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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

Correspondence: E-mail: dave{at}ess.washington.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

 
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 outflow channels. The morphology of the Thaumasia Plateau is typical of thin-skinned 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 defined 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 flows conventionally thought to compose the flanks 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 large-scale gravity spreading. We propose that the generally linear chasmata of Valles Marineris reflect 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 defined 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 inflation or extrusively through lava flow and/or ash fall emplacement. Higher heat flow during Noachian time, or geothermal heating due to burial by Tharsis-derived volcanic rocks, would have contributed to flow 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 outflow channels of Echus, Coprates, and Juventae chasmata at relatively uniform source elevations along the northern margin of the "mega-slide," where regional groundwater flow 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 outflow channels.

Key Words: Mars • salts • sulfates • salt tectonics • Thaumasia • Valles Marineris • gravity spreading • volcanic plume • outflow channels

	INTRODUCTION

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

 
Ongoing debate surrounds the relations, if any, between the Tharsis volcanic province, the deformation of the Thaumasia Plateau, the opening of Valles Marineris, and the associated outburst floods (Fig. 1). The wide range of explanations for the formation of Valles Marineris, which exposes the deepest sections through the martian crust, includes structural collapse and/or tectonic rifting based on analogy to terrestrial continental rifts (Table 1). Explanations offered to explain carving of the associated outflow channels include incision by wind, glaciers, debris flows and lava, or catastrophic floods due to thermal response to dike emplacement, discharge from overpressured aquifers, and dewatering of extensive deposits of hydrated salts, as well as explosive depressurization of clathrates and combinations of these mechanisms (Table 2). Although evidence for each explanation accounts for a subset of the observations, no single process or combination of processes has satisfactorily explained connections between the growth of the Tharsis volcanic province, deformation of the Thaumasia Plateau, formation of the Valles Marineris chasmata, and the associated outburst flooding that created Echus, Juventae, and Coprates chasmata.

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).

Montgomery and Gillespie (2005) hypothesized that heating and dewatering of extensive buried evaporite deposits generated the great discharges that carved channels as outflows from Valles Marineris and vicinity. Elaborating upon that idea to propose a single mechanism to form Valles Marineris, Noctis Labyrinthus, and the compressional highlands ringing the Thaumasia Plateau, we draw on prior research, analyses of Mars Orbiter Laser Altimeter (MOLA; Zuber et al., 1992) topography, and interpretation of Thermal Emission Imaging System (THEMIS; Christensen et al., 2004) and Mars Orbiter Camera (MOC; Malin et al., 1991) images to propose regional deformation moving down a gentle topographic slope. This gravity-driven deformation produced a linked extensional-contractional system detached on flowing salts or salt-rich deposits heated from below and loaded from above by burial during the rise of Tharsis and Syria Planum. Within and at the toe of the deforming region, fractures connected an overpressured, confined aquifer below the cryosphere to the surface and triggered outburst floods. Such a large-scale detachment, or "mega-slide," explains otherwise perplexing aspects of relations between the growth of Tharsis, deformation of the Thaumasia Plateau, and the origin of Valles Marineris and its outburst floods.

