Continental-scale salt tectonics on Mars and the origin of Valles Marineris and associated outflow channels

Abstract

A synthesis of deformation patterns within and around the Thaumasia Plateau, Mars, points to a new interpretation for regional deformation and the origin of Valles Marineris and associated outfl ow channels. The morphology of the Thaumasia Plateau is typical of thinskinned deformation, akin to a "mega-slide," in which extensional deformation in Syria Planum and Noctis Labyrinthus connects via lateral zones of transtension and strike-slip-Claritas Fossae and Valles Marineris-to a broad zone of compressional uplift and shortening defi ned by truncated craters and thrust faults along the Coprates Rise and Thaumasia Highlands. However, the low regional slope (∼1°) results in gravitational body forces that are too small to deform the basaltic lava fl ows conventionally thought to compose the fl anks of the Tharsis volcanic province. Instead, we conclude that geothermal heating and topographic loading of extensive buried deposits of salts and/ or mixtures of salts, ice, and basaltic debris would allow for weak detachments and largescale gravity spreading. We propose that the generally linear chasmata of Valles Marineris refl ect extension, collapse, and excavation along fractures radial to Tharsis, either forming or reactivated as part of one lateral margin of the Thaumasia gravity-spreading system. The other, dextral, lateral margin is a massive splay of extensional faults forming the Claritas Fossae, which resembles a trailing extensional imbricate fan. The compressional mountain belt defi ned by the Coprates Rise and Thaumasia Highlands forms the toe of the "mega-slide." Topographic observations and previous structural analyses reveal evidence for a failed volcanic plume below Syria Planum that could have provided further thermal energy and topographic potential for initiating regional deformation, either intrusively through infl ation or extrusively through lava fl ow and/or ash fall emplacement. Higher heat fl ow during Noachian time, or geothermal heating due to burial by Tharsis-derived volcanic rocks, would have contributed to fl ow of salt deposits, as well as formation of groundwater from melting ice and dewatering of hydrous salts. We further propose that connection of overpressured groundwater from aquifers near the base of the detachment through the cryosphere to the martian surface created the outfl ow channels of Echus, Coprates, and Juventae chasmata at relatively uniform source elevations along the northern margin of the "mega-slide," where regional groundwater fl ow would have been directed toward the surface. Our hypothesis provides a unifying framework to explain perplexing relationships between the rise of the Tharsis volcanic province, deformation of the Thaumasia Plateau, and the formation of Valles Marineris and associated outfl ow channels.

A volcanic origin for the outflow channels of Mars: Key evidence and major implications

“…Though alternative channel-forming processes have received consideration in subsequent decades (e.g., Hoffman, 2000;Tanaka et al, 2001Tanaka et al, , 2002, widespread acceptance of aqueous interpretations of the Martian outflow channels has been maintained to the present (e.g., Baker et al, 1991;Zimbelman et al, 1992;Carr, 1996;Komatsu and Baker, 1997;Baker, 2001;Clifford and Parker, 2001;Wilson et al, 2009;Burr, 2010). Modern hypotheses of aqueous channel development continue to predominantly feature incision by liquid water (e.g., Burr, 2003;Andrews-Hanna and Phillips, 2007;Keszthelyi et al, 2007;Harrison and Grimm, 2008;Montgomery et al, 2009;Warner et al, 2009), though the possibility of associated erosional or depositional contributions by mudflows or debris flows (e.g., Tanaka, 1999;Wilson and Mouginis-Mark, 2003;Williams and Malin, 2004;Rodriguez et al, 2005) or by the flow of glaciers (e.g., Lucchitta, 2001;Head et al, 2004;Pacifici et al, 2009;Chapman et al, 2010) is also incorporated into many models.…”

“…However, regardless of the exact nature of water release, the relatively small volumes of basins at the heads of many systems strongly suggest that floodwaters would have necessarily been derived from much larger subsurface reservoirs (e.g., Carr, 1979Carr, , 1996Clifford and Parker, 2001;Jakosky and Phillips, 2001;Burr et al, 2002a;Andrews-Hanna and Phillips, 2007); in this perspective, areas of disturbance or collapse at the heads of outflow channels mark the locations of outbursts rather than the full lateral dimensions of subsurface reservoirs (Clifford and Parker, 2001;Andrews-Hanna and Phillips, 2007). The high estimated discharge rates of flood events have suggested the need for highly porous and well-connected aquifer systems (Carr, 1979), with required porosities at 0-7 km depth of at least~5-20% (e.g., Clifford, 1981;MacKinnon and Tanaka, 1989;Hanna and Phillips, 2005) and required permeabilities as great as~10 − 6 and 10 − 7 m 2 Montgomery et al, 2009;Wilson et al, 2009). Permeabilities such as these are not typical of aquifers on the Earth, but are believed by some workers to have possibly been produced on Mars by widespread disturbance of the megaregolith by impacts during the heavy bombardment phase (e.g., Wilson et al, 2009).…”

“…Martian porosities in excess of 40% have been hypothesized for depths of~2-7 km in regions substantially underlain by fractured and vesicular basalt units . Large increases in hydrologic connectivity are additionally hypothesized to have developed in the subsurface as a result of processes such as the dewatering of hydrous salts (Montgomery and Gillespie, 2005;Montgomery et al, 2009), gradual formation of caverns by hydrothermal processes (Rodriguez et al, 2003(Rodriguez et al, , 2005, or physical disruption of rock units during outburst events (Carr, 1996).…”

“…In recent decades, the Mars community has mainly favored aqueous interpretations involving catastrophic floods from large subsurface reservoirs (Baker and Milton, 1974;Carr, 1974Carr, , 1979Mars Channel Working Group, 1983;Clifford, 1993;Carr, 2000;Clifford and Parker, 2001;Coleman, 2003;Carr, 2007;Keszthelyi et al, 2007;Leask et al, 2007;Montgomery et al, 2009;Warner et al, 2009;Wilson et al, 2009). Aqueous interpretations of the Martian outflow channels have led to inferences of the past existence of large bodies of standing water at channel mouths (e.g., Baker et al, 1991;Parker et al, 1993;Kargel et al, 1995;Gulick et al, 1997;Clifford and Parker, 1999;Baker, 2001;Clifford and Parker, 2001;Ivanov and Head, 2001;Fairén et al, 2003;Baker, 2006;Harrison and Chapman, 2008;Dohm et al, 2009), inferences of major past changes in global climate (Sagan et al, 1973;Sharp and Malin, 1975;Baker, 1979;Toon et al, 1980;Baker et al, 1991;Kasting, 1991;Schaefer, Fig.…”

Mars Sedimentary Geology: Key Concepts and Outstanding Questions

“…Putting aside the northern plains of Mars, the origin of which is controversial (e.g., Marinova et al, 2008), the Valles Marineris rift valley system is a spectacular, but unique, case of a massive sediment accumulation (several-kilometer-thick interior layered deposits) that occurred after gravity-related, thin-skin tectonics (Montgomery et al, 2008). Otherwise, large-scale topographic lows evidently resulted from the removal of uppermost crust during impacts that created multi-ring crater basins.…”