Groundwater flow induced collapse and flooding in Noctis Labyrinthus, Mars

Author’s Accepted Manuscript Groundwater flow induced collapse and flooding in Noctis Labyrinthus, Mars J. Alexis P. Rodriguez, Mario Zarroca, Rogelio Linares, Virginia Gulick, Catherine M. Weitz, Yan Jianguo, Alberto G. Fairén, Hideaki Miyamoto, Thomas Platz, Victor Baker, Jeffrey Kargel, Natalie Glines, Kana Higuchi

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To appear in: Planetary and Space Science Received date: 15 June 2015 Revised date: 12 December 2015 Accepted date: 20 December 2015 Cite this article as: J. Alexis P. Rodriguez, Mario Zarroca, Rogelio Linares, Virginia Gulick, Catherine M. Weitz, Yan Jianguo, Alberto G. Fairén, Hideaki Miyamoto, Thomas Platz, Victor Baker, Jeffrey Kargel, Natalie Glines and Kana Higuchi, Groundwater flow induced collapse and flooding in Noctis Labyrinthus, Mars, Planetary and Space Science, http://dx.doi.org/10.1016/j.pss.2015.12.009 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 galley proof before it is published in its final citable 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.

Groundwater flow induced collapse and flooding in Noctis Labyrinthus, Mars J. Alexis P. Rodriguez1,2*, Mario Zarroca3, Rogelio Linares3, Virginia Gulick2,4, Catherine M. Weitz1, Yan Jianguo5, Alberto G. Fairén6,7, Hideaki Miyamoto8 , Thomas Platz1,9, Victor Baker10 , Jeffrey Kargel10, Natalie Glines2,4, and Kana Higuchi6 1

Planetary Science Institute, 1700 East Fort Lowell Road, Suite 106, Tucson, AZ 85719-2395,

USA. 2

NASA Ames Research Center, MS 239-20, Moffett Field, CA 94035.

3

Department of Geology, Autonomous University of Barcelona, 08193 Bellaterra, Barcelona,

Spain. 4

SETI Institute, 189 Bernardo Avenue, Mountain View, CA 94043, USA.

5

State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing,

Wuhan University, Wuhan 430070, China. 6

Centro de Astrobiología, Madrid 28850, Spain.

7

Department of Astronomy, Cornell University, Ithaca, NY 14850, USA.

8

The University Museum, University of Tokyo, 113-0033, Japan.

9

Planetary Sciences and Remote Sensing, Institute of Geological Sciences, Freie Universität

Berlin, 12249 Berlin, Germany. 10

Department of Hydrology & Water Resources, University of Arizona, Tucson, AZ 85721,

USA.

*Corresponding author: E-mail: [email protected]. Tel: 520-622-6300. FAX: 520-622-8060.

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Catastrophic floods of enormous proportions are thought to have played a major role in the excavation of some of the Solar System’s largest channels; the circum-Chryse outflow channels. The generation of the floods has been attributed to both the evacuation of regional highland aquifers and ancient paleo-lakes. Numerous investigators indicate that these source regions were likely recharged and pressurized by eastward groundwater flow via conduits extending thousands of kilometers from an elevated groundwater table in the Tharsis volcanic rise. This hypothesis remains controversial, largely because subsequent stages of Valles Marineris development and enlargement would have resulted in the widespread destruction of the proposed groundwater paths. Here, we show that Noctis Labyrinthus, a unique canyon system connecting the Tharsis volcanic rise and western Valles Marineris, retains geologic evidence of conduit development associated with structurally-controlled groundwater flow through salt-rich upper crustal deposits, consistent with aquifer drainage from the Tharsis volcanic rise region. Our investigation indicates that subsequent surface collapse over these conduits during the Hesperian Period resulted in the generation of large basins in the central and eastern regions of Noctis Labyrinthus, and contributed to the chasmata formation in the western portion of Valles Marineris. The lava-covered floors of these basins, dated by previous workers as Late Amazonian, contain hydrated mineral deposits occurring in spatial coexistence with decameter-scale features that we interpret to be lacustrine and periglacial in origin. The proposed paleo-lake sites also include chaotic terrains, which could comprise groundwater discharge zones, pointing to regional hydrologic processes that could have operated from the Early Hesperian until a few tens of millions of years ago. Episodic fluidized discharges from eastern Noctis Labyrinthus troughs delivered vast volumes of sediments and volatiles

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into western Valles Marineris, contributing to the construction of a regional volatile-rich stratigraphy. Episodic groundwater discharges and the resulting intermittent formation of lakes within regional tectono-volcanic basins could have lasted hundreds of millions of years, making the study region of prime interest for astrobiological exploration.