	BACKGROUND

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

 
Despite the wide range of proposed explanations for the formation of Valles Marineris, few attempts have been made to link its growth to the regional setting and evolution of the Thaumasia Plateau beyond the general recognition of the volcanic province of Tharsis as a logical source of both heat and topographic potential. The Thaumasia Plateau extends almost 3000 km across and rises >4 km above the surrounding plains. It is now generally accepted that formation of the Thaumasia Plateau involved: (1) increased mass from intrusion and extrusion of volcanic rocks; and (2) contractional deformation from tectonic activity (Schultz and Tanaka, 1994). However, the mechanisms responsible for the deformation of the Thaumasia Plateau are still debated. For example, Dohm and Tanaka (1999) attributed formation of the plateau to deformation comparable to intracontinental plateaus like the Rocky Mountains fold-and-thrust belts. Anguita et al. (2001) proposed that the Thaumasia Plateau was an independent lithospheric block that buckled in the Late Noachian–Early Hesperian under E-W compression, forming the Valles Marineris chasmata by transtensional deformation (inferred by them to be dextral) along the northern margin of the Thaumasia block. They also concluded that deformation of the Thaumasia Plateau involved an early thick-skinned event in the Late Noachian–Early Hesperian (ca. 3.6 Ga) and later thin-skinned deformation in the Amazonian (<3 Ga). More recently, Anguita et al. (2006) reexamined tectonic structures at Thaumasia Plateau and neighboring Daedalia Planum and Aonia Planum, and proposed that these areas formed part of a larger Noachian orogen.

Perhaps the most globally prominent of martian landforms—other than large volcanoes and impact basins—is Valles Marineris, an immense, remarkably linear, canyon system of distinct troughs that extends more than 1500 km along the northern margin of the Thaumasia Plateau. Conventional explanations favoring rifting have focused on the linearity of individual troughs comprising Valles Marineris and on prominent fault scarps in many of the chasmata as evidence for a tectonic origin (e.g., Frey, 1979; Peulvast and Masson, 1993; Peulvast et al., 2001). Conversely, those inferring structural collapse typically point to the closed depression of Hebes Chasma as compelling evidence for removal of subsurface support (e.g., Spencer and Fanale, 1990; Schultz, 1998).

The stratigraphy of the region has also been debated, in particular whether the layered deposits exposed in Valles Marineris formed before or after the present chasmata. Some hold that the layered deposits formed in lakes (e.g., Nedell et al., 1987; Komatsu et al., 1993; Lucchitta et al., 1994; Quantin et al., 2005; Dromart et al., 2007) or as sub-ice volcanoes (Chapman and Tanaka, 2001) within Valles Marineris, and thus that the layered deposits are younger than the chasmata walls. These walls are widely thought to consist of basaltic lava flows, based on local exposures of layering extending through the entire section (McEwen et al., 1999). Although contacts between the layered materials and wall rocks are obscure throughout most of the Valles Marineris chasmata, Malin and Edgett (2000) reported that MOC images reveal layered deposits within Valles Marineris that extend, in places, beneath the darker wall-forming material, and thus predate the chasmata there. Williams et al. (2003) showed that layering in the wall rock extends deeper in the western parts than in the eastern parts of the main trough of Valles Marineris, and proposed that both extrusive and intrusive magmatism formed the layering. However, examination of additional MOC images of areas where layered deposits are exposed in Melas Chasma (Montgomery and Gillespie, 2005) and Juventae Chasma (Catling et al., 2006) also show light-toned layered formations extending under darker rocks that form the canyon walls. Montgomery and Gillespie (2005) further posited that Tharsis-related volcanism buried extensive deposits of hydrous salts. Bigot-Cormier and Montgomery (2007) reported additional evidence for a contact between wall-forming material and an underlying weaker layer in the walls of Candor, Ophir, and Melas chasmata within Valles Marineris. Together these observations challenge the widely held view that the crust exposed in the walls of the Valles Marineris chasmata consists entirely of stacked basalt flows, even if this may be so in some locations (e.g., McEwen et al., 1999).

