions are observed at ~1.9, 2.3, and 2.5 µm. With these absorptions olivine and
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carbonate are interpreted to be present, as with the Fractured Unit. Spatially extensive regions are mapped as FRU–DT, and erosional windows through this unit reveal
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additional Fe/Mg-smectite and low-Ca spectral signature concentrations.
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4.2.2 Raised Linear Ridges In addition to the Large Linear Features, linear features of smaller scale are
present in NE Syrtis. These ridges are most abundant in the Depression region and are meters in width, hundreds of meters in length, and are high-standing above a relatively smooth surface (Figure 19a). The ridges are observed in two stratigraphic positions: in linear networks stratigraphically beneath the olivine-rich Fractured Unit (mapped as
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 35 Raised Linear Ridges), or in polygonal networks of short linear ridges interspersed with light-toned banded formations above the Capping Unit (mapped as Raised Boxwork Ridges, see Section 4.3).
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CTX DEM topography shows that the exposures of Raised Linear Ridges (RLR) are commonly topographically higher than the Dark-Toned Fractured Unit in elevation, but visual analysis suggests that they are stratigraphically lower (Figure 19b). The
topographical relationship appears to follow broader trends, where at higher elevations
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the overlying material may have been eroded away, exposing the stratigraphically lower Raised Linear Ridges. The greatest vertical relief appears to be correlated with smooth mounds a few 100 m across that form within the confines of the Raised Linear Ridges. Conversely, the Dark-Toned Fractured Unit and Capping Unit may not have been
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emplaced in a manner that allowed these units to fully drape the entire region and cover all of the local topographic highs. Regardless, the Raised Linear Ridges present
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erosional windows through the olivine-rich unit and into the Basement Unit. Small
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mounds of capping material akin to remnants of the Capping Unit or Fractured Unit are observed in select locations where the ridges intersect.
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The Raised Linear Ridges appear to vertically cross-cut the host unit and
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terminate at the upsection Fractured Unit, an observation similarly reported for the breccia dikes studied by Head and Mustard (2006). The raised ridges of Saper and Mustard (2013) closely correspond with the regions we mapped as Raised Linear Ridges, but this previous study also included the bounding ridges along the Large Linear Features, ridges tapering downsection of Fractured Unit, and the Raised Boxwork Ridges (see below).
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 36 The basement layer from which the Raised Linear Ridges cross-cut bears absorptions in CRISM images at ~1.4, 1.9, and 2.3 µm interpreted to be a Fe/Mgsmectite-bearing basement (Figure 19c). The ~1.4 µm band is relatively broad in comparison to single-phase laboratory spectra and centered at ~1.41 µm. Broad
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absorptions suggest a mixture of phyllosilicates or variance in cation by substitution in the Mg/Fe solid solution of smectites (Ehlmann et al., 2009). Small doublets are
observed at 2.298 and 2.318 µm for the ~2.3 µm band, or one single band is observed with a band minima at 2.305 or 2.318 µm, suggesting a degree of heterogeneity or
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variation in the phyllosilicate composition, corroborating the broad ~1.4 µm band. A minima at ~2.39 µm is variably present in this unit (similar to the ~2.39 µm minima of the Fractured Unit discussed above), but its magnitude is on the order of instrument and
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data-processing artifacts and the low signal-to-noise ratio suggests using it in mineral identification is questionable (Figure 19c). A broad absorption (or set of absorptions) at
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~1.4 µm and a narrow ~2.39 µm absorption may be indicative of the mineral saponite (Figure 19c). We categorized the Raised Linear Ridges as a subunit of the Basement
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Unit as they appear stratigraphically below the Fractured Unit, and due to the observed
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VNIR mineralogy matching the Basement Unit.
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4.2.3 Megabreccia
Exposures of megabreccia are observed throughout the mapping area but are
most common in the Depression region. One particularly interesting exposure is observed at a boundary between Capping Unit and Dark-Toned Fractured Unit (Figure 20). This megabreccia exposure consists of blocks meters to 10s of meters in size and of multiple tonalities that are dispersed amongst a dark-toned supporting material. The
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 37 blocks bear rounded edges, but are variably fractured. This outcrop appears to have a horizontal progression in block size; trending towards the northwest, the breccia blocks become larger and more spread out in the dark supporting material. These breccia blocks, some 10s of meters in size, appear to have an internal fabric, suggesting
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layering or banding prior to disruption and amalgamation into breccia (Figure 20a).
Here, angular blocks are observed in close proximity with heavily rounded fragments, and one angular fragment appears to have a fault with a visible offset of ~10 m (Figure 20a). The megabreccia in this region bear a relatively smooth surface as is evidenced
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by meter-sized boulders casting shadows on the surface (Figure 20). The megabreccia appears to be stratigraphically equal with the olivine-carbonate-bearing Dark-Toned Fractured Unit, and is contacted on most edges with this material (Figure 20b), though
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the exact stratigraphic position of the megabreccia is difficult to constrain from orbit. If this breccia is stratigraphically equal, then it is of a different provenance than the
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distributed megabreccia observed in the Basement Unit in the Plains region, and may have been emplaced contemporaneously with the Fractured Unit. While this
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megabreccia was mapped as part of the Dark-Toned Fractured Unit, other
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interpretations based on the imagery could include cross-cutting, underlying, or embaying relationships with the Fractured Unit. The megabreccia were not mapped as
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distinct geomorphic units as they are not of comparable spatial extent to the other mapped units. 4.2.4 Undifferentiated Terrain The Undifferentiated Terrain (UNT) incorporates regions lacking sufficient highresolution image coverage for the mapping of the geomorphic units characterized at
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 38 HiRISE resolution. CTX imagery was used to map unit contacts where they were identifiable at CTX resolution, but the fidelity required to map these units is absent, and as a result the Undifferentiated Terrain contains multiple geomorphic units. Further hindering high-resolution mapping in this region is the seemingly greater spatial
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heterogeneity of the geomorphic units. The general surface observed in the CTX
images is composed of a lighter toned knobby terrain with small knobs and ridges in close proximity, and amongst isolated exposures of other mapped units, such as craterpreserving surfaces or light-toned fractured blocks with contacts obfuscated at this
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resolution (Figure 21a). Where possible, comparing the appearance of Plains region units in overlapping HiRISE and CTX images was used to extend mapping into the regions without HiRISE coverage. The olivine-rich units identified in CRISM OLINDEX2
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parameter images were also used to confirm Fractured Unit exposures and extend mapping in this region where HiRISE image data was not available (Salvatore et al.,
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2010). The Undifferentiated Terrain provides a unit representative of the overall geology of NE Syrtis as it incorporates diversity of units exposed in the region. It bears a mottled
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Basement Unit expressed as a knobby slopes and dissected crust, dispersed light-
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toned, polygonally-fractured surfaces (likely Fractured Unit), crater-preserving surfaces, and smooth cover. The surface flow features present in the Depression region,
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described by Skok and Mustard (2014), are contained within this unit. 4.2.5 The valley Cutting east-west across the mapping area is a valley bearing a narrow channel that ends in an ill-defined fan-shaped structure south of Jezero crater. This valley is the main morphologic feature dividing the Plains region from the Depression region (Figure
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 39 1c). For the final ~17 km of the channel prior to the fan, the channel is a few 100s of meters wide and filled with light-toned aeolian cover forming dunes perpendicular to the channel. Westward up the valley, the channel widens in sections along its length with large dune fields present at these locations. While features along the track of the
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channel bear some degree of sculpting towards streamlined features, the geomorphic units in the vicinity of the valley appear to largely preserve their primary depositional shape and extent. The western terminus of this valley opens onto large expanses of
highest elevated lengths of the channel.
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Capping Unit and Dark-Toned Fractured Unit at elevations 100 to 200 m lower than the
4.3 The contact with the Syrtis Major Volcanics
The border region between NE Syrtis and the Syrtis Major Volcanics has been
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studied (Ehlmann and Mustard, 2012; Quinn and Ehlmann, 2014a, 2014b). Our
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geomorphic mapping incorporates the Syrtis Major Volcanics, providing additional observations and descriptions. Summaries of these units, the Syrtis Major Volcanics
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unit (SMV), the Feature-bearing Slope Unit (FSU), and the Raised Boxwork Ridges
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(RBR) are provided in Table 2. Large contiguous regions in the west and south were mapped as the Syrtis Major
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Volcanics (SMV) unit where an elevated, flat, and relatively smooth surface preserving craters was observed. The Syrtis Major Volcanics are high-standing in the mapping area with elevations greater than or equal to that of the Plains region. A scarp face is observed with a sharp elevation drop of ~500 m towards the Depression region. The thickness of the Syrtis Major Volcanics, when exposed at this scarp face, is measured via a HiRISE DEM to be ~10 m. The steep slopes off of the Syrtis Major Volcanics are
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 40 largely smooth, but variable exposures of light-toned layers are observed on the slopes, and at the base knobs are observed high-standing above a smooth cover. The slopes are mapped separately as Feature-bearing Slope Unit (FSU) due to these morphological (and spectral, see below) characteristics. Multiple amphitheater-headed
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canyons are observed along this scarp wall with subtle fluvial channels that follow the topographic gradient across the volcanic surface and terminate at the amphitheater heads (Figure 21a). The flow features continue from the amphitheater heads
downwards into the Depression region and on into the valley (cf. Figure 21a, Lamb et
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al., 2014, and Skok and Mustard, 2014).
Strong but isolated high-Ca pyroxene detections, validated in ratioed spectra, are observed in CRISM images covering the Syrtis Major Volcanics, and ratioed spectra
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collected from these regions bear a broad absorption centered on ~2.2 µm, interpreted to be high-Ca pyroxene. We observe spectral signatures indicative of olivine-carbonate-
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bearing materials associated with mounds and knobs at the base of the Feature-bearing
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Slope Unit and appear to rise above the smooth slope cover. This suggests that the smooth slope of the Feature-bearing Slope Unit extends past the lower contact of this
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unit and obscures the downsection Capping Unit and Fractured Unit at the base. Lighttoned, layered outcrops of the Feature-bearing Slope Unit exhibit absorptions at 1.4 and
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1.9 µm and an inflection at 2.4 µm (Figure 21d). Variably present is a weak absorption with a minima at 2.26 µm (Figure 21g), and this absorption is furthermore variably paired with a weaker absorption at 2.21 µm that is on the order of the spectral noise. These absorption bands are consistent with laboratory spectra of polyhydrated sulfates and the mineral jarosite, but the variability of the 2.21 µm absorption may suggest some
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 41 degree of mixing or cation variance between jarosite and natrojarosite, which does not have a prominent absorption at this position (Figure 21f). These observations confirm the previous identification and interpretation by Ehlmann and Mustard (2012) in NE
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Syrtis, and the spatial variability of the sulfate mineral phases. Observed stratigraphically below the Syrtis Major Volcanics and within the
Feature-bearing Slope Unit are raised ridges meters in width and 10s to 100s of meters in length that frequently intersect at orthogonal angles forming a boxwork texture. Saper
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and Mustard (2013) previously mapped these ridges, and combined these Raised
Boxwork Ridges (RBR) and our Raised Linear Ridges together. Here, we separate the two as they have distinct morphological characteristics and stratigraphic positions. The ridges are high-standing above a smooth basement and interspersed with light-toned
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outcrops. The Raised Boxwork Ridges are inferred to be in a subsurface stratigraphic layer beneath the capping Syrtis Major Volcanics and are exposed subsequent to
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erosion of the Syrtis Major capping material (Quinn and Ehlmann, 2014b; Ebinger and
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Mustard, 2015).
The Syrtis Major Volcanics unit displays comparatively low thermal inertia values.
resulting in a modeled particle size of ~200 µm. These values correlate with a regolith
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0.5
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The analyzed locations have a measured thermal inertia of 238.98 ± 11.15 J m–2 K–1 s–
at the surface corroborated by the etched retained craters and influence from the darktoned cover. A dichotomy is observed in the thermal inertia values of the Featurebearing Slope Unit, which have a split distinction between higher- and lower-TI areas. The low TI areas produce a measured value of 207.43 ± 11.39 J m–2 K–1 s–0.5 and a modeled effective particle size of ~100 µm. These lower values correlate to large areas
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 42 of the thick, dark-toned cover. The high TI areas produce a measured value of 304.64 ± 30.11 J m–2 K–1 s–0.5 and a modeled effective particle size of ~560 µm. The higher values correlate to local high-standing features, including high-standing exposures of olivine-carbonate-bearing materials as identified using CRISM. The THEMIS DCS
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mosaic similarly displays this TIR dichotomy as an overall equal distribution of colors with minor enhancements of yellow and is observed for large portions of the Featurebearing Slope Unit (Figure 8f). This unit also displays strong isolated concentrations of purple (Figure 8g), correlating to mounds high-standing above the smooth cover. These
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data corroborate the interpretation of the high-standing mounds being exposures of the olivine-carbonate-bearing Fractured Unit. 5. Discussion
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5.1 Regional and local stratigraphy
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Our coordinated regional geomorphic mapping and VNIR and TIR mineralogical analyses allow NE Syrtis to be placed in context with other orbital analyses throughout
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Nili Fossae. Mapping by Goudge et al. (2015) of Jezero crater and its associated
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watershed to the north of our mapping area provides an excellent opportunity to place the stratigraphy of NE Syrtis in a larger context. The largest spatial overlap between our
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mapping effort and that of Goudge et al. (2015) occurs with their mapped Altered basement (Bal). In our analysis, we subdivided this Altered basement region into three units, Basement Unit, Fractured Unit, and Capping Unit. This observation reflects the fact that our mapping was performed at higher spatial resolution utilizing significant coverage of HiRISE images, allowing for more extensive morphological subdivision. Our mapping scale was 1:1,000 versus 1:100,000 for the Jezero crater paleolake system.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 43 Altered basement (and the Ridged and Dusty massive subunits) of Goudge et al. (2015) morphologically corresponds with the Basement Unit (particularly the Raised Linear Ridges subunit). These units display small, light-toned knobs, variable topography, and no obvious structure in select locations, matching observations of the
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dissected exposures of the Crustal Mounds, Knobby Plains, and Smooth Plains
subunits. The Altered basement units and the Basement Unit share a CRISM-inferred mineralogy of Fe/Mg-smectite. These units also share many morphological similarities with the Undifferentiated Terrain. Additionally, the valley cutting east-west through NE
the watershed as Valley network floor.
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Syrtis (Section 4.2.5) matches the spatial size and morphology of a valley mapped in
Morphological and mineralogical similarity is observed for the Mottled terrain
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(MT, and subunits) and the Light-toned floor unit (LTF) of Goudge et al. (2015) with our Fractured Unit. The Mottled terrain displays a similar, though less degraded,
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hummocky, corrugated surface as the Fractured Unit. The Light-toned floor unit displays
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light-toned blocky materials, as does our Fractured Unit. CRISM spectra collected of the Mottled terrain and Light-toned floor unit show absorptions indicative of a mixture of
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olivine and Mg-rich carbonate (Goudge et al., 2015), matching the mineralogy observed for the Fractured Unit. The Jezero crater watershed does not appear to have preserved
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the Capping Unit to the same extent as in NE Syrtis, though perhaps the Capping Unit was not emplaced this far northward or was emplaced in a patchy distribution. The Thin dark capping unit (Tcu) mapped on the rim of Jezero morphologically matches our Capping Unit.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 44 Recent mapping by Quinn and Ehlmann (2014a; 2014b) of the NE Syrtis sulfate unit shows that this unit, mapped here as the Feature-bearing Slope Unit, is laterally contiguous beneath the Syrtis Major Volcanics and continues ~55 km south of our
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mapping area. Paired morphology observations and TIR analysis allows NE Syrtis to be
compared with similar mapping results in Nili Fossae and the Isidis basin. As discussed above, our Fractured Unit and Capping Unit share thermophysical properties and
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morphology with the olivine/carbonate-bearing materials and low-olivine capping unit of Edwards and Ehlmann (2015), respectively. Combining high-resolution morphological and spectral investigations of others (Ehlmann and Mustard, 2012; Edwards and Ehlmann, 2015; Goudge et al., 2015) with our work suggests that these capping units
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may have been laterally contiguous, or that materials with these properties and compositions are common throughout this region. If these units were emplaced via
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similar or the same processes, then the units mapped in NE Syrtis would have been
north-northeast.
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contiguous throughout the Jezero watershed and continue further more towards the
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The observations of olivine-rich bedrock mapped by Hamilton and Christensen (2005) in Nili Fossae do not extend southward to include NE Syrtis, but the thermal
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inertias of ~455 J m–2 K–1 s–0.5 reported for the strongest olivine signatures are similar for our measurements of the Fractured Unit. Global maps of olivine by Ody et al. (2012) using OMEGA data show that NE Syrtis resides at the southernmost edge of the large Nili Fossae olivine-rich terrain. These two observations along with the similarities of the olivine-rich materials described by Goudge et al. (2015) and Edwards and Ehlmann
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 45 (2015) confirm that the NE Syrtis olivine-rich Fractured Unit is part of this regional unit, though perhaps the most degraded. These observations show that the geology and stratigraphy of NE Syrtis, while
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exceptionally exposed and bearing diverse alteration products, is associated with the geology of the larger Nili Fossae region as observed throughout are units bearing
comparable compositional and morphological characteristics in analogous stratigraphic succession. Our geomorphic and spectral analyses confirm that NE Syrtis exhibits
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considerable geomorphic and mineralogic diversity within a relatively small geographic area that is representative of the geologic processes occurring throughout the broader Nili Fossae region during the Noachian and Hesperian. The overall stratigraphy matches that of the regional Nili Fossae group of Mangold et al. (2007); a stratigraphy of
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basement phyllosilicates overlain by an olivine-rich unit and capped by the Syrtis Major lavas with a trend of decreasing alteration upsection. NE Syrtis adds to this
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lithostratigraphic sequence by exposing the diverse environmental history of this region
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as observed through the presence of alteration minerals not present in this fidelity or proximity elsewhere in Nili Fossae.
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5.2 Geological history of Northeast Syrtis Major
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We constructed a detailed geological history of NE Syrtis on the basis of our high-resolution geomorphic mapping and integration with previous mapping efforts and spectral characterizations. The history began with the pre-Noachian formation of the primitive martian crust and the large basin-forming impacts with the exception of Isidis. Primitive crustal materials may be observed in the region, perhaps identifiable through mafic signatures such as low-Ca pyroxene (Mustard et al., 2005), and large
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 46 megabreccia blocks displaying layering, hint at active geological processes prior to the basin-forming impacts affecting NE Syrtis. During the Noachian, the crust was aqueously altered, perhaps by subsurface
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hydrothermal circulation (Ehlmann et al., 2011) or top-down pedogenic weathering, producing Fe/Mg-smectites as the dominant hydrated silicate mineral. Additional
phyllosilicate formation hypotheses, including formation during the last stages of a magma ocean (Cannon et al., 2016) or via magmatic precipitation (Meunier et al.,
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2012), would be applicable here. The Isidis basin formed in the Middle-to-Late Noachian at 3.96 Ga (Werner, 2005), resetting the surface morphology with extensive ejecta deposits and excavated material in the mapping area. Megabreccia observed in the Basement Unit may be due to this impact and/or the pre-impact substrate. The overall
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topography of the region was likely a result of the cratering process and the subsequent crustal readjustment of the basin as evidenced by the formation of the Nili Fossae and
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the potential provenance of the large crustal mounds as part of a basin ring structure
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(Wichman and Schultz, 1989; Mustard et al., 2007). Contemporaneous or following the basin formation was a period of heavy
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cratering. Craters ranging from 100s of meters to a few kilometers in size were formed,
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and their influence may be observed in the present by the quasi-circular mesa-forming expressions of the Capping and Fractured Units and are suggestive of pooling into the topographic lows of pre-existing small cavities. The nearby Jezero crater formed subsequent to Isidis and would have covered the Plains region of NE Syrtis with a continuous ejecta blanket. The dynamic topography of the mapping area likely completed major structural alterations not long after the period of heavy cratering. The
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 47 Depression region and Jezero crater were formed prior to the emplacement of the olivine-rich Fractured Unit, as well as the unnamed 6 km crater that has departed from circularity, suggesting minor crustal movements or preferential erosion may have
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affected it. The major stratigraphic marker of the olivine-rich Fractured Unit was emplaced subsequent to the formation of the dynamic topography of the region. The olivine-rich Fractured Unit embays the Basement Unit and drapes the topography on both local and
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region scales. It is observed draping small topographic lows on the scale of 100s of meters, as well as covering the regional topography from on top of the southwest rim of Jezero crater, across the sloping plains, and draping the walls of the Depression region towards the Syrtis Major Volcanics. The draping of the olivine-rich Fractured Unit could
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additionally be explained by emplacement on an undulating crust, emplacement as an ash-fall deposit, or emplacement contemporaneous with basin formation as a sheet that
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adjusted with the large-scale crustal movement. The presence of megabreccia possibly
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within the Dark-toned Fractured Unit in the Depression region suggests that this olivinecarbonate-bearing Fractured Unit may be impact related, as suggested by Mustard et
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al. (2007).
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The Large Linear Features were also emplaced at this point as they are observed branching off of the Fractured Unit, though the process leading to their emplacement is difficult to deduce at an orbital scale. The bifurcation of the Large Linear Features at angles forming triple or orthogonal junctions hints that this subunit displaced the Basement Unit during emplacement of the Fractured Unit. The raised bounding ridges, and their sharp contacts, may suggest a contact metamorphism
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 48 interaction occurred between the Large Linear Features and the host Basement Unit. The raised bounding ridges and lower internal material indicate that the bounding ridges are hardened and less susceptible to erosion. The Large Linear Features are also observed in present-day troughs and appear to have conformed to the shape of these
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troughs, which may indicate that the olivine-rich melts followed topographic gradients on the surface during emplacement and pooled in topographic lows. These observations suggest an emplacement relationship exists between fracturing of the Basement Unit and the olivine-rich material, though here multiple hypotheses can exist. The Large
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Linear Features may be remnants of a network of fractures that supported the
mobilization of olivine-rich melts in the near-surface crust. The high-temperature fluids may have altered the host wall rock along its interface and, following subsequent
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erosion, this less erosion-susceptible rock became the high-standing bounding ridges. The sharp bifurcation angles seen in the Large Linear Features and the long lengths not
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deviating from a straight line may support this crustal-interaction hypothesis. Similarly, these observations may suggest emplacement occurred cotemporaneous with a
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pervasive fracturing of the crust, such as during impact basin formation. Therefore, the
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Large Linear Features could be indicative of extrusive volcanic dykes or an impact melt sheet emplaced and mobilized upon on a heavily fractured surface subsequent to the
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formation of the Isidis basin. These features do not exclude any of the current hypotheses for emplacement of the Fractured Unit. Enigmatically, the presence of the olivine-rich Fractured Unit on the rim and floor
of Jezero crater and its presence as quasi-circular features suggest a significant period of time elapsed between the formation of the Isidis basin and the emplacement of the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 49 Fractured Unit. Possible implications include the emplacement of the Fractured Unit subsequent to the formation of Jezero crater by a volcanic process, the olivine-rich material superposing Jezero crater being of different provenance than in NE Syrtis, or the formation of Jezero crater as a secondary crater formed by the fallback of ejecta
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during the formation of the Isidis basin. The formation age of the Nili Fossae is poorly constrained, although it is argued that they formed subsequent to the olivine-rich unit emplacement (Mustard et al., 2009), likely shortly after Isidis was formed (Wichman and
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Schultz, 1989), or in the Middle Noachian (Tanaka et al., 2005).