1. Introduction Valles Marineris (VM) and Noctis Labyrinthus (NL) comprise a system of interconnected canyons extending ~4000 km from the central portions of the Tharsis volcanic rise to the upstream-most parts of the southern circum-Chryse outflow channels (Masursky et al., 1978; Scott and Tanaka, 1986; Witbeck et al., 1991). During periods of high obliquity, ice sheets likely formed and covered vast extensions of the Tharis rise (Harrison and Grimm, 2004). Widespread meltwater generation and infiltration along the basal contacts of these ice sheets produced by high geothermal heat, is thought to have generated, and/or recharged, aquifers (Harrison and Grimm, 2004). Episodic drainage of these aquifer systems via conduits, proposed to have developed along upper crustal tectonic fractures generated during the formation of the Tharsis volcanic rise (Lucchitta et al., 1994; Rotto and Tanaka, 1995), has been linked to the origin of the circum-Chryse outflow channels (Harrison and Grimm, 2004; Andrews-Hanna and Phillips, 2005; Harrison and Grimm, 2009; Montgomery et al., 2009). This hypothesized groundwater migration route could have also inundated the chasmata of central VM (Lucchitta et al., 1994; Rotto and Tanaka, 1995; Lucchitta 2010) and Capri/Eos chasmata in eastern VM (Warner et al., 2013). These chasmata include thick (a kilometer or more) Interior Layered Deposits (ILDs) containing abundant hydrated sulfates and hydroxyl minerals (Gendrin et al., 2005b; Murchie et al., 2009; Weitz et al., 2012), which are thought to 3

consist of minerals precipitated within groundwater-fed paleo-lakes (McCauley, 1978; Nedell et al., 1987; Malin and Edgett, 2000, 2001; Gendrin et al., 2005a; Murchie et al., 2009), spring mound discharges (Rossi et al., 2008) or saturation of aeolian mantles (Murchie et al., 2009; Andrews-Hanna et al., 2010) [-a detailed review of evidence for lakes and alternative possibilities for the compositions and origins of ILDs was provided by Lucchitta (2010)-]. Other hypotheses that do not involve groundwater upwelling include the trapping of dust and sulfur aerosols within relict glacial ice deposits (Niles and Michalski, 2009; Michalski and Niles, 2012) and sub-ice volcanism (Chapman and Tanaka, 2001). Groundwater migration paths have been extensively documented in the southern circumChryse region on the bases of subsidence and collapse surface structures and patterns that surround deeper outflow channel canyons and chaotic terrains (Rodriguez et al., 2003; Rodriguez et al., 2005a; Rodriguez et al., 2005b; Meresse et al., 2008; Rodriguez et al., 2015b). However, these type of terrains are absent within the plateaus bounding most of the VM chasmata, which typically consist of deep canyon floors separated from adjoining highland surfaces by abrupt scarps. This morphologic setting suggests that VM chasmata development and integration likely removed older subsidence and collapse structures related to tectonically-controlled conduit systems (Lucchitta et al., 1994; Rotto and Tanaka, 1995) connecting the Tharsis volcanic rise to head regions of the Chryse outflow channels proposed by Harrison and Grimm (2004). In this investigation, we have used recently acquired remote sensing image and topographic data to examine the spatial, morphometric and morphologic characteristic of Noctis Labyrinthus, an intricate system of Late Hesperian and Early Amazonian linear troughs and rounded pits (Tanaka and Davis, 1988; Tanaka et al., 2014) distributed in alignment to pre-existing faults and grabens oriented concentrically and radially to the elevated volcanic plains of Syria Planum (Fig. 4

1A and B). The NL troughs and pits are cut by the eastern flank of the Tharis volcanic rise to depths of up to ~8 km (Fig. 1C), and thus their floors and lower scarps comprise likely zones of elevated hydraulic head development, which could have driven groundwater flow as well as surface discharges. In addition, this region is unique in that it constitutes the only VM boundary terrain thought to retain collapsed structures produced by groundwater flow-induced conduit formation (Tanaka and Davis, 1988; Witbeck et al., 1991), or alternatively, by the withdrawal of subsurface magmatic reservoirs (Mège et al., 2003). 2. Morphologic and topographic characterization of the Noctis Labyrinthus canyons Utilizing a Context Camera CTX (5.91 m/pixel) image mosaic from the Mars Reconnaissance Orbiter (MRO) spacecraft and the Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) surface topography (~460 m/pixel horizontal and 1 m vertical resolution), we have produced a morphologic map that includes large-scale depressions in the western, central, and eastern parts of NL, as well as the western-most zone of VM (Fig. 1A and B). Western NL exhibits closely-spaced pits and troughs (Fig. 1A and B) with floors that range in elevation from ~6500 to ~4000 m (Fig. 1C). The margins of these depressions align with regional faults and grabens (Fig. 2A) and their floors retain regional tectonic structures (Fig. 2B). In contrast, central NL is characterized by larger and deeper pits and troughs that are mostly spatially integrated into four large enclosed basins (labels (i), (ii), (iii), (iv) in Fig. 1B) encircling floor surfaces ranging in elevation between ~4000 to ~2000 m (Fig. 1C). Basin (i) is covered by texturally smooth materials (Figs. 3A-E and 4) that appear locally faulted (Fig. 4), and which embay older chaotic terrains located along the basin’s eastern and western margins (e.g., Figs. 3A and D). Where not mantled by aeolian drift (e.g., Fig. 3C), the smooth deposits appear distinctively bright in THEMIS night IR images (Figs. 3B and 4A). 5

