Evolution of the Navua Valles region: Implications for Mars' paleoclimatic history

Accepted Manuscript Evolution of the Navua Valles region: Implications for Mars' paleoclimatic history

H.I. Hargitai, V.C. Gulick, N.H. Glines PII: DOI: Reference:

S0019-1035(18)30016-2 https://doi.org/10.1016/j.icarus.2019.04.024 YICAR 13289

To appear in:

Icarus

Received date: Revised date: Accepted date:

11 January 2018 17 April 2019 24 April 2019

Please cite this article as: H.I. Hargitai, V.C. Gulick and N.H. Glines, Evolution of the Navua Valles region: Implications for Mars' paleoclimatic history, Icarus, https://doi.org/ 10.1016/j.icarus.2019.04.024

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Evolution of the Navua Valles Region: Implications for Mars’ Paleoclimatic History H. I. Hargitai1, V. C. Gulick2,3, and N. H. Glines 2,3 1

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NASA Ames Research Center, NPP, MS 239-20, Moffett Field, CA 94035, USA, [email protected] 2

SETI Institute, 189 Bernardo Ave, Suite 200, Mountain View, CA 94043

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NASA Ames Research Center, MS 239-20, Moffett Field, CA 94035, USA, [email protected]

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1 INTRODUCTION

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The Navua Valles are a complex system of channels and valleys on the inner northeastern rim of Hellas Basin, the largest impact basin on Mars. Channel formation may have been controlled by the available thermal energy from regional volcanic systems, surface and ground-water reserves, surface and subsurface properties (e.g.: rates of infiltration, permeability, and erodibility), topography, lithology, and paleoclimate.

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The highly degraded rim of Hellas basin contains two densely dissected regions, which are both associated with ancient volcanic centers, including Peneus and Amphitrites Paterae in south Hellas, and Hadriacus and Tyrrhenus Montes in northeastern Hellas (this study). A 1500-km-long continuous slope extends from the highest point of Tyrrhena Terra to the floor of Hellas Basin resulting in a relief of 9.5 km. The Tyrrhena Terra highland region, northeast of the Navua Valles, is dissected by Vichada Valles, which is a valley network separated from the Navua Valles by a 450-km-long area featuring only few isolated channels.

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Martian valley networks typically display a dendritic pattern and are located in isolated drainage basins in the intercrater terrains of the highlands. In contrast, the main discontinuous channels of the Navua Valles drainage systems cut into a smooth plains material on the lower interior rim of Hellas Basin with a slope of 0.34°. These plains contain highly degraded impact craters with no evidence for Noachian highland massifs as in adjacent Hellas basin rim regions. Drainage networks in Navua Valles have a uniform slope orientation and grade into subparallel systems with steepening slopes along the interior basin rim. We have previously mapped channels (Hargitai et al. 2017), lakes (Hargitai et al. 2018b) and geologic units (Hargitai et al. 2018a) in this region. We observe three main drainage patterns: 1) dense network of short (100-200 km) channels in the vicinity of Crater 21-35, which originate on elevated impact materials; 2) dense network of channels on the Hesperian dissected plains (Hpd) unit (Navua A, B, Fig. 1) where channels disappear and emerge downslope but maintain uniform channel widths; and 3) interior channels limited to within 1

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main channels. Most drainages are poorly or not integrated but do occur over a large region. This suggests that only a limited amount of water was available over a large region at the time of channel formation period(s). The westernmost drainage system (Navua A, marked “A” Fig. 1) transported water from a heavily cratered plateau region down the plains to the Hellas basin floor with little additional input from independent tributaries. Hargitai et al. (2017) used stratigraphy and superposition to identify and map channel morphologies and deposits, and determined that some channels within the same system formed at significantly different times. They identified at least five separate fluvial events ranging from catastrophic flows (~106 m3/s) to more persistent, lower discharge (~100 m3/s) fluvial activity.

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The goal in our current study is to better understand the processes and environments which resulted in these wide range of channel discharges and morphologies. Were they produced by several different fluvial modes at different times, or simultaneously, responding to different local conditions of available hydrothermal activity, lithology or topography? Were the channel and valley segments, which were based on crater age-dated channel floors and terminal deposits, formed or reactivated during the Hesperian or during the more recent time of outflow channel formation? Was channel formation controlled by past hydrothermal activity from the nearby volcanic centers or was it controlled by more global processes?

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To answer these questions, we determined: 1) if channel formation correlated with the volcanic activity at Hadriacus and Tyrrhena Montes; 2) if channels formed only on certain surface units or lithology; and 3) if these drainage systems formed at the same time or more episodically over different periods. Knowing when the channels and valleys were active and how nearby volcanic centers control fluvial activity can help reconstruct the climate and water history of Mars.

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Hargitai et al. (2018a) produced a 1:1 million scale geologic map of the Navua Valles region and determined the ages of the channel floors, channel associated deposits and several individual crater ejecta crossed by channels based on crater densities and superposition relations. In this paper we merge the geomorphologic knowledge with timescales to develop a timeline of activity throughout this region and discuss the implications of the mapped past fluvial events for paleoclimatic change on Mars.

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1.1 NOMENCLATURE

The Navua Valles drainage system includes those channels that terminate in northeast Hellas Basin, but are not connected to channels on the slope directly below Hadriacus Mons. The southernmost channel in our mapped area (Channel “D”, Fig. 1) is therefore not part of the Navua Valles, because it originates directly on Hadriacus Mons, even though it terminates in the mapped region. The informal nomenclature of the drainage systems in the Navua Valles region is taken from Hargitai et al. (2017). The delineation and a name for this region (originally proposed: ‘Spree Valles’) was submitted to the IAU by M. Voelker (Voelker, personal communication, 10/20/2016). We refer to unnamed craters by their Robbins Crater Database designation (Robbins and Hynek 2012).