An active cryosphere in the martian regolith has long been recognized (Sharp, 1973), but recent discovery of extensive salt deposits on Mars (Squyres et al., 2004; Gendrin et al., 2005; Osterloo et al., 2008) suggests that much of the martian crust is composed of three end-member materials: (1) basaltic ash, flows, and rubble, (2) liquid water or ice, and (3) salts and hydrated salts. Hence, the different physical properties of exposed or buried salt and salt-rich deposits provide a potentially underappreciated third component that could influence martian landforms. In particular, extensive deposits of buried salts, or salt-cemented deposits, could provide both sub-surface ductile layers and sources for diapiric or extrusive salt tectonics. In contrast to both liquid water and ice, which are unstable under ambient surface conditions and are lost over time, many salts are stable at martian surface conditions but may dewater (e.g., Montgomery and Gillespie, 2005) or readily deform viscously (e.g., Schreiber and Helman, 2005). Saline deposits on Mars may include both chloride and sulfate salts, and perhaps other compounds. At least three potential mechanisms could account for such deposits: (1) precipitation at the ground surface due to evaporative concentration, such as within a shallow or ephemeral lake; (2) sub-surface precipitation as cements and displacive salts within porous sediments through evaporation, brine cooling, and/or brine mixing; and (3) precipitation from pressure release of salt-saturated supercritical waters ("outsalting" in the sense of Hovland et al., 2006). Although ice could persist near the surface of Mars if buried by dust or rubble (Mellon et al., 1997), and the depth to the martian cryosphere varies latitudinally, the origin, extent, and geomorphic influence of a martian "halosphere" remains to be established. In this context, a ternary diagram with the end-members referred to above provides a formal way to conceptualize the influence of the three primary landscap-forming materials on Mars.

	OBSERVATIONS

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

 
Spectroscopic and Stratigraphic Evidence for Salts Exposed in Valles Marineris
Gendrin et al. (2005) identified sulfate salts from OMEGA spectra of light-toned layered deposits in Ius, Hebes, Juventae, and Capri chasmata. It is not yet clear whether all light-toned deposits exposed in the Valles Marineris chasmata can be interpreted as containing salts. The absence of salt spectra for some light-toned deposits may be due to dust cover, masking by other spectral components, or concentrations of salt minerals below the OMEGA detection threshold. Other possibly common salts such as halite do not have diagnostic spectral features in the near-infrared wavelength range sampled by OMEGA. Although salts might be expected to have higher albedos than surrounding materials, albedo is not a reliable indicator of composition. Salts readily darken when they contain even trace amounts of optically absorbing components such as metal oxides and sulfides or organic compounds (e.g., Talbot and Aftabi, 2004) or when they are subjected to ionizing radiation. For example, dark gypsum dunes have been identified in the north-polar region of Mars (Langevin et al., 2005). Even though the proportion of salt minerals in the Valles Marineris chasmata is not known, the spectroscopic evidence confirms that locally some layers are salt rich and may even be pure enough to fall near the salt end-member in our proposed ternary diagram.

If salts are concentrated in the light-toned layered deposits, evidence cited above indicates that they occur over a wide range of stratigraphic positions throughout the Valles Marineris chasmata from the rims of chasmata to the base of cliff walls 6–8 km below (Malin and Edgett, 2000; Montgomery and Gillespie, 2005; Bigot-Cormier and Montgomery, 2007). However, light-toned layered rocks are not restricted to the sides and floors of the chasmata. MOC images at Ius and Candor chasmata within Valles Marineris, as well as Juventae Chasma (Catling et al., 2006), show them on the highland plains (at Ius and west Candor the light-toned layered deposits rim the chasmata). Other images show them in crater floors and highland plains far from the chasmata. Hence, potential salt-rich deposits appear to span the extent of Valles Marineris, extend beneath the surrounding plains, and be locally concentrated in the martian crust (Kargel et al., 2007). Also spectral detection of halite shows it to be widely distributed (Osterloo et al., 2008).

Structural and Geomorphological Analyses of MOLA Data
In contrast to ongoing debate about the stratigraphy, there appears to be less debate over the structural geology around Tharsis and the Thaumasia Plateau. Consistent styles of deformation in different areas around the Thaumasia Plateau indicate coherent local and regional spatial patterns. Extension in Syria Planum and Noctis Labyrinthus in the NW transitions to shortening along the Coprates Rise and Thaumasia Highlands in the SE. In addition, models of stress patterns resulting from growth of the Tharsis volcanic province describe the observed pattern of radial tension cracks and grabens, including the general alignment of the chasmata composing Valles Marineris (Banerdt et al., 1982; Mége and Masson, 1996a).