Ridges ranging within ~140 m of elevation bound the northern and southern sections of this basin (white pointers in Fig. 3D and E). The ridges in the southern part of the basin are cut by a channel that extends onto the basin’s floor and which exhibits overflow deposit morphology (Figs. 3E and 5). Our examination of the basin’s smooth floor using HiRISE images reveals bumpy and pitted terrains (Fig. 6A), shallow irregular depressions (Fig. 6D), and polygonally patterned ground (Fig. 6G). Eastern NL includes five extensive troughs with floors that share the same elevation range as the interior basins of central NL (i.e., ~4000 to ~2000 m, labeled 1-5 in Fig. 1B). However, these canyons are not enclosed; rather, they open and converge into the lower-lying and wider floors of western Tithonium chasma, which range in elevation between ~2000 and ~-1000 m (Fig. 1B and C). At some locations these troughs show margins that are extensively modified by prominent alcoves (e.g., black arrow in Fig. 1C). The contact between the eastern NL troughs and Tithonium Chasma includes isolated mesas and ridges, which reach common maximum elevations to those of adjoining highlands and inter-trough ridge divides (e.g., Fig. 7A). One of these troughs (labeled 1 in Fig. 1B, and Figs. 8A and B) is particularly interesting because it comprises the source region of a knobby southwest-trending (black lines in Fig. 8A and B) and a smoother northeast-trending (red and yellow lines in Fig. 8A and B) sedimentary deposits, which, respectively, exited the trough via an upper (white arrow labeled 1 in Fig. 8B) and a lower (white arrow labeled 2 in Fig. 8B) breached wall sections. Knobs comprising the southwest-trending deposit are embayed by the northeast-trending one at a location in southern Tithonium Chasma (Fig. 8C) as well as by another deposit extending northward from the Oudemans Crater floor (blue line in Fig. 8A).

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The northeast-trending deposit extends as much as ~250 km into the floors of Tithonium and Ius Chasmata. The distal margins of this deposit exhibit lobate fronts, which are located ~3.5 km lower than the source’s canyon floor (Fig. 9A). The deposit’s source trough is mostly covered by layered medium-toned materials (labeled 1 in Fig. 1B, Fig. 10A). These materials exhibit etched textural characteristics (Fig. 10B, D, and F) similar to those of some sedimentary deposits in Melas Chasma (Fig. 10C, E, and G). Eastern NL transitions east into Tithonium Chasma, an immense trough that forms the westernmost part of VM, and which exhibits extensively fractured floor materials (blue arrow in Fig. 9A, Fig. 11A) as well as zones of subsidence (Fig. 11 B) and collapsed pits (Fig. 11C) that are spatially associated with regional faults (Fig. 11A). A HiRISE view of the pit’s margin shows jarosite-bearing layers (Weitz et al., 2014) ruptured by a cross-cutting structure oriented at a steep angle to the general stratigraphy (Fig. 11D). The floors surrounding the pit include domical structures, which have been eroded into circular and elliptical craters in some places (e.g., black arrow in Fig. 11C). Northwestern Tithonium Chasma includes a channel that follows the margin of the basin’s sedimentary deposit (black arrow in Fig. 1A and B, Fig. 12). 3. Interpretative synthesis 3.1. Formation and inundation of enclosed basins in Noctis Labyrinthus We propose that the tectonically-aligned troughs, pits and enclosed basins of central and eastern NL (Fig. 1A and B) likely developed as a consequence of surface collapse over conduits generated by fault-controlled groundwater flows, sourced from an aquifer system in NE Syria Planum and discharged into the floors of ancestral basins (Fig. 13A panel 1 and 13B) produced during the early rift stages of western VM (Lucchitta et al., 1994). Syria Planum includes two channels that have initiation zones at ~6000 m (white arrows labeled 1 and 2 in Fig. 1). These 7

channels source from discrete depressions and do not exhibit distinct lava flows extending from their margins or termini (e.g., Fig. 14B), which would be consistent with a fluvial origin resulting from groundwater discharges. Furthermore, the presence of impact craters with rampart ejecta blankets in Syria Planum (black arrows in Fig. 14A and B) is also indicative of the former existence of a groundwater table that was locally excavated by impacts. Central NL’s basin (iii) and Tithonium Chasma in western VM are bounded to the south and to the southeast by the ~120-km diameter Oudemans impact crater (Figs. 1 and 7C). At the contact zone of these regions, the crater’s rim is absent (Figs. 7C, 15A). The impact crater has a wellpreserved ejecta blanket spreading over the surrounding highland plateau surfaces. However, this ejecta blanket is not present on the walls and floors of adjoining troughs in NL and Tithonium Chasma (Fig. 15A). Consequently, regional conduit collapse appears to have modified Oudemans crater, and thus post-dated its formation. In addition, the emplacement of the Tithonium Chasma deposits (Fig. 8) must have also taken place after the crater’s formation. Sizefrequency distributions of impact craters superimposed on Oudemans’ ejecta blanket return an Early Hesperian modeled age (~3.69 Ga, plot in Fig. 15B), which is consistent with an upper age value for the bulk development of VM as estimated by Witbeck et al. (1991). Thus, the formation of the basins in central and eastern NL most likely took place during the Hesperian Period. The conduits would have been generated by fault-controlled groundwater flows (Fig. 13A panels 1 and 2). The implied magnitude of subsurface cavity space would be consistent with the dissolution of hydrated salt deposits buried within the regional upper crust (Montgomery et al., 2009). The contact between the tectonic pits and troughs of western NL and the hypothesized zone of conduit collapse in central NL (Fig. 1A and B) is characterized by a sudden deepening in