Subsequent to the emplacement of the Fractured Unit was the deposition of the Capping Unit. The origin of this material may have been volcanic flows into the region or an ash-fall deposit of volcanic or impact-generated debris. The layered nature of the
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Capping Unit suggests that either multiple deposits were emplaced or the unit was emplaced and, then during solidification, differentiated into two layers of differing
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physical properties. The layered nature is not always observed for the Capping Unit, but
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this may be the result of varying degrees of erosion and exposure. The layers may conversely represent temporally and spatially separated episodes of deposition.
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Multiple formation mechanisms are hypothesized for the carbonate observed in
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conjunction with the olivine-rich Fractured Unit. The timing of its formation is both important and difficult to constrain. If the carbonate formed in the process of hydrothermal serpentinization, a carbonatization system, or a talc-carbonate system, then the carbonate would have formed in the Late Noachian near-contemporaneously with the emplacement of the Fractured Unit. These mechanisms have been invoked to explain the Nili Fossae carbonate. The water would percolate through the olivine-rich
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 50 Fractured Unit, and the heat driving the reaction would be from the emplacement of the unit, the geothermal gradient, or burial digenesis resulting from the Syrtis Major Volcanics (Ehlmann et al., 2008; Brown et al., 2010; Viviano et al., 2013). If the carbonate formed via low-temperature alteration induced by overland flow of water into
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the region, then it may have been contemporaneous with the formation of the valley networks, regional rises in groundwater, regional glaciation, or an Isidis ice sheet in the Hesperian or early Amazonian (see below). If the carbonate formed from surface
weathering of olivine-rich rocks in a manner akin to evaporites observed on meteorites
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found in Antarctica (e.g., Jull et al., 1988; Velbel et al., 1991), then the carbonate may have formed at any time throughout martian history, including in recent times at the cold and dry surface conditions. This mechanism could activate on olivine-rich rocks
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exposed at the surface during transitory periods of limited surface water, or perhaps even thin films of water as was suggested for the carbonate observed by the Phoenix
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lander (Boynton et al., 2009).
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The depositional event subsequent to the emplacement of the Capping Unit, and after a period of non-deposition as suggested by its cratered surface, was the
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emplacement of the sulfate-bearing Feature-bearing Slope Unit. Raised Boxwork Ridges are confined to, and mapped as a subunit of, the Feature-bearing Slope Unit.
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Regional mapping and structural analysis favors a sedimentary emplacement origin for this unit and filled volume-loss fractures as the origin of the ridges (Quinn and Ehlmann, 2014b). If the sulfate-bearing layers were produced in a sedimentary depositional environment, the aqueous environment producing the Feature-bearing Slope Unit may have been relatively long-lived, as it produced ~500 m of layers. Further running
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 51 hypotheses for the emplacement of this unit include ash layers, volcanic flows, or sedimentary deposits that developed sulfate-bearing mineralogy via subsequent digenesis, perhaps in a manner similar to that observed at Meridiani Planum (e.g., McLennan et al., 2005). Sulfate formation in this manner could suggest that this deposit
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is tied to regional groundwater circulation (e.g., Andrews‐Hanna and Lewis, 2011),
therefore the formation timing of these sulfates may be similarly linked to global sulfateforming phenomena.
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Initiating at ~3.7 Ga (Hiesinger and Head, 2004; Ivanov et al., 2012) and
subsequent to the deposition of the Feature-bearing Slope Unit, the Hesperian volcanic flows from Syrtis Major began to enter the region. If these flows extended onto the Plains region of NE Syrtis, then a significant amount of subsequent erosion has
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occurred as there is no morphological evidence for remnant flows on the Plains region. The one exception being the Smooth Sloped Mounds with their possible dark-toned,
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high-Ca pyroxene cover; for if high-Ca pyroxene was inherent to the Smooth Sloped
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Mounds and not the cover, then this could suggest the flows crested the Plains region. The volcanic flows from Syrtis Major likely halted not much further than their present
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location and the subsequent erosion developed the scarp of Feature-bearing Slope Unit. At the contact with the Nili Fossae, the Syrtis Major Volcanics may have
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encountered ice deposits as evidenced by the highly irregular margins of lava flows in the region (Ivanov and Head, 2003). If the valley of NE Syrtis (Section 4.2.5) was formed by overland fluvial processes, then the presence of glaciers and surface waters could explain the flow patterns from the Syrtis Major Volcanics into the Depression region and eastward out through the valley, as opposed to following the main
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 52 topographic gradient south (Skok and Mustard, 2014). The valley needs to be reconciled with the observation that the geomorphic units remain relatively intact with minor sculpting towards streamlines shapes, and therefore the duration or intensity of fluvial activity may have been limited. Numerical simulations suggest that an ice sheet
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may have covered the entire Isidis basin and formed the thumbprint terrain on Isidis Planitia dated at 2.8 to 3.5 Ga (Souček et al., 2015). If the formation of the valley was in conjunction with the presence of an Isidis basin ice sheet, then the valley would have formed in the Late Hesperian or Early Amazonian, likely from runoff related to the ice
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sheet. Fluvial, alluvial, or glacial processes throughout the Hesperian and early
Amazonian may have been responsible for the erosion and removal of significant amounts of material from NE Syrtis, as suggested by the absence of continuous ejecta
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blankets for Jezero crater or the unnamed 6 km crater in the northwest quadrant of the mapping area, the high-standing nature and erosional scarp faces of the mesa-forming
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expressions of the Capping and Fractured Units, and the significant removal of the
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Capping Unit from the Fractured Unit.
It is difficult to pinpoint the time of formation of the Al-phyllosilicate clay-bearing
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deposits, as they appear distributed in isolated patches within the Fe/Mg-smectitebearing Basement Unit and not as a distinct stratum. Several hypotheses exist for the
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formation of Al-phyllosilicates. The deposits may have formed as a result of the alteration of somewhat heterogeneous parent rocks or pedogenic-type leaching of phyllosilicate-bearing parent rocks (Ehlmann et al., 2009), long-term weathering (Gaudin et al., 2011), or deep hydrothermal alteration (Ehlmann et al, 2011). The emplacement of a hot capping rock may have allowed the Al-phyllosilicate to form from
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 53 hydrothermal fluids circulating in a Fe/Mg-smectite parent rock (Ehlmann et al., 2009), and subsequent erosion exposing the Al-phyllosilicates at the surface. Our observations at the contact of the upsection Fractured Unit do not support the presence of a distinct stratum bearing Al-phyllosilicate in NE Syrtis. As we observe scattered detections of Al-
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phyllosilicates at the surface of the crustal basement, we favor the formation of this material via isolated leaching of the crustal rocks, perhaps influenced by heterogeneous initial composition or the result of localized surface weathering.
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Due to the presence of the parallel north-south trending dunes in NE Syrtis, it is suggested that the Amazonian period witnessed the slow and continual erosion of the surface by aeolian denudation, with the Isidis basin governing the motion of local air masses. The complexity of the landscape results from the differing mechanical
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properties of the various units, and the multiple events of deposition, aqueous alteration,
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and erosion. 6. Conclusions
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NE Syrtis exposes a stratigraphic sequence spanning the Noachian-Hesperian
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boundary, encapsulating ~250 Myr of geological history. Between the bookend stratigraphic markers of the Isidis-forming impact and the Syrtis Major lavas at 3.96 and
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~3.7 Ga, respectively, exists great geomorphic heterogeneity that is unifiable under five distinct units. They are, ascending the stratigraphic column: (1) the Basement Unit, (2) the Fractured Unit, (3) the Capping Unit, (4) the Featuring-bearing Slope Unit, and (5) the Syrtis Major Volcanics unit. NE Syrtis bears a local variation on the regional Nili Fossae group (Mangold et al., 2007) where basement phyllosilicates are overlain by an olivine-rich unit and topped by the Syrtis Major lavas, displaying a trend of decreasing
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 54 alteration. NE Syrtis adds to this lithostratigraphic sequence by exposing the diverse environmental history of this region as observed through the presence of alteration minerals that are not exposed or apparent in this fidelity elsewhere in Nili Fossae.
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NE Syrtis bears extensive coverage of the Nili Fossae regional olivine-rich unit that is variably altered to carbonate. Mapped as the Fractured Unit, it is identified here from the contact with the Syrtis Major Volcanics to the southwest rim of Jezero crater. The Large Linear Features of the Fractured Unit are intriguing and enigmatic landforms.
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Their geomorphic features suggest that the olivine-rich melts were mobilized in a highlyfractured surface or near-surface crust, with contact metamorphism being suggested by the raised bounding ridges. Megabreccia textures are observed within NE Syrtis, both stratigraphically below and within the olivine-rich Fractured Unit. In the Plains region,
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large and spatially distributed megabreccia appear stratigraphically older than the Fractured Unit and, in the Depression region, clast-rich breccias sharply contact the
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Fractured Unit and appear to be stratigraphically cotemporaneous. Despite the
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compilation of additional orbital observations presented herein, the continuation of both a volcanic and impact melt emplacement hypotheses remains viable.
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The presence of Fe/Mg-smectites, kaolin-group minerals, carbonates, and
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sulfates distributed amongst three of our geomorphic units (Basement Unit, Fractured Unit, Feature-bearing Slope Unit, respectively) confirms initial work by Ehlmann and Mustard (2012) that as many as four temporally distinct aqueous environments existed in NE Syrtis. These three units span the Noachian-Hesperian and Phyllosian-Theiikian boundaries and are capped by the Syrtis Major flows, suggesting an aqueous history of at least 250 Myr and extending further into the Noachian past. Spectral signatures of a
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 55 kaolin-group mineral are observed in isolated patches within the Basement Unit and not as a distinct stratigraphic unit. Their location and geomorphic characteristics argue for formation via isolated leaching or heterogeneous initial crustal composition, rather than extended weathering profiles or magma precipitation. The aqueous history of the region
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extends from the Late Noachian into the Hesperian and, depending on the formation mechanism for these alteration minerals, may extend to the Late Hesperian or Amazonian.
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Perhaps no other location on Mars offers the diversity of geology, mineralogy, and geomorphology in such spatial proximity than NE Syrtis. The geomorphic diversity is revealed through the variety of landforms observed in the mapped geomorphic units, the fluvial valley cutting through the mapping area, the geomorphic remnants from
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impact and volcanic processes, and from the eons of denudation of the landforms and aeolian processes. The chronology of the region is relatively well constrained as it is
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bookended by the impact processes that resulted from the formation of the Isidis basin
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to the later emplacement of the Syrtis Major Volcanics. In close spatial proximity are igneous mineralogies spanning from potential primitive crust to those from later
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volcanism and impact processes, in addition to alteration mineralogies alluding to multiple distinct aqueous environments. The stratigraphically-bounded alteration
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minerals within NE Syrtis suggests the presence of multiple, potentially habitable (Ehlmann and Mustard, 2012), environments located in this region in the martian past. Understanding the detailed formation mechanisms of these alteration minerals, the geological history of the geomorphic units, and the chronology of active processes in this ancient terrain will greatly constrain the likelihood of these environments as being
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 56 conducive to life. As we turn our eyes to the next target for in situ exploration on the martian surface, no location offers better access of the gamut of geological processes active at Mars than Northeast Syrtis Major.
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Acknowledgments: We thank Chris Edwards, Bethany Ehlmann, Tim Goudge, Ralph Milliken, and Sandra Wiseman for insightful discussions on the geology of the Nili
Fossae, and Chris, Tim, and Sandra for producing data products analyzed in this study. The quality of this paper was greatly improved by detailed and constructive reviews
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provided by Sheridan Ackiss, Briony Horgan, and an anonymous reviewer. We also thank the Mars Reconnaissance Orbiter and Mars Odyssey teams for their data
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collection efforts.
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ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 70 Figure 1: Northeast Syrtis Major, Mars. North is up in all images. (a) The location of Northeast Syrtis Major at the nexus of the western edge of the Isidis basin, the northeastern Syrtis Major volcanic province, and the southeastern extent of the Nili Fossae. The mapping area is delineated by the black rectangle, and (b) is indicated with
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the dashed white rectangle. The background is MOLA colored topography overlain on the ~100 m/pixel THEMIS global daytime infrared mosaic. (b) Local context for the
mapping area (black rectangle). Jezero crater is immediately to the northeast, and the Syrtis Major volcanic province begins at the south and west of the mapping area. To the
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north is the watershed for the western fan of Jezero crater (Goudge et al., 2015). The background is the THEMIS mosaic as in (a), and the location of 1c is indicated by the dashed white rectangle. (c) Northeast Syrtis Major. The mapped area (black rectangle)
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is divided into three regions: the Plains, the Depression, and the Syrtis Major Volcanics. The unnamed 6 km crater discussed in the text is the topographic depression observed
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in the top-left quadrant. The basemap is a CTX-derived DEM overlain on a mosaic of
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Table S2.
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CTX images. The CTX images comprising the mosaic are listed in the supplementary
Figure 2: HiRISE and CRISM coverage of Northeast Syrtis Major. (a) Blue polygons outline the extent of HiRISE image coverage of the mapping area (black rectangle). In these regions, the geomorphic mapping was performed using the HiRISE imagery. In the remaining regions, the mapping was performed using the CTX mosaic, which is seen here below the blue polygons. The CTX and HiRISE images utilized in this
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 71 analysis are listed in the supplementary Table S1 and Table S2, respectively. (b) Orange polygons display the footprints of CRISM images used in our analysis and their coverage of the mapping area (back rectangle). The CRISM images utilized in this
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analysis are listed in the supplementary Table S3.
Figure 3: Stylized stratigraphic column of Northeast Syrtis Major. Relative relationships
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of the geomorphic units are shown, as well as the stratigraphic location of select
geomorphic subunits. Depicted are the Basement Unit (BAU) and Raised Linear Ridges (RLR) subunit, the Fractured Unit (FRU) and Large Linear Feature (LLF) subunit, the
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Capping Unit (CAU), the Feature-bearing Slope Unit (FSU) and Raised Boxwork Ridges (RBR) subunit, and the Syrtis Major Volcanics (SMV) unit. See main text and Table 2 for
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description of units. Colors and legend correspond to that of the geomorphic map
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(Figure 4).
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Figure 4: (a) Geomorphic map of Northeast Syrtis Major. Colors and map symbols for (a) and (b) correspond to the legend as shown in (c). See main text and Table 2 for description of units. Black lines indicate mapped contacts between geomorphic units while dashed black lines are interpreted contacts. The latter are exemplified by contacts that disappear and reemerge from aeolian cover, contacts characterized at HiRISE resolution (~0.25 m/pixel) but mapped in lower-resolution CTX images (~7 m/pixel), and
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 72 contacts that were mapped with the guidance of map-projected CRISM images. (b) Geomorphic map as in (a) but transparent and overlain on the CTX mosaic (CTX images used are listed in Table S1). HiRISE images (listed in Table S2) are rectified to and overlain on the CTX mosaic. Locations are indicated for subsequent figures shown
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throughout the paper. Note that Figure 11a continues further north than indicated by the white rectangle. (c) Geomorphic map legend depicting color and symbols of the mapped units and relative stratigraphic positions. Geomorphic subunits and minor units in the
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paper are shifted to the right in the legend. North is up in both images.
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Figure 5: The Plains of Northeast Syrtis Major. (a) As in Figure 4b, geomorphic map overlain on a mosaic of HiRISE images rectified and overlain on the CTX mosaic. This
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focus view of the Plains region shows the fine-scale geomorphic heterogeneity. The overall stratigraphic sequence of Basement Unit, Fractured Unit, and Capping Unit can
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be observed. Large Linear Features are observed bifurcating at distinct angles. (b)
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Corresponding area to (a) without the geomorphic map overlain. Locations are indicated for subsequent figures shown throughout the paper. The spatial scale of (b) is the same
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as (a). North is up in both images.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 73 Figure 6: Geomorphic example of the Crustal Mounds (CMD) of the Basement Unit (BAU). (a) Observed are high-standing mounds with a peak, plateau, or elevated ridge from which lineations on the slopes fan away. The CMD are embayed, and here surrounded, by the Fractured Unit (FRU). Subframe of HiRISE image
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ESP_016219_1980 with geomorphic unit contacts drawn. The arrow shows the location for the numerator spectrum used for the CRISM ratioed reflectance spectrum (g). (b-c) Geomorphic example of the Knobby Plains (KNP) of the BAU. (b) Geomorphic map subframe depicting the KNP and the complex relationship with the Capping Unit and
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FRU. (c) HiRISE mosaic subframe of corresponding area to (b) with unit contacts
drawn. Circular light-toned features are observed in the plains unit (arrow i) that may be indicative of megabreccia rounding or light-toned material infilling small circular
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depressions. The transition between the visibly discernable plains surface textures is shown (arrow ii); the lowermost is a darker-toned plain, and meters above this is a
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smoother, lighter-toned plain. The transition between the KNP and CMD in this scene bears strong spectral signatures for a kaolin-group mineral (arrow iii; also showing the
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location for the numerator spectrum used for the CRISM ratioed reflectance spectrum in
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Figure 7a). North is up in all images. (d-g) Laboratory and CRISM ratioed spectra indicative of the dominant spectra observed in the BAU. The saponite spectrum (d) is
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compared here with the narrow absorptions centered at ~1.4, 1.9, and 2.3 μm, indicative of Fe/Mg-smectites, collected from the BAU (e). The laboratory low-Ca pyroxene spectrum (f) matches the spectrum collected from the BAU with (g) broad absorption centered at ~2 μm and second dip in the VNIR towards broad absorption short of ~1 μm. The saponite (LASA53) and orthopyroxene (C2PE30) spectra are taken
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 74 from the CRISM spectral library (CRISM Science Team, 2006). Vertical lines are
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located at 1.4, 1.91, and 2.3 μm.
Figure 7: Geomorphic context for the kaolin-group mineral spectral signatures. (a) A kaolin-group mineral is identified in CRISM ratioed spectra by absorptions at ~1.4, 1.9, and 2.2 μm. The laboratory kaolinite spectrum (top; LAKA02), taken from the CRISM
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spectral library (CRISM Science Team, 2006), is shown in comparison to the ratioed CRISM spectrum (bottom). Vertical lines are located at 1.41, 1.91, and 2.2 μm. (b-c) The kaolin-group mineral spectral signatures are observed in light-toned, isolated
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patches in the Basement Unit (BAU). These locations display light-toned features on the surface with no internal fabric at the highest resolution apart from dark-toned fractures
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and cover. The outcrops bearing these signatures commonly have sharp contacts with
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neighboring features in the BAU. North is up in both images.
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Figure 8: Mosaicked THEMIS decorrelation stretch of Northeast Syrtis Major, with a redgreen-blue combination of band 8 (11.79 μm), band 7 (11.04 μm), and band 5 (9.35 μm), respectively. The image displays the mapping area (cf. Figure 8 with Figure 4). The contacts of the mapped geomorphic units are overlain. The Basement Unit (BAU) and its geomorphic subunits are predominately green (a). Regions mapped as Fractured Unit (FRU) (b) display a predominately purple color, corroborating the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 75 enrichment in olivine observed by CRISM. The Capping Unit (CAU) displays a greenyellow color with isolated spots of purple (c). Outcrops of the Capping Unit (CAU) in the Depression region bearing significant dark-toned cover display an orange and green color (d). The Syrtis Major Volcanics (SMV) (e) display an equal distribution of all the
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colors (green, blue, and magenta), with the exception of orange. The Feature-bearing Slope Unit (FSU) displays an overall equal distribution of colors with minor
enhancements of yellow (f), but this unit also displays strong isolated concentrations of purple (g) correlating to mounds high-standing above the smooth cover. The
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Undifferentiated Terrain (UNT) unit displays a patchy mosaic of purples and greens with
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coherent features a few pixels in size (h). North is up in the image.
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Figure 9: The Fractured Unit (FRU). (a) Geomorphic map transparent and overlain on HiRISE image ESP_016786_1980. The subset of geomorphic map symbols and colors
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visible in this frame are included. The locations of (b) and (c) are indicated by the black
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rectangles and are subframes of HiRISE image ESP_016786_1980. (b) Geomorphic example of the FRU. Light-toned blocks of various sizes are seen closely packed in a
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darker supporting material. The dark-toned material forms polygonal shapes around the light-toned block, giving this unit a fractured appearance. (c) Geomorphic example of the FRU and Smooth-Sloped Mounds (SSM). The SSM are stratigraphically-equivalent variants on the Capping Unit (CAU) that bear a dark-toned mantle on their smooth, gradual, slope. A deposit of the SSM is observed in the lower-left of this subframe and is interpreted to be a CAU bearing a dark-toned cover. North is up in all images.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 76 Figure 10: Geomorphic examples of banding of the Fractured Unit. (a) Context for the banding of the Fractured Unit (FRU) concentric to the Capping Unit (CAU). Subframe of a HiRISE mosaic constructed from images listed in Table S2. (b) Banding observed within the FRU meters in width and approximately concentric to the contact with the
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CAU. This banding appears to occur on a relatively flat surface. (c) Context for the
banding of the Fractured Unit with a layered appearance. Subframe of HiRISE image ESP_027902_1980. (d) Alternating light- and dark-toned bands of the Fractured Unit of non-uniform thickness and displaying a wavy appearance. (e) Outcrop of the Fractured
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Unit displaying a layered appearing on a scarp face 10s of meters in height. North is up
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in all images.
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Figure 11: Draping of the Fractured Unit. (a) Subframe of a Digital Elevation Model (DEM) made using HiRISE stereo images ESP_024513_1980 and ESP_025370_1980.
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The DEM is colorized so that reds indicate high elevations and blues low elevations.
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Profiles a-a’ and b-b’ are indicated, and elevation data extracted for these profiles are the basis of the cross sections of (b). North is up in the image. (b) Colorized and
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interpreted cross sections corresponding to HiRISE DEM topography geographically correlated with profiles a-a’ and b-b’. The mesa-forming expressions of Capping Unit (CAU; red) and Fractured Unit (FRU; green) display a draping nature where the units are observed to rise up multiple slopes of a Basement Unit (BAU; blue) depression, as shown in the cross sections. The draping nature is further demonstrated by the inability
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 77 to fit a flat plane along the mapped contact between the FRU and the CAU. Colors
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correlate with those of the geomorphic map in Figure 4.
