Erosional valleys in the Thaumasia region of Mars: Hydrothermal and seismic origins

Analysis of erosional valleys, geologic materials and features, and topogra phy through time in the Thaumasia region of Mars using co-registered digita l spatial data sets reveals significant associations that relate to valley origin. Valleys tend to originate (1) on Noachian to Early Hesperian (stage s I and 2) large volcanoes, (2) within 50-100 km of stages 1 and 2 rift sys tems, and (3) within 100 km of Noachian (stage 1) impact craters >50 km in diameter. These geologic preferences explain observations of higher valley- source densities (VSDs) in areas of higher elevations and regional slopes ( >1 degrees) because the volcanoes, rifts, and craters form high, steep topo graphy or occur in terrain of high relief. Other stage 1 and stage 2 high, steep terrains, however, do not show high VSDs. The tendency for valleys to concentrate near geologic features and the overall low drainage densities in Thaumasia compared to terrestrial surfaces rule out widespread precipita tion as a major factor in valley formation (as is proposed in warm, wet cli mate scenarios) except perhaps during the Early Noachian, for which much of the geologic record has been obliterated. Instead, volcanoes and rifts may indicate the presence of shallow crustal intrusions that could lead to loc al hydrothermal circulation, melting of ground ice and snow, and groundwate r sapping. However, impact-crater melt would provide a heat source at the s urface that might drive away water, forming valleys in the process. Post-st age I craters mostly have low nearby VSDs, which, for valleys incised in ol der rocks, suggests burial by ejecta and, for younger valleys, may indicate desiccation of near-surface water and deepening of the cryosphere. Later H esperian and Amazonian (stages 3 and 4) valleys originate within 100-200 fu n of three young, large impact craters and near rifts systems at Warrego Va lles and the southern part of Coprates rise. These valleys likely developed when the cryosphere was a couple kilometers or more thick, inhibiting vall ey development by hydrothermal circulation, However, eruption of,groundwate r may have occurred from impact-induced fracturing and lateral and perhaps minor upward transport of water due to seismic pumping. The two smaller cra ters formed along the plateau margin where the highest potential hydraulic head would occur in aquifers beneath the plateau. In the case of the larger crater (Lowell, 200 km in diameter), potential aquifers would likely be at depths of kilometers below the cryosphere; Seismic energy generated by the Lowell impactor would have been much greater, pumping both groundwater and perhaps fluidized slurry to the surface from beneath the cryosphere to for m the young valleys and flow deposit. Along the margin of Thaumasia, tecton ic pressurization of groundwater also may have contributed to valley format ion. Dissection of rim materials of the Argyre impact may relate to tectoni c activity and the unconsolidated state of basin ejecta.