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baseline elevation from ~4 km to ~2 km, which extends into eastern NL troughs (Fig. 1C).This baseline elevation likely defines the typical maximum depth of regional conduit formation. The presence of widespread hydrated salt deposits within the central NL basins (Weitz et al., 2013; Weitz and Bishop, 2014) is consistent with a conduit origin linked to the dissolution of regional upper crustal salt deposits, aided perhaps by groundwater suffusion along regional faults extending across buried unconsolidated sedimentary deposits. The present elevation difference between the surface of Syria Planum and the floor of Tithonium Chasma is ~7 km. Assuming that the proposed conduits between the Syria Planum aquifer and the floors of the ancestral basins in western VM interconnected a similar relief difference, then this elevation range likely provided hydrostatic pressures of ~ 26 MPa [calculated as (h (relief in meters) × g (for Mars 3.711 m/s²) × ρ (water density, ~1 g/cm3))]. This value of regional aquifer pressurization exceeds the upper estimate proposed to have led to outflow channel activity in southern circum-Chryse [10 MPa] (Andrews-Hanna and Phillips, 2007), and thus, it would have been sufficient to drive the proposed large-scale groundwater flow and discharge. However, there is no evidence for catastrophic floods extending from NL, which could have been the result of groundwater outbursts occurring within a densely troughed topography and would have promoted the development of paleolake systems within the immense enclosed basins (Fig. 13A panel 3). Late Amazonian (~50 to ~100 Ma) volcanic resurfacing occurred within central NL (Mangold et al., 2009; Weitz et al., 2013). In addition, the identification of numerous hydrated minerals within the depressions of NL, including opal, gypsum, monohydrated and polyhydrated sulfates, Fe/Mg-smectites, and Al-clays suggests coeval episodes of aqueous activity (Weitz et al., 2011; Thollot et al., 2012; Weitz et al., 2013; Weitz and Bishop, 2014). We interpret the ridges that 9

mark the northern and southern margins of basin (i) (Fig. 3D and E) as possible lacustrine paleoshorelines that were likely produced as sediments were pushed outwards during expansion/freezing cycles of the lake’s surface (Fig. 3F) (Lee et al., 2009). This shoreline type does not necessary follow an equipotential elevation, and thus, the proposed hypothesis is consistent with the ridges’ ~140 m elevation range (white pointers in Fig. 3D and E). We have also identified numerous decameter-scale landforms within this basin which, based on assemblage, comparative geomorphology and the proposed paleo-lacustrine geologic context, we interpret to have likely developed in periglacial settings. These landforms include: (1) domes that locally exhibit summit pits (Fig. 6A), which we interpret as pingos (Fig. 6B) or mud volcanoes (Fig. 6C), (2) shallow irregular depressions (Fig. 6D), which we interpret as produced by thermokarst melting (Fig. 6E and F), (3) patterned ground (Fig. 6G), which we interpret as produced by expansion-contraction episodes associated with ice freeze-thaw/condensationsublimation cycles (Fig. 6H) (Costard and Kargel 1995; Seibert and Kargel, 2001) or by sediment desiccation (Fig. 6I). As observed in terrestrial polygons developed in ice-rich permafrost regions (Levy et al., 2009) and salt/playa lake environments (Hunt and Wasburn, 1966), the documented Martian polygons range from a few to ~20 m in diameter (Fig. 6G). To reconcile the pyroxene-rich composition (Mangold et al., 2009) of the floor of basin (i), the spectral evidence consistent with aqueous alteration (Weitz et al., 2011; Thollot et al., 2012; Weitz et al., 2013; Weitz and Bishop, 2014), and our discovery of possible lacustrine and periglacial features, we propose that the basin’s surface might consist of dark basaltic sand mantles (e.g., Fig. 3C) that became entrapped within the paleo-lake(s) (Murchie et al., 2009; Andrews-Hanna et al., 2010).

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Water at temperatures close to the freezing point could have hindered the alteration of these sediments to hydrated minerals such as phyllosilicates (Fairén et al., 2012). Alteration driven by higher subsurface temperatures, however, could have resulted in the generation of possible Alclays that appear mostly covered by the upper pyroxene-rich floor deposits (Weitz et al, 2013). As the paleo-lake(s) dried up, they were resurfaced by periglacial processes. Precipitation of hydrated mineral deposits (Weitz et al., 2013) could have contributed to bedrock induration as indicated by the basin’s surface high thermal inertia (Figs. 3B and 4A, (Christensen et al., 2003)). Late Amazonian inundation phases in central NL are indicative of a second stage of regional groundwater flow from the Syria Planum aquifer (Fig. 13A panel 3), which might have perhaps been triggered by coeval volcanic processes (Mangold et al., 2009; Flahaut et al., 2011; Weitz et al., 2013) or by recent tectonic activity (e.g., faulting in Fig. 4). Surface groundwater discharges likely involved (1) highly pressurized outbursts as indicated by the presence of chaotic terrains within the floor of basin (i) (Fig. 3D), and (2) sapping activity leading to the formation of prominent alcoves (e.g., black arrow in Fig. 1C). In addition, melting of snow and ice along the troughs’ flanks could have also contributed to the history of regional flooding (Weitz et al., 2013). 3.2. Basin enlargement in eastern Noctis Labyrinthus and integration into Valles Marineris Although the floors of the central and eastern troughs of NL share the same elevation ranges (Fig. 1C), the eastern ones are not enclosed and instead they open into Tithonium Chasma (labels 1-5 in Fig. 1B). The contact between eastern NL and Tithonium Chasma includes mesas and ridges, which respectively share the same maximum elevation as adjoining highland surfaces and ridge divides (Fig. 7). These spatial relationships suggest that the eastern troughs were initially enclosed and subsequently opened into Tithonium Chasma as their eastern divides were removed. 11