Figure 12: CRISM ratioed reflectance spectra form the Fractured Unit. Laboratory reflectance of (a) magnesite (KACB03A), (b) saponite (LASA53), and (c) fayalite
(C1PO58) taken from the CRISM spectral library (CRISM Science Team, 2006) are
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included for comparison. (d) Observed spectral signatures are consistent with a mixture of olivine (broad ~1 μm) and carbonate (paired ~2.3 and 2.5 μm). The ~1.9 μm absorption is indicative of either hydrous Mg-rich carbonate or anhydrous carbonate and the admixture of a hydrated silicate (e.g., Fe/Mg-smectite). (e) Spectra are consistent
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with Mg-rich carbonate with paired absorptions at approximately 2.31 and 2.51 μm or
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2.32 and 2.52 μm, though band minima approach paired positions at 2.33 and 2.53, indicative of possible Ca in the crystal structure, such as a dolomite (Hunt and
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Salisbury, 1971). Vertical lines are located at 1.91, 2.31, 2.39, and 2.51 μm.
Figure 13: The Large Linear Features (LLF). (a) Geomorphic map subframe, as in Figure 4, depicting the LLF. The LLF seamlessly join with the Fractured Unit, and the orthogonal and triple junctions formed by the LLF are visible. (b) HiRISE mosaic subframe of corresponding area to (a). Images are subframes of the mosaic of HiRISE images listed in Table S2, and is the background of (a). The location of (c) is shown in
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 78 the white rectangle. (c) Focus view of the LLF depicting the light-toned and blocky material of the LLF in a dark-toned matrix. (d) Alternate view of the LLF depicting the topographic variability with the geomorphic unit contacts overlain on a colorized HiRISE stereo-derived DEM from image pair ESP_016575_1980 and ESP_016509_1980. The
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LLF supporting an upsection Capping Unit is observed at topographic highs, but the LLF are also observed recessed into the Basement Unit and present in topographically low
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troughs. North is up in all images.
Figure 14: The raised bounding ridges of the Large Linear Features (LLF). Raised linear
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features are observed along the contact between the LLF and the downsection Basement Unit (BAU). (a-c) The bounded LLF bear a center of light-toned blocky
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material with a darker supporting material, and the bounding features are lighter in tone and form a sharp contact with the BAU. Three profiles are identified and correspond to
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the profiles in (d). Images are subframes of the mosaic of HiRISE images listed in Table
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S2, and north is up in all images. (d) Topographic profiles perpendicular to the LLF with raised ridges measured from HiRISE DEMs derived from stereo image pairs
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ESP_016219_1980 and ESP_016364_1980, and ESP_016575_1980_ESP 016509_1980. The profiles show the bounding ridges to be meters in height when the LLF are high-standing above the BAU. The topography drops towards the center of the LLF, but the center remains a few meters above the level of the host unit.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 79 Figure 15: Mesa-forming expressions of the basal Fractured Unit (FRU) and the upper Capping Unit (CAU). (a) These units are mapped throughout the study area with the FRU more spatially extensive than the CAU. Geomorphic map overlain on a subframe of HiRISE image ESP_016443_1980. (b) Subframe of HiRISE image
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ESP_016443_1980, with corresponding area to (a), depicting the local context. The crater-preserving CAU is observed as well as the separation into small mesas. The location of (c) is indicated by the white rectangle. The approximate vantage point for Figure 16a is shown with the white ‘>’ symbol. (c) The layered nature of the CAU. Here,
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the top crater-preserving layer of the CAU is clearly distinguished from the lower lighttoned, fractured, and blocky layer. The lowermost outer rim, beneath the light-toned,
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fractured CAU layer, is the olivine-carbonate-bearing FRU. North is up in all images.
Figure 16: Mesa-forming expressions of the basal Fractured Unit (FRU) and the upper
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Capping Unit (CAU). (a) HiRISE DEM perspective view produced from stereo pair
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images ESP_015942_1980 and ESP_016443_1980. The Fractured Unit (green) and Capping Unit (red) are outlined and colored with their corresponding map unit color in
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(a) and (b). The HiRISE DEM topographic profile A–A’ is shown, and an interpretive stratigraphic cross section of this profile is shown in (b). The vantage point for this image is shown on Figure 15b. (b) The two characteristics of the CAU include a craterretaining upper surface and a lower layer of light-toned blocks bearing boulder-shedding slopes. The olivine-carbonate-bearing FRU is stratigraphically between this unit and the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 80 phyllosilicate-bearing Basement Unit (BAU). The contacts in this two-dimensional profile
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are interpreted and illustrative.
Figure 17: The quasi-circular mesa-forming expressions of the Capping and Fractured Units. Subframes of a HiRISE mosaic constructed from images listed in Table S2. Numerous occurrences display a quasi-circular appearance formed by both the
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Fractured Unit (FRU) and Capping Unit (CAU). (a-d) Quasi-circular exposures of
approximately equal spatial extent. The local context of (c) can be observed in Figure 10a. (e) Quasi-circular shapes are common at the scale of a few 100 meters, but
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expressions on the scale of several kilometers also display approximately equal
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perpendicular diameters and some rounded edges. North is up in all images.
Figure 18: The Dark-Toned Fractured Unit (FRU-DT). (a) The crater-preserving surface
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of the Capping Unit (CAU) is observed in the Depression region with a darker
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appearance than on the Plains region. The CAU transitions to a smooth, dark surface with the light-toned, fractured, and blocky appearance of the FRU–DT observed at the outer edges. The location of (b) is indicated with the white rectangle. (b) The outer reaches of the FRU-DT. Here the light-toned, fractured, and blocky component of the FRU is observed at a contact with the Basement Unit. Away from the contact and
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 81 towards the center of the FRU-DT is an increasing dark-toned cover. North is up in both
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images.
Figure 19: Geomorphic example of the Raised Linear Ridges (RLR). (a) Areas mapped as RLR bear ridges meters in width, hundreds of meters in length, and are highstanding above a relatively smooth basement. Subframe of HiRISE image
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ESP_038029_1980 overlain by a corresponding subframe of the geomorphic map
(Figure 4). North is up in the image. (b) As in (a), but without the geomorphic map. The RLR are observed stratigraphically below the Fractured Unit (FRU) and are categorized
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as a subunit of the Basement Unit. (c) CRISM ratioed reflectance spectrum (d) collected of the RLR through a FRU erosional window. Narrow absorptions observed centered at
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~1.4, 1.9, and 2.3 μm are indicative of Fe/Mg-smectites. A weak absorption at ~2.39 μm may be present, and can be indicative of Fe/Mg-smectites or phyllosilicates, including
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saponite (e) or talc (f). The saponite spectrum (LASA53) taken from the CRISM spectral
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library (CRISM Science Team, 2006), and the talc spectrum (GDS23) is from the USGS digital spectral Library (Clark et al., 2007). Vertical lines are located at 1.41, 1.91, 2.31,
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and 2.39 μm.
Figure 20: Breccia-bearing terrain in the Depression region. (a) Large megabreccia blocks 10s of meters in size are observed at a flat surface as evidenced by small
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 82 boulders casting shadows (arrow i). Some blocks display an internal texture suggestive of layering or banding present prior to disruption via impact (arrow ii). One breccia block appears to have been offset ~10 m by faulting (arrow iii). (b) Smaller breccia blocks are observed in a clast-rich outcrop that sharply contacts the olivine-rich Fracture Unit (light-
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toned fractured outcrops surrounding the clast-rich breccia). (b) is to the southwest of (a) showing the northwest trend of breccia blocks become larger and more spread out in the dark supporting material. Subframes of HiRISE image ESP_018988_1980. North is
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up in both images.
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Figure 21: The Syrtis Major Volcanics (SMV) unit, Feature-bearing Slope Unit (FSU), and Undifferentiated Terrain (UNT). (a) Subframe of the CTX mosaic (Table S1)
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displaying the amphitheater heads and subtle channels leading towards the Depression region. The Raised Boxwork Ridges (RBR) of the FSU are visible where polygonal
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networks of ridges meters in width are stratigraphically below the SMV and within the
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FSU. (b) CRISM laboratory spectra of the Feature-bearing Slope Unit. Laboratory spectra are of (c) polyhydrated sulfate (799F366), (e) jarosite (C1092F53), and (f)
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natrojarosite (LASF28A) taken from the CRISM spectral library (CRISM Science Team, 2006). CRISM spectra of this unit match absorptions at 1.4 and 1.9 µm and an inflection at 2.4 µm (d) matching the shape of polyhydrated sulfates. An absorption at 2.26 µm (vertical line) is variably observed in this unit (d and g). If paired with an asymmetrical 2.21 µm absorption, jarosite would be the matching spectrum, but this absorption, if present, is on order of the noise. As a result, natrojarosite also matches absorptions in
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 83 this spectrum, suggesting spatial variability of the sulfate minerals present in this unit as jarosite has been identified in nearby outcrops by Ehlmann and Mustard (2012). The spectral shape and absorption at ~1.4 and 1.9 µm, respectively, suggest that these spectra may not be representative of a single phase, and that multiple sulfates or other
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minerals including phyllosilicates may be intermixed, or alternatingly stratified in these
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ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 104 Table 1. Compendium of Running Hypotheses Applicable to Northeast Syrtis Major
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Impact melt or volcanics mobilized on a fractured surface; Dyke-fed volcanism
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Large Linear Features emplacement
Carbonate formation
References Hamilton et al., 2003; Hoefen et al., 2003; Hamilton and Christensen, 2005; Mustard et al., 2005, 2007, 2009; Tornabene et al., 2008; Poulet et al., 2009 this work Fractured Unit, Large Linear Features Ehlmann et Fractured al., 2008; Unit Murchie et al., 2009; Mustard et al., 2009; Brown et al., 2010; Ehlmann and Mustard, 2012; Viviano et al., 2013; Edwards and Ehlmann, 2015
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Feature and Process Hypotheses Olivine-rich unit emplacement Basaltic or picritic lava flows; Impact melt from Isidis basin formation
Relevant Northeast Syrtis Major Geomorphic Unit(s) Fractured Unit
Hydrothermal alteration of the ultramafic host rock at slightly elevated temperatures; Contact metamorphism; Precipitation from transitory shallow lakes; Weathering of olivine-rich rocks
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 105 Surface weathering; Hydrothermal alteration in the shallow crust; Magmatic precipitation; Impact origin; Alteration by supercritical fluids during magma ocean
Kaolin-group mineral formation
Pedogenic-type leaching; Hydrothermal alteration; Heterogeneous initial composition Serpentinization of the olivinerich unit in a hydrothermal environment; Contact metamorphism set up by emplacement of hot olivine-rich rocks; Deep burial by Syrtis Major lavas
ElkinsTanton, 2008; Ehlmann et al., 2009, 2011; Gaudin et al., 2011; Ehlmann et al., 2011; Meunier et al., 2012; Tornabene et al., 2013; Cannon et al., 2016 Ehlmann et al., 2009; Gaudin et al., 2011 Brown et al., 2010; Ehlmann et al., 2010; Michalski and Niles, 2010; Viviano et al., 2013 McLennan et al., 2005; Wray et al., 2011; Ehlmann and Mustard, 2012; Quinn and Ehlmann, 2014b Mustard et al., 2007, 2009; Tornabene
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Crustal phyllosilicate formation
Altered layered volcanic flows; Altered ash flows or falls; Lacustrine groundwater and evaporite sediments
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Sulfate formation
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Megabreccia emplacement
Isidis basin megaregolith; PostIsidis impact gardening; Zones of heterogeneous composition; Igneous intrusions
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Fractured Unit
Featurebearing slope Unit
Basement Unit, Fractured Unit
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 106 et al., 2013
Mineralized fracture zones; Breccia dykes and crater-related faults; Sedimentary filled volumeloss fractures
Fluvial feature formation
Formation with regional fluvial activity; Formation with regional glaciation
Head and Mustard, 2006; Saper and Mustard, 2013; Quinn and Ehlmann, 2014b Skok and Mustard, 2014; Souček et al., 2015; this work Elkins‐ Tanton et al., 2005; Skok et al., 2012; Grott et al., 2013
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Formation and exposure of primitive crust
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ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 107 Table 2. Geomorphic Units Mapped in Northeast Syrtis Major
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Crater – Young
Crater with circular rim and a discernab le ejecta blanket obscuring contacts of other geomorp hic units.
THEMIS Therma l Inertia
Modeled Effective Particle Sizes
Unit Interpr etatio Relevant n Figure(s) Surfici al debris, mobili zed by aeolia n proces ses in a genera l eastwest motio n (down slope towar ds Isidis Basin). Geolog icallyrecent crater forme d subseq uent to the extensi ve region al erosio n.
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Interpret ed CRISM Signature s
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Aeolian Debris Cover
Patches of dune fields obscuring contacts of other geomorp hic units. Dune crests track approxim ately northsouth.
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Unit Namea
Unit Descripti on
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Unit Map Symb ol
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 108
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CE AC FSU
highCa 238.9 pyroxe 8 +/ne 11.15
~200 µm
304.6 4 +/polyhy 30.11 drated (high sulfate TI) / , 207.4 jarosit 3 +/e, 11.39 natroja (low rosite TI)
~560 µm (high TI) / ~100 µm (low TI)
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Syrtis Major Volcani SMV cs Unit
Elevate d, flat, relative ly smooth surface preserv ing craters. Steep slopes off of the SMV variabl y exposi ng lighttoned layers, crustal knobs,
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Lava flows from the Syrti s Majo r volca nic provi nce halti ng in the vicini ty of Nort heast Syrti s Majo r. 3, 21 Layer ed sedi ment s depo sited in an aque ous or volca nic syste 3, 21
Feature -bearing Slope Unit
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 109
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— Raised boxwork ridges
Raised ridges meters wide and 10s-100s meters long in a boxwork texture highstanding above a smooth basement with intersper sed lighttoned outcrops.
m, unde rwen t sulfa te preci pitati on, then erod ed and cover ed givin g smoo th appe aran ce.
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or FRU highstandin g above the smooth cover.
Highstandi ng ridges forme d as filled volum e-loss fractur es (Quinn and Ehlma nn, 2014b) .
3, 21
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 110
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Volca nic flow or ashfall depo sit (volc anic or impa ctgene rated ). Lacki ng abun dant Fe2+ in mafic mine rals or poorl y cryst alline and/ or glass y. Curre nt ~375 µm expr
AC
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Unit with an upper most craterpreserv ing surface and a variabl yexpose d lower layer of lighttoned fractur unrem ed arkabl blocks. e
CAU
Capping Unit
276.9 +/18.5
3, 5, 6, 9–11, 13, 15– 18
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 111
AC
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essio n gove rned by exte nt and mech anica l prop ertie s of unde rlyin g FRU.
SSM
— Smoothsloped mounds
CAU variant lacking craterpreservat ion, and bearing smooth, gradual, darkertoned slopes.
high-Ca pyroxene
Varian t of CAU with weake r mecha nical proper ties or erosio nal history produc ing smoot h slopes and covere d with darktoned highCa
5, 9
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 112 pyroxe nebearin g deposi ts.
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Corrug ated surface of lighttoned, fractur ed blocks surrou nded by darktoned materi al. Sparsel y cratere d. Variabl e bandin g.
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A coars elycryst alline olivin erich unit empl aced as impa ct melt or volca nism, rapid ly cover ed by the CAU. It has been varia bly alter ed to carb 3, 5, 6, onat 9–13, e. 15–19
FRU
Fracture d Unit
olivine, Mgrich carbon ate, ~1.9 µm hydrati on feature , Fe/Mg- 420.2 smectit 9 +/e 20.74
> 1 mm
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 113
AC
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Olivine -rich materi al forme d into linear featur es via mobili zation within crust or at the surfac e during empla cemen t as dykes or mobili zation of an impact -melt sheet. Meta morph ism along contac t genera ted erosio nresista nt walls now highstandi ng followi ng
LLF
— Large Linear Features
Linear features 10s-100s meters wide and 1000s of meters long compose d of lighttoned fractured blocks surround ed by darktoned material.
3, 5, 13, 14
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 114
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erosio n.
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Baseme nt Unit
AC
CE
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BAU
low-Ca pyroxe ne, Fe/Mgsmectit e, kaolingroup 255.1 minera 1 +/l 18.05 ~260 µm
CMD
— Crustal Mounds
High relief peak, plateau, or elevated ridge from which lineations fan away.
3, 5–7, 9, 11, 13–16 Remna nts of ancien t crust. Expres sion govern ed by topogr aphy forme d by Isidis impact and distrib ution of FRU. Highly erode d and variabl y covere d with
5, 6, 9, 13, 15
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 115 debris.
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Highstandi ng ridges forme d throug h the mobili zation and crystall ization of fluids, with lighttoned phyllos ilicatebearin g outcro ps expose d on the floor in betwe en. 3, 19 Expans ive plains with numer ous mound s of ancien t 5, 6, 9, crustal 13,15
PT
ED
Rectilinea r features highstanding above smooth basement . Meters in width and 100s of meters in length. Expansive smooth surface with many intersper sed knobs, mounds, and ridges.
CE
AC
RLR
— Raised Linear Ridges
KNP
— Knobby Plains
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 116
AC
CE
PT
ED
M
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materi al, erosio nal debris, and isolate d FRU remna nts. Patchy erosio n produc t distrib ution. Surfac e exposi ng ancien t basem ent. Large blocks likely megab reccia or accum ulation and erosio n of materi als when showin ga circula r appear ance. Planar flows 5, 9, 13, of 15
SMP
— Smooth Plains
Smooth, darktoned surface. Heteroge neous internal structure. Abundant , but dispersed , decamete r lighttoned blocks.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 117
AC
CE
PT
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LCM
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— Large Crustal Mounds
Similar morpholo gy to CMD but with elevation s of 100s of meters.
CRE
Crater – Eroded
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volcani c or erosio n produc ts may be presen t. Moun ds forme d nearconte mpora neousl y with Isidis. Heavil y erode d but mainta ining high topogr aphy. Ancien t craters forme d during the early heavy bomba rdmen t. Subse quentl y erode d and slightly defor med by
Crater of significan t size (≳ 1km) with no discernab le eject blanket dominati ng the local morpholo gy.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 118
AC
CE
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Mixed expressio ns of the above units. Unit contacts below Undifferen CTX tiated resolutio UNT Terrain n. a Geomorphic units are indicated in bold, subunits of the geomorphic units are indicated in italics, and minor units not significant to the text are indicated in plain letters.
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minor tectoni c move ments.
21
The Geological History of Northeast Syrtis Major, Mars Michael S. Bramble , John F. Mustard , Mark R. Salvatore PII: DOI: Reference:
S0019-1035(16)30349-9 10.1016/j.icarus.2017.03.030 YICAR 12424
To appear in:
Icarus
Received date: Revised date: Accepted date:
2 July 2016 28 March 2017 30 March 2017
Please cite this article as: Michael S. Bramble , John F. Mustard , Mark R. Salvatore , The Geological History of Northeast Syrtis Major, Mars, Icarus (2017), doi: 10.1016/j.icarus.2017.03.030
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 1 Highlights
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We use HiRISE observations to characterize and map geomorphic units at Northeast Syrtis Major, Mars. Mineralogy and physical properties of the geomorphic units are characterized with CRISM and THEMIS. Stratigraphy unifiable with 5 geomorphic units despite heterogeneity at decameter scale. We identify intriguing large linear features and circular landforms of the olivine-rich unit. Synthesizing mapping results with the literature, a detailed geological history is constructed.