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ACCEPTED MANUSCRIPT 1.2 PHYSIOGRAPHIC OVERVIEW

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The Navua Valles region is characterized by topographically distinct terrains, each of which has a complex but distinct geological history (Fig. 1). The densely cratered highland terrain in the Navua Valles region (1) contains outcrops of subdued mountain peaks and short and scattered intercrater valleys concentrated around Crater 21-35. The rolling, smooth plains with wrinkle ridges (2) protrude into the highlands from the Hadriacus Mons. This terrain contains no major channels and is somewhat higher than the surroundings. The wedge-shaped, smoothsurfaced plains (3) cut deeply into the cratered highlands, and do not contain Noachian-aged peaks or crater rims. These plains contain the Navua A system that descends to the basin floor in a complex series of discontinuous segments (Fig. 4), composed of subparallel valleys and channels separated by deposits. East from the Navua A plains is an isolated block of highland terrain (4) dissected by the Navua B channel system. Highland peaks are absent in a band adjacent to the channels. The highlands of Navua B’s source region transition to a steep, smooth, densely dissected slope (5) towards the floor of Hellas basin, with major and minor channels terminating at the foot of this slope. The basin floor has two major units: a transitional zone (“ramp”) (6) containing outcrops of the underlying rolling topography on which both the short, complex and prominent Navua C (“C”, Fig. 1) has formed and where Channel D (“D”, Fig. 1) enters the ramp region from Hadriacus Montes. Finally, the smooth basin floor (7) terrain contains broad wrinkle ridges with lobate features and no channels.

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Stratigraphically, the Navua Valles region consists of Noachian-aged crustal highland terrains, which are overlain by Hesperian-aged relatively smooth volcanic units. These units may consist of interbedded lava flows and pyroclastic materials that were emplaced during the second Late Hesperian to Early Amazonian active phase of Hadriaca Patera (Williams et al. 2007). Most channels within Navua Valles are eroded into these plains and ejecta.

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1.3 PREVIOUS GEOLOGIC MAPPING STUDIES

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Prior to 2018, the most detailed available geological map of the region was the 1:4.3 million scale map of Leonard and Tanaka (2001). In this map, only major Navua channels are shown and those are contained within a single, Hesperian–Noachian unit appropriate in scale for resolving units > 9 km wide (see Tanaka et al. 2011). At this scale, it would not be possible to resolve the smaller channel-associated subunits that could help constrain the individual channel system’s evolution. Previous work in East–Northeast Hellas focused on the two low-relief volcanoes, Hadricaus and Tyrrhenus Montes and the four major outflow-like, 5–20 km wide channels, Dao, Niger, Harmakhis, and Reull Valles, which formed in the Amazonian–Hesperian boundary (Price 1998). In contrast, the Navua Valles is comprised of 1–2 km wide channels that are smaller than typical outflow channels but larger than <500 m wide Fresh shallow valleys (FSVs) (Wilson et al. 2016). Similar sized channels, such as Sungari Vallis, formed southeast of the Navua Valles. Previous 1:1 million scale geologic mapping covered different parts of the region dissected by Dao, Niger, Harmakhis and Reull outflow channels (southeast: Price 1998, southwest: 3

ACCEPTED MANUSCRIPT Bleamaster and Crown 2010, northeast: Mest and Crown 2015), the Hadriacus Mons volcanic center (Crown and Greeley 2007), and the Vichada Valles region of Terra Tyrrhena (Mest and Crown 2006, Mest et al. 2010) (see overview map in Fig. 1 of the Supplementary Material). Navua Valles, the northwestern part of the dissected region, was only covered at smaller scales: Greeley and Guest (1987) mapped the area at 1:15 million scale, and Leonard and Tanaka (2001) at 1:4.3 million scale.

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More recently, the region was mapped at 1:20 million scale in the global geologic map of Mars (Tanaka et al. 2014), and Bernhardt et al. (2016) mapped the Hellas Basin floor units, but not the slope units, although they traced all major Navua channels, extending their mapping beyond their geologic units, at 1:2 million scale (see our Supplementary Material Fig. 2c).

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Recent studies by several groups have focused on the Eastern Hellas region (Crown et al. 1997, 2004, 2005a, 2005b; Pierce and Crown 2003, Bleamaster and Crown 2004, 2005; Mest and Crown 2009, Mest et al. 2015). Crown and Mest (2015) mapped a terrain south of Tyrrenus Mons where fluvial and lava channels co-exist. Zuschneid et al. (2013, 2014) identified Early to Late Amazonian activities on the floodplain of Dao Vallis; Kostama et al. (2007, 2009, 2010), Ivanov et al. (2010), Lahtela et al. (2011), Korteniemi and Kukkonen (2013, 2014), Kukkonen et al. (2013, 2015) showed that the floor of Harmakhis Vallis is completely covered with viscous flow units of Middle to Late Amazonian age. Mest and Crown (2001) concluded that the discontinuous Reull Vallis has had a long and complex history and inferred that the system was once continuous. Kukkonen and Kostama (2016, 2017) identified repeated active episodes along the Dao–Reull Vallis systems with some channel segments formed independently, at different times; Bernhardt et al. (2014, 2016) mapped the Hellas basin floor and put the age of the Dao–Harmakhis Valles to 3.7 Ga; and Glamoclija et al (2011) geologically mapped Harmakhis Vallis. In summary, while these previous studies did not cover the northwesternmost margin of the East Hellas channeled region, they do confirm repeated channel activity in this region from the Hesperian and extending until the Middle to Late Amazonian (Kukkonen 2018 and references therein).