One of these troughs (location and context in Fig. 1B indicated by label 1, Fig. 8) comprises the initiation zone of two sedimentary flow deposits covering vast portions of Tithonium Chasma (Fig. 8). These deposits extend to the SE (black lines in Fig. 8A and B) and to the NE (red and yellow lines in Fig. 8A and B) via breached sections of a former wall divide (white arrows labeled 1 and 2 in Fig. 8B). These observations are consistent with an erosional history of the eastern troughs’ western divides, involving surface flow discharges, leading to their spatial integration with Tithonium Chasma (Fig. 13A panel 4). The deposit that extends from the higher breach (black line in Fig. 8A and B) is embayed by (and thus predates) (Fig. 8C), the one extending from the lower breach (red and yellow lines in Fig. 8A and B) as well as another sedimentary deposit that extends from the Oudemans Crater floor (blue line in Fig. 8A and B). Thus, regional trough integration was likely linked to a history of intermittent large-scale mass flow discharges from the NL canyons into Tithonium Chasma. The trough comprising the source of the sedimentary flows covering the floor of Tithonium Chasma (labeled 1 in Fig. 1B, Fig. 10A) contains widespread layered medium-toned deposits that exhibit etched textural characteristics (Fig. 10A, B, D, and F) similar to those found within a proposed paleo-lake site in Melas Chasma (Metz et al., 2009) (Fig. 10C, E, and G). Thus, it is plausible that lacustrine deposits within formerly enclosed basins comprised important contributors to the sedimentary history of Tithonium Chasma in western Valles Marineris. Breaching of the eastern-confining divide could have been facilitated by sub-surface seepage, sapping, and weakening within the confining wall rock, initiating collapse of the overlying wall material. The deposit that extends from the trough’s lower breach (red and yellow lines in Fig. 8A and B) propagated as much as ~250 km into the floors of Tithonium and Ius Chasmata and its distal

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margins exhibit lobate fronts located ~3.5 km below the source region (Fig. 9A, elevation profile). The lobate morphology indicates that the sedimentary deposit was likely emplaced by a high viscosity flow. If VM was, at least partly, inundated during the Late Hesperian (Wezel and Baioni, 2010), subaqueous flow propagation could have contributed to its long run-out distance (Lipman et al., 1988). Although we are not able to date the deposit’s surface using crater counts, because it appears highly resurfaced, its eastern-most parts occupy the floor of a trough in western Ius Chasma, which comprises the only surface route interconnecting the NL/eastern Tithonium Chasma canyon system to central VM. The deposit’s well-preserved lobate flow fronts within this region exhibit no signs of modification by more recent eastwards flow discharges (Fig. 9B). Thus, we can establish with a high degree of certainty that this deposit comprises a stratigraphic marker defining the end of large-scale discharges from NL to central VM, perhaps resulting from a thickening of the regional aquifer-confining cryosphere during the Amazonian (Clifford, 1993; Clifford and Parker, 2001). Fractures and pits within the floor of Tithonium Chasma (e.g., blue arrow in Fig. 9A) reveal a buried upper stratigraphy that shows rhythmic sequences of layered light-toned deposits separated by dark beds (white arrow in Fig. 11). The identification of hydrous sulfate jarosite in these deposits (Weitz and Bishop, 2014) would be consistent with these strata, including sequences of paleo-lacustrine sediments. Furthermore, the presence of a possible fluvial channel, which shows topographically inverted sections in its lower reaches (black arrow in Fig. 1A and B, Fig. 12), is consistent with a regional fluvial activity that could have contributed to paleo-lake formation as well as to the erosion and removal of topographic divides that formerly separated the eastern NL troughs and Tithonium Chasma. Resurfacing of these materials appears to have 13

resulted in widespread fracturing (Figs. 9A and 11A), subsidence (Fig. 11B) and collapse (Fig. 11C). Possible salt diapirism (e.g., eroded domical structures indicated by black arrows in Fig. 11C) and fluidized emissions (e.g., cross-sectional view of a possible conduit infilled with the remnants of a fluidized intrusion, Fig. 11D) likely participated in the removal of subsurface materials within the deposit. 4. Hydrologic and astrobiological implications Our findings are consistent with tectonically controlled groundwater drainage from the Syria Planum region of the Tharsis volcanic rise producing extensive network of tectonically aligned conduits within NL. Subsequent phases of widespread collapse resulted in trough growth and integration in central and eastern NL, as well as chasmata formation in western VM. We attribute conduit formation to the erosion and dissolution of buried evaporitic deposits by the groundwater flows. The collapsed terrains retain the tectonic orientation of the precursor conduit networks (Fig. 1A and B), indicating that no significant scarp retreat took place following their development. Consequently, ice-rich strata and aquifers are not likely significant constituents of those crustal regions, which would have experienced widespread collapse and subsidence over the proposed conduits as well as along regional high-relief scarps, as inferred to have been the case in southern circum-Chryse (Rodriguez et al., 2005a; Rodriguez et al., 2005b; Rodriguez et al., 2015b). A key implication is that upper crustal salts appear to have played very important roles in the hydrologic history of Mars. Some already discussed in the literature include salt deposits that could have (1) rapidly released groundwater to produce the catastrophic floods that formed the Martian outflow channels (Kargel et al., 2007; Montgomery et al., 2009), (2) triggered high thermal anomalies capable of driving surface and near surface hydrologic processes (Kargel et al., 14