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ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 2 History of Northeast Syrtis Major, Mars Michael S. Bramble a,*, John F. Mustard a, and Mark R. Salvatore b a
Department of Earth, Environmental, and Planetary Sciences, Brown University,
b
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Providence, RI Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI
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*Corresponding Author Contact Information: Michael S. Bramble
Department of Earth, Environmental, and Planetary Sciences
324 Brook Street, Box 1846
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Providence, RI 02912, USA
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Brown University
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Email: [email protected]
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Third revision date: 16 March 2017 Second revision date: 16 January 2017
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First revision date: 16 November 2016 Original submission date: 1 July 2016 Pages: 82 – Figures: 21 – Tables: 2 (4 supplementary) – References: 101 Keywords: Mars, geomorphology, mineralogy, spectroscopy
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 3 Abstract As inferred from orbital spectroscopic data, Northeast Syrtis Major bears considerable mineral diversity that spans the Noachian-Hesperian boundary despite its
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small geographic area. In this study we use observations from the High Resolution Imaging Science Experiment camera, supplemented with Context Camera imagery, to characterize and map the lateral extent of geomorphic units in Northeast Syrtis Major, and constrain the geomorphic context of the orbital-identified mineral signatures. Using
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recent observations, we confirm previous mineralogy identified with the Compact
Reconnaissance Imaging Spectrometer for Mars, and greatly extend the lateral extent of visible to near-infrared investigation utilizing the greater coverage. Analysis of Thermal Emission Imaging System observations reveals further physical properties and
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distribution of the geomorphic units. The stratigraphy, which spans the NoachianHesperian boundary, displays significant morphological heterogeneity at the decameter
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scale, but it is unifiable under five distinct geomorphic units. Our paired morphological
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and mineralogical analysis allows us to construct a detailed geological history of Northeast Syrtis Major. Several geological events that occurred in Northeast Syrtis
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Major—including the formation of the post-Isidis crust, the emplacement of an olivinerich unit, the formation of sulfate minerals, and the emplacement of the Syrtis Major
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Volcanics—can be related to regional and global processes constraining the local chronology. Other mineralogical indicators, particularly the formation of Alphyllosilicates, are difficult to place in the temporal sequence. They are observed in isolated patches on the post-Isidis crust, not as a distinct stratigraphic unit as observed elsewhere in Nili Fossae, suggesting their formation via isolated leaching or through
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 4 alteration of initial compositional heterogeneities within the crust. Exposures of an olivine-rich unit are intermittently observed to form quasi-circular landforms, suggestive of emplacement in circular depressions, which may indicate a period of cratering between the formation of the Isidis basin and the deposition of the olivine-rich unit. We
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identify and discuss intriguing large linear features of the olivine-rich unit, reminiscent of dyke-fed volcanism, that have raised bounding ridges suggestive of contact
metamorphism with the crust. We compile, review, and discuss many of the outstanding questions and running hypotheses relevant to our mapping area. A synthesis of our
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geomorphic mapping with recent literature reveals a well-defined geological history with extensive aqueous activity at Northeast Syrtis Major that is amassed in a stratigraphic sequence spanning a time likely greater than 250 million years of geological history. Our
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geomorphic and spectral analyses confirm that Northeast Syrtis Major exhibits considerable geomorphic and mineralogic diversity within a relatively small geographic
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area that is representative of the geologic processes occurring throughout the broader Nili Fossae region during the Noachian and Hesperian. Northeast Syrtis Major adds to
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this sequence by exposing the diverse environmental history of this region as observed
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through the presence of alteration minerals not present in this fidelity or proximity
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elsewhere in Nili Fossae.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 5 1. Introduction At the nexus of the northwestern border of the Isidis basin and the northeastern extent of the Syrtis Major volcanic province lies the region informally known as
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Northeast Syrtis Major (Figure 1; ~ 17–18 °N and 76–77 °E), a diverse martian landscape with varied geology and mineralogy that spans the Late Noachian and Early Hesperian boundary (Ehlmann and Mustard, 2012). At Northeast Syrtis Major
(henceforth NE Syrtis), four distinct aqueous environments were inferred via the
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presence of orbital-identified minerals: shallow crustal Fe/Mg-phyllosilicate-bearing terrains, carbonate-bearing olivine-rich terrains, Al-phyllosilicate-bearing terrains, and layered sulfate-bearing terrains (Mustard et al., 2007, 2009; Ehlmann et al., 2009; Ehlmann and Mustard 2012). The stratigraphy of key mineral-bearing strata are
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interpreted to show a trend from early neutral-pH water environments in the Late Noachian to acidic conditions in the Early Hesperian (Ehlmann and Mustard, 2012),
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though the paradigm of alkaline mineralogy inferred from Fe/Mg-smectites has recently
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started to be questioned as these minerals have been shown to form at mildly acidic pH conditions (e.g., Peretyazhko et al., 2016). Previous investigators examined the
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mineralogy and stratigraphy at a regional scale (e.g., Ehlmann et al., 2009; 2011; Ehlmann and Mustard, 2012), but this left unexamined important geomorphic
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characteristics and stratigraphic relationships that can only be revealed through detailed orbital analyses at the meter to decameter scale. Here, using the highest spatial resolution data available, we investigate the geomorphic context of the previously identified orbital mineral signatures to further our understanding of the relative age relationships and geomorphic textures. Our objective
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 6 is to investigate the lateral extent of the mineral stratigraphy first identified in the area by Ehlmann and Mustard (2012) over the broader NE Syrtis region, and observe how the morphology and composition of units identified in NE Syrtis varies spatially and through time by correlating and characterizing multiple exposures throughout the region at a
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high level of detail. NE Syrtis has a complicated but rich geological history that is
expressed over a large area (~2500 km2) with a unique sequence of units. We aim to correlate stratigraphic contacts and mineralogy mapped in NE Syrtis to the surrounding Nili Fossae, including the Jezero crater watershed, as the spatial proximity of these
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analyses to our study region provide valuable points of comparison. Lastly, we aim to construct a detailed geologic history of NE Syrtis using the most comprehensive mapping of the region to date. Unraveling this history through the detailed, high-
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resolution analysis of the morphology and mineralogy presented here will further our understanding of the Late Noachian to Early Hesperian transition, and the context of NE
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Syrtis in the geological history of Mars.
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2. Background
The age of the key geomorphic units around NE Syrtis are reasonably well
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constrained due to two stratigraphic markers: the Isidis basin and the Syrtis Major
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volcanic province. The region lies near the 1500 km ring of the Isidis impact basin (Frey et al., 2000), halfway between the 1100 and 1900 km diameters rings (Schultz and Frey, 1990), and the crater size-frequency distribution of impacts on the basin and ejecta generates a formation age of 3.96 Ga (Werner, 2005). An olivine-rich unit lies directly on the terrain excavated by the Isidis basin-forming event, establishing its age as contemporaneous with or immediately post-Isidis (Mangold et al., 2007; Mustard et
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 7 al., 2007). The younger stratigraphic marker is the volcanics from Syrtis Major capping layered rocks that are sulfate-bearing. Crater retention ages date the lavas to the Early Hesperian, with an age range between ~3.5–3.8 Ga (Hiesinger and Head, 2004; Ivanov et al., 2012). Additionally, the formation of the Nili Fossae fractures occurred after both
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the formation of the Isidis basin and the emplacement of the regionally-extensive
olivine-rich unit, and before the emplacement of the Syrtis Major volcanic province
(Mustard et al., 2009). This is well constrained as the fractures cut the olivine-rich unit but are embayed by the Syrtis Major volcanic province (Mustard et al., 2009). Therefore,
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approximately 250 Myr of geological history is recorded between these two stratigraphic markers.
The regionally-extensive olivine-rich unit, which is well exposed in NE Syrtis, has
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been studied with multiple orbital spectroscopic instruments including the Thermal Emission Spectrometer (TES) (Hamilton et al., 2003; Hoefen et al., 2003; Tornabene et
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al., 2008), the Thermal Emission Imaging System (THEMIS) (Hamilton and Christensen,
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2005; Tornabene et al., 2008), l’Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) (Mustard et al., 2005; Mustard et al., 2007; Ody et al., 2013), and
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the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) (Mustard et al., 2009). Two hypotheses have been proposed for the emplacement mechanism of the
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Nili Fossae olivine-rich unit. The first hypothesis is that the olivine-rich unit was emplaced as either basaltic volcanic flows in-place when Isidis formed (Hamilton and Christensen, 2005) or as picritic lava flows emplaced subsequent to the Isidis basin formation (Tornabene et al., 2008). The clear stratigraphic context for the olivine-rich unit emplaced post-Isidis precludes a pre-Isidis intrusive igneous origin (Hoefen et al.,
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 8 2003). The second hypothesis is that the unit comprises impact melt produced during the formation of the Isidis basin (Mustard et al., 2007, 2009). Supporting evidence includes the observation that the spatially-extensive unit ranges across ~3 km of elevation (Hamilton and Christensen, 2005), drapes the post-Isidis topography, and is
which are morphologically consistent with impact melt.
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distributed across the ring structures of the Isidis Basin (Mustard et al., 2007), all of
The olivine-rich unit is of particular significance because of its high olivine content
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(up to ~40%, Poulet et al., 2009), large areal extent (>113,000 km2, Hamilton and Christensen, 2005; Mustard et al., 2009), and intimate association with Mg-rich
carbonate (Ehlmann et al., 2008; Ehlmann and Mustard, 2012; Edwards and Ehlmann, 2015). The variable alteration to Mg-rich carbonate and isolated observations of
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serpentine are particularly intriguing aspects of the olivine-rich unit (Ehlmann et al., 2008, 2010; Murchie et al., 2009; Mustard et al., 2009; Ehlmann and Mustard, 2012;
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Mustard et al., 2014). Proposed carbonate formation mechanisms include hydrothermal
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alteration of the ultramafic host rock at slightly elevated temperatures, contact metamorphism, precipitation from transitory shallow lakes, or weathering of the olivine-
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rich rocks (Ehlmann et al., 2008; Brown et al., 2010; Viviano et al., 2013; Bramble and Mustard, 2016). The olivine-rich unit is observed throughout the western Isidis basin
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(Mustard et al., 2007; 2009) including on the floor of Jezero crater (Goudge et al., 2015), in the vicinity of the large Nili Fossae graben (Ehlmann et al., 2008; Mustard et al., 2009), to the north in a region informally called the Nili Fossae carbonate plains (Ehlmann et al., 2008; Edwards and Ehlmann, 2015), and in the southern Isidis basin at Libya Montes (Mustard et al., 2009; Bishop et al., 2013a). Furthermore, the carbonate-
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 9 bearing unit is significant as it may be a potential reservoir of atmospheric carbon, though the degree to which carbonate-bearing rocks may have acted as a carbon sink is difficult to constrain from orbit (Wray et al., 2016), however recent efforts have begun addressing this question (Edwards and Ehlmann, 2015). The Nili Fossae olivine-rich
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unit has been hypothesized as a source region for the plumes of methane observed in the martian atmosphere (Mumma et al., 2009; Wray and Ehlmann, 2011) and the
fracturing of the olivine-rich unit may allow methane, perhaps diffusely produced in the
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subsurface from serpentinization of the olivine-rich unit, to reach the surface.
Orbital visible to near-infrared spectroscopic investigations have identified varied aqueous alteration and igneous mineralogies in the Nili Fossae and NE Syrtis. Based on regional-scale stratigraphic relationships, previous studies have deduced a
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sequence of aqueous mineral formation, suggesting it progresses from phyllosilicate, to carbonate, to kaolinite, and terminating with sulfate formation, as distinct layers rich in
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these minerals are isolated in well-defined ascending stratigraphic positions (Ehlmann
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and Mustard, 2012). Many phyllosilicate minerals have been identified (e.g., Mangold et al., 2007; Mustard et al., 2008; Ehlmann et al., 2009), with the dominant clay mineral in
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the region identified as Fe-rich smectites and observed to correlate with exposures of the Noachian crust (Mangold et al., 2007; Mustard et al., 2008; Ehlmann et al., 2009).
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Al-bearing clays were identified in NE Syrtis and were suggested as an alternative alteration pathway of the Noachian low-Ca pyroxene crustal materials (Ehlmann and Mustard, 2012). Serpentine was identified in conjunction with the Noachian plains olivine-rich unit that has been variably altered to Mg-rich carbonate (Ehlmann et al., 2010; Ehlmann and Mustard, 2012), and was suggested to be a product of
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 10 serpentinization of the olivine-rich unit in a hydrothermal aqueous environment (Ehlmann et al., 2010), perhaps set up by the emplacement of hot olivine-rich material (Viviano et al., 2013), or by deep burial by Syrtis Major lavas (Michalski and Niles, 2010). Mechanisms are under consideration for the formation of the crustal
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phyllosilicates for Mars in general and in our mapping region include formation as
surface weathering products incorporated into the megaregolith by impact processes, hydrothermal alteration in the shallow crust (Ehlmann et al., 2011), magmatic
precipitation (Meunier et al., 2012), impact origin (Tornabene et al., 2013), and
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alteration in the presence of supercritical fluids during the cooling of the martian magma ocean (Elkins-Tanton, 2008; Cannon et al., 2016). Kaolin-group minerals may suggest that pedogenic-type leaching or hydrothermal alteration occurred (Gaudin et al., 2011),
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and these outcrops may be the result of formation under circum-neutral pH waters, or localized acidic leaching (Ehlmann et al., 2009).
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Sulfates are observed at the boundary of NE Syrtis and the Syrtis Major volcanic
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province (Ehlmann and Mustard, 2012; Quinn and Ehlmann, 2014a, 2014b). They are hypothesized to record a transition in the aqueous environments towards bearing acidic
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waters in both a regional and global context (Bibring et al., 2006; Ehlmann and Mustard, 2012). Spectral analysis resolved diagnostic absorptions in multiple locations that were
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consistent with the jarosite, polyhydrated sulfates, and/or a mixture of jarosite with other hydrated phases (Ehlmann and Mustard, 2012). The sulfate-bearing unit is ~500 m thick and displays meter-scale layering (Quinn and Ehlmann, 2014a, 2014b). The morphology and structure of raised boxwork ridges, erosion-resistant ridges in a polygonal network confined to the sulfate-bearing unit, suggests formation via filled
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 11 volume-loss fractures in a sedimentary setting (Quinn and Ehlmann, 2014b). Ehlmann and Mustard (2012) offered two formation hypotheses for the sulfates at NE Syrtis. First was a volcanic-hydrothermal hypothesis where acidic fluids circulate through layered volcanic flows altering the rock and precipitating sulfate. A variation on this hypothesis
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would be the deposition of the layers as ash flows or falls (Quinn and Ehlmann, 2014b). Second was a lacustrine-groundwater hypothesis where the layers are deposited as evaporite sediments in a geologic settings similar to those observed at Meridiani Planum (McLennan et al., 2005) or Terra Sirenum (Wray et al., 2011), and the
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circulation of fluids may be related to extensive groundwater aquifers (Andrews-Hanna and Lewis, 2011) or a hot volcanic layer leading to contact metamorphism of underlying sediments. The abundant carbonate and sulfate minerals in Nili Fossae and NE Syrtis
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corroborate the presence of diverse aqueous environments, as carbonates require near-neutral conditions and some sulfates, such as jarosite, require acidic conditions.
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The assemblages bearing phyllosilicate, carbonate, and sulfate minerals present in NE Syrtis illustrate that spatially and temporally distinct environments of aqueous alteration
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were active in the region (e.g., Poulet et al., 2005). This mineralogy is hypothesized to
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indicate a global trend in the planet’s chemical history suggestive of the transition of waters from circum-neutral pH towards acidic conditions (Bibring et al., 2006; Ehlmann
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et al., 2011; Ehlmann and Mustard, 2012), and also shows that the stratigraphy of NE Syrtis spans the Phyllosian-Theiikian boundary (Bibring et al., 2006). Additional intriguing geological observations in Nili Fossae and NE Syrtis
generate further outstanding questions about the evolution of Mars. Unaltered crystalline igneous units in Noachian-aged crust, enriched in low-Ca pyroxene, would
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 12 provide testable constraints on geochemical and petrological formation models for the early crust of Mars (Elkins‐Tanton et al., 2005; Skok et al., 2012; Grott et al., 2013). Raised ridges in Nili Fossae and NE Syrtis are an outstanding question as running hypotheses for their formation includes mineralized fracture zones (Saper and Mustard,
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2013), as well as breccia dykes and crater-related faults (Head and Mustard, 2006). Megabreccia bearing internal fabric, including layering and possible phyllosilicates, demonstrate active geological processes in the region prior to the basin-forming
impacts, and may bear geochemical remnants of the primitive and hydrothermally-
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altered crust of Mars and provide access to the subsurface (Mustard et al., 2009;
Tornabene et al., 2013). The question remains whether the altered megabreccia blocks originated as primary ancient crust, impact processes, or were produced by later
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igneous intrusions.
The greater Nili Fossae region has recently been under intense analysis,
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indicative of the exceptional exposure of geologic units in this region, the
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unprecedented wealth of orbital data, and its importance in understanding the evolution of Mars. Immediately north of our study area is the watershed for the deltas observed in
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Jezero crater to the northeast (Goudge et al., 2015), further north are the carbonate plains with the planet’s largest exposure of carbonate-bearing rocks (Edwards and
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Ehlmann, 2015), and to the northwest is the Nili Fossae trough (Ryan et al., 2016). The spatial proximity of these analyses to our study provides valuable points of comparison for the discussion of NE Syrtis. In their geomorphic mapping of the Jezero crater and watershed, Goudge et al. (2015) demonstrated that the mineralogy of the fan deposits is detrital in nature and reflects the composition of their associated watersheds. They
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 13 mapped 28 geomorphic units and observed a mottled olivine- and carbonate-bearing unit overlying basement units bearing Fe/Mg-rich smectite, similarly reflecting broader mineralogical and morphological trends in Nili Fossae (Mangold et al., 2007). Edwards and Ehlmann (2015) performed a joint thermal infrared and visible to near-infrared
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analysis of the Nili Fossae carbonate plains where several geomorphic units were
identified and their mineralogy and physical properties constrained. They demonstrated that carbonation processes at low temperatures likely altered olivine-enriched basalts to compositions bearing at most ~20% Fe/Mg carbonate, which suggests carbonate-
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bearing rocks in Nili Fossae only sequestered <12 mbar of CO2.
The diverse geomorphic units in a stratigraphic sequence at NE Syrtis demonstrate the variety of testable hypotheses addressable via future exploration or
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return of samples to the Earth. These hypotheses are compiled in Table 1. It is beyond the scope of a single paper to address each of these hypotheses, but our geomorphic
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map and combined geomorphic and spectroscopic analyses presented here enable
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findings that constrain some of the above hypotheses. Additionally, our detailed mapping, which pushes the highest resolution orbital data available to its limits, helps to
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clarify what can and cannot be resolved with orbital analysis alone. Certain unavoidable limitations—image resolution or non-uniqueness of geomorphic indicators as observed
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in orbital data, for example—similarly limit the ability to address outstanding hypotheses with the investigation herein. In this contribution, we investigate the morphological characteristics of NE Syrtis and the stratigraphic setting of the multiple mineralogical signatures identified by orbital spectroscopy using the available high-resolution orbital
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 14 imagery, in an aim to constrain the spatial, stratigraphic, and temporal relationships in NE Syrtis. 3. Data and methods
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To investigate the characteristics and lateral extent of identified geomorphic units and the geomorphic context of the orbital-identified mineral signatures, a geomorphic map was created utilizing high-resolution image data. Tandem analyses in the visible to near-infrared (VNIR) and thermal infrared (TIR) further constrained the properties of the
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geomorphic units. 3.1 Morphological mapping and analysis
The foundation of the mapping of NE Syrtis was two basemaps, created using
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the high-resolution imagery of the region acquired by the Mars Reconnaissance Orbiter (MRO) spacecraft (Zurek and Smrekar, 2007). The first basemap was a mosaic of
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images from the Context Camera (CTX) (Malin et al., 2007) produced using the USGS Integrated Software for Imagers and Spectrometers (ISIS) and rendered at a resolution
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of ~7 m/pixel (Anderson et al., 2004). Capitalizing on the excellent coverage of the
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region with images from the High Resolution Imaging Science Experiment (HiRISE) (McEwen et al., 2007), a second basemap was created utilizing HiRISE images and
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Environmental Systems Research Institute’s (ESRI) ArcMap geographic information system (GIS) software. HiRISE images were spatially referenced to the CTX mosaic prior to mosaicking. Digital Elevation Models (DEMs) were produced using CTX and HiRISE stereo image pairs with the NASA Ames Stereo Pipeline (Broxton and Edwards, 2008; Moratto et al., 2010). The DEMs were map-projected and referenced to the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 15 martian areoid. The DEMs allowed for constraints on both regional and localized topographic and stratigraphic relationships, and for morphometric analysis of geologic units. The CTX and HiRISE images utilized in this analysis are listed in the
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supplementary Table S1 and Table S2, respectively. The geomorphic mapping area was bounded to the north by the Jezero
watershed and the region investigated by Goudge et al. (2015), to the east by the
unnamed 7 km crater south of Jezero crater, and to the south and west by the first
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contact with the Syrtis Major volcanic flows (Figure 1). The region investigated has
near-contiguous HiRISE coverage allowing for the meter-scale geomorphology to be studied. In regions with HiRISE coverage, the surface was investigated at a mapping scale of 1:1,000, and unit contacts were drawn at this scale. The mapped geomorphic
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units were initially characterized at HiRISE resolution, and the transition to regions without HiRISE coverage results in the loss of the fidelity required to accurately map
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these units. The contacts between these units occur below the resolution of CTX. In
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regions without HiRISE coverage, CTX images were investigated and unit contacts were drawn where geomorphic units were discernable at a mapping scale of 1:5,000.
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Regions where the resolution was too low to identify the HiRISE-characterized units as the boundaries were below the resolution of CTX, or regions where multiple geomorphic
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units were in close spatial proximity hindering mapping at CTX resolution, were incorporated into the Undifferentiated Terrain (UNT) unit. Figure 2a identifies the area in NE Syrtis where HiRISE imagery was used in the geomorphic unit mapping. The geomorphic mapping was performed without regard to the mineralogical analysis. An exception was made for regions without HiRISE coverage where CRISM spectral
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 16 summary parameters for the mineral olivine were utilized to confirm suspected geomorphic units matching other spectrally-confirmed olivine-rich exposures. In general, geomorphic units were defined by their characteristics identifiable in the HiRISE images and derived DEMs. These characteristics include, but were not limited to, the qualitative
unit, DEM topography, and stratigraphic position. 3.2 CRISM spectral analysis
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albedo, overall two-dimensional geomorphic shape, texture and patterns internal to a
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The VNIR spectral characteristics of the identified geomorphic units were
investigated using hyperspectral images from CRISM aboard MRO (Murchie et al., 2007). Full-resolution (~18 m/pixel) and half-resolution (~36 m/pixel) targeted observations made by the CRISM infrared detector (1.00 to 3.92 μm) were processed
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by dividing the I/F image by the cosine of the incidence angle, where I/F is the radiance
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at the sensor divided by the solar irradiance divided by π (Murchie et al., 2007). The image was divided by a scaled volcano scan observation empirically optimized for each
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observation as a first-order correction for atmospheric gases. Spectral summary parameters were calculated to guide the spatial identification of mineral signatures
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(Pelkey et al., 2007; Salvatore et al., 2010; Viviano-Beck et al., 2014). Images were
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map-projected and georeferenced to a Mars Orbiter Laser Altimeter (MOLA; Smith et al., 2001) digital terrain model to correlate with the geomorphic mapping. A vector image of the geomorphic map was overlain on the projected CRISM images to spatially compare the spectral features identified with the mapped units. To identify mineralogies of the mapped units, CRISM image pixels that displayed a strong signal in the parameter bands were collected from the map-projected images, spatially averaged
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 17 within the mapped unit, and then the spectra were divided with in-scene, spectrallybland spectra to diminish residual atmospheric and instrumental artifacts. For comparison with laboratory spectra of mineral samples, the map-projected pixels with the strongest spectral features were located in the non-projected camera-space images
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and the collected spectral pixels were divided by a spectrally-bland region spanning the same along-track detector width. CRISM observations utilized in this analysis are listed in supplementary Table S3 and their coverage of the mapping area is shown in Figure 2b. The spatial locations of CRISM spectra presented in the figures below are listed in
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supplementary Table S4.
Absorption features observed in the CRISM spectra were characterized by their shape and wavelength position and were compared to a suite of laboratory-measured
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mineral spectra. Laboratory VNIR spectra of pyroxenes display broad absorptions centered near ~1 and ~2 µm, and the centers of these features vary primarily as a
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function of the mineral’s Ca content, with increasing Ca content shifting these
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absorptions towards longer wavelengths (Adams, 1974; Burns, 1993; Klima et al., 2007, 2011). Olivine has a prominent ~1 µm feature composed of three absorptions produced
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from octahedrally coordinated Fe2+ that varies its band center with changes in olivine
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Fe2+ content (King and Ridley, 1987; Burns, 1993). Fe/Mg-smectites, including Fe-rich nontronite or Mg-rich saponite, bear
prominent vibrational absorptions at ~1.4 (first OH stretching overtone and H2O structural combination tone), 1.9 (structural H2O combination tone of OH bend and H-OH stretch), and 2.29-2.31 µm (combination tone of OH stretch and Fe/Mg-OH bend; Clark et al., 1990; Bishop et al., 1999, 2002; Frost et al., 2002). A suite of absorptions at
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 18 ~1.4, 1.9, and 2.2 µm suggest the presence of a kaolin-group mineral, with the first two absorptions matching the sources in Fe/Mg-smectites and the 2.2 µm doublet absorption resulting from a OH stretch and Al-OH bend combination tone (Clark et al., 1990). The mineral talc has a sharp and deep 1.39 µm absorption from OH vibration
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and a broader 1.44 µm absorption (Clark et al., 2007). Narrow talc absorptions at 2.32 and 2.39 µm are of comparable width and the latter is ~50% the depth of the former (Clark et al., 2007). Similarly, an absorption at 2.47 µm is ~50% the band depth of the 2.39 µm absorption. Talc does not have H2O in its crystal structure and therefore is
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lacking an absorption at ~1.9 µm.
