Detailed Study of an Hydrological System of Valleys, a Delta and Lakes in the Southwest Thaumasia Region, Mars

Icarus 180 (2006) 75–87 www.elsevier.com/locate/icarus Detailed study of an hydrological system of valleys, a delta and lakes in the Southwest Thaumasia region, Mars Nicolas Mangold ∗, Véronique Ansan Lab. IDES, Bat. 509, Université Paris-Sud et CNRS, 91405 Orsay, France Received 20 December 2004; revised 26 April 2005 Available online 9 November 2005 Abstract The occurrence of fluvial activity and standing bodies of water on early Mars is the subject of debate. Using MOC, MOLA, and THEMIS data, we identify a whole set of landforms in the Thaumasia region which attest to water flows during geologically long periods of more than thousand years. A thick fan-delta is identified within an impact crater at the outlet of a deep valley. Ponded water filled and overflowed this crater’s rim, creating entrance and exit breaches and an outlet valley. These landforms show that the 25-km diameter impact crater contained a lake up to 600 m deep. At the head of this crater’s deep contributing valley, a closed depression may have contained another lake, but depositional landforms are not evident in this headward basin. Alternatively, groundwater discharge may have supplied the valley, but the observed landforms are not consistent with a sudden release of water, as is usually invoked for the large martian outflows channels. Stratigraphic relationships show that this hydrological activity occurred during the Hesperian period, thus relatively late in the history of martian valley network development. 2005 Elsevier Inc. All rights reserved. Keywords: Mars, surface; Mars, climate 1. Introduction et al., 2002) but the longevity of these putative standing bodies of water remains uncertain. A recent study shows the most The climate of the early Mars has been a subject of debates convincing evidence for a potential paleolake in the “north–east for 30 years centered around two end-member models theories Holden crater” which contains a distributary fan or delta with involving a warm and wet climate or cold and dry processes meandering inverted channels (Malin and Edgett, 2003).Al- only (e.g., Pollack et al., 1987). Groundwater flows driven by though valley network development occurred primarily during early high geothermal gradient are often invoked to explain val- the Noachian period perhaps associated with warmer climate ley networks in cases where paleoclimate models do not reach (Craddock and Maxwell, 1990), some paleolakes could have warm temperatures (Kasting, 1991; Clifford, 1993). Neverthe- existed during the Hesperian and Amazonian periods (e.g., Ori less, recent climate models (Forget and Pierrehumbert, 1997; et al., 2000a). Mischna et al., 2000; Colaprete and Toon, 2000) and recent ob- In this study, we identify two potential paleolakes using servations of valley networks (Craddock and Howard, 2002; topographic data of Mars Global Surveyor (MGS) Mars Ob- server Laser Altimeter (MOLA, Smith et al., 1999), wide an- Grant, 2000; Mangold et al., 2004) or potential paleolakes gle and narrow angle visible Mars Observer Camera images (Cabrol and Grin, 1999; Ori et al., 2000a; Malin and Edgett, (MOC, Malin et al., 1998) acquired at different spatial reso- 2003) renew the case for a warmer early Mars. Potential pa- lution (∼250 m and 3 m/pixel) and thermal imagery acquired leolakes were identified inside impact craters on the basis of with the Mars Odyssey Thermal Emission Imaging System in- geomorphic criteria and/or topographic data (de Hon, 1992; strument (THEMIS, Christensen et al., 2003). Maps, volume Newson et al., 1996; Cabrol and Grin, 1999, 2001, 2002; Irwin and slope measurements are done using a Geographic Infor- mation System (GIS) software after referencing all data in a * Corresponding author. Fax: +33 1 69 15 63 48. martian standard. The two potential paleolakes are located in E-mail address: [email protected] (N. Mangold). the southern part of the Tharsis region inside high-standing ter- 0019-1035/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2005.08.017 76 N. Mangold, V. Ansan / Icarus 180 (2006) 75–87 rains dated to the Late Noachian epoch. The two basins are (Fig. 3b). This depression is similar to basins observed in rift connected by a deep valley which shows a 600-m thick delta zones on Earth (e.g., Needham et al., 1976). at its mouth (Ansan and Mangold, 2005). Chronological relationships show that this fluvial activity postdates terrains dated 2.2. Fluvial features of the Noachian–Hesperian boundary indicating Hesperian age water flows (Dohm et al., 2001), a period thus relatively late in Branching valleys similar to terrestrial fluvial networks the history of martian valley network development. are locally observed mainly on the southern slope of the Noachian period Thaumasia highlands bounding the smooth 2. Tectonic and fluvial landforms in West Thaumasia plain (Fig. 3b). Valleys show a subparallel drainage