2007), (3) produced briny flows capable of surface flow under the currently extremely cold and dry Martian surface environments (Ojha et al., 2015). Our investigation suggests that conduit formation within salt-rich crust could have contributed to the development of regionally integrated hydrologic systems lasting hundreds of millions of years. The same tectonic geometry characterizes most of VM (Tanaka et al., 2014), suggesting that perhaps VM did not rift into aquifers. Such relatively stable upper crustal materials would have facilitated the development of the conduit networks proposed to have extended thousands of kilometers and interconnected the Tharsis aquifers to the circum-Chryse outflow channels (Harrison and Grimm, 2004; Andrews-Hanna and Phillips, 2005; Harrison and Grimm, 2009; Montgomery et al., 2009). Collapsed conduits, briny groundwater emissions and inundations, and lava eruptions within central NL appear to have episodically taken place during the Hesperian and Amazonian Epochs. The documented stages in geologic activity within NL are consistent with a mode of gradual chasmata development spanning hundreds of millions of years. The long-lived integration of subsurface and surface hydrologic environments, perhaps enriched in nutrients by hydrothermal processes, could have favored the development of potentially habitable environments, thus this region of Mars comprises a prime candidate for astrobiological exploration. Episodic sedimentary (e.g., Fig. 8) and fluvial (e.g., Fig. 12) discharges from eastern NL into VM might have constituted an important supply of sediments, salts, and volatiles to the interior deposits of VM such as the ILDs (Gendrin et al., 2005b; Murchie et al., 2009; Weitz et al., 2012) and relict glacial ice (Gourronc et al., 2014).

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We have identified a Himalayan zone in western Tibet (Fig. S1) that comprises an outstanding analog geologic setting to that of central NL. The Tibetan region of interest includes lakes contained within grabens that form part of an extensively tectonized high mountain plateau (Taylor and Yin, 2009). The lacustrine basins exhibit surface mineral deposits formed by aqueous mineral precipitation (Wang et al., 2002; Kapp et al., 2008) under a dominantly cold and dry climate (Lee et al., 2009). Our survey of this region reveals that, as proposed to have occurred in central and eastern NL, inter-basin drainage, and perhaps groundwater circulation to lower terrains, appear to have reduced the relief of tectonic basin divides (Fig. S2). Within the Tibetan study region we have identified various landforms that are analogous to those documented within basin (i) (Figs. S3-5). Further investigations of this region of Earth would be helpful to better understand the geologic environments of central NL, and thus their potential to host life.

Acknowledgements

Funding provided by NASA's NPP program to J. Alexis P. Rodriguez and by MRO HiRISE CoInvestigator and SETI Institute’s NAI Co-Investigator funds to V.C. Gulick. T. Platz was supported by a DFG grant (PL613/2-1) and the Helmholtz association through the research alliance “Planetary Evolution and Life”. A. G. Fairén was supported by the European Research Council under the European Union's Seventh Framework Programme (FP7/20072013), ERC Grant agreement no. 307496. Figure captions Figure 1 (A) Geologic feature map showing the canyons of western (red), central (light blue), and eastern (dark blue) Noctis Labyrinthus. Western VM is shown in black. White lines mark the 16

outlines of troughs and broader depressions enclosed by topographic divides reaching elevations ≥2000 m. White arrows 1 and 2 show high elevation channels in western Syria Planum, and the black arrow shows a channel located on the NW floor of Tithonium Chasma. Part of THEMIS day-time infrared image mosaic (100 m per pixel) centered at 7°20'S, 99°37'W. (B) Close-up view on panel A showing labelling for the central and eastern trough systems. (C) Regional view showing the elevation ranges characterizing each of the mapped zones in panel A. The elevations of the adjacent plateaus are shown, but only within the range 4000-6500 m. Higher elevations, which range up to 9000 m, are not color-coded as to avoid excessive detail. Part of THEMIS day-time infrared image mosaic (100 m per pixel) centered at 7°20'S, 99°37'W underlying classified MOLA topography (460 m/pixel). Elevation profiles a-a' and b-b' (y axes in meters, x axes in kilometers) show that the topographic transitions between various zones are abrupt. Figure 2 View of western NL showing tectonically aligned troughs. For example, the black arrows in panel (A) indicate the locations of a trough and a contiguous graben bounded to the north by the same fault scarp. Context and location in Figure 1A. (B) Close-up view on the trough floors reveals retention of pre-existing tectonic fabrics (white arrows). (A and B, perspective CTX (5.91 m/pixel) views centered at 6°54'S, 104° 2'W). Figure 3 (A) Perspective CTX (5.91 m/pixel) view of an enclosed trough in central NL labelled as (i) in Figure 1B. The black arrows show the locations of chaotic terrains embayed by the floor materials. Image centered at 6°51'S, 98°59'W. (B) Part of THEMIS night-time infrared mosaic (100 m/pixel) showing that the basin’s floor exhibits high thermal inertia (bright). (C) CTX (5.91 m/pixel) view of basin’s floor south-western margin that is partly covered by dark aeolian dunes. Light-toned hydrated deposits are visible adjacent to some chaotic blocks. (D) and (E) are close-up views on panel (A) showing the north-eastern and southern parts of the basin, 17