Narrow absorptions observed at ~2.3 and 2.5 µm can match fundamental vibrational mode overtones of the CO3 in the carbonate mineral structure, and paired
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band centers observed at ~2.31 and 2.51 µm or 2.32 and 2.52 µm are indicative of Mgrich carbonate (Hunt and Salisbury, 1971; Gaffey, 1987). Orbital carbonate
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identifications on Mars also bear an absorption at ~1.9 µm (e.g., Ehlmann et al., 2008;
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Ehlmann and Mustard, 2012), which may indicate the Mg-rich phase is hydrated (Calvin et al., 1994) or is mixed with another hydrated phase, such as a smectite (Bishop et al.,
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2013b).
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Sulfate minerals have been identified in NE Syrtis (Ehlmann and Mustard, 2012) and the particular phases identified were jarosite, based on the identification of a Fe-OH asymmetric doublet centered at 2.21 and 2.26 µm (Swayze et al., 2008), and polyhydrated sulfates, identified by structural H2O absorptions at 1.4 and 1.9 µm and a diagnostic inflection at 2.4 µm. 3.3 Thermal infrared analysis
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 19 Thermal infrared (TIR) data from the THEMIS aboard the Mars Odyssey spacecraft (Christensen et al., 2004) were investigated in the mapping of NE Syrtis to analyze variations in the thermal inertia (TI) as well as broad trends in the dominant surface mineralogy, which influences the shape of the primary Reststrahlen features in
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the TIR wavelength region (Christensen et al., 2004). THEMIS images I27043003,
I39820004, and I44288005 were the focus of our analyses and together these images cover the entire mapping area. For compositional analyses, THEMIS TIR data were processed using the techniques described by Salvatore et al. (2016). Summarized here,
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the data were collected by THEMIS with 10 spectral bands located between 6.8 μm and 14.9 μm and a surface spatial resolution of 100 m/pixel. These data were calibrated to top-of-atmosphere effective emissivity (Christensen, 1998; Rogers et al., 2005) and
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atmospherically corrected (Bandfield et al., 2004) to produce surface emissivity data. Windowed decorrelation stretch (DCS) images were produced using THEMIS radiance
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data (Edwards et al., 2011), and THEMIS DCS images with red-green-blue combinations of band 8 (11.79 μm), band 7 (11.04 μm), and band 5 (9.35 μm),
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respectively, were utilized in the mapping and the corroboration of geomorphic units
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identified using HiRISE and CTX images. Decorrelation stretch images using this particular band combination have been shown to be effective in differentiating spectrally
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distinct surface units, especially variations in mafic compositions (e.g., Hamilton and Christensen, 2005; Rogers et al., 2005; Edwards et al., 2008, 2011). Quantitative TI measurements, which indicate surface properties including the
effective particle size and induration of the geomorphic units, were collected from the mapped geomorphic units using a global TI map (Fergason et al., 2006; Christensen
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 20 and Fergason, 2013). Modeled effective particle sizes for each unit were generated as relationships between TI and the effective particle size of materials have been shown to be applicable on Mars (e.g., Kieffer et al., 1973). TI is related to the degree of induration and the grain size properties of the surface; for example, competent rock outcrops have
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a high TI while surfaces covered in fine-grained dust have low TI. As a means of
minimizing dust cover and quantifying only the best exposures, measurements of the TI were made within each of the mapped units at locations exhibiting the highest TI values
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and in an area large enough to be confirmed within the bounds of the mapped unit. 4. Results
Despite NE Syrtis bearing great morphologic heterogeneity at meter to decameter scales, the geology of the area is unifiable under five distinct units of paired
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morphology and mineralogy (Figure 3, Table 2). Ascending the stratigraphic column
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(Figure 3) they are: (1) a Basement Unit (BAU) that exposes areas of low-Ca pyroxene and Fe/Mg-smectite and is observed in a range of morphologies including crustal
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mounds, smooth and knobby plains, and raised linear ridges; (2) an extensive Fractured Unit (FRU) that bears olivine-rich VNIR and TIR spectra, is variably altered to
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carbonate, and tapers off into Large Linear Features (LLF) that appear to descend
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downsection; (3) a Capping Unit (CAU) with an unremarkable VNIR spectrum that preserves craters and sheds boulders; (4) a 100s of meters thick slope unit (FSU) exposing layered sulfates and raised boxwork ridges; and (5) the dated Syrtis Major Volcanics unit (SMV) bearing a mineralogy of high-Ca pyroxene. Isolated and distributed outcrops in the Basement Unit display spectral signatures of a kaolin-group
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 21 mineral, and while these locations are at the surface of the Basement Unit they are not a significant stratigraphic unit as seen elsewhere in Nili Fossae (Ehlmann et al., 2009). Raised linear features that are meters in width are observed in the mapping area
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and in two stratigraphic positions. Some of these ridges have been previously mapped by Saper and Mustard (2013) as raised ridges. Ridges meters in width are observed at the edges of the olivine-rich Fractured Unit descending into the downsection Basement Unit. Raised Linear Ridges (RLR) are observed in coherent outcrops in the Depression
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region (see below), are mapped as a subunit of the Basement Unit, and are most
comparable to those discussed by Saper and Mustard (2013). Raised Boxwork Ridges (RBR) are stratigraphically equivalent to the ~500 m thick Feature-bearing Slope Unit (FSU), and are high-standing above light-toned outcrops (Figure 3).
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From the geomorphic mapping and spectral analysis of NE Syrtis (Figure 4) we
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identify three regions (Figure 1c): (1) the Plains region, consisting of the relatively flatlying region from 17.6–18.0 °N and 76.8–77.3 °E, (2) the Depression region to the south
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and west of the Plains region that transitions from the Plains region towards the Syrtis Major Volcanics, and (3) the Syrtis Major Volcanics region. Our mapped geomorphic
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units are discussed in detail below and summarized in Table 2.
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4.1 The Plains of Northeast Syrtis Major The topographically subdued region outside the southwest rim of Jezero crater is
termed here as the Plains region of NE Syrtis (Figure 5). The Plains region has a dynamic topographical range of <100 m, and a shallow regional slope towards the south and east, away from a topographically-high large crustal mound. The Plains region is
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 22 additionally differentiated via the large contiguous area of HiRISE coverage, where ~30×30 km of HiRISE stereo images have been acquired and used to generate highresolution topography.
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4.1.1 Basement Unit The stratigraphically lowest geomorphic unit identified in the mapping area is the Basement Unit (BAU), which is differentiated into five geomorphic subunits (Table 2). No unit is identified underlying the Basement Unit, and therefore it is of undetermined
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thickness. The Crustal Mounds (CMD) subunit of the Basement Unit is observed as mounds exhibiting significant vertical relief with a peak, plateau, or elevated ridge from which lineations on the slopes fan away (Figure 6a). The slopes are slightly darker in tone and do not exhibit boulder shedding. A subunit differentiation is made for Large
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Crustal Mounds (LCM), which share many morphological similarities with the Crustal
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Mounds, with the exception of their heights reaching of 100s of meters of elevation. These mounds are light-toned with lineations fanning away from the topographic high.
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The Crustal Mounds are more sparsely cratered than the Fractured Unit or Capping Unit (see below), likely due in part to a combination of its physical properties and amount of
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debris cover.
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Two plains units are mapped in NE Syrtis: Smooth Plains and Knobby Plains, both subunits of the Basement Unit. Smooth Plains are characterized by an expansive, relatively dark-toned smooth surface. Isolated ridges or spatially-insignificant mounds akin to the Crustal Mounds or the other mapped units are observed in the Smooth Plains but are relatively uncommon. Knobby Plains are extensive smooth regions interspersed with frequent ridges or mounds akin to the Crustal Mounds or other
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 23 mapped units imparting a knobby texture in the HiRISE and CTX data. The Crustal Mounds, Knobby Plains, and Smooth Plains may be indicative of an erosional progression, with the Crustal Mounds being the most intact and the Smooth Plains representing the greatest amount of degradation. An intermediary step would be the
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variably dissected exposures of the Crustal Mounds that are spatially related in
seemingly coherent structures with light-toned surfaces and dark-toned smooth gradual slopes.
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Dispersed exposures of massive blocks or megabreccia are observed in the subunits of the Basement Unit. The megabreccia or massive blocks exhibit a diversity of morphologies including both angular and rounded textures, internal structure including possible layering, and disaggregation into seemingly drawn-out filaments. The
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megabreccia are up to 10s of meters in diameter, and are of similar characteristics to those observed elsewhere in Nili Fossae (e.g., Mustard et al., 2009; Goudge et al.,
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2015). The megabreccia are spatially distributed throughout the Basement Unit as
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small, isolated occurrences of light-toned blocks that are meters in size with no topographic signature. Multiple surface textures are observed within the Smooth Plains,
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where the lowermost is a darker-toned plain bearing abundant megabreccia, and meters above is a smoother, lighter-toned plain (Figures 6b and 6c). These textures and
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the megabreccia are not mapped as separate units as they are not spatially significant at the mapping scale. CRISM pixels displaying strong signals in the parameter images were analyzed from the Basement Unit. Narrow absorptions at approximately 1.4, 1.9, and 2.3 μm are observed in isolated patches of the Basement Unit (Figure 6e) where the dust cover is
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 24 low. These absorptions are consistent with the prominent vibrational absorptions of Fe/Mg-smectite, including Fe-rich nontronite or Mg-rich saponite, and are interpreted as such. Broad absorptions are observed at ~2 μm consistent with the presence of low-Ca pyroxene (Figure 6g). Low-Ca pyroxene is common and pervasive in the Basement Unit
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and subunits via this feature identified in several exposures.
Distinct absorptions at ~1.4, 1.9, and 2.2 µm correlated with isolated light-toned features 10s of meters in size are diagnostic of the presence of a kaolin-group mineral
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(Figure 7). Outcrops exhibiting these absorptions are only observed in small and
isolated patches in the two plains subunits, and are associated with light-toned features on the surface that are qualitatively among the lightest-toned features in a given HiRISE scene. The light-toned surfaces display a massive texture with no internal fabric at the
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highest resolution apart from dark-toned fractures and cover (Figures 7b and 7c). No positive or negative topographic irregularities are observed correlating with these light-
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toned features using a HiRISE DEM, and the features appear part of the basement and
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not a separate stratigraphic unit eroded to its current exposure. As the geomorphic units were identified independently of CRISM data, outcrops bearing a kaolin-group mineral
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could not be identified on morphology alone and therefore were not mapped as separate subunits. While some of the circular light-toned massive blocks could be
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interpreted as rounded megabreccia, they may also be interpreted as circular depressions formed by small craters filled with light-toned materials that were later eroded to form a level surface (Figure 7b). TIR investigation of the Basement Unit produced a TI value of 255.11 ± 18.05 J m–2 K–1 s–0.5, which generated a modeled effective particle size of ~260 µm. These
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 25 values likely correlate more closely to the regolith and debris observed covering the Basement Unit, rather than inherent thermophysical properties of the basement itself, consistent with the patchy distribution of strong CRISM signatures of low-Ca pyroxene, Fe/Mg-smectite, and a kaolin-group mineral. The Basement Unit is predominately green
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in the THEMIS DCS mosaic (Figure 8a). The green color is indicative of generally higher values in band 7 than the surrounding materials due to more silica-rich
compositions shifting the main Reststrahlen features towards shorter wavelengths. Green units in this THEMIS DCS band combination are generally consistent with
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basaltic compositions with alteration phases present (Salvatore et al., 2016), and this observation is corroborated here by the identification of Fe/Mg-smectites and Alphyllosilicates in CRISM pixels correlated with the Basement Unit.
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4.1.2 The Fractured Unit
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The Fractured Unit (FRU) is a spatially extensive unit with a hummocky, corrugated surface of light-toned blocks on the scale of 10s of meters seen closely
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packed in a darker supporting material stratigraphically above the Basement Unit (Figure 9). The dark-toned material forms polygonal shapes around the light-toned
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blocks giving this unit a fractured appearance, and a fractal pattern of smaller fractured
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blocks is observed within the larger blocks (Figure 9). A sharp contact is seen between this unit and the Basement Unit, and the Fractured Unit is observed to embay the Basement Unit (Figure 6a). Fields of light- and dark-toned boulders are observed, and interspersed are small crater-preserving mesa mounds and small mounds resembling the other units identified but not spatially significant. The Fractured Unit is commonly covered with patches of aeolian cover forming dune fields with an approximate north-
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 26 south dune orientation. The dune fields are commonly located in the topographic lows and craters of the Fractured Unit. Parallel lineations are occasionally observed in the Fractured Unit. The lineations
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most frequently observed are a variable banding that is concentric and parallel to the upsection Capping Unit (Figure 10a). This banding appears to occur on a relatively flat surface. Other less-frequently observed lineations are suggestive of layering. These lineations are most commonly found in the Depression region (see below) and display
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alternating light- and dark-toned bands (Figure 10c). The bands are not of uniform
thickness and display a wavy appearance. The bands can span 20 to 30 m of elevation when outcropping on slopes.
The Fractured Unit conforms to the underlying topography on both local and
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regional scales. The unit follows the topographic gradient and, over 100s of meters,
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remnants are observed in linear topographic lows and form circular features (Figure 11). The Fractured Unit embays the slopes of large mounds with the underlying unit often
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exposed at the crest. The unit is also observed to conform to, or drape, the larger-scale regional topography, being observed at elevations from –2000 m (relative to the MOLA
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datum) in the north of the mapping area and descending to –2450 m in the Depression
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region (see below) and disappearing under the Syrtis Major Volcanics to the south. This draping characteristic has also been noted by Mustard et al. (2009) at elevations as high as –500 m in their investigation ~250 km north of our mapping area, and suggests that the Fractured Unit is the local expression of the Nili Fossae olivine-rich unit (e.g., Hamilton and Christensen, 2005; Mustard et al., 2009).
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 27 The thickness of the Fractured Unit is variable within the Plains region. At the contact between the Fractured Unit and the Basement Unit, the Fractured Unit is commonly ~5–10 m thick. This thickness can range from a few meters to a few tens of meters, and may be governed in part by variable thin regolith cover or preferential
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erosion of the Basement Unit.
CRISM spectra collected of the Fractured Unit bear a broad absorption at ~1 μm and narrower, less prominent absorptions at 1.9, 2.3, and 2.5 μm (Figure 12). We
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interpret this spectrum to indicate a mixture of olivine and carbonate minerals, and
possibly an additional mixture phase of smectite, consistent with the interpretations of Ehlmann et al. (2009) and Ehlmann and Mustard (2012). In NE Syrtis, the carbonate spectral signatures appear more spatially pervasive and less variable in the olivine-rich
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unit than observed in other locations in Nili Fossae (e.g., Ehlmann et al., 2009). This may suggest that aqueous alteration was more extensive and complete in this region
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than elsewhere in Nili Fossae. Possible unaltered olivine-rich outcrops lacking 1.9, 2.3,
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and 2.5 µm absorptions are observed in a limited number of exposures relatively free of aeolian cover, not only in dunes as previously reported (Ehlmann et al., 2010; Ehlmann
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and Mustard, 2012).
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Several exposures of the olivine-carbonate-bearing Fractured Unit appear to bear a weak absorption at ~2.39 µm (Figure 12). This absorption is often approaching the level of the spectrum noise, and the approximate CRISM band minima varies between the 2.3840, 2.3906, 2.3972, and 2.4104 µm channels. While it has been suggested that talc, with an absorption at 2.386 µm (Clark et al., 2007), would be an expected mineral phase from contact metamorphism of the olivine-rich unit (Brown et
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 28 al., 2010; Viviano et al., 2013), it is not clear that talc is supported by the available data, as no 1.39 µm absorptions are observed, and the 2.39 µm absorption, consistent with talc (Clark et al., 1990; Brown et al., 2010; Viviano et al., 2013), is also present in Fe/Mg-phyllosilicates. We interpret spectra bearing a broad ~1 µm absorption, narrow
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1.9, 2.3, and 2.5 absorptions, and a weak 2.39 µm absorption as a mixture of olivine and Mg-rich carbonate with lesser amounts of a phyllosilicate, possibly saponite (Figure 12). The previous identification of serpentine in NE Syrtis (Ehlmann et al., 2010;
Ehlmann and Mustard, 2012) is in an outcrop located at the southernmost extent of the
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Fractured Unit in our mapping area, and is not a widespread spectral component.
Regions mapped as Fractured Unit display a predominately purple color in the THEMIS DCS mosaic (Figure 8b). This observation corroborates the enrichment in
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olivine observed by CRISM, as purple tones are indicative of olivine in this band combination (e.g., Edwards et al., 2008). The Fractured Unit displays the highest
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thermal inertia of the mapping area at 420.29 ± 20.74 J m–2 K–1 s–0.5, and a modeled
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effective particle size of > 1 mm. This value correlates well with values produced by a similar scale joint TIR–VNIR analysis by Edwards and Ehlmann (2015), where thermal
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inertia values of ~400–500 J m–2 K–1 s–0.5 were reported from the modeling of olivine-
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and carbonate-bearing materials on the more-northern Nili Fossae carbonate plains. 4.1.2.1 Large Linear Features There are multiple linear features observed in NE Syrtis ranging from linear
raised ridges (e.g., Saper and Mustard, 2013) to previously undocumented Large Linear Features (LLF) of distinct morphologic character and size. These Large Linear Features typically: (1) are 10s to 100s of meters wide and 1000s of meters long (Figure 13a); (2)
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 29 join seamlessly with edges of the Fractured Unit (Figure 13b) and share the same olivine-carbonate-bearing VNIR spectral signature; (3) the material comprising the Large Linear Features is light-toned and blocky in a dark-toned supporting material, and raised rims commonly bound the edges of the Large Linear Features (Figure 13c); (4)
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they appear to follow the trend of the local topography and are sometimes exposed as raised ridges or recessive troughs (Figure 13d). These linear features are distinct from the raised ridges mapped in the Nili Fossae region discussed below (Saper and
Mustard, 2013; Ebinger and Mustard, 2015). A select number of Large Linear Features
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are mapped as Capping Unit (see below) as a material consisting of rough surfaces with dark-toned boulder-shedding slopes is supported on, and conforms to, the Large Linear Feature shape (Figure 5).
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A cross-cutting relationship between the Large Linear Features and the Basement Unit is observed in the Plains region. The Large Linear Features are
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commonly observed sharply bifurcating at angles into multiple Large Linear Features,
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including orthogonal intersections and triple junctions (Figure 13a). Large Linear Features are observed in present-day troughs. These troughs are a few meters in depth
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with the Large Linear Features in the topographic low at the end of smooth gradual slopes of Basement Unit material on the sides of the trough, as measured by a HiRISE
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DEM. Additionally, smaller raised ridges meters in width and 10s to 100s of meters in length are observed extending off the edges of Fractured Unit and Large Linear Features and tapering into the Basement Unit. Some of these ridges tapering off of the Large Linear Features were mapped by Saper and Mustard (2013), and, while they appear innately related to the Large Linear Features, for select ridges it is difficult to
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 30 constrain whether the ridges are a seamless part of the Large Linear Features, or downsection ridges rising and halting at the Fractured Unit or Large Linear Feature contact.
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Raised ridges or linear mounds are commonly observed along the contact of the Large Linear Features and the Basement Unit. Topographic profiles across the Large Linear Features measured from the HiRISE DEMs (Figure 14) show the bounding
ridges to be meters in height when the Large Linear Feature is present on the two plains
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subunits and the center remains a few meters above the level of the host unit. The
bounded Large Linear Features bear a center comprised of light-toned blocky material with a darker supporting material. The light-toned bounding rims are interpreted as erosion-resistant rims of the Large Linear Feature or possible aeolian cover lapping
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onto the linear feature.
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Fractures that follow the track of the linear feature are observed at the center of select Large Linear Features. These fractures are ~1 m wide and 10s to 100s of meters
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in length and have dark, shadowed crevices suggesting that they are relatively free of dust cover. Fractures of a similar scale are also observed in the mapping area in the
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other mapped units, including the units identified by CRISM to be olivine-rich. The dark
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shadows formed by these small fractures suggest they may be meters in depth. 4.1.3 The Capping Unit The Capping Unit (CAU) is stratigraphically above the Fractured Unit (Figure 15)
and is characterized by: (1) an uppermost rough, crater-preserving surface. On finer scales, this surface bears abundant meter-scale light- and dark-toned boulders
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 31 surrounded by a dark-toned material (Figure 16). (2) Lower dark-toned slopes shedding both light- and dark-toned boulders variably exposing internal light-toned fractured blocks. In select expressions where the crater-retaining cap appears to have been significantly eroded, light-toned and angular blocks are observed within a fractured and
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polygonal dark supporting material (Figure 15c). Some expressions of the Capping Unit display coarse banding in this lower slope and/or a lack of boulder shedding. The
uppermost surface and underlying boulder-shedding slope were mapped together as the Capping Unit (Figure 16), as they exist in various states of preservation and
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exposure. An erosional progression is observed for the Capping Unit where the unit is removed in successive steps and the end stage is a cap of light-toned angular blocks with no crater-retaining cap or boulders (Bramble and Mustard, 2015). Smooth-Sloped
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Mounds (SSM), a subunit of the Capping Unit, have equal stratigraphic position and were mapped where the capping material displays smoother slopes, subdued boulder
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shedding, darker-toned slopes, and no evidence of crater preservation (Figure 9). The Capping Unit commonly forms mesa-like landforms in conjunction with the downsection
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Fractured Unit (Figure 16). The Capping Unit bears a greater crater density relative to
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the Fractured Unit.
The thickness of the Capping Unit is also variable within the Plains region. The
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Capping Unit is commonly 10 m in thickness as measured from HiRISE DEMs, and the thickness is observed to vary within a range of ~5–15 m. An approximately meter-scale benching is observed (commonly in the absence of discernable layering) across the Capping and Fractured Units. Select mesa-forming expressions of the Capping and Fractured Units have vertical expressions approaching the ~1 m resolution of the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 32 HiRISE DEMs, in particular where the contacts occur on a gradual slope. The SmoothSloped Mounds subunit is notable due to its significant topographic relief, with some mounds reaching 60 m in height.