Structural evidence around Tharsis points to early radial fracturing followed by uplift along the Coprates Rise and Thaumasia Highlands and shortening in the form of well-developed wrinkle ridges on the Thaumasia Plateau (Anderson et al., 2001; Borraccini et al., 2007). In particular, Schultz and Tanaka (1994) mapped deformation at two distinct scales within and on the margins of the Thaumasia Plateau. They concluded that a well-defined zone of shortening extending from the Coprates Rise (60°W 15°S) along the Thaumasia Highlands to the western edge of Solis Planum (110°W 30°S) resulted from buckling and thrust faulting that drove 2–4 km of uplift of Noachian and perhaps Early Hesperian units. Schultz and Tanaka (1994) held that the Thaumasia Plateau was pushed over the adjacent foreland, in a manner that Dohm and Tanaka (1999) later interpreted to be consistent with crustal underplating.

The distribution of thrust faults, normal faults, and wrinkle ridges were mapped onto a gray-scale MOLA, shaded-relief base map, drawing on a MOLA-registered THEMIS global map (http://jmars.asu.edu/data) and using infrared, elevation, and visible data (Fig. 2). Each mapped line represents two conjugate faults because normal fault traces are the width of a typical graben. Only a few faults are shown where they are densely spaced; we did not try to map all apparent fractures but instead chose to map illustrative features where we had high interpretive confidence. Wrinkle-ridge traces were mapped along the middle of both sharp-crested ridges and flat-crested ridges, either of which may have faults on one or both sides. Probable thrust traces were inferred mostly from truncated craters or scarp morphology.

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.

East and north of these grabens, a continuous zone of extension reaches from Syria Planum to Noctis Labyrinthus at the western end of Valles Marineris. Tanaka and Davis (1988) documented normal faulting and extension in Syria Planum on the NW end of the Thaumasia Plateau and argued that closely spaced faults in Noctis Labyrinthus and Claritas Fossae evolved at the same time. Based on graben widths, they calculated that the faulted thickness ranged from 0.5 to 4.5 km. Arguing in favor of response to local uplift of Syria Planum rather than Tharsis-wide loading, they inferred radial fracturing in response to local mantle diapirism and uplift. Later, concentric faulting was attributed to crustal relaxation, and perhaps magma withdrawal as lava flows erupted onto Syria Planum. They further inferred that the troughs and grabens of Noctis Labyrinthus were roughly coeval with the formation of Valles Marineris.

Our mapping of extensional grabens on the Thaumasia Plateau shows two distinctive areas and styles of deformation: Claritas Fossae and Noctis Labyrinthus. The linear grabens in the western margin in Claritas Fossae resemble the lateral margin of a gravity-spreading fan. Dextral deformation at Claritas Fossae is indicated by (1) a splay geometry resembling a trailing extensional imbricate fan (Fig. 2), (2) apparent R shears and P shears on the western margin (Fig. 4A), (3) left-stepping, en echelon fault arrays farther east (Fig. 4B), and (4) curvature of wrinkle ridges near this boundary (Fig. 2), as described below.

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.

Mangold et al. (1998) investigated whether wrinkle ridges formed by deep thrusting, buckling of lava plains, or thrusting on shallow décollements. They showed that structural relations between wrinkle ridges and faulted craters were inconsistent with both buckling and deeply rooted faulting, but were consistent with shallow-rooted thrusting. They further argued that the décollement depth was not related to the thickness of lava flows, but instead to the depth to ground ice, which deforms ductilely and therefore can provide a detachment surface. Based on a simple geometric model for the depth to a décollement for a compressional pop-up structure, Mangold et al. (1998) estimated the depth to the detachment beneath wrinkle ridges elsewhere on Mars to range from 1 to almost 4 km.