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2 METHODS OF MAPPING Our study region consisted of a rectangular area of 267,000 km2 (445 km × 600 km) (~ equivalent to the area of the state of Colorado) between 79°E, 28°S and 87°E, 38°S (Fig. 1). To determine the timing and sequence of channelization, we calculated crater ages, not only for geologic units at 1:1 million map scale, but also for individual channel segments within the channel floors and on the terminal deposits at a scale of hundreds of meters, and for crater ejecta crossed by channels. Details of the age assessment procedure of the geologic units and individual channel segments are discussed in the Supplementary Materials in this paper. The 1:1 scale geologic map, data and mapping methodology is presented in Hargitai et al. (2018a), and its Supplementary Materials. In this paper, we use unit names from the Geologic Map of the Navua Valles region of Mars (Supplemental material of Hargitai et al. 2018a and Supplementary Material Fig. 3 of this paper), also presented in simplified form in Fig. 1. 4

ACCEPTED MANUSCRIPT 3 GEOLOGICAL EVOLUTION AND UNITS OF THE MAPPED REGION

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We reconstructed the evolution of the Navua Valles region based on the results of our geologic mapping (Fig. 1), crater counting (Figs. 2–3), and stratigraphic relations between the units and surface features. The geologic events along the Dao–Niger–Harmakhis–Reull outflow channels correlate with the active phases of Tyrrhenus Mons (see Kukkonen et al. 2018), while the unit formation and resurfacing periods in the Navua Valles area correlate with the active episodes of the more proximal Hadriacus Mons volcanic center (Fig. 3). Depression-filling deposits and blanketing materials formed in both the outflow channels and the Navua Valles, independently from the volcanic centers.

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3.1 NOACHIAN UNITS

(Epoch boundaries based on the Neukum system in Michael (2013): Early >3.94 Ga, Middle 3.94–3.83 Ga, Late 3.83–3.71 Ga)

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The evolution of exposed materials in the mapping area began 4.08 Ga ago with the formation of Hellas Basin (Robbins et al. 2013), a degraded, elliptical multi-ring impact basin.

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Isolated patches of mountain peaks (Nm) (crater counting model age: 4.07 Ga) are common above approximately -5000 m in the mapping area, along the interior rim of Hellas Basin. These peaks typically exhibit a muted broad topography and a rough textured rock surface. The mountain peaks unit typically forms 1300–1600 m high elongate ranges that are either isolated peaks or form in groups. Craters 7–20 km in diameter are depleted in Nm (see Hargitai et al. 2018a for crater size frequency diagrams), indicating a possible resurfacing at 3.81 Ga and total destruction of craters <20 km by that time (e.g., Boyce and Garbeil 2007) partly due to a >1 km general resurfacing concentrated in the Noachian (Robbins et al. 2013). Mountain peaks are the oldest units on our geologic map, formed in the Early Noachian and represent either heavily degraded (e.g., Carr and Head 2010 a,b) remnants of ancient impact crater rims or remnants of one the peaks of the Hellas Basin rim, composed of crustal rocks. However, mountain peaks are not present on the plain of Navua A at elevations where these peaks would be expected, and so it is likely that this plain was resurfaced several times in the Noachian, eventually reaching a linear longitudinal profile (Hargitai et al. 2017). In this process, volcanically induced debris flows (Tanaka et al. 2002), lava flows, tsunamis (e.g. Rodriguez et al. 2016), and large-scale fluvial processes may have played roles in leveling the topography and removing craters and any remaining peaks. The morphology and age of the oldest, smooth crater fill units (f) (3.9 Ga) suggest that it originates from airfall from the earliest volcanic episodes at Hadriacus and Tyrrhena Montes. The dissected apron (HNa, 3.85 Ga) of one Noachian mountain range forms a distinct terrain with a stepped margin. Its easily dissected materials likely accumulated from a volcanic episode at Hadriacus Mons (its formation coincides with the first, shield building volcanic episodes at Hadriacus and Tyrrhenus Montes, Williams et al. 2007, 2008) and were eroded during the next local volcanic episode, in the Hesperian (resurfacing: 3.68 Ga).

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ACCEPTED MANUSCRIPT Intermontane terrains (Nmi, HNmi, in two patches) (3.81 Ga) are situated between mountain peaks and impact crater rims. We interpret this material to be megabrecciadominated Noachian sediments and volcanic material. This unit marks the southern margin of Tyrrhena Terra. This unit is similar to the Npld unit of Crown and Greeley (2007) interpreted as formed during an interval of intense cratering.

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Anseris Mons’ sharp-crested isolated peak (HNms) (3.7 Ga) is apparently similar to the features that Xiao et al. (2012) termed ancient volcanic remnants. Crater counting (Hargitai et al. 2018a) showed that this unit formed significantly later than the Mountain peaks unit, in the Late Noachian. The peak of Anseris is only 45×25 km wide and has steep (at the crest approximately 26.6°) slopes. It is the highest peak in the broader region reaching a height of 6 km above the surroundings. Its southern flank has since collapsed, and the resulting slopes are dissected by deep valleys that terminate in deposits.