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An additional observed characteristic of select mesa-forming expressions of the Capping and Fractured Units is a quasi-circular nature of these two units. These
Capping Unit mesas are observed with a quasi-circular rim of Fractured Unit and
variably isolated from the larger expanses of the Fractured Unit (Figure 17). The quasi-
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circular mesas are of a relatively uniform size, all being a few 100 m in diameter, though even much larger mesa-forming expressions of these two units, on the order of several kilometers in diameter, appear to have approximately equal perpendicular diameters (Figure 17e).
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CRISM spectra collected of Capping Unit surfaces are unremarkable in the
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spectral parameter images and spectra collected from the Capping Unit display a flat, featureless spectrum. Select slopes of Smooth-Sloped Mounds bear a broad absorption
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centered at ~2.2 μm indicative of high-Ca pyroxene materials in these deposits. These spectral signatures appear correlated to the mantling material and are not an inherent
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property of the Smooth-Sloped Mound subunit. Analyzing spectra collected of the
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Capping Unit or Smooth-Sloped Mounds subunit do not reveal absorptions indicative of high-Ca pyroxene, corroborating the association of the ~2.2 µm absorption with the dark-toned mantling material. The Capping Unit displays a green-yellow color with isolated spots of purple (Figure 8c), indicative of higher values in band 7 likely due to the main Reststrahlen features being centered towards shorter wavelengths, showing slightly more silica-rich
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 33 compositions. Our observation of a TI of 276.9 ± 18.5 J m –2 K–1 s–0.5 for the Capping Unit similarly correlates with the TI range of 250–300 J m–2 K–1 s–0.5 reported for the lowolivine capping unit by Edwards and Ehlmann (2015). Our modeled particle size of ~375 µm for the selected Capping Unit outcrops is at the lower end of the ~300–700 μm
4.2 The Depression southwest of the Plains
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range reported for their capping unit (Edwards and Ehlmann, 2015).
To the south and west of the Plains region is a topographic depression
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containing variants of the geomorphic units described above (Figure 1c). CTX DEM topography shows the Depression region’s lowest elevations lie ~500 m below the average elevation of the Plains region. From the Depression region northwards to the 6 km crater is a rise in elevation towards a Large Crustal Mound outside the mapping
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area to the north of this crater. The Depression region lacks extensive HiRISE
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coverage. The isolated HiRISE imagery in this region provides insight to the units present, but the ability to map geomorphic units characterized at HiRISE resolution is
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hindered.
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4.2.1 Depression variations on Capping Unit and Fractured Unit Large, rough, crater-preserving surfaces matching morphological characteristics
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of the Capping Unit are observed in the Depression region, but here they are commonly larger in area (few kilometers across versus few 100 m) and bearing an extensive, smooth, dark-toned cover (Figure 18a). Similarly, in the Depression region there is a variant of the Fractured Unit termed the Dark-Toned Fractured Unit (FRU–DT) that is of matching morphology but the light-toned, polygonally-fractured surface bears a patchy
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 34 but smooth, dark-toned cover. This unit is commonly identified by exposures near the contacts with neighboring units, where the surface of light-toned blocks closely packed in a darker matrix is observed (Figure 18b). Beginning within a few 100s of meters from the contact and increasing in apparent abundance towards the center of these units is a
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dark-toned cover. The same dark-toned cover appears to be partially obfuscating both the Fractured Unit and Capping Unit. The lateral extent of the dark-toned cover matches the orange color in the THEMIS DCS image (Figure 8d). Orange in this band
combination suggests the presence of exceptionally silica-rich compositions or sulfates,
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though little can be definitively said about this orange color as the color may be
explained by coatings, alteration rinds, or sulfates that are generally invisible to VNIR detectors.
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CRISM analysis of the FRU–DT corroborates its relationship to the Fractured Unit as a strong and broad absorption is observed at ~1 µm and narrower and weaker
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absorptions are observed at ~1.9, 2.3, and 2.5 µm. With these absorptions olivine and
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carbonate are interpreted to be present, as with the Fractured Unit. Spatially extensive regions are mapped as FRU–DT, and erosional windows through this unit reveal
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additional Fe/Mg-smectite and low-Ca spectral signature concentrations.
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4.2.2 Raised Linear Ridges In addition to the Large Linear Features, linear features of smaller scale are
present in NE Syrtis. These ridges are most abundant in the Depression region and are meters in width, hundreds of meters in length, and are high-standing above a relatively smooth surface (Figure 19a). The ridges are observed in two stratigraphic positions: in linear networks stratigraphically beneath the olivine-rich Fractured Unit (mapped as
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 35 Raised Linear Ridges), or in polygonal networks of short linear ridges interspersed with light-toned banded formations above the Capping Unit (mapped as Raised Boxwork Ridges, see Section 4.3).
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CTX DEM topography shows that the exposures of Raised Linear Ridges (RLR) are commonly topographically higher than the Dark-Toned Fractured Unit in elevation, but visual analysis suggests that they are stratigraphically lower (Figure 19b). The
topographical relationship appears to follow broader trends, where at higher elevations
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the overlying material may have been eroded away, exposing the stratigraphically lower Raised Linear Ridges. The greatest vertical relief appears to be correlated with smooth mounds a few 100 m across that form within the confines of the Raised Linear Ridges. Conversely, the Dark-Toned Fractured Unit and Capping Unit may not have been
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emplaced in a manner that allowed these units to fully drape the entire region and cover all of the local topographic highs. Regardless, the Raised Linear Ridges present
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erosional windows through the olivine-rich unit and into the Basement Unit. Small
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mounds of capping material akin to remnants of the Capping Unit or Fractured Unit are observed in select locations where the ridges intersect.
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The Raised Linear Ridges appear to vertically cross-cut the host unit and
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terminate at the upsection Fractured Unit, an observation similarly reported for the breccia dikes studied by Head and Mustard (2006). The raised ridges of Saper and Mustard (2013) closely correspond with the regions we mapped as Raised Linear Ridges, but this previous study also included the bounding ridges along the Large Linear Features, ridges tapering downsection of Fractured Unit, and the Raised Boxwork Ridges (see below).
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 36 The basement layer from which the Raised Linear Ridges cross-cut bears absorptions in CRISM images at ~1.4, 1.9, and 2.3 µm interpreted to be a Fe/Mgsmectite-bearing basement (Figure 19c). The ~1.4 µm band is relatively broad in comparison to single-phase laboratory spectra and centered at ~1.41 µm. Broad
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absorptions suggest a mixture of phyllosilicates or variance in cation by substitution in the Mg/Fe solid solution of smectites (Ehlmann et al., 2009). Small doublets are
observed at 2.298 and 2.318 µm for the ~2.3 µm band, or one single band is observed with a band minima at 2.305 or 2.318 µm, suggesting a degree of heterogeneity or
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variation in the phyllosilicate composition, corroborating the broad ~1.4 µm band. A minima at ~2.39 µm is variably present in this unit (similar to the ~2.39 µm minima of the Fractured Unit discussed above), but its magnitude is on the order of instrument and
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data-processing artifacts and the low signal-to-noise ratio suggests using it in mineral identification is questionable (Figure 19c). A broad absorption (or set of absorptions) at
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~1.4 µm and a narrow ~2.39 µm absorption may be indicative of the mineral saponite (Figure 19c). We categorized the Raised Linear Ridges as a subunit of the Basement
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Unit as they appear stratigraphically below the Fractured Unit, and due to the observed
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VNIR mineralogy matching the Basement Unit.
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4.2.3 Megabreccia
Exposures of megabreccia are observed throughout the mapping area but are
most common in the Depression region. One particularly interesting exposure is observed at a boundary between Capping Unit and Dark-Toned Fractured Unit (Figure 20). This megabreccia exposure consists of blocks meters to 10s of meters in size and of multiple tonalities that are dispersed amongst a dark-toned supporting material. The
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 37 blocks bear rounded edges, but are variably fractured. This outcrop appears to have a horizontal progression in block size; trending towards the northwest, the breccia blocks become larger and more spread out in the dark supporting material. These breccia blocks, some 10s of meters in size, appear to have an internal fabric, suggesting
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layering or banding prior to disruption and amalgamation into breccia (Figure 20a).
Here, angular blocks are observed in close proximity with heavily rounded fragments, and one angular fragment appears to have a fault with a visible offset of ~10 m (Figure 20a). The megabreccia in this region bear a relatively smooth surface as is evidenced
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by meter-sized boulders casting shadows on the surface (Figure 20). The megabreccia appears to be stratigraphically equal with the olivine-carbonate-bearing Dark-Toned Fractured Unit, and is contacted on most edges with this material (Figure 20b), though
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the exact stratigraphic position of the megabreccia is difficult to constrain from orbit. If this breccia is stratigraphically equal, then it is of a different provenance than the
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distributed megabreccia observed in the Basement Unit in the Plains region, and may have been emplaced contemporaneously with the Fractured Unit. While this
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megabreccia was mapped as part of the Dark-Toned Fractured Unit, other
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interpretations based on the imagery could include cross-cutting, underlying, or embaying relationships with the Fractured Unit. The megabreccia were not mapped as
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distinct geomorphic units as they are not of comparable spatial extent to the other mapped units. 4.2.4 Undifferentiated Terrain The Undifferentiated Terrain (UNT) incorporates regions lacking sufficient highresolution image coverage for the mapping of the geomorphic units characterized at
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 38 HiRISE resolution. CTX imagery was used to map unit contacts where they were identifiable at CTX resolution, but the fidelity required to map these units is absent, and as a result the Undifferentiated Terrain contains multiple geomorphic units. Further hindering high-resolution mapping in this region is the seemingly greater spatial
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heterogeneity of the geomorphic units. The general surface observed in the CTX
images is composed of a lighter toned knobby terrain with small knobs and ridges in close proximity, and amongst isolated exposures of other mapped units, such as craterpreserving surfaces or light-toned fractured blocks with contacts obfuscated at this
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resolution (Figure 21a). Where possible, comparing the appearance of Plains region units in overlapping HiRISE and CTX images was used to extend mapping into the regions without HiRISE coverage. The olivine-rich units identified in CRISM OLINDEX2
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parameter images were also used to confirm Fractured Unit exposures and extend mapping in this region where HiRISE image data was not available (Salvatore et al.,
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2010). The Undifferentiated Terrain provides a unit representative of the overall geology of NE Syrtis as it incorporates diversity of units exposed in the region. It bears a mottled
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Basement Unit expressed as a knobby slopes and dissected crust, dispersed light-
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toned, polygonally-fractured surfaces (likely Fractured Unit), crater-preserving surfaces, and smooth cover. The surface flow features present in the Depression region,
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described by Skok and Mustard (2014), are contained within this unit. 4.2.5 The valley Cutting east-west across the mapping area is a valley bearing a narrow channel that ends in an ill-defined fan-shaped structure south of Jezero crater. This valley is the main morphologic feature dividing the Plains region from the Depression region (Figure
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 39 1c). For the final ~17 km of the channel prior to the fan, the channel is a few 100s of meters wide and filled with light-toned aeolian cover forming dunes perpendicular to the channel. Westward up the valley, the channel widens in sections along its length with large dune fields present at these locations. While features along the track of the
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channel bear some degree of sculpting towards streamlined features, the geomorphic units in the vicinity of the valley appear to largely preserve their primary depositional shape and extent. The western terminus of this valley opens onto large expanses of
highest elevated lengths of the channel.
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Capping Unit and Dark-Toned Fractured Unit at elevations 100 to 200 m lower than the
4.3 The contact with the Syrtis Major Volcanics
The border region between NE Syrtis and the Syrtis Major Volcanics has been
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studied (Ehlmann and Mustard, 2012; Quinn and Ehlmann, 2014a, 2014b). Our
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geomorphic mapping incorporates the Syrtis Major Volcanics, providing additional observations and descriptions. Summaries of these units, the Syrtis Major Volcanics
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unit (SMV), the Feature-bearing Slope Unit (FSU), and the Raised Boxwork Ridges
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(RBR) are provided in Table 2. Large contiguous regions in the west and south were mapped as the Syrtis Major
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Volcanics (SMV) unit where an elevated, flat, and relatively smooth surface preserving craters was observed. The Syrtis Major Volcanics are high-standing in the mapping area with elevations greater than or equal to that of the Plains region. A scarp face is observed with a sharp elevation drop of ~500 m towards the Depression region. The thickness of the Syrtis Major Volcanics, when exposed at this scarp face, is measured via a HiRISE DEM to be ~10 m. The steep slopes off of the Syrtis Major Volcanics are
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 40 largely smooth, but variable exposures of light-toned layers are observed on the slopes, and at the base knobs are observed high-standing above a smooth cover. The slopes are mapped separately as Feature-bearing Slope Unit (FSU) due to these morphological (and spectral, see below) characteristics. Multiple amphitheater-headed
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canyons are observed along this scarp wall with subtle fluvial channels that follow the topographic gradient across the volcanic surface and terminate at the amphitheater heads (Figure 21a). The flow features continue from the amphitheater heads
downwards into the Depression region and on into the valley (cf. Figure 21a, Lamb et
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al., 2014, and Skok and Mustard, 2014).
Strong but isolated high-Ca pyroxene detections, validated in ratioed spectra, are observed in CRISM images covering the Syrtis Major Volcanics, and ratioed spectra
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collected from these regions bear a broad absorption centered on ~2.2 µm, interpreted to be high-Ca pyroxene. We observe spectral signatures indicative of olivine-carbonate-
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bearing materials associated with mounds and knobs at the base of the Feature-bearing
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Slope Unit and appear to rise above the smooth slope cover. This suggests that the smooth slope of the Feature-bearing Slope Unit extends past the lower contact of this
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unit and obscures the downsection Capping Unit and Fractured Unit at the base. Lighttoned, layered outcrops of the Feature-bearing Slope Unit exhibit absorptions at 1.4 and
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1.9 µm and an inflection at 2.4 µm (Figure 21d). Variably present is a weak absorption with a minima at 2.26 µm (Figure 21g), and this absorption is furthermore variably paired with a weaker absorption at 2.21 µm that is on the order of the spectral noise. These absorption bands are consistent with laboratory spectra of polyhydrated sulfates and the mineral jarosite, but the variability of the 2.21 µm absorption may suggest some
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 41 degree of mixing or cation variance between jarosite and natrojarosite, which does not have a prominent absorption at this position (Figure 21f). These observations confirm the previous identification and interpretation by Ehlmann and Mustard (2012) in NE
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Syrtis, and the spatial variability of the sulfate mineral phases. Observed stratigraphically below the Syrtis Major Volcanics and within the
Feature-bearing Slope Unit are raised ridges meters in width and 10s to 100s of meters in length that frequently intersect at orthogonal angles forming a boxwork texture. Saper
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and Mustard (2013) previously mapped these ridges, and combined these Raised
Boxwork Ridges (RBR) and our Raised Linear Ridges together. Here, we separate the two as they have distinct morphological characteristics and stratigraphic positions. The ridges are high-standing above a smooth basement and interspersed with light-toned
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outcrops. The Raised Boxwork Ridges are inferred to be in a subsurface stratigraphic layer beneath the capping Syrtis Major Volcanics and are exposed subsequent to
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erosion of the Syrtis Major capping material (Quinn and Ehlmann, 2014b; Ebinger and
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Mustard, 2015).
The Syrtis Major Volcanics unit displays comparatively low thermal inertia values.
resulting in a modeled particle size of ~200 µm. These values correlate with a regolith
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0.5
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The analyzed locations have a measured thermal inertia of 238.98 ± 11.15 J m–2 K–1 s–
at the surface corroborated by the etched retained craters and influence from the darktoned cover. A dichotomy is observed in the thermal inertia values of the Featurebearing Slope Unit, which have a split distinction between higher- and lower-TI areas. The low TI areas produce a measured value of 207.43 ± 11.39 J m–2 K–1 s–0.5 and a modeled effective particle size of ~100 µm. These lower values correlate to large areas
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 42 of the thick, dark-toned cover. The high TI areas produce a measured value of 304.64 ± 30.11 J m–2 K–1 s–0.5 and a modeled effective particle size of ~560 µm. The higher values correlate to local high-standing features, including high-standing exposures of olivine-carbonate-bearing materials as identified using CRISM. The THEMIS DCS
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mosaic similarly displays this TIR dichotomy as an overall equal distribution of colors with minor enhancements of yellow and is observed for large portions of the Featurebearing Slope Unit (Figure 8f). This unit also displays strong isolated concentrations of purple (Figure 8g), correlating to mounds high-standing above the smooth cover. These
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data corroborate the interpretation of the high-standing mounds being exposures of the olivine-carbonate-bearing Fractured Unit. 5. Discussion
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5.1 Regional and local stratigraphy
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Our coordinated regional geomorphic mapping and VNIR and TIR mineralogical analyses allow NE Syrtis to be placed in context with other orbital analyses throughout
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Nili Fossae. Mapping by Goudge et al. (2015) of Jezero crater and its associated
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watershed to the north of our mapping area provides an excellent opportunity to place the stratigraphy of NE Syrtis in a larger context. The largest spatial overlap between our
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mapping effort and that of Goudge et al. (2015) occurs with their mapped Altered basement (Bal). In our analysis, we subdivided this Altered basement region into three units, Basement Unit, Fractured Unit, and Capping Unit. This observation reflects the fact that our mapping was performed at higher spatial resolution utilizing significant coverage of HiRISE images, allowing for more extensive morphological subdivision. Our mapping scale was 1:1,000 versus 1:100,000 for the Jezero crater paleolake system.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 43 Altered basement (and the Ridged and Dusty massive subunits) of Goudge et al. (2015) morphologically corresponds with the Basement Unit (particularly the Raised Linear Ridges subunit). These units display small, light-toned knobs, variable topography, and no obvious structure in select locations, matching observations of the
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dissected exposures of the Crustal Mounds, Knobby Plains, and Smooth Plains
subunits. The Altered basement units and the Basement Unit share a CRISM-inferred mineralogy of Fe/Mg-smectite. These units also share many morphological similarities with the Undifferentiated Terrain. Additionally, the valley cutting east-west through NE
the watershed as Valley network floor.
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Syrtis (Section 4.2.5) matches the spatial size and morphology of a valley mapped in
Morphological and mineralogical similarity is observed for the Mottled terrain
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(MT, and subunits) and the Light-toned floor unit (LTF) of Goudge et al. (2015) with our Fractured Unit. The Mottled terrain displays a similar, though less degraded,
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hummocky, corrugated surface as the Fractured Unit. The Light-toned floor unit displays
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light-toned blocky materials, as does our Fractured Unit. CRISM spectra collected of the Mottled terrain and Light-toned floor unit show absorptions indicative of a mixture of
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olivine and Mg-rich carbonate (Goudge et al., 2015), matching the mineralogy observed for the Fractured Unit. The Jezero crater watershed does not appear to have preserved
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the Capping Unit to the same extent as in NE Syrtis, though perhaps the Capping Unit was not emplaced this far northward or was emplaced in a patchy distribution. The Thin dark capping unit (Tcu) mapped on the rim of Jezero morphologically matches our Capping Unit.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 44 Recent mapping by Quinn and Ehlmann (2014a; 2014b) of the NE Syrtis sulfate unit shows that this unit, mapped here as the Feature-bearing Slope Unit, is laterally contiguous beneath the Syrtis Major Volcanics and continues ~55 km south of our
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mapping area. Paired morphology observations and TIR analysis allows NE Syrtis to be
compared with similar mapping results in Nili Fossae and the Isidis basin. As discussed above, our Fractured Unit and Capping Unit share thermophysical properties and
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morphology with the olivine/carbonate-bearing materials and low-olivine capping unit of Edwards and Ehlmann (2015), respectively. Combining high-resolution morphological and spectral investigations of others (Ehlmann and Mustard, 2012; Edwards and Ehlmann, 2015; Goudge et al., 2015) with our work suggests that these capping units
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may have been laterally contiguous, or that materials with these properties and compositions are common throughout this region. If these units were emplaced via
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similar or the same processes, then the units mapped in NE Syrtis would have been
north-northeast.
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contiguous throughout the Jezero watershed and continue further more towards the
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The observations of olivine-rich bedrock mapped by Hamilton and Christensen (2005) in Nili Fossae do not extend southward to include NE Syrtis, but the thermal
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inertias of ~455 J m–2 K–1 s–0.5 reported for the strongest olivine signatures are similar for our measurements of the Fractured Unit. Global maps of olivine by Ody et al. (2012) using OMEGA data show that NE Syrtis resides at the southernmost edge of the large Nili Fossae olivine-rich terrain. These two observations along with the similarities of the olivine-rich materials described by Goudge et al. (2015) and Edwards and Ehlmann
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 45 (2015) confirm that the NE Syrtis olivine-rich Fractured Unit is part of this regional unit, though perhaps the most degraded. These observations show that the geology and stratigraphy of NE Syrtis, while
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exceptionally exposed and bearing diverse alteration products, is associated with the geology of the larger Nili Fossae region as observed throughout are units bearing
comparable compositional and morphological characteristics in analogous stratigraphic succession. Our geomorphic and spectral analyses confirm that NE Syrtis exhibits
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considerable geomorphic and mineralogic diversity within a relatively small geographic area that is representative of the geologic processes occurring throughout the broader Nili Fossae region during the Noachian and Hesperian. The overall stratigraphy matches that of the regional Nili Fossae group of Mangold et al. (2007); a stratigraphy of
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basement phyllosilicates overlain by an olivine-rich unit and capped by the Syrtis Major lavas with a trend of decreasing alteration upsection. NE Syrtis adds to this
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lithostratigraphic sequence by exposing the diverse environmental history of this region
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as observed through the presence of alteration minerals not present in this fidelity or proximity elsewhere in Nili Fossae.