respectively. The white pointers in panels D and E identify marginal ridges and note elevation ranges obtained from longitudinal MOLA topographic profiles. (F) Ridge-forming shorelines in a high elevation lake in the Tibetan plateau (image modified from Lee et al. (2009)). Other examples are shown in Fig. S3. Figure 4 Views of a NW-trending tectonic fracture developed within the floor of basin (i) in central NL (see Fig. 1B for location and context). (A) The fracture appears relatively dark in THEMIS night IR images, indicating that it contains low thermal conductivity materials. (B) [part of CTX (5.91 m/pixel) mosaic] and (C) [part of HiRISE ESP_019878_1730, (25 cm/pixel)] are close-up views that show aligned tectonic depressions (black arrows in (B)) infilled with aeolian mantles responsible for the low thermal conductivity signature (white arrows in (C)). Black arrows in (A) and (B) point to the same locations. Figure 5 (A-C, parts of HiRISE image ESP_017465_1730, (25 cm/pixel)). (A) View of channel in the southern part of basin (i) (see figure 3E for context and location). The dashed white line shows the margin of an overflow deposit. Close-up views (B, C) reveal lobate raised margins (black arrows). The flow's surface morphology does not exhibit characteristic lava flow textures. Figure 6 (A) Lobate front along the southern floor margin of basin (i) showing clusters of pitted domes. Part of HiRISE view EPS_017465_1730 (25 cm/pixel) centered at 7°4' S, 98°55' W. Location indicated in Figure 3E. (B,C) Terrestrial analog pingos and mud volcanoes, respectively. (D) Thermokarst-like depressions within the basin’s floor. Part of HiRISE view ESP_019878_1730 (25 cm/pixel) centered at 6°52' S, 98°55' W. Location indicated in figure 3A. (E, F) Terrestrial analog thermokarst features. (G) Patterned ground within the basin’s floor. Part of HiRISE view ESP_019878_1730 (25 cm/pixel) centered at 6°55' S, 98°53' W. Location

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indicated in figure 3A. (H, I) Terrestrial analog permafrost and playa polygonally patterned ground sites, respectively. Figure 7 (A) North-western part of Tithonium Chasma. MOLA topography elevation profiles (y and x axes in meters) show interior mesas and ridges with the same maximum elevation as various adjoining highland surfaces. (B) Reconstruction of likely former basin divides (yellow) based on elevation continuity and proximity between promontories and adjoining highlands. (C) South-western part of Tithonium Chasma and Oudemans Crater showing reconstruction of likely basin divides prior to the removal of the impact crater’s rim (white arrow) and the margins of canyons in eastern NL (black arrow). Panels show MOLA (460 m/pixel) DEMs over topographic shaded reliefs. Figure 8 (A) View of part of the eastern NL and Tithonium Chasma. The red and yellow lines delineate the periphery of a vast deposit that extends from an upland trough (black arrow). The margins are clearly recognizable where they overlap older canyon floor materials (red lines). However, their traceability is locally obscured where mantled by aeolian deposits (yellow lines). The black lines outline the margins of another deposit that sourced from a higher breach in the trough’s walls, and the blue line those of another deposit that extends from the northern margin of Oudemans Crater (we used single colors for these deposits because their margins are mostly clearly recognizable). Numbers 1-3 illustrate the inferred emplacement sequence and the white arrows the overall flow directions. MOLA DEM (460 m/pixel) over part of a THEMIS day-time infrared image mosaic (100 m per pixel). (B) Close-up view on panel (A) showing the higher (arrow 1, elevation profile) and a lower (arrow 2) trough breaches, which respectively channeled debris discharges towards the SE and NE. (C) CTX (5.91 m/pixel) view of distal reaches of the deposit labeled (1) in Fig. 1A, which consists of mostly prominent knobs. The knobs are 19

embayed by the deposit extending from Oudemans Crater (red dots) as well as by the vast deposit that covers most of Tithonium Chasma (white dots). Figure 9 (A) View of the most extensive deposit shown in Fig. 8A (outlined by black lines). The elevation profile shows a ~3.5 km elevation drop over ~250 km from the source region to the deposit’s eastern-most reaches. Image inset close-up view shows how the central part of the deposit is extensively modified by fractures. Part of THEMIS night-time infrared mosaic (100 m/pixel) underlying MOLA (460 m/pixel) topography. (B) Perspective CTX (5.91 m/pixel) view of eastern-most lobe. The white arrows show how the deposits margin follows the pre-existing erosional form of the encasing canyon walls. Figure 10 (A) Perspective CTX (5.91 m/pixel) view centered at 8°5' S, 94°12' W of the upland trough shown in Fig. 8B. (B) Irregular depressions (black arrow) with interior knobs developed within smooth surfaces (CTX view, context in panel A). (C) Similar terrain located along the southern scarp of Melas Chasma (CTX, centered at 12°29'50.13"S, 72°44'44.30"W). (D) Closely-spaced depressions surrounded marginal ridges (black arrows, CTX view, context in panel A). (E) Similar terrain located along the southern scarp of Melas Chasma (CTX, centered at 13°12'51.91"S, 69°51'59.19"W). (F) Knobby deposits (white arrow) that are locally modified by irregular depressions (black arrow). (G) Similar terrain located along the southern scarp of Melas Chasma (CTX, centered at 12° 0'23.28"S, 74°20'18.05"W). Figure 11 (A) Perspective CTX (5.91 m/pixel) view of the floor of Tithonium Chasma showing the locations of a subsided depression and a large pit, which are both spatially connected to a regional graben. (B) Part of CTX image centered at 7°46' S, 92°32' W. Close-up on subsided depression reveals a densely fractured margin, produced be extension during downward warping.