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Early Hesperian

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(Epoch boundary: 3.71–3.61 Ga; Michael 2013)

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3.2 HESPERIAN UNITS

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Most of the Navua Valles channels are cut into the surface of the Dissected plains (Hpd) unit that formed in the Early Hesperian (3.66 Ga) and is consistent with previous mapping (Leonard and Tanaka 2001). The Hpd is the oldest smooth flow unit exposed in the area characterized by heavily degraded “ghost” craters (Arvidson 1974). Stratigraphic relations suggest that both the formation of Hpd wrinkle ridges and the degradation of these ghost craters here were completed before the formation of Navua A channels. Crater 28-44 is the most degraded large crater in the Navua A Hpd plains. It is approximately 58 km in diameter and only 250 m deep from rim to floor on average. Boyce and Garbeil (2007) determined the depth-Diameter ratio for complex impact craters on Mars to be dr = 0.381 D0.52

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where dr is the rim-to-floor crater depth after the modification phase, and D is the crater diameter. Applying this equation to Crater 28-44, we determined an initial crater depth of 3.14 km. This suggests that approximately 2.9 km of height difference was lost due to the combined effect of rim erosion and infilling, by the time the channels breached its rim. Late Hesperian

(Epoch boundary: 3.61–3.37 Ga; Michael 2013) Basin basement. The oldest, “basement” material in the basin ramp, Hmib (3.62 Ga), is approximately 0.5 Gyr older than the Hellas rim basement and is probably comprised of a substrate that formed in situ within the basin. Layered mountains (Crown et al. 2005b, Bleamaster and Crown 2010) (Hml, 3.51 Ga) likely represent layered strata within the Hmib materials. The outliers of Hml and inliers (erosional windows) of Light Toned Deposits (e) overlap in the Late Hesperian, which suggests that both were produced from volcanic materials from the then-active Hadriacus or Tyrrhenus Montes. 6

ACCEPTED MANUSCRIPT The oldest calderas of Hadriaca Patera formed at 3.5–3.6 Ga (Williams et al. 2007, Werner 2009, Robbins et al. 2011). Crater counting suggests that Hpd (3.66 Ga) preceded this event and is related to the shield-building phase, while AHp postdated this event and occurred just following the Hesperian-Amazonian transition. The units that best fit the patera formation age are the Hmib-Hml units in the basin whose origin may be related to this event.

3.3 UNITS OF THE HESPERIAN–AMAZONIAN TRANSITION

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The Formation Age of the Navua Valles

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3.34–3.36 Ga marks the emplacement of the youngest plains materials, which consists of smooth, high thermal inertia plains (AHpb) (3.36 Ga) in the floor of Hellas Basin, and distinct, wrinkle ridged plains (AHp) (3.34 Ga) on the upper basin rim in the mapped region. Older, plains (Hpd) and rolling intermontane slopes materials (HNmi) were both resurfaced during the transition (Hpd: 3.36 Ga, HNmi: 3.38 Ga), which shows that this event affected large areas. Contemporaneously, Tyrrhenus and Hadriacus Montes volcanoes both ended their modification phases, and the Dao–Reull Valles formed (Price 1998). Although this would suggest that these volcanoes triggered a regional outflow episode, ‘Fresh shallow valley’ formation in the Arabia region also occurred during this time period (Wilson et al. 2016), which suggests a regionally or globally wet climate. This, in turn, may be linked to effects of widespread volcanism.

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The Navua Valles drainage networks are composed of V-shaped valleys, and flat-floored channels with in-channel and terminal deposits (cm, Fig. 1). In places, the channels are connected to depressions we interpret as potential paleolakes (Hargitai et al. 2018b). The drainage systems on the interior Hellas basin slopes cut into Late Hesperian flow (Hpd) and ejecta (HclM) units, which suggest that at least some early Navua channels formed during or after the Late Hesperian. Navua B’s main channel is deflected at the boundary of basin flow units (AHpb) that formed at the Hesperian–Amazonian boundary, which suggests that these flow units existed before the time of channel formation. It is possible that a paleo-Navua B channel initially cut across Crater 346’s ejecta but was flooded by AHpb and later Navua B fluvially rejuvenated the pathway. Navua A and E valleys predate the Mid-Early Amazonian (2.6 Ga), because they are filled or covered with low thermal inertia distal material of Crater 28-336’s 2.6 Ga ejecta blanket (Fig. 1). However, crater counting shows a relatively younger age for Navua A floor and deposits, which puts the initial formation age of the Navua Valles drainage systems around the Late Hesperian–Early Amazonian transition. Channel surfaces. Crater ages scattered from the Early to Late Amazonian in different drainage systems and channel reaches suggest that channels were infilled or resurfaced periodically until recent times. These age data probably do not indicate initial channel formation, because the original channel floor may have been covered by deposits (e.g., Kukkonen et al. 2015). Additionally, channel segment ages were determined by crater counts in small areas that may only resolve more recent resurfacing events.

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ACCEPTED MANUSCRIPT The role of AHp. The only plains unit in the mapped basin rim that is not dissected by wide channels is the wrinkle-ridged AHp, which is also the youngest plains unit. The absence of channels on AHp and its age suggest that channel formation was controlled by either (1) the timing of the emplacement of AHp and wet conditions, or (2) the lithology of AHp.

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In a time-controlled channelization model, major channel forming events did not continue after the emplacement of this unit on the upper basin slopes. Wrinkle ridges, many only crenulations, are common in this unit along with several leveed narrow channels that we interpret as lava channels similar to the narrow channels in the Tyrrhenus Mons lava flow field (Crown and Mest 2015) that originated from the Tyrrhena Patera caldera. These features suggest that AHp is most likely the volcanic flow material from Hadriacus Mons. Channelforming fluvial erosion in upper Navua B only took place prior to 3.34 Ga when the emplacement of AHp from Hadriacus and Tyrrhenus volcanic event(s) buried any previously existing channels.

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In a lithology-controlled channelization model, AHp was emplaced prior to significant fluvial activity. However, channels did not form in the permeable volcanic lava flows of AHp. Because of high infiltration rates, water flowed downslope along less permeable layers within the subsurface until those layers outcropped further downslope.

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AHpb in the basin formed 3.36 Ga ago, and is dissected by channels, which suggests that the emplacement of AHpb unit predates fluvial activity. This would constrain channel formation to the period between 3.36 and 3.34 Ga, unless fluvial activity took place at different times on the upper versus lower slopes of the basin margin between the AHp and AHpb units, respectively.