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5.2 Geological history of Northeast Syrtis Major
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We constructed a detailed geological history of NE Syrtis on the basis of our high-resolution geomorphic mapping and integration with previous mapping efforts and spectral characterizations. The history began with the pre-Noachian formation of the primitive martian crust and the large basin-forming impacts with the exception of Isidis. Primitive crustal materials may be observed in the region, perhaps identifiable through mafic signatures such as low-Ca pyroxene (Mustard et al., 2005), and large
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 46 megabreccia blocks displaying layering, hint at active geological processes prior to the basin-forming impacts affecting NE Syrtis. During the Noachian, the crust was aqueously altered, perhaps by subsurface
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hydrothermal circulation (Ehlmann et al., 2011) or top-down pedogenic weathering, producing Fe/Mg-smectites as the dominant hydrated silicate mineral. Additional
phyllosilicate formation hypotheses, including formation during the last stages of a magma ocean (Cannon et al., 2016) or via magmatic precipitation (Meunier et al.,
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2012), would be applicable here. The Isidis basin formed in the Middle-to-Late Noachian at 3.96 Ga (Werner, 2005), resetting the surface morphology with extensive ejecta deposits and excavated material in the mapping area. Megabreccia observed in the Basement Unit may be due to this impact and/or the pre-impact substrate. The overall
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topography of the region was likely a result of the cratering process and the subsequent crustal readjustment of the basin as evidenced by the formation of the Nili Fossae and
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the potential provenance of the large crustal mounds as part of a basin ring structure
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(Wichman and Schultz, 1989; Mustard et al., 2007). Contemporaneous or following the basin formation was a period of heavy
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cratering. Craters ranging from 100s of meters to a few kilometers in size were formed,
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and their influence may be observed in the present by the quasi-circular mesa-forming expressions of the Capping and Fractured Units and are suggestive of pooling into the topographic lows of pre-existing small cavities. The nearby Jezero crater formed subsequent to Isidis and would have covered the Plains region of NE Syrtis with a continuous ejecta blanket. The dynamic topography of the mapping area likely completed major structural alterations not long after the period of heavy cratering. The
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 47 Depression region and Jezero crater were formed prior to the emplacement of the olivine-rich Fractured Unit, as well as the unnamed 6 km crater that has departed from circularity, suggesting minor crustal movements or preferential erosion may have
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affected it. The major stratigraphic marker of the olivine-rich Fractured Unit was emplaced subsequent to the formation of the dynamic topography of the region. The olivine-rich Fractured Unit embays the Basement Unit and drapes the topography on both local and
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region scales. It is observed draping small topographic lows on the scale of 100s of meters, as well as covering the regional topography from on top of the southwest rim of Jezero crater, across the sloping plains, and draping the walls of the Depression region towards the Syrtis Major Volcanics. The draping of the olivine-rich Fractured Unit could
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additionally be explained by emplacement on an undulating crust, emplacement as an ash-fall deposit, or emplacement contemporaneous with basin formation as a sheet that
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adjusted with the large-scale crustal movement. The presence of megabreccia possibly
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within the Dark-toned Fractured Unit in the Depression region suggests that this olivinecarbonate-bearing Fractured Unit may be impact related, as suggested by Mustard et
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al. (2007).
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The Large Linear Features were also emplaced at this point as they are observed branching off of the Fractured Unit, though the process leading to their emplacement is difficult to deduce at an orbital scale. The bifurcation of the Large Linear Features at angles forming triple or orthogonal junctions hints that this subunit displaced the Basement Unit during emplacement of the Fractured Unit. The raised bounding ridges, and their sharp contacts, may suggest a contact metamorphism
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 48 interaction occurred between the Large Linear Features and the host Basement Unit. The raised bounding ridges and lower internal material indicate that the bounding ridges are hardened and less susceptible to erosion. The Large Linear Features are also observed in present-day troughs and appear to have conformed to the shape of these
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troughs, which may indicate that the olivine-rich melts followed topographic gradients on the surface during emplacement and pooled in topographic lows. These observations suggest an emplacement relationship exists between fracturing of the Basement Unit and the olivine-rich material, though here multiple hypotheses can exist. The Large
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Linear Features may be remnants of a network of fractures that supported the
mobilization of olivine-rich melts in the near-surface crust. The high-temperature fluids may have altered the host wall rock along its interface and, following subsequent
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erosion, this less erosion-susceptible rock became the high-standing bounding ridges. The sharp bifurcation angles seen in the Large Linear Features and the long lengths not
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deviating from a straight line may support this crustal-interaction hypothesis. Similarly, these observations may suggest emplacement occurred cotemporaneous with a
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pervasive fracturing of the crust, such as during impact basin formation. Therefore, the
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Large Linear Features could be indicative of extrusive volcanic dykes or an impact melt sheet emplaced and mobilized upon on a heavily fractured surface subsequent to the
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formation of the Isidis basin. These features do not exclude any of the current hypotheses for emplacement of the Fractured Unit. Enigmatically, the presence of the olivine-rich Fractured Unit on the rim and floor
of Jezero crater and its presence as quasi-circular features suggest a significant period of time elapsed between the formation of the Isidis basin and the emplacement of the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 49 Fractured Unit. Possible implications include the emplacement of the Fractured Unit subsequent to the formation of Jezero crater by a volcanic process, the olivine-rich material superposing Jezero crater being of different provenance than in NE Syrtis, or the formation of Jezero crater as a secondary crater formed by the fallback of ejecta
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during the formation of the Isidis basin. The formation age of the Nili Fossae is poorly constrained, although it is argued that they formed subsequent to the olivine-rich unit emplacement (Mustard et al., 2009), likely shortly after Isidis was formed (Wichman and
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Schultz, 1989), or in the Middle Noachian (Tanaka et al., 2005).
Subsequent to the emplacement of the Fractured Unit was the deposition of the Capping Unit. The origin of this material may have been volcanic flows into the region or an ash-fall deposit of volcanic or impact-generated debris. The layered nature of the
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Capping Unit suggests that either multiple deposits were emplaced or the unit was emplaced and, then during solidification, differentiated into two layers of differing
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physical properties. The layered nature is not always observed for the Capping Unit, but
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this may be the result of varying degrees of erosion and exposure. The layers may conversely represent temporally and spatially separated episodes of deposition.
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Multiple formation mechanisms are hypothesized for the carbonate observed in
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conjunction with the olivine-rich Fractured Unit. The timing of its formation is both important and difficult to constrain. If the carbonate formed in the process of hydrothermal serpentinization, a carbonatization system, or a talc-carbonate system, then the carbonate would have formed in the Late Noachian near-contemporaneously with the emplacement of the Fractured Unit. These mechanisms have been invoked to explain the Nili Fossae carbonate. The water would percolate through the olivine-rich
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 50 Fractured Unit, and the heat driving the reaction would be from the emplacement of the unit, the geothermal gradient, or burial digenesis resulting from the Syrtis Major Volcanics (Ehlmann et al., 2008; Brown et al., 2010; Viviano et al., 2013). If the carbonate formed via low-temperature alteration induced by overland flow of water into
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the region, then it may have been contemporaneous with the formation of the valley networks, regional rises in groundwater, regional glaciation, or an Isidis ice sheet in the Hesperian or early Amazonian (see below). If the carbonate formed from surface
weathering of olivine-rich rocks in a manner akin to evaporites observed on meteorites
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found in Antarctica (e.g., Jull et al., 1988; Velbel et al., 1991), then the carbonate may have formed at any time throughout martian history, including in recent times at the cold and dry surface conditions. This mechanism could activate on olivine-rich rocks
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exposed at the surface during transitory periods of limited surface water, or perhaps even thin films of water as was suggested for the carbonate observed by the Phoenix
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lander (Boynton et al., 2009).
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The depositional event subsequent to the emplacement of the Capping Unit, and after a period of non-deposition as suggested by its cratered surface, was the
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emplacement of the sulfate-bearing Feature-bearing Slope Unit. Raised Boxwork Ridges are confined to, and mapped as a subunit of, the Feature-bearing Slope Unit.
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Regional mapping and structural analysis favors a sedimentary emplacement origin for this unit and filled volume-loss fractures as the origin of the ridges (Quinn and Ehlmann, 2014b). If the sulfate-bearing layers were produced in a sedimentary depositional environment, the aqueous environment producing the Feature-bearing Slope Unit may have been relatively long-lived, as it produced ~500 m of layers. Further running
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 51 hypotheses for the emplacement of this unit include ash layers, volcanic flows, or sedimentary deposits that developed sulfate-bearing mineralogy via subsequent digenesis, perhaps in a manner similar to that observed at Meridiani Planum (e.g., McLennan et al., 2005). Sulfate formation in this manner could suggest that this deposit
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is tied to regional groundwater circulation (e.g., Andrews‐Hanna and Lewis, 2011),
therefore the formation timing of these sulfates may be similarly linked to global sulfateforming phenomena.
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Initiating at ~3.7 Ga (Hiesinger and Head, 2004; Ivanov et al., 2012) and
subsequent to the deposition of the Feature-bearing Slope Unit, the Hesperian volcanic flows from Syrtis Major began to enter the region. If these flows extended onto the Plains region of NE Syrtis, then a significant amount of subsequent erosion has
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occurred as there is no morphological evidence for remnant flows on the Plains region. The one exception being the Smooth Sloped Mounds with their possible dark-toned,
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high-Ca pyroxene cover; for if high-Ca pyroxene was inherent to the Smooth Sloped
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Mounds and not the cover, then this could suggest the flows crested the Plains region. The volcanic flows from Syrtis Major likely halted not much further than their present
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location and the subsequent erosion developed the scarp of Feature-bearing Slope Unit. At the contact with the Nili Fossae, the Syrtis Major Volcanics may have
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encountered ice deposits as evidenced by the highly irregular margins of lava flows in the region (Ivanov and Head, 2003). If the valley of NE Syrtis (Section 4.2.5) was formed by overland fluvial processes, then the presence of glaciers and surface waters could explain the flow patterns from the Syrtis Major Volcanics into the Depression region and eastward out through the valley, as opposed to following the main
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 52 topographic gradient south (Skok and Mustard, 2014). The valley needs to be reconciled with the observation that the geomorphic units remain relatively intact with minor sculpting towards streamlines shapes, and therefore the duration or intensity of fluvial activity may have been limited. Numerical simulations suggest that an ice sheet
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may have covered the entire Isidis basin and formed the thumbprint terrain on Isidis Planitia dated at 2.8 to 3.5 Ga (Souček et al., 2015). If the formation of the valley was in conjunction with the presence of an Isidis basin ice sheet, then the valley would have formed in the Late Hesperian or Early Amazonian, likely from runoff related to the ice
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sheet. Fluvial, alluvial, or glacial processes throughout the Hesperian and early
Amazonian may have been responsible for the erosion and removal of significant amounts of material from NE Syrtis, as suggested by the absence of continuous ejecta
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blankets for Jezero crater or the unnamed 6 km crater in the northwest quadrant of the mapping area, the high-standing nature and erosional scarp faces of the mesa-forming
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expressions of the Capping and Fractured Units, and the significant removal of the
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Capping Unit from the Fractured Unit.
It is difficult to pinpoint the time of formation of the Al-phyllosilicate clay-bearing
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deposits, as they appear distributed in isolated patches within the Fe/Mg-smectitebearing Basement Unit and not as a distinct stratum. Several hypotheses exist for the
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formation of Al-phyllosilicates. The deposits may have formed as a result of the alteration of somewhat heterogeneous parent rocks or pedogenic-type leaching of phyllosilicate-bearing parent rocks (Ehlmann et al., 2009), long-term weathering (Gaudin et al., 2011), or deep hydrothermal alteration (Ehlmann et al, 2011). The emplacement of a hot capping rock may have allowed the Al-phyllosilicate to form from
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 53 hydrothermal fluids circulating in a Fe/Mg-smectite parent rock (Ehlmann et al., 2009), and subsequent erosion exposing the Al-phyllosilicates at the surface. Our observations at the contact of the upsection Fractured Unit do not support the presence of a distinct stratum bearing Al-phyllosilicate in NE Syrtis. As we observe scattered detections of Al-
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phyllosilicates at the surface of the crustal basement, we favor the formation of this material via isolated leaching of the crustal rocks, perhaps influenced by heterogeneous initial composition or the result of localized surface weathering.
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Due to the presence of the parallel north-south trending dunes in NE Syrtis, it is suggested that the Amazonian period witnessed the slow and continual erosion of the surface by aeolian denudation, with the Isidis basin governing the motion of local air masses. The complexity of the landscape results from the differing mechanical
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properties of the various units, and the multiple events of deposition, aqueous alteration,
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and erosion. 6. Conclusions
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NE Syrtis exposes a stratigraphic sequence spanning the Noachian-Hesperian
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boundary, encapsulating ~250 Myr of geological history. Between the bookend stratigraphic markers of the Isidis-forming impact and the Syrtis Major lavas at 3.96 and
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~3.7 Ga, respectively, exists great geomorphic heterogeneity that is unifiable under five distinct units. They are, ascending the stratigraphic column: (1) the Basement Unit, (2) the Fractured Unit, (3) the Capping Unit, (4) the Featuring-bearing Slope Unit, and (5) the Syrtis Major Volcanics unit. NE Syrtis bears a local variation on the regional Nili Fossae group (Mangold et al., 2007) where basement phyllosilicates are overlain by an olivine-rich unit and topped by the Syrtis Major lavas, displaying a trend of decreasing
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 54 alteration. NE Syrtis adds to this lithostratigraphic sequence by exposing the diverse environmental history of this region as observed through the presence of alteration minerals that are not exposed or apparent in this fidelity elsewhere in Nili Fossae.
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NE Syrtis bears extensive coverage of the Nili Fossae regional olivine-rich unit that is variably altered to carbonate. Mapped as the Fractured Unit, it is identified here from the contact with the Syrtis Major Volcanics to the southwest rim of Jezero crater. The Large Linear Features of the Fractured Unit are intriguing and enigmatic landforms.
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Their geomorphic features suggest that the olivine-rich melts were mobilized in a highlyfractured surface or near-surface crust, with contact metamorphism being suggested by the raised bounding ridges. Megabreccia textures are observed within NE Syrtis, both stratigraphically below and within the olivine-rich Fractured Unit. In the Plains region,
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large and spatially distributed megabreccia appear stratigraphically older than the Fractured Unit and, in the Depression region, clast-rich breccias sharply contact the
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Fractured Unit and appear to be stratigraphically cotemporaneous. Despite the
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compilation of additional orbital observations presented herein, the continuation of both a volcanic and impact melt emplacement hypotheses remains viable.
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The presence of Fe/Mg-smectites, kaolin-group minerals, carbonates, and
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sulfates distributed amongst three of our geomorphic units (Basement Unit, Fractured Unit, Feature-bearing Slope Unit, respectively) confirms initial work by Ehlmann and Mustard (2012) that as many as four temporally distinct aqueous environments existed in NE Syrtis. These three units span the Noachian-Hesperian and Phyllosian-Theiikian boundaries and are capped by the Syrtis Major flows, suggesting an aqueous history of at least 250 Myr and extending further into the Noachian past. Spectral signatures of a
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 55 kaolin-group mineral are observed in isolated patches within the Basement Unit and not as a distinct stratigraphic unit. Their location and geomorphic characteristics argue for formation via isolated leaching or heterogeneous initial crustal composition, rather than extended weathering profiles or magma precipitation. The aqueous history of the region
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extends from the Late Noachian into the Hesperian and, depending on the formation mechanism for these alteration minerals, may extend to the Late Hesperian or Amazonian.
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Perhaps no other location on Mars offers the diversity of geology, mineralogy, and geomorphology in such spatial proximity than NE Syrtis. The geomorphic diversity is revealed through the variety of landforms observed in the mapped geomorphic units, the fluvial valley cutting through the mapping area, the geomorphic remnants from
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impact and volcanic processes, and from the eons of denudation of the landforms and aeolian processes. The chronology of the region is relatively well constrained as it is
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bookended by the impact processes that resulted from the formation of the Isidis basin
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to the later emplacement of the Syrtis Major Volcanics. In close spatial proximity are igneous mineralogies spanning from potential primitive crust to those from later
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volcanism and impact processes, in addition to alteration mineralogies alluding to multiple distinct aqueous environments. The stratigraphically-bounded alteration
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minerals within NE Syrtis suggests the presence of multiple, potentially habitable (Ehlmann and Mustard, 2012), environments located in this region in the martian past. Understanding the detailed formation mechanisms of these alteration minerals, the geological history of the geomorphic units, and the chronology of active processes in this ancient terrain will greatly constrain the likelihood of these environments as being
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 56 conducive to life. As we turn our eyes to the next target for in situ exploration on the martian surface, no location offers better access of the gamut of geological processes active at Mars than Northeast Syrtis Major.
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Acknowledgments: We thank Chris Edwards, Bethany Ehlmann, Tim Goudge, Ralph Milliken, and Sandra Wiseman for insightful discussions on the geology of the Nili
Fossae, and Chris, Tim, and Sandra for producing data products analyzed in this study. The quality of this paper was greatly improved by detailed and constructive reviews
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provided by Sheridan Ackiss, Briony Horgan, and an anonymous reviewer. We also thank the Mars Reconnaissance Orbiter and Mars Odyssey teams for their data
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collection efforts.
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ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 70 Figure 1: Northeast Syrtis Major, Mars. North is up in all images. (a) The location of Northeast Syrtis Major at the nexus of the western edge of the Isidis basin, the northeastern Syrtis Major volcanic province, and the southeastern extent of the Nili Fossae. The mapping area is delineated by the black rectangle, and (b) is indicated with
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the dashed white rectangle. The background is MOLA colored topography overlain on the ~100 m/pixel THEMIS global daytime infrared mosaic. (b) Local context for the
mapping area (black rectangle). Jezero crater is immediately to the northeast, and the Syrtis Major volcanic province begins at the south and west of the mapping area. To the
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north is the watershed for the western fan of Jezero crater (Goudge et al., 2015). The background is the THEMIS mosaic as in (a), and the location of 1c is indicated by the dashed white rectangle. (c) Northeast Syrtis Major. The mapped area (black rectangle)
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is divided into three regions: the Plains, the Depression, and the Syrtis Major Volcanics. The unnamed 6 km crater discussed in the text is the topographic depression observed
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in the top-left quadrant. The basemap is a CTX-derived DEM overlain on a mosaic of
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Table S2.
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CTX images. The CTX images comprising the mosaic are listed in the supplementary
Figure 2: HiRISE and CRISM coverage of Northeast Syrtis Major. (a) Blue polygons outline the extent of HiRISE image coverage of the mapping area (black rectangle). In these regions, the geomorphic mapping was performed using the HiRISE imagery. In the remaining regions, the mapping was performed using the CTX mosaic, which is seen here below the blue polygons. The CTX and HiRISE images utilized in this
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 71 analysis are listed in the supplementary Table S1 and Table S2, respectively. (b) Orange polygons display the footprints of CRISM images used in our analysis and their coverage of the mapping area (back rectangle). The CRISM images utilized in this
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analysis are listed in the supplementary Table S3.
Figure 3: Stylized stratigraphic column of Northeast Syrtis Major. Relative relationships
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of the geomorphic units are shown, as well as the stratigraphic location of select
geomorphic subunits. Depicted are the Basement Unit (BAU) and Raised Linear Ridges (RLR) subunit, the Fractured Unit (FRU) and Large Linear Feature (LLF) subunit, the
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Capping Unit (CAU), the Feature-bearing Slope Unit (FSU) and Raised Boxwork Ridges (RBR) subunit, and the Syrtis Major Volcanics (SMV) unit. See main text and Table 2 for
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description of units. Colors and legend correspond to that of the geomorphic map
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(Figure 4).
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Figure 4: (a) Geomorphic map of Northeast Syrtis Major. Colors and map symbols for (a) and (b) correspond to the legend as shown in (c). See main text and Table 2 for description of units. Black lines indicate mapped contacts between geomorphic units while dashed black lines are interpreted contacts. The latter are exemplified by contacts that disappear and reemerge from aeolian cover, contacts characterized at HiRISE resolution (~0.25 m/pixel) but mapped in lower-resolution CTX images (~7 m/pixel), and
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 72 contacts that were mapped with the guidance of map-projected CRISM images. (b) Geomorphic map as in (a) but transparent and overlain on the CTX mosaic (CTX images used are listed in Table S1). HiRISE images (listed in Table S2) are rectified to and overlain on the CTX mosaic. Locations are indicated for subsequent figures shown
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throughout the paper. Note that Figure 11a continues further north than indicated by the white rectangle. (c) Geomorphic map legend depicting color and symbols of the mapped units and relative stratigraphic positions. Geomorphic subunits and minor units in the
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paper are shifted to the right in the legend. North is up in both images.
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Figure 5: The Plains of Northeast Syrtis Major. (a) As in Figure 4b, geomorphic map overlain on a mosaic of HiRISE images rectified and overlain on the CTX mosaic. This
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focus view of the Plains region shows the fine-scale geomorphic heterogeneity. The overall stratigraphic sequence of Basement Unit, Fractured Unit, and Capping Unit can
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be observed. Large Linear Features are observed bifurcating at distinct angles. (b)
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Corresponding area to (a) without the geomorphic map overlain. Locations are indicated for subsequent figures shown throughout the paper. The spatial scale of (b) is the same
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as (a). North is up in both images.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 73 Figure 6: Geomorphic example of the Crustal Mounds (CMD) of the Basement Unit (BAU). (a) Observed are high-standing mounds with a peak, plateau, or elevated ridge from which lineations on the slopes fan away. The CMD are embayed, and here surrounded, by the Fractured Unit (FRU). Subframe of HiRISE image
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ESP_016219_1980 with geomorphic unit contacts drawn. The arrow shows the location for the numerator spectrum used for the CRISM ratioed reflectance spectrum (g). (b-c) Geomorphic example of the Knobby Plains (KNP) of the BAU. (b) Geomorphic map subframe depicting the KNP and the complex relationship with the Capping Unit and
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FRU. (c) HiRISE mosaic subframe of corresponding area to (b) with unit contacts
drawn. Circular light-toned features are observed in the plains unit (arrow i) that may be indicative of megabreccia rounding or light-toned material infilling small circular
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depressions. The transition between the visibly discernable plains surface textures is shown (arrow ii); the lowermost is a darker-toned plain, and meters above this is a
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smoother, lighter-toned plain. The transition between the KNP and CMD in this scene bears strong spectral signatures for a kaolin-group mineral (arrow iii; also showing the
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location for the numerator spectrum used for the CRISM ratioed reflectance spectrum in
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Figure 7a). North is up in all images. (d-g) Laboratory and CRISM ratioed spectra indicative of the dominant spectra observed in the BAU. The saponite spectrum (d) is
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compared here with the narrow absorptions centered at ~1.4, 1.9, and 2.3 μm, indicative of Fe/Mg-smectites, collected from the BAU (e). The laboratory low-Ca pyroxene spectrum (f) matches the spectrum collected from the BAU with (g) broad absorption centered at ~2 μm and second dip in the VNIR towards broad absorption short of ~1 μm. The saponite (LASA53) and orthopyroxene (C2PE30) spectra are taken
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 74 from the CRISM spectral library (CRISM Science Team, 2006). Vertical lines are
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located at 1.4, 1.91, and 2.3 μm.
Figure 7: Geomorphic context for the kaolin-group mineral spectral signatures. (a) A kaolin-group mineral is identified in CRISM ratioed spectra by absorptions at ~1.4, 1.9, and 2.2 μm. The laboratory kaolinite spectrum (top; LAKA02), taken from the CRISM
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spectral library (CRISM Science Team, 2006), is shown in comparison to the ratioed CRISM spectrum (bottom). Vertical lines are located at 1.41, 1.91, and 2.2 μm. (b-c) The kaolin-group mineral spectral signatures are observed in light-toned, isolated
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patches in the Basement Unit (BAU). These locations display light-toned features on the surface with no internal fabric at the highest resolution apart from dark-toned fractures
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and cover. The outcrops bearing these signatures commonly have sharp contacts with
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neighboring features in the BAU. North is up in both images.
