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

David R. Montgomery^1,, Sanjoy M. Som¹, Martin P. A. Jackson², B. Charlotte Schreiber³, Alan R. Gillespie⁴ and John B. Adams⁵

¹ Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA, Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA
² Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78713-8924, USA
³ Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA
⁴ Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA Astrobiology Program, University of Washington, Seattle, Washington 98195-1310, USA
⁵ Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA

View larger version (112K): [in this window] [in a new window]   Figure 1. Mars Orbiter Laser Altimeter (MOLA)-derived shaded relief location map of the setting of SE Tharsis, Valles Marineris, and the Thaumasia Plateau, showing place names and the locations of Figures 3–5, 7, and 8, as well as the traces of the topographic profiles shown in Figure 10 (yellow lines). Asterisks show locations of grabens discussed by Grott et al. (2007).

View larger version (156K): [in this window] [in a new window]   Figure 2. Shaded-relief map of the Thaumasia Plateau (Mars Orbiter Laser Altimeter [MOLA] base) showing mapped distribution of thrust faults (thick red), normal faults (yellow), wrinkle ridges (thin red), and dikes (green) on the Thaumasia Plateau and immediate vicinity.

View larger version (158K): [in this window] [in a new window]   Figure 3. Shaded-relief map of the Thaumasia Highlands (Thermal Emission Imaging System infrared [THEMIS IR] base) showing the distribution of normal faults (yellow) and thrust faults (thin red), trace of frontal anticlinal (thick red), and truncated craters along the southern edge of the Thaumasia Plateau.

View larger version (363K): [in this window] [in a new window]   Figure 4. Shaded-relief maps of western margin of Thaumasia Plateau (Mars Orbiter Laser Altimeter [MOLA] base) showing (A) grabens (yellow) indicative of dextral transtension, as well as R- and P-shear and (B) en echelon faulting indicative of dextral shear (locations shown by ellipses).

View larger version (148K): [in this window] [in a new window]   Figure 5. Shaded-relief map of Syria Planum and Noctis Labyrinthus (Thermal Emission Imaging System infrared [THEMIS IR] base) showing intersecting sets of N-S–striking and annular normal faults (yellow) that form a regional polygonal chaos.

View larger version (144K): [in this window] [in a new window]   Figure 6. Rose-diagram map showing the distribution of aspects for topography steeper than 5° for a 5° x 5° grid across the Thaumasia Plateau.

View larger version (171K): [in this window] [in a new window]   Figure 7. Shaded-relief map of Melas Chasma (Thermal Emission Imaging System infrared [THEMIS IR] base) showing wrinkle-ridge axes (red) and grabens (yellow), indicating extensional unloading revealed by arcuate grabens on the S side and stepped grabens on the N side of the chasma.

View larger version (144K): [in this window] [in a new window]   Figure 8. Shaded-relief map of Thaumasia Minor (Thermal Emission Imaging System infrared [THEMIS IR] base) showing structural features: thrust faults (thick red), normal faults (yellow), wrinkle-ridge (thin red), and inferred dikes (green).

View larger version (127K): [in this window] [in a new window]   Figure 9. Shaded-relief map of the Thaumasia Plateau (Thermal Emission Imaging System infrared [THEMIS IR] base) showing the broad-scale, inferred sense of translation during gravity spreading, and the boundaries of "microplates" (dashed lines) and outflow channels (open circles).

View larger version (47K): [in this window] [in a new window]   Figure 10. Longitudinal profiles across Thaumasia Plateau through Solis Planum (locations shown in Fig. 1). Vertical exaggeration is 100:1. Shaded area is the "excess area" of the Coprates Rise–Thaumasia Highlands projecting above the regional elevation trend. Dots mark locations of craters intersected by the profiles.

View larger version (19K): [in this window] [in a new window]   Figure 11. Calculated martian geotherms based on a surface temperature of –55 °C, a heat flux of 100 mW/m2 (Hesperian, upper curve) or 30 mW/m2 (present-day, lower curve), a crustal density of 3000 kg/m3, thermal conductivity of 1.7 W/mK), and a heat capacity of 840 J/kgK. Also shown are effective viscosity-temperature curves for rock salt (halite), indicating decreasing viscosity (more effective detachment) with increasing temperature and decreasing mean grain sized (after van Keken et al., 1993). Gray shading shows temperatures <0 °C.

View larger version (69K): [in this window] [in a new window]   Figure 12. Whiting Dome as a smaller-scale terrestrial analog for gravity spreading detaching on salt. This dome comprises a Quaternary sedimentary veneer detaching on an allochthonous salt sheet. (A) Shaded-relief map having 2x vertical exaggeration, with highs and lows shown as warm and cool colors, respectively. Structural interpretation, based on a deeper map from Peel et al. (1995). (C) Location on the continental slope of the northern Gulf of Mexico. (D) Profile down regional dip; line of section shown in (B). A small part of the diapir crest has sagged below the regional datum because of extension. However, most of the diapir crest has risen asymmetrically several hundred meters above regional because of salt inflation due to downdip salt flow. Bathymetric data in (C) from Smith and Sandwell (1997). All other bathymetric data from Divins and Metzger (2007).

View larger version (75K): [in this window] [in a new window]   Figure 13. Schematic illustration (not to scale) of the deformation of the Thaumasia Plateau as a "mega-slide" formed by continental-scale salt tectonics. In stage 1, discontinuous layers of salts, ice, and impact breccia accumulate in Noachian time as megaregolith before the growth of Tharsis, perhaps as surface evaporites, or subsurface precipitates during impact-driven generation of the brecciated martian megaregolith. In stage 2, the rise of the Tharsis volcanic province and later arching of Syria Planum above a suspected plume increased both the regional heat flux and local topographic slope, loading the region with ash and lava flows and producing radial extension faults that may have penetrated through the regional cryosphere. In stage 3, layers of salts, ice, and basaltic debris within the regolith provided multiple detachments for the gravity spreading southeastward, possibly in response to further intrusion under Syria Planum. In addition, the aquifer below the cryosphere found outlets to the surface along the chasmata of Valles Marineris where the basal décollement cuts upward to intersect radial extension cracks projecting from Tharsis. This connection rapidly drained the extensive confined aquifer, which had substantial topographic head within the deforming zone. Drainage then carved the outflow channels.