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Channel D (1.1 Ga) cuts into the 3.29 Ga old Poti crater ejecta (AcrP). Either (1) Channel D formed after 3.29 Ga and resurfaced 1.1 Ga, or (2) AHpb remained unchanneled until 1.1 Ga when a fluvial channel directly from Hadriacus Mons emptied into Hellas Basin.

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The most likely scenario is that major fluvial activity at Navua predated the formation of AHp (3.34 Ga) that was emplaced from Hadriacus Mons, and the last pulse of fluvial activity originated directly from Hadriacus Mons ~1.1 Ga, dissecting only those plains units that were downslope from the base of Hadriacus Mons. Liquid water thus originated from the southern flank of Hadriacus Mons for Channel D, while the source of Navua A and the other Navua channels lied more to the West, confined to one single location around Crater 21-35, perhaps also associated with Hadriacus’ activity that was more energetic at its first active episode and could affect farther locations. The exact formation period for interior channels could not be determined with crater counting but relative formation timing could be determined stratigraphy, specifically by the impacts of several km-sized craters between the time of channel and interior channel formation which strongly suggest a long gap between the primary channel formation and its dissection by the interior channel (Hargitai et al. 2017). Interior channels preferentially formed within the main channels and are discontinuous even within parent channel segments Unlike their parent channel systems, interior channels follow a single down slope pathway, the discontinuous main channel of the Navua A system and two separate paths within Navua B system, but are 8

ACCEPTED MANUSCRIPT absent in tributaries, isolated channels and subparallel, partially integrated channels (Fig. 5c). This suggests that the sources of the main channels reactivated in a single event while tributaries remained inactive, and lithology or climate-supported alternating surface and subsurface flow with minimal water loss as external ground water sources are not expected for the interior channels. Interior channels are more fragmented then the parent channels likely because water was less available.

3.4 EARLY AND MIDDLE AMAZONIAN UNITS

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(Epoch boundaries: Early 3.37–1.23 Ga, Middle 1.23–0.328 Ga; Michael 2013)

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3.3–1.6 Ga: From 3.3 Ga, there is no evidence for significant geologic or fluvial activity in the Navua Valles region for more than 1 Gyr except for new impacts and the exhumation of the layered erosional window (e) (Mustard et al. 2009, Wray et al. 2011, Salese et al. 2016) units. Geologic activity resumed approx. 2.2–1.6 Ga, but was confined to the Navua channels and some channel filling materials. The fluvial resurfacing of the flanks of Hadriaca Patera in the Early Amazonian (2.6 and 1.5 Ga, Williams et al. 2007 or 2.1, 1.6 and 1.8 Ga, Robbins et al. 2011) coincided with and may have triggered fluvial activity across the entire Navua region.

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<1.5 Ga: Resurfacing activity terminated at Hadriacus Mons at 1.5 Ga, when its southern flanks were ultimately, fluvially dissected (Williams et al. 2007). Werner (2009) determined the age of last resurfacing of an unspecified process at Hadriaca Patera to be 1.08 Ga. These ages, within error bars, are consistent with the age of the floor of Channel D (1.1 Ga) and Navua C (1.3 Ga), suggesting that these channel deposits and perhaps the channel too, may have formed when or after Hadriacus Mons’ slopes were eroded fluvially. Throughout the Navua Valles, the majority of in-channel deposit surfaces formed or resurfaced in the Early and Middle Amazonian (confluence site of Navua B: 0.73 Ga, for details see Hargitai et al. 2017). Bajadas (b) formed (0.54 Ga) in the Middle Amazonian showed that the intra-crater fans’ formation coincided with channel erosion, which suggests widespread fluvial activity across the entire region.

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Valley-blocking (or channel-filling) materials (db) are found at 6 sites scattered in the mapped area. These deposits are confined to 4–30 km long valley sections. Their surfaces are smooth and show very few craters. One such deposit is older than Early Amazonian as it is blanketed by lobate material from the Early Amazonian impact crater 28-336. The crater counting age of these channel-fill deposits is 12 Ma (the youngest individual deposit is 7 Ma). We interpret valley blocking materials as debris associated with channels and nearby impacts.

3.5 LATE AMAZONIAN UNITS (Epoch boundary: <0.328 Ga; Michael 2013) Late Amazonian channel floor resurfacing events are likely glacial or fluvio-glacial, consistent with Late Amazonian glaciations (e.g., Kadish et al. 2014), or aeolian. Crater counting suggests that at least some of these recent modifications were most significant in the upper reaches of the main channels (Hargitai et al. 2018a). In contrast, the terminal reaches show the 9

ACCEPTED MANUSCRIPT oldest formation model ages. This suggests that the active regions progressed upslope and newer resurfacing events are confined to the source regions. 0.5–0.2 Ga: The latest resurfacing episodes occurred in the Upper Navua A and B channels (commonly knobby), and in the knobby terrain of the Navua B shallow basin at its upper discontinuity. Intra-crater bajadas were also resurfaced during this period. Terminal deposits of the short Navua G and E (Hargitai et al. 2017) formed at 0.5 Ga, and these deposits were not modified when Navua A and B experienced more recent resurfacing episodes.

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0.4– Ga: Rumpled, complex, lineated (fl) and complex (fc) crater and valley filling deposits (e.g., Howard 2003, Morgenstern et al., 2007, Baker and Head 2015) formed by sublimation of ice-rich deposit.

4 THE FORMATION PROCESSES OF THE NAVUA DRAINAGE SYSTEMS

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The spatial relation of geologic units, drainage divides and the locations of channel start and terminus locations can be used as an indicator of lithologic control on channel formation. In this section we introduce this tool and discuss potential implications on paleoclimate.