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Figure 8: Mosaicked THEMIS decorrelation stretch of Northeast Syrtis Major, with a redgreen-blue combination of band 8 (11.79 μm), band 7 (11.04 μm), and band 5 (9.35 μm), respectively. The image displays the mapping area (cf. Figure 8 with Figure 4). The contacts of the mapped geomorphic units are overlain. The Basement Unit (BAU) and its geomorphic subunits are predominately green (a). Regions mapped as Fractured Unit (FRU) (b) display a predominately purple color, corroborating the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 75 enrichment in olivine observed by CRISM. The Capping Unit (CAU) displays a greenyellow color with isolated spots of purple (c). Outcrops of the Capping Unit (CAU) in the Depression region bearing significant dark-toned cover display an orange and green color (d). The Syrtis Major Volcanics (SMV) (e) display an equal distribution of all the
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colors (green, blue, and magenta), with the exception of orange. The Feature-bearing Slope Unit (FSU) displays an overall equal distribution of colors with minor
enhancements of yellow (f), but this unit also displays strong isolated concentrations of purple (g) correlating to mounds high-standing above the smooth cover. The
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Undifferentiated Terrain (UNT) unit displays a patchy mosaic of purples and greens with
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coherent features a few pixels in size (h). North is up in the image.
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Figure 9: The Fractured Unit (FRU). (a) Geomorphic map transparent and overlain on HiRISE image ESP_016786_1980. The subset of geomorphic map symbols and colors
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visible in this frame are included. The locations of (b) and (c) are indicated by the black
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rectangles and are subframes of HiRISE image ESP_016786_1980. (b) Geomorphic example of the FRU. Light-toned blocks of various sizes are seen closely packed in a
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darker supporting material. The dark-toned material forms polygonal shapes around the light-toned block, giving this unit a fractured appearance. (c) Geomorphic example of the FRU and Smooth-Sloped Mounds (SSM). The SSM are stratigraphically-equivalent variants on the Capping Unit (CAU) that bear a dark-toned mantle on their smooth, gradual, slope. A deposit of the SSM is observed in the lower-left of this subframe and is interpreted to be a CAU bearing a dark-toned cover. North is up in all images.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 76 Figure 10: Geomorphic examples of banding of the Fractured Unit. (a) Context for the banding of the Fractured Unit (FRU) concentric to the Capping Unit (CAU). Subframe of a HiRISE mosaic constructed from images listed in Table S2. (b) Banding observed within the FRU meters in width and approximately concentric to the contact with the
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CAU. This banding appears to occur on a relatively flat surface. (c) Context for the
banding of the Fractured Unit with a layered appearance. Subframe of HiRISE image ESP_027902_1980. (d) Alternating light- and dark-toned bands of the Fractured Unit of non-uniform thickness and displaying a wavy appearance. (e) Outcrop of the Fractured
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Unit displaying a layered appearing on a scarp face 10s of meters in height. North is up
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in all images.
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Figure 11: Draping of the Fractured Unit. (a) Subframe of a Digital Elevation Model (DEM) made using HiRISE stereo images ESP_024513_1980 and ESP_025370_1980.
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The DEM is colorized so that reds indicate high elevations and blues low elevations.
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Profiles a-a’ and b-b’ are indicated, and elevation data extracted for these profiles are the basis of the cross sections of (b). North is up in the image. (b) Colorized and
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interpreted cross sections corresponding to HiRISE DEM topography geographically correlated with profiles a-a’ and b-b’. The mesa-forming expressions of Capping Unit (CAU; red) and Fractured Unit (FRU; green) display a draping nature where the units are observed to rise up multiple slopes of a Basement Unit (BAU; blue) depression, as shown in the cross sections. The draping nature is further demonstrated by the inability
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 77 to fit a flat plane along the mapped contact between the FRU and the CAU. Colors
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correlate with those of the geomorphic map in Figure 4.
Figure 12: CRISM ratioed reflectance spectra form the Fractured Unit. Laboratory reflectance of (a) magnesite (KACB03A), (b) saponite (LASA53), and (c) fayalite
(C1PO58) taken from the CRISM spectral library (CRISM Science Team, 2006) are
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included for comparison. (d) Observed spectral signatures are consistent with a mixture of olivine (broad ~1 μm) and carbonate (paired ~2.3 and 2.5 μm). The ~1.9 μm absorption is indicative of either hydrous Mg-rich carbonate or anhydrous carbonate and the admixture of a hydrated silicate (e.g., Fe/Mg-smectite). (e) Spectra are consistent
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with Mg-rich carbonate with paired absorptions at approximately 2.31 and 2.51 μm or
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2.32 and 2.52 μm, though band minima approach paired positions at 2.33 and 2.53, indicative of possible Ca in the crystal structure, such as a dolomite (Hunt and
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Salisbury, 1971). Vertical lines are located at 1.91, 2.31, 2.39, and 2.51 μm.
Figure 13: The Large Linear Features (LLF). (a) Geomorphic map subframe, as in Figure 4, depicting the LLF. The LLF seamlessly join with the Fractured Unit, and the orthogonal and triple junctions formed by the LLF are visible. (b) HiRISE mosaic subframe of corresponding area to (a). Images are subframes of the mosaic of HiRISE images listed in Table S2, and is the background of (a). The location of (c) is shown in
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 78 the white rectangle. (c) Focus view of the LLF depicting the light-toned and blocky material of the LLF in a dark-toned matrix. (d) Alternate view of the LLF depicting the topographic variability with the geomorphic unit contacts overlain on a colorized HiRISE stereo-derived DEM from image pair ESP_016575_1980 and ESP_016509_1980. The
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LLF supporting an upsection Capping Unit is observed at topographic highs, but the LLF are also observed recessed into the Basement Unit and present in topographically low
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troughs. North is up in all images.
Figure 14: The raised bounding ridges of the Large Linear Features (LLF). Raised linear
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features are observed along the contact between the LLF and the downsection Basement Unit (BAU). (a-c) The bounded LLF bear a center of light-toned blocky
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material with a darker supporting material, and the bounding features are lighter in tone and form a sharp contact with the BAU. Three profiles are identified and correspond to
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the profiles in (d). Images are subframes of the mosaic of HiRISE images listed in Table
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S2, and north is up in all images. (d) Topographic profiles perpendicular to the LLF with raised ridges measured from HiRISE DEMs derived from stereo image pairs
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ESP_016219_1980 and ESP_016364_1980, and ESP_016575_1980_ESP 016509_1980. The profiles show the bounding ridges to be meters in height when the LLF are high-standing above the BAU. The topography drops towards the center of the LLF, but the center remains a few meters above the level of the host unit.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 79 Figure 15: Mesa-forming expressions of the basal Fractured Unit (FRU) and the upper Capping Unit (CAU). (a) These units are mapped throughout the study area with the FRU more spatially extensive than the CAU. Geomorphic map overlain on a subframe of HiRISE image ESP_016443_1980. (b) Subframe of HiRISE image
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ESP_016443_1980, with corresponding area to (a), depicting the local context. The crater-preserving CAU is observed as well as the separation into small mesas. The location of (c) is indicated by the white rectangle. The approximate vantage point for Figure 16a is shown with the white ‘>’ symbol. (c) The layered nature of the CAU. Here,
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the top crater-preserving layer of the CAU is clearly distinguished from the lower lighttoned, fractured, and blocky layer. The lowermost outer rim, beneath the light-toned,
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fractured CAU layer, is the olivine-carbonate-bearing FRU. North is up in all images.
Figure 16: Mesa-forming expressions of the basal Fractured Unit (FRU) and the upper
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Capping Unit (CAU). (a) HiRISE DEM perspective view produced from stereo pair
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images ESP_015942_1980 and ESP_016443_1980. The Fractured Unit (green) and Capping Unit (red) are outlined and colored with their corresponding map unit color in
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(a) and (b). The HiRISE DEM topographic profile A–A’ is shown, and an interpretive stratigraphic cross section of this profile is shown in (b). The vantage point for this image is shown on Figure 15b. (b) The two characteristics of the CAU include a craterretaining upper surface and a lower layer of light-toned blocks bearing boulder-shedding slopes. The olivine-carbonate-bearing FRU is stratigraphically between this unit and the
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 80 phyllosilicate-bearing Basement Unit (BAU). The contacts in this two-dimensional profile
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are interpreted and illustrative.
Figure 17: The quasi-circular mesa-forming expressions of the Capping and Fractured Units. Subframes of a HiRISE mosaic constructed from images listed in Table S2. Numerous occurrences display a quasi-circular appearance formed by both the
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Fractured Unit (FRU) and Capping Unit (CAU). (a-d) Quasi-circular exposures of
approximately equal spatial extent. The local context of (c) can be observed in Figure 10a. (e) Quasi-circular shapes are common at the scale of a few 100 meters, but
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expressions on the scale of several kilometers also display approximately equal
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perpendicular diameters and some rounded edges. North is up in all images.
Figure 18: The Dark-Toned Fractured Unit (FRU-DT). (a) The crater-preserving surface
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of the Capping Unit (CAU) is observed in the Depression region with a darker
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appearance than on the Plains region. The CAU transitions to a smooth, dark surface with the light-toned, fractured, and blocky appearance of the FRU–DT observed at the outer edges. The location of (b) is indicated with the white rectangle. (b) The outer reaches of the FRU-DT. Here the light-toned, fractured, and blocky component of the FRU is observed at a contact with the Basement Unit. Away from the contact and
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 81 towards the center of the FRU-DT is an increasing dark-toned cover. North is up in both
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images.
Figure 19: Geomorphic example of the Raised Linear Ridges (RLR). (a) Areas mapped as RLR bear ridges meters in width, hundreds of meters in length, and are highstanding above a relatively smooth basement. Subframe of HiRISE image
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ESP_038029_1980 overlain by a corresponding subframe of the geomorphic map
(Figure 4). North is up in the image. (b) As in (a), but without the geomorphic map. The RLR are observed stratigraphically below the Fractured Unit (FRU) and are categorized
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as a subunit of the Basement Unit. (c) CRISM ratioed reflectance spectrum (d) collected of the RLR through a FRU erosional window. Narrow absorptions observed centered at
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~1.4, 1.9, and 2.3 μm are indicative of Fe/Mg-smectites. A weak absorption at ~2.39 μm may be present, and can be indicative of Fe/Mg-smectites or phyllosilicates, including
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saponite (e) or talc (f). The saponite spectrum (LASA53) taken from the CRISM spectral
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library (CRISM Science Team, 2006), and the talc spectrum (GDS23) is from the USGS digital spectral Library (Clark et al., 2007). Vertical lines are located at 1.41, 1.91, 2.31,
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and 2.39 μm.
Figure 20: Breccia-bearing terrain in the Depression region. (a) Large megabreccia blocks 10s of meters in size are observed at a flat surface as evidenced by small
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 82 boulders casting shadows (arrow i). Some blocks display an internal texture suggestive of layering or banding present prior to disruption via impact (arrow ii). One breccia block appears to have been offset ~10 m by faulting (arrow iii). (b) Smaller breccia blocks are observed in a clast-rich outcrop that sharply contacts the olivine-rich Fracture Unit (light-
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toned fractured outcrops surrounding the clast-rich breccia). (b) is to the southwest of (a) showing the northwest trend of breccia blocks become larger and more spread out in the dark supporting material. Subframes of HiRISE image ESP_018988_1980. North is
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up in both images.
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Figure 21: The Syrtis Major Volcanics (SMV) unit, Feature-bearing Slope Unit (FSU), and Undifferentiated Terrain (UNT). (a) Subframe of the CTX mosaic (Table S1)
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displaying the amphitheater heads and subtle channels leading towards the Depression region. The Raised Boxwork Ridges (RBR) of the FSU are visible where polygonal
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networks of ridges meters in width are stratigraphically below the SMV and within the
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FSU. (b) CRISM laboratory spectra of the Feature-bearing Slope Unit. Laboratory spectra are of (c) polyhydrated sulfate (799F366), (e) jarosite (C1092F53), and (f)
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natrojarosite (LASF28A) taken from the CRISM spectral library (CRISM Science Team, 2006). CRISM spectra of this unit match absorptions at 1.4 and 1.9 µm and an inflection at 2.4 µm (d) matching the shape of polyhydrated sulfates. An absorption at 2.26 µm (vertical line) is variably observed in this unit (d and g). If paired with an asymmetrical 2.21 µm absorption, jarosite would be the matching spectrum, but this absorption, if present, is on order of the noise. As a result, natrojarosite also matches absorptions in
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 83 this spectrum, suggesting spatial variability of the sulfate minerals present in this unit as jarosite has been identified in nearby outcrops by Ehlmann and Mustard (2012). The spectral shape and absorption at ~1.4 and 1.9 µm, respectively, suggest that these spectra may not be representative of a single phase, and that multiple sulfates or other
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minerals including phyllosilicates may be intermixed, or alternatingly stratified in these
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outcrops.
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ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 104 Table 1. Compendium of Running Hypotheses Applicable to Northeast Syrtis Major
ED
PT CE AC
CR IP T
Impact melt or volcanics mobilized on a fractured surface; Dyke-fed volcanism
M
Large Linear Features emplacement
Carbonate formation
References Hamilton et al., 2003; Hoefen et al., 2003; Hamilton and Christensen, 2005; Mustard et al., 2005, 2007, 2009; Tornabene et al., 2008; Poulet et al., 2009 this work Fractured Unit, Large Linear Features Ehlmann et Fractured al., 2008; Unit Murchie et al., 2009; Mustard et al., 2009; Brown et al., 2010; Ehlmann and Mustard, 2012; Viviano et al., 2013; Edwards and Ehlmann, 2015
AN US
Feature and Process Hypotheses Olivine-rich unit emplacement Basaltic or picritic lava flows; Impact melt from Isidis basin formation
Relevant Northeast Syrtis Major Geomorphic Unit(s) Fractured Unit
Hydrothermal alteration of the ultramafic host rock at slightly elevated temperatures; Contact metamorphism; Precipitation from transitory shallow lakes; Weathering of olivine-rich rocks
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 105 Surface weathering; Hydrothermal alteration in the shallow crust; Magmatic precipitation; Impact origin; Alteration by supercritical fluids during magma ocean
Kaolin-group mineral formation
Pedogenic-type leaching; Hydrothermal alteration; Heterogeneous initial composition Serpentinization of the olivinerich unit in a hydrothermal environment; Contact metamorphism set up by emplacement of hot olivine-rich rocks; Deep burial by Syrtis Major lavas
ElkinsTanton, 2008; Ehlmann et al., 2009, 2011; Gaudin et al., 2011; Ehlmann et al., 2011; Meunier et al., 2012; Tornabene et al., 2013; Cannon et al., 2016 Ehlmann et al., 2009; Gaudin et al., 2011 Brown et al., 2010; Ehlmann et al., 2010; Michalski and Niles, 2010; Viviano et al., 2013 McLennan et al., 2005; Wray et al., 2011; Ehlmann and Mustard, 2012; Quinn and Ehlmann, 2014b Mustard et al., 2007, 2009; Tornabene
Basement Unit
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Crustal phyllosilicate formation
Altered layered volcanic flows; Altered ash flows or falls; Lacustrine groundwater and evaporite sediments
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Sulfate formation
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Serpentine formation
Megabreccia emplacement
Isidis basin megaregolith; PostIsidis impact gardening; Zones of heterogeneous composition; Igneous intrusions
Basement Unit
Fractured Unit
Featurebearing slope Unit
Basement Unit, Fractured Unit
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 106 et al., 2013
Mineralized fracture zones; Breccia dykes and crater-related faults; Sedimentary filled volumeloss fractures
Fluvial feature formation
Formation with regional fluvial activity; Formation with regional glaciation
Head and Mustard, 2006; Saper and Mustard, 2013; Quinn and Ehlmann, 2014b Skok and Mustard, 2014; Souček et al., 2015; this work Elkins‐ Tanton et al., 2005; Skok et al., 2012; Grott et al., 2013
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Magma ocean; Volcanism
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Formation and exposure of primitive crust
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Basement Unit, Raised Linear Ridges, Featurebearing Slope Unit, Raised Boxwork Ridges Basement Unit, Fractured Unit, Capping Unit
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Erosion-resistant ridge formation
Basement Unit
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 107 Table 2. Geomorphic Units Mapped in Northeast Syrtis Major
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Crater – Young
Crater with circular rim and a discernab le ejecta blanket obscuring contacts of other geomorp hic units.
THEMIS Therma l Inertia
Modeled Effective Particle Sizes
Unit Interpr etatio Relevant n Figure(s) Surfici al debris, mobili zed by aeolia n proces ses in a genera l eastwest motio n (down slope towar ds Isidis Basin). Geolog icallyrecent crater forme d subseq uent to the extensi ve region al erosio n.
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Interpret ed CRISM Signature s
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Aeolian Debris Cover
Patches of dune fields obscuring contacts of other geomorp hic units. Dune crests track approxim ately northsouth.
PT
ADC
Unit Namea
Unit Descripti on
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Unit Map Symb ol
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 108
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CE AC FSU
highCa 238.9 pyroxe 8 +/ne 11.15
~200 µm
304.6 4 +/polyhy 30.11 drated (high sulfate TI) / , 207.4 jarosit 3 +/e, 11.39 natroja (low rosite TI)
~560 µm (high TI) / ~100 µm (low TI)
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Syrtis Major Volcani SMV cs Unit
Elevate d, flat, relative ly smooth surface preserv ing craters. Steep slopes off of the SMV variabl y exposi ng lighttoned layers, crustal knobs,
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Lava flows from the Syrti s Majo r volca nic provi nce halti ng in the vicini ty of Nort heast Syrti s Majo r. 3, 21 Layer ed sedi ment s depo sited in an aque ous or volca nic syste 3, 21
Feature -bearing Slope Unit
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 109
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— Raised boxwork ridges
Raised ridges meters wide and 10s-100s meters long in a boxwork texture highstanding above a smooth basement with intersper sed lighttoned outcrops.
m, unde rwen t sulfa te preci pitati on, then erod ed and cover ed givin g smoo th appe aran ce.
CR IP T
or FRU highstandin g above the smooth cover.
Highstandi ng ridges forme d as filled volum e-loss fractur es (Quinn and Ehlma nn, 2014b) .
3, 21
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 110
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Volca nic flow or ashfall depo sit (volc anic or impa ctgene rated ). Lacki ng abun dant Fe2+ in mafic mine rals or poorl y cryst alline and/ or glass y. Curre nt ~375 µm expr
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Unit with an upper most craterpreserv ing surface and a variabl yexpose d lower layer of lighttoned fractur unrem ed arkabl blocks. e
CAU
Capping Unit
276.9 +/18.5
3, 5, 6, 9–11, 13, 15– 18
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 111
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essio n gove rned by exte nt and mech anica l prop ertie s of unde rlyin g FRU.
SSM
— Smoothsloped mounds
CAU variant lacking craterpreservat ion, and bearing smooth, gradual, darkertoned slopes.
high-Ca pyroxene
Varian t of CAU with weake r mecha nical proper ties or erosio nal history produc ing smoot h slopes and covere d with darktoned highCa
5, 9
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 112 pyroxe nebearin g deposi ts.
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Corrug ated surface of lighttoned, fractur ed blocks surrou nded by darktoned materi al. Sparsel y cratere d. Variabl e bandin g.
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A coars elycryst alline olivin erich unit empl aced as impa ct melt or volca nism, rapid ly cover ed by the CAU. It has been varia bly alter ed to carb 3, 5, 6, onat 9–13, e. 15–19
FRU
Fracture d Unit
olivine, Mgrich carbon ate, ~1.9 µm hydrati on feature , Fe/Mg- 420.2 smectit 9 +/e 20.74
> 1 mm
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 113
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Olivine -rich materi al forme d into linear featur es via mobili zation within crust or at the surfac e during empla cemen t as dykes or mobili zation of an impact -melt sheet. Meta morph ism along contac t genera ted erosio nresista nt walls now highstandi ng followi ng
LLF
— Large Linear Features
Linear features 10s-100s meters wide and 1000s of meters long compose d of lighttoned fractured blocks surround ed by darktoned material.
3, 5, 13, 14
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 114
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erosio n.
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Baseme nt Unit
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BAU
low-Ca pyroxe ne, Fe/Mgsmectit e, kaolingroup 255.1 minera 1 +/l 18.05 ~260 µm
CMD
— Crustal Mounds
High relief peak, plateau, or elevated ridge from which lineations fan away.
3, 5–7, 9, 11, 13–16 Remna nts of ancien t crust. Expres sion govern ed by topogr aphy forme d by Isidis impact and distrib ution of FRU. Highly erode d and variabl y covere d with
5, 6, 9, 13, 15
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 115 debris.
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Highstandi ng ridges forme d throug h the mobili zation and crystall ization of fluids, with lighttoned phyllos ilicatebearin g outcro ps expose d on the floor in betwe en. 3, 19 Expans ive plains with numer ous mound s of ancien t 5, 6, 9, crustal 13,15
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Rectilinea r features highstanding above smooth basement . Meters in width and 100s of meters in length. Expansive smooth surface with many intersper sed knobs, mounds, and ridges.
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RLR
— Raised Linear Ridges
KNP
— Knobby Plains
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 116
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materi al, erosio nal debris, and isolate d FRU remna nts. Patchy erosio n produc t distrib ution. Surfac e exposi ng ancien t basem ent. Large blocks likely megab reccia or accum ulation and erosio n of materi als when showin ga circula r appear ance. Planar flows 5, 9, 13, of 15
SMP
— Smooth Plains
Smooth, darktoned surface. Heteroge neous internal structure. Abundant , but dispersed , decamete r lighttoned blocks.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 117
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LCM
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— Large Crustal Mounds
Similar morpholo gy to CMD but with elevation s of 100s of meters.
CRE
Crater – Eroded
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volcani c or erosio n produc ts may be presen t. Moun ds forme d nearconte mpora neousl y with Isidis. Heavil y erode d but mainta ining high topogr aphy. Ancien t craters forme d during the early heavy bomba rdmen t. Subse quentl y erode d and slightly defor med by
Crater of significan t size (≳ 1km) with no discernab le eject blanket dominati ng the local morpholo gy.
ACCEPTED MANUSCRIPT Northeast Syrtis Major – Revision 3 – 118
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Mixed expressio ns of the above units. Unit contacts below Undifferen CTX tiated resolutio UNT Terrain n. a Geomorphic units are indicated in bold, subunits of the geomorphic units are indicated in italics, and minor units not significant to the text are indicated in plain letters.
CR IP T
minor tectoni c move ments.
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