Okubo and Schultz (2004) analyzed the morphology of wrinkle ridges in the western hemisphere of Mars. From maps of back-thrusts, they determined that the crust beneath Solis and Thaumasia plana varies in strength, as would arise from interstratified weak layers or reservoirs of near-surface volatiles (e.g., ice). In contrast, most areas outside the Thaumasia Plateau lacked backthrusts, indicating stronger detachments having more frictional resistance. Okubo and Schultz (2004) predicted that the backthrusts associated with wrinkle ridges in the Thaumasia Plateau indicated weak layers in the upper 2–5 km. They further invoked a primary thrust beneath the Solis Planum wrinkle ridges at >10 km depth.

Anguita et al. (2006) noted that the spacing of wrinkle ridges in Nectaris Fossae, the easternmost part of the Thaumasia Plateau, decreases from ~60 km to <20 km in the easternmost margin of the highlands. The inferred depth to the detachment beneath the Thaumasia Plateau decreases from 8 to 15 km under the main plateau to 2–6 km beneath its easternmost edge. They interpreted this decrease to indicate that the décollement beneath the eastern edge of the Thaumasia Plateau dips to the west at 0.6°–3.7°, a range of dips comparable to detachments beneath terrestrial thrust wedges.

Vectors orthogonal to the wrinkle ridges are used to map the flow patterns that formed them. Dividing the Thaumasia Plateau into a 5° x 5° grid (i.e., a 300-km grid at the equator that reduces to a 221-km grid at the southern end of the study area) allows compiling the distribution of aspects (i.e., directions normal to the ground surface) for slopes steeper than 5°. For each grid cell so defined, a rose diagram can show the distribution of vectors orthogonal to the dominant wrinkle-ridge orientation. The resulting map pattern reveals downslope and outward tectonic transport across much of the plateau (Fig. 6). On the western and eastern margins of the plateau, the rose diagrams indicate extension caused by outward spreading, whereas in Solis Planum the rose diagrams show NW-SE shortening. On the plains beyond the Thaumasia Plateau, the rose diagrams indicate no directional bias to slopes >5°.

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.

Canyon Wall Unloading
The pattern of younger chasmata-parallel fault scarps on the edges of Valles Marineris provides further evidence for mobility on layers at depth after the chasmata opened. In particular, the pattern of grabens bordering Melas Chasma indicates lateral extension due to canyon-wall unloading (Fig. 7). The normal faults cut the wrinkle ridges, so Melas Chasma must have widened after the wrinkle ridges formed. Similarly, the projection of some normal faults into the present canyon walls shows that the canyon widened after extension began. Particularly telling are arcuate scarps set back from and sub-parallel to the arcuate southern edge of Melas Chasma in the heart of Valles Marineris. The strikes of these fractures indicate that they postdate formation of the chasma and hence reveal further extension across a broad area toward the free face of the chasma walls. Similarly, grabens on the northern side of Melas Chasma record post-chasmata normal faulting indicative of broad deformation and movement of the surrounding upland toward the chasma walls.

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).

	INTERPRETATION OF COMPOSITE DEFORMATION

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

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).

In this context, there are at least two possible origins for the frontal anticline bounding the Thaumasia Plateau on the SE and E. One possibility is that this is merely a deformation front where gravity spreading ground to a halt but could have advanced farther if the gravity drive had been stronger or the rocks weaker. Alternatively, the frontal anticline marks a stratigraphic limit of the detachment. Similarly, in some terrestrial evaporite basins the frontal shortening structures coincide exactly with the limits of the evaporite basin (e.g., Perdido and Mississippi Fan fold belts in Gulf of Mexico; Lower Congo Basin of offshore Angola; Ebro Basin in Spain; and Melville Island fold belt in Arctic Canada).

The SW margin of the Thaumasia spreading system is well defined as a zone of dextral shear by three consistent criteria: (1) en echelon fault relays, (2) a fault splay (a trailing extensional imbricate fan) forming the Claritas Fossae structural province, and (3) curvature of wrinkle ridges and normal faults. Failing to support (or contradict) either of the conflicting interpretations of Anguita et al. (2001) and Borraccini et al. (2007), we found no kinematic indicators for the sense of shear along the northern boundary of Thaumasia, which we assume to involve sinistral deformation because of the downslope direction of gravity spreading. However, evidence for sinistral shear on the northern margin of the Thaumasia Plateau could have been eroded during excavation of Valles Marineris because the troughs have widened significantly since their initial excavation (e.g., Schultz, 1991; Peulvast and Masson, 1993; Lucchitta et al., 1994; Peulvast et al., 2001).

	ANALYSES

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

 
Structural Analysis
The amount of lateral shortening on the margins of the Thaumasia Plateau, particularly the Coprates Rise and the Thaumasia Highlands, as well as the finer-scale wrinkle ridges of Solis Planum can be constrained by the equal-area method (Mitra and Namson, 1989). Assuming conservation of cross-sectional area, one can solve for the combinations of depth to the detachment and the displacement needed to raise material in individual wrinkle ridges or in the uplifted frontal anticline defined by the Coprates Rise and the Thaumasia Highlands.

MOLA-derived topographic profiles across the Thaumasia Plateau were used to project the regional far-field slope through the Coprates Rise and the Thaumasia Highlands (Fig. 10). The "excess" cross-sectional area measured from these profiles reveals areas of 340–730 km² above the regional topographic trend across these uplands (Mitra and Namson, 1989). A wide range of combinations of depth to detachment and net displacement distance are compatible with the excess cross-sectional area of the Coprates Rise and the Thaumasia Highlands. Judging from the maximum local depth of the extensional features in Noctis Labyrinthus, the basal detachment is >8 km deep. The floor of Hebes Chasma also lies ~8 km below the surrounding plains. Hence, following Mitra and Namson (1989), deformation consistent with an 8- to 10-km-deep basal décollement corresponds to 30–90 km of translation for the deeper structure responsible for the compressional uplands. Across the 3500 km width of shortening, from the frontal escarpment to the wrinkle ridges farthest updip, this entails an overall shortening strain of <3%.

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.

Even the greatest displacement and uplift implied by this analysis represents little finite strain. Deformation need not have occurred evenly with depth and is likely to have been concentrated as shear zones in particularly weak layers. Indeed, the deformation represented by the compressional highlands on the southern margin of the Thaumasia Plateau and the wrinkle ridges on the plateau imply décollements at different depths.

Stability Analysis
Despite the striking morphological similarity, a key problem with interpreting the Thaumasia Plateau as a coherent continental-scale landslide is that the general slope across Solis Planum is only ~1° from Syria Planum across the Thaumasia Plateau to the far side of either the Coprates Rise or the Thaumasia Highlands. From the high point on Syria Planum, the slope decreases gradually until the topography rises several km across the Coprates Rise and the Thaumasia Highlands before descending to the plains of Aonia Terra surrounding the Thaumasia Plateau.

Simple slope-stability analysis shows that groundwater pressure far greater than lithostatic (i.e., artesian) would be required to cause gravity failure of basaltic material on the low topographic gradient across the Thaumasia Plateau. Even the simplest model, the infinite-slope stability model (e.g., Selby, 1993) based on the balance of driving to resisting forces for failure of an infinitely wide surface, predicts unreasonably high pore-water pressures to trigger failure of Coulomb materials on such low-gradient slopes. The model is typically expressed as

where FS is the factor of safety (which equals 1 at failure); and are, respectively, the shear and normal stresses acting across the failure plane; _s is the density of the slop-forming material; _w is the density of the pore fluid; g is the acceleration due to gravity (3.72 m s^–2); C is cohesion; is the friction angle; z is the depth to the slide plane; is the ground slope; and µ is the pore-water pressure in void spaces at the slide plane (µ = _wgh, where h is pore-water pressure head).

Rearranging and solving Equation 1 for FS = 1 provides the critical pore-water pressure, µ_c, for slope failure from

Solving for the critical hydraulic head required to trigger slope failure [given by h_c = µ _c/(_w g)] of a 10-km-thick section of crust typical of basaltic lava flows (C = 2 MPa; = 30°; = 3000 kg m^–3) down a 1° regional slope, Equation 2 predicts that almost 22 km of artesian head (i.e., h_c = 31.7 km) would be required. Drag along the lateral boundaries would increase the frictional resistance to gravity spreading even more than this simple analysis indicates. Hence, either the material forming the slope must be failing on highly (and unrealistically) overpressured material, or parts of the plateau-forming material are very weak, or viscous. In contrast to even saturated piles of granular basalt, salt deforms viscously at ambient temperatures on both Earth and Mars, as discussed further below.

	SALT TECTONICS

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

 
Mechanical Properties of Salts
The observations described above are consistent with continental-scale gravity spreading involving thin-skinned (i.e., 10-km-thick) deformation of part or all of the megaregolith, consisting of ancient impact breccia, salts, ice, overlying and/or interbedded ash, volcanic breccia and lava flows. The two distinct styles of deformation in and around the Thaumasia Plateau—large compressional highlands and small wrinkle ridges—indicate a likelihood of décollements at multiple levels, and perhaps several deformation phases. In addition, a regional gravity low that corresponds to the Thaumasia Plateau (Zuber et al., 2000) is consistent with burial of a thick pile of materials having lower than average density (like salts). Finally, the high concentration of rampart craters on Solis Planum would also be consistent with impacts into shallowly buried and potentially hydrous salts. Such craters have halos of granular material that flowed away from the impact and have been interpreted as evidence for near-surface ice (Barlow et al., 2001) or hydrous salts (Komatsu et al., 2007).

Most rocks (other than shales) have relatively similar frictional strengths in the upper crust (Byerlee, 1978), but salts are several orders of magnitude weaker and thus prone to viscous creep (Weijermars et al., 1993). An equally striking difference is that on Earth the brittle-ductile transition for salts can be as shallow as a few meters, as shown by salt glaciers in Iran (Talbot, 1998; Talbot and Aftabi, 2004). Salts deform ductilely as a power-law or Newtonian viscous fluid at temperatures and pressures ambient on Earth. They have virtually no yield strength and flow subaerially at rates that can exceed 1 m yr^–1 under small gravitational shear stresses provided by topographic slopes of <5° (Talbot et al., 2000; Talbot and Aftabi, 2004). The effective viscosity of rock salt is estimated as ~10¹⁸ Pa s from rock mechanics (van Keken et al., 1993) and modeling of Interferometric Synthetic Aperture Radar (InSAR)-derived uplift data from the Mount Sedom diapir, Israel (Weinberger et al., 2006). Rock salt's viscosity decreases as grain size decreases and trace water content and temperature increase. Salts are extremely weak and can support little shear traction—an ideal property for a basal detachment for fold-and-thrust belts (Davis and Engelder, 1985; Letouzey et al., 1995; Rowan et al., 2004). Indeed, terrestrial fold and thrust belts over salt detachments can have very low (<1°) topographic dips. Salt-based fold-and-thrust belts typically widen downslope, support regularly spaced folds of weak vergence, and exhibit abrupt changes in deformational style at their margins (Davis and Engelder, 1985)—features that characterize the Thaumasia Plateau. In particular, a complex of thrusts (like those along the Coprates Rise) commonly forms along frontal anticlines (such as the Coprates Rise and the Thaumasia Highlands) at the edge of salt basins where resistance to translation climbs abruptly. In addition, strata above a mobile salt layer often appear