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Sources and termini. In Figure 4, we noted the source and terminus of each channel segment in the mapped region. These locations may indicate where ground water emerged from and subsequently re-entered the subsurface. Between these points may indicate areas where the infiltration rates are low and the surface is mantled with ash or sediment allowing surface runoff (Gulick and Baker 1989, 1990; Gulick 1998, 2001). Ground-water outflow onto these regions from more permeable subsurface units could eventually erode these surfaces forming channels (Gulick 1998, 2001). Regions where surface infiltration rates are higher enable water to flow in more permeable subsurface units (e.g., permeable basalt or lava flows) until these aquifers outcrop at the surface further downslope.

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Most channels are short and singular and therefore result in numerous source and sink locations (334 source points with 225 end points: 1.2 sources and 0.8 channel termini per 1000 km2 on average). The highest concentration of channels is near Crater 21-35, and features a large number of isolated short valleys and channels (4.6 sources and 3.5 channel termini per 1000 km2) (Fig. 5). Equidistant zones. The Navua and the Hadriacus regions have two landforms associated with dense channels and valleys: an impact crater for the Navua systems and a volcano for the Hadriacus systems. We calculated the density of channel start and end locations in 100 km wide concentric zones away from these inferred water sources. We observe no linear geographic trend in source or terminus density away from the Navua or Hadriacus source regions (Fig. 6), which suggests that while Early Amazonian volcanic center activity is tied to late fluvial activity, proximity to Hadriacus may not have affected channel formation intensity within the Navua Valles region. The density of source and sink locations remains remarkably constant at a distance of 200 to 500 km from the Navua source region calculated for 20–50 thousand km2 large areas (proximity measured in Figure 6b). At the region where channels are short and isolated, sources and termini are similar in density to regions with significant, 10

ACCEPTED MANUSCRIPT continuous channels. Channel pattern and discharges are different but not the number of sources and sinks. This suggests that water was scarcely but relatively uniformly available and that water was flowing through similar, but alternating permeable and less permeable units (e.g., permeable basalt or lava flows, alternating with interbedded ash units) in the subsurface throughout the region.

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Aligned sources. Sources are most densely aligned at two locations, both near and parallel to geologic unit boundaries: on the apron unit HNa (marked “a” on Fig. 4) and in the Navua C channel system (marked “b” on Fig. 4). The NHa channels likely emerged at sources fed by abundant precipitation, and not hydrothermal systems, because this region is elevated in position and is situated in the region with the largest number of sources. There is a significant dichotomy between the two slopes of this massif. On the northern slope facing away from Hellas Basin has an integrated valley network with interior channels, which transported sediments from the foot of the massif into a large crater and produced a prominent, delta-like deposit. The southern flank, however, shows subparallel singular valleys and valley systems that terminate at the foot of the massif’s apron (Fig 5d). This difference may be due to different lithology (the northern slope’s network formed on crater ejecta, the southern channels on the massif’s apron) or slope orientation. The Navua C system on the basin floor was likely fed by ground water, because it is in a plains region down slope from the termini of other channels, and the channels exhibit a sapping morphology (e.g., theater-headed tributaries). Apart from these two locations, there is no obvious correlation between the geologic units or their boundaries and the channel sources.

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Sources in valleys. Channels down slope from the inferred source region typically emerge on the plains and not at the bases of steep massif slopes at the margins of those valleys. This suggests that those channels were fed by water collected or emerged in valleys.

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Role of lithology. Channels source and terminate on the ejecta of a few large craters and on Hpd plains units. This suggests that channels preferentially formed on erodible materials such as ash or ejecta.

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Watersheds and drainage pattern. All topographic drainage basins contain channel sources and termini except those entirely located on the Hellas basin floor. This suggests that water sources were available across the entire region, from precipitation or ground water. Subparallel branches along the same drainage system on Earth characterize channel networks formed on arid climates dominated by surface runoff (Seybold et al. 2018), which is consistent with the Navua Valles paleoclimatic scenario. The role of slope angles. Channels are concentrated on steep slopes, especially on the 0.8– 1.4° slopes at the margin of the basin floor, but terminate on gentle slopes where they typically deposited sediments. These suggest that only the highest-energy conditions favored channel formation. On non-channelized regions water could be available as sheet flows or thinner snow coverage. Alternatively, channels may have formed on steeper slopes if ground water outcrops had higher concentrations there. Concentration of sources. The region with the highest concentration of channels, sources, and termini is located on the highest altitude region of the Navua Valles and channelized 11

ACCEPTED MANUSCRIPT regions occur only down slope from this region. This suggests that the entire Navua system was fed from this single 200-km-diameter region where orographic precipitation (snowfall or possibly rain) occurred.

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Lack of sources. Sources or termini are absent on the layered Hml and Hmib basin floor units, erosional windows (unit “e”) and ancient, degraded but locally elevated massif units such as HNmi and Nm. HNmi is crossed by a drainage divide where ground water fed channels are not expected (Fig. 5). Additionally, sources are absent on the volcanic AHp unit (discussed earlier in section 3.3.1.).

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Lava channel scenario. Source and sink locations are scattered over various geologic units, including megabreccia and volcanic plains. Neither of the Navua channels have lobate margins as observed in lava flows elsewhere on Mars and pits are not observed along the channels, as in discontinuous channels on lava flows (Hargitai and Gulick 2018). These geologic relations make it unlikely that any of the Navua channels (A, B, C, and E) or Channel D are lava channels.

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5 THE PROPOSED SEQUENCE OF WATER-RELATED EVENTS Based on the results of our mapping studies, we propose the following sequence of events in the history of the Navua Valles region with special attention to water related features (Fig. 7).