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(C) Part of CTX image centered at 7°41' S, 92°48' W. Close-up on the pit reveals light-toned jarosite-bearing deposit (white arrow) buried beneath dark mantles. The black arrow shows a domical structure with an eroded elliptical summit area. (D) Part of HiRISE image PSP_005980_1725, 25 m/pixel. Close-up on light-toned deposit strata reveals the cross-section of a brecciated intrusion (margins indicated by black arrows). Figure 12 (A) View of northern Tithonium Chasma showing the northern part of the most extensive deposit shown in Fig. 8A (red lines, certain; yellow lines, uncertain) and a possible fluvial channel (blue line). Base is a MOLA (460 m/pixel) shaded relief. (B) CTX (5.91 m/pixel) close-up views of the contact between the channel and deposit. The lower part of the channel, which is infilled to the rim and locally exhibits inverted relief where the surrounded materials have been eroded away, follows the lobate deposit’s margin. Figure 13 (A) Sketches illustrating the proposed model for collapse and inundation in central and eastern Noctis Labyrinthus (line labeled a to a' in panel (B) shows the location of the proposed upper crustal reconstructions). (1) Groundwater flow dominated by a radial trend from Syria Planum to ancestral basins in western Valles Marineris generate extensive systems of nested conduits along pre-existing faults. (2) Collapse over the excavated conduits forms enclosed basins in central and eastern NL. (3) The floors of the basins accumulate widespread lacustrine sediments and lavas. (4) Surface erosion associated with eastward large-scale mass flow discharges results in the merging of eastern NL basins and Tithonium Chasma. Tithonium Chasma accumulates large volumes of sedimentary deposits, including possible lacustrine materials. (B) MOLA (460 m/pixel) shaded relief map showing the proposed drainage paths of an aquifer in Syria Planum (red shading, blue arrows show inferred flow direction). The aquifer’s approximate location is marked by the blue shading. The white question marks delineate 21

uncertain boundaries. The red question marks show boundaries beyond which the existence of the aquifer, or groundwater flow conduits, is unlikely because the densely tectonized crust shows no evidence of groundwater discharges into the regional troughs. Figure 14 (A) Portions of Oudemans’ ejecta blanket that was dated at ~3.69 Ga. The blue outline notes the area where craters were counted. Part of THEMIS day-time infrared image mosaic (100 m per pixel)). (B). Crater statistics were performed with CraterTools (Kneissl et al., 2011) and Craterstats (Michael and Neukum, 2010) following the description presented in Platz et al. (2013). The chronology (Hartmann and Neukum, 2001) and production (Ivanov, 2001) functions were used to derive crater model ages. The measured crater population was tested for randomness (cf. Michael et al., 2012) to validate whether secondary craters are included in the population. Figure 15 (A) MOLA DEM (460 m/pixel) over THEMIS day-time infrared image mosaic (100 m per pixel) view of Syria Planum centered at 11°32' S, 105°23' W. (B) Close-up CTX (5.91 m/pixel) view on channel that extends from a depression. The black arrows in both panels show the locations of rampart impact craters. References Andrews-Hanna, J. C., Phillips, R. J., 2005. Hydrological modeling of the Martian crust with application to the pressurization of aquifers. J. Geophys. Res. 110, E01004. Andrews-Hanna, J. C., Phillips, R. J., 2007. Hydrological modeling of outflow channels and chaos regions on Mars. J. Geophys. Res. 112, E08001.

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Weitz, C. M., Noe Dobrea, E. Z., Lane, M. D., Knudson, A., 2012. Geologic relationships between gray hematite, sulfates, and clays in Capri Chasma. J. Geophys. Res. 117, E11, 10.1029/2012JE004092. Weitz, C.M., J.L. Bishop, P. Thollot, N. Mangold, and L.H. Roach (2011) Diverse mineralogy in two troughs of Noctis Labyrinthus, Mars, Geology, 39;899-902, doi: 10.1130/G32045.1. Wezel, F. C., Baioni, D., 2010. Evidence for subaqueously resedimented sulphate evaporates on Mars. Planetary and Space Science. 58, 1500-1505. Witbeck, N. E., Tanaka, K. L., Scott, D. H., 1991. Geologic map of the Valles Marineris region, Mars (east half and west half) , scale 1:2,000,000.. In: p. M. I.-. U.S. Geol.Surv. Misc. Invest. Ser., (Ed.). Series Map I-2010. McCauley, J. F., 1978. Geologic Map of the Coprates Quadrangle of Mars, scale 1:5,000,000. U.S. Geol. Surv. Misc. Inv. Series Map I-897. Highlights Late Noachian groundwater flow through upper crustal salt deposits likely generated extensive conduits in the Noctis Labyrinthus region of Mars. Subsequent phases of collapse are interpreted to have produced enclosed basin systems, which were episodically occupied by lakes and lavas as young as Late Amazonian. The groundwater discharges leading to regional inundation events likely sourced from an aquifer that was, at least partly, located in Syria Planum. Episodic groundwater discharges and the resulting intermittent formation of lakes within regional tectono-volcanic basins could have lasted hundreds of millions of years, making the study region of prime interest for astrobiological exploration.

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