pattern highlands, South Claritas Fossae region rather than a tree-like pattern, indicating control by the topographic slope (Schumm et al., 1987). THEMIS images show 2.1. Geologic context and tectonic features that fluvial valleys are incised less deeply than are the grabens. The topography of fluvial valleys is usually not visible at the Claritas Fossae and Thaumasia highlands are located 39◦ S MOLA resolution (463 m/cell). Several valleys identified on and 103◦ W at the southern end of the Tharsis bulge (Fig. 1). the nighttime THEMIS images converge into the eastern closed This region consists of heavily cratered Noachian highlands depression although these valleys are sparse (Fig. 3b). Val- that were uplifted during the Noachian period, which ended 3.8 leys could have developed in two or more stages from the to 3.6 Ga ago (Dohm and Tanaka, 1999) and surrounded by Noachian into the Hesperian periods (Dohm and Tanaka, 1999). smooth Hesperian plains along their western edge (Dohm et al., Such valleys were first attributed to rainfall-fed fluvial ero- 2001). This uplift has been attributed to outward-verging fold sion (Sagan et al., 1973). Hypotheses involving hydrothermal and thrust margins (Schultz and Tanaka, 1994) or magmatic ac- activity associated with impacts, volcanism or tectonics have tivity and crustal underplating (Dohm and Tanaka, 1999).The been proposed to explain morphological differences between most abundant geomorphic features over these rugged high- martian valleys and terrestrial valleys (Tanaka et al., 1998; lands consist of (1) extensional tectonic faults mainly oriented Gulick, 2001), but limited atmospheric precipitation could also in the north–south direction and (2) valley networks such as explain the immature development of martian valleys, because Warrego Valles located to the southeast of the studied area the valley heads are distributed over a wide range of eleva- (Dohm et al., 2001; Gulick, 2001; Ansan and Mangold, 2003, tions (Mangold and Ansan, 2004; Stepinski et al., 2004).No 2005). These two kinds of geologic features are well expressed detailed analyses are presented here to discriminate between in the mosaic of daytime THEMIS images, in which the con- these modes of formation. trast is due primarily to albedo and slope (Fig. 2a). The sim- In addition to small valleys, a larger east–west oriented val- ilarity in tone on the visible image (Fig. 1) indicates a pre- ley of 130-km length and up to 10-km width (DV in Fig. 1) dominance of topographic effect in the THEMIS image. In the connects the eastern depression in the Thaumasia highlands nighttime THEMIS mosaic (Fig. 2b), spatial differences in the to an impact crater located at the plain–highlands boundary. thermal inertia of material are evident but do not organize into MOLA data show that this valley is about 400 m deep on aver- laterally extensive specific geologic units. Bright areas in the age. This deep valley exhibits two about linear reaches arranged image represent the rocky slopes of faults or craters and other with an angle of 120◦ (Figs. 1, 2, and 3b). The lower section is contrasts may indicate regional differences in the properties of more sinuous with three tributary valleys than the straight upper aeolian mantles. section. This linear pattern raises the question of whether the Well-developed tectonic faults are evident in all context im- origin of this valley was volcanic or fluvial. No volcanic flow ages. The predominant north–south direction is composed of is associated with this valley, so it seems unlikely that it was normal faults forming narrow grabens of a few kilometers in formed by lava tubes. The deep valley orientation is not con- width and up to 100 km long (Fig. 1). West of the studied area, cordant with the strike of most normal faults. In contrast, the some of these narrow grabens (Fig. 1) continue into the smooth valley mouth occurs 250 m below the valley head, indicating plains southwest of the highland boundary. The part of these a low longitudinal gradient of 0.15◦ on average. Such a gradi- plains visible in Fig. 1 is dated to the Noachian–Hesperian tran- ent is within the range of fluvial valley gradients (e.g., Leopold sition (HNf on the map of Dohm et al., 2001). The extensional et al., 1992). MOC images cannot provide more information tectonic activity, partially cutting these terrains, therefore began such as the presence of inner channels because the valley floor in the Noachian period and continued in the Hesperian epoch is mantled by transverse aeolian dunes. The deep valley cross- or later. Minor east–west trending normal faults or fractures ex- cut two N–S normal faults of the plateau. The fact that the deep ist locally in the southern and western parts of the study area. valley is not crosscut by any north–south faults shows that it These grabens are relatively shallow and more degraded, which formed after the main tectonic episodes (Fig.

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