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1) Early Noachian: A giant impact produced Hellas Basin, resulting in the formation of thick ejecta and an extensive network of subsurface fractures. Basin embayments (wedge shaped plains protruding into the basin rim) formed during the impact. Basin walls were modified in catastrophic mass wasting events, changing it in places into a gently sloping terrain. Substantial material was eroded from the highlands and filled the basin likely involving water. However, no channel forms were preserved from this period.

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2) Noachian to Hesperian: Volcaniclastic materials and lavas covered the slopes originating from Hadriacus Mons or other volcanic centers, burying smaller landforms and flooding some large craters. Craters were filled with airfall materials.

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3) Late Hesperian: Layered deposits accumulated at the margin of Hellas Basin by airfall, lacustrine or mass wasting processes. Layered deposits also formed in the highlands. 4) Late Hesperian to Early Amazonian: Volcanism, Precipitation/Melt, Channel Initiation a. Channels formed in the Circum-Hellas region, including the Navua and the Dao– Reull systems, associated with volcanism. Precipitation, likely as snow (Head and Marchant 2014), occurred in patches in the basin rim highlands (Hargitai et al. 2017). Discontinuous drainage networks were produced by the melting of snowpacks, similar to isolated Fresh shallow valleys (FSVs) elsewhere (Wilson et al. 2006). Channels may have been formed from hydrothermal ground-water outflow onto more erodible ash-covered or pyroclastic flow units with lower infiltration rates (Gulick and Baker 1989, 1990; Gulick 1998, 2001) from the nearby Hadriacus Mons that was active at the time of channel formation (Fig. 3). 12

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b. Accumulated snowfall melted in some areas either by heat from Hadriacus from hydrothermal activity (Gulick 2001, 1998, Gulick and Baker 1989, 1990); newly formed impacts (near 21-35) (Mangold 2012), or a climatic event, suddenly releasing high quantities of water, carving channels that reached long distances, producing the main, longest reaches of the channel systems. Channels of the Hesperian–Amazonian episode were fed by highland precipitation and emptied into Hellas basin floor. While the drainage systems situated in the densely cratered highlands are similar to dendritic networks, the valleys and channels that cut Hesperian terrains are typically subparallel and large channels resemble single outflow channels. Channels are neither present on the flat interior of the basin floor, nor in steep basin margins that are distant from the volcanic centers. The floods that produced the channels, especially Navua A, originated from a single region, around crater 21-35, from the crater rim and its slopes. This regional-scale precipitation produced a variety of features inside and outside of this crater suggesting longlasting or recurring wet periods.

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c. Liquid water carved valleys and channels downslope, deposited sediments, and likely infiltrated on gentle slopes, and incised again further down slope.

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d. Repeated volcanic pulses and/or climatic events may have produced several active episodes over the same channels, but they may also have produced new channels or segments. Multiple lobes of terminal and intermediate deposits, terraces and crosscutting channels are evidence for repeated activity along these channels. The large number of discontinuous channel segments, some isolated, others down slope from other channel segments suggests that fluvial activity was widespread geographically but limited to short segments on the surface.

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e. New volcanic activity emplaced volcanic materials in some parts of the Hellas slopes and the basin.

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5) Early Amazonian: older channels resurfaced in Navua Valles and new channels formed on the flanks of Hadriacus Mons. When Hadriacus’ southern flanks resurfaced (Williams et al. 2017), these channels (Channel D) also reactivated and new channels formed or resurfaced on the basin ramp (Navua C). Water here originated from the upper slope of the caldera, suggesting precipitation associated with volcanism. 6) Middle to Late Amazonian: After a long cessation of fluvial activity, one or several episodic climatic changes produced small discharge interior channels in the sedimentfilled reaches in single events. Narrow channels formed in intra-crater bajada slopes. As water-dominated processes waded, ice-dominated processes started shaping the surface (Crown et al. 2005). Lineated material filled depressions in both the Navua and Dao– Reull channels. Several types of recent features required some form and quantity of H2O during their formation. These include valley-filling deposit packs, mound fields, and knobby terrain. 7) Late Amazonian: Channels were mantled by an intermediate albedo material that in places eroded into a pitted surface (Schon et al. 2012a, Pierce and Crown 2003), 13

ACCEPTED MANUSCRIPT preferentially in the interior channels. Most recently, transverse dunes occupied several channels. These again link the Navua Valles evolution to processes common globally on Mars.

6 CONCLUSIONS

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Most of the water-related features in the Navua Valles region can be explained by localized precipitation triggered by global climate excursions, local or global volcanism, impact, or external effects such as solar events. In the Hesperian, valley networks formed isolated in time and space (Fassett and Head 2008) by scattered, localized precipitation that, however, occurred globally on a longer time scale. While the unit dissected by the Navua A channels formed in the early Hesperian, as mapped previously, stratigraphic considerations suggest that the Navua Valles channels formed later, at the Hesperian–Amazonian transition when both Hadriacus and Tyrrhenus Montes were volcanically active. Subsurface water was available down slope from two main source regions, one in the Navua Valles region and another on Hadriacus Montes. Since both source regions are situated on elevated landforms, it is unlikely that water originated from a deep subsurface reservoir. The drainage systems are composed of few-tens-of-km channel segments that suggest low energy conditions and limited availability of water. In the dissected terrain, on both crater ejecta and Hesperian volcanic plains units, a new channel emerged within every 1000 km2 on average, but after about a few tens of km downslope, water infiltrated again. This may be caused by relatively uniform distribution of surface or subsurface water sources or from subsurface lithologic or permeability controls. Eventually, water reached Hellas Basin where it was lost into the subsurface and produced terminal deposits.

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Channels originating directly from the middle flanks of the Hadriacus Mons volcano formed or resurfaced later than the Navua Valles channels, in the Early to Middle Amazonian. This suggests a localized event in the Hadriacus region at that time within the error bars of channelization age proposed by Williams et al. (2017). The latest fluvial episodes reactivated the major channels producing smaller-discharge interior channels and narrow intra-crater fan channels, not resolved in previous mapping studies. This suggests an Amazonian paleoclimate capable of supporting liquid surface water, which was available in less quantity than before. The formation of some other features, such as knobby terrain, mound fields, and valley blocking deposits in Amazonian channel reaches likely also involved water but was dominated by ice-related processes. The observed evidence for repeated but decreasing intensity fluvial episodes during the earlier Amazonian transitioning to more ice-dominated processes is consistent with earlier studies in East Hellas and along other channel systems (e.g., Jaumann et al. 2015, Kukkonen and Kostama 2016). While Late Hesperian fluvial and Late Amazonian ice-related processes fit into global climatic scenarios, Early to Middle Amazonian active geologic episodes, including channelization, were probably local, related to the possible reactivation of the Hadriacus Mons volcanic center.

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ACCEPTED MANUSCRIPT 7 ACKNOWLEDGEMENTS

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This research was supported by a senior postdoctoral research fellowship awarded to H. Hargitai by the NASA Postdoctoral Program (NPP) at Ames Research Center, administered initially by ORAU and later by Universities Space Research Association through a contract with NASA. V. Gulick was partially supported by MRO HiRISE Co-I funds and by funds from the NASA Astrobiology Institute under Grant No. NNX15BB01A. N. Glines was also supported by funds from the NASA Astrobiology Institute under Grant No. NNX15BB01A. This research has made use of the USGS Integrated Software for Imagers and Spectrometers (ISIS). We thank David Crown for the useful discussion about the area. We are grateful for the reviews by Corey Fortezzo and Jim Zimbelman, whose suggestions greatly improved the content and readability of the manuscript.

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Williams, D. A., Greeley, R., Zuschneid, W., Werner, S. C., Neukum, G., Crown, D. A., Gregg, T. K. P., Gwinner, K., Raitala, J. 2007. Hadriaca Patera: Insights into its volcanic history from Mars Express HighResolution Stereo Camera, J. Geophys. Res., 112, E10004, doi:10.1029/2007JE002924. Williams, D. E., Yingst, R. A., Garry, W. B , 2014. Introduction: The geologic mapping of Vesta. Icarus 244, 1–12. Wilson, S. A., Howard, A. D., Moore, J. M., Grant, J. A. 2016. A Cold-Wet Mid-Latitude Environment on Mars during the Hesperian-Amazonian Transition: Evidence from Northern Arabia Valleys and Paleolakes. J. Geophys. Res. Planets, 121, doi:10.1002/2016JE005052. Wray, J. J., et al., 2011. Columbus crater and other possible groundwater‐ fed paleolakes of Terra Sirenum, Mars, J. Geophys. Res., 116, E01001, doi:10.1029/2010JE003694. Xiao, L., Huang, J., Christensen, P.R., Greeley, R., Williams, D.A., Zhao, J., He, Q., 2012. Ancient volcanism and its implication for thermal evolution of Mars. Earth and Planetary Science Letters 323–324, 9–18. Zuschneid, W., van Gasselt, S., 2013. Fluvial Processes in Eastern Hellas Planitia, Mars: New Stratigraphic Insights. 44th Lunar and Planetary Science Conference abstract #2191. Zuschneid, W., van Gasselt, S., 2014. Episodic Fluvial Processes in Eastern Hellas Planitia, Mars. 1120.pdf Eighth International Conference on Mars abstract #1120.

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ACCEPTED MANUSCRIPT Abstract

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The Navua Valles are comprised of a system of channels and valleys on the inner Northeastern rim of Hellas Basin, which is a 1500-km-long sloping terrain. Drainage systems and regional geology in this unique setting were not previously mapped in detail. We mapped this region using CTX (6m/px) as the base map and assessed surface unit ages resulting from our crater counting. We found that the timing of the deposit-forming episodes in this region during the Hesperian and Early to Middle Amazonian largely correlated to active phases of the Hadriacus Mons volcanic center. We found evidence for several episodes of fluvial activity Hesperian to the Amazonian with declining intensity, and transitioning to icedominated processes. The channels in the Navua Valles region erode into deposits dating from the Noachian to Early Amazonian, including the Noachian highlands, Noachian to early Amazonian crater ejecta, and likely volcanic plains formed from the Hesperian to the Hesperian–Amazonian transition. Channels directly originating from Hadriacus Mons are younger, while precipitation-fed channels at larger distance from the volcanic center are older, indicating different triggers for fluvial activity. Crater counting results indicate that almost all channel floors were at least partially resurfaced during the Amazonian and that several channel deposits formed during the last 0.5 Gyr. Water pathways likely included surface channels, lakes, and subsurface flow. The Navua Valles channel system is discontinuous, and the number of terminal deposits (sink locations) is almost as high as the number of channel sources, which is unusual for valley networks elsewhere on Mars. Interior channels formed only in the major Navua channels, they are even more fragmented than their parent channels, but occur along their entire length. Channels and valley systems within the Navua Valles are potential targets for in situ astrobiological studies, as they could have provided potential habitats at least periodically, from the Late Hesperian to the Late Amazonian.

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ACCEPTED MANUSCRIPT Highlights * Eastern Hellas Basin surface terrains formed by episodic volcanic and fluvial activity * Fluvial episodes occurred simultaneously to active phases at Hadriacus Mons * There are almost as many terminal deposits as sources within the channel systems * Channels located on the basin slopes are older than those on the basin floor * Floods and sustained flows periodically formed primary or interior channels

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* Channel activity occurred periodically from the Noachian until the Late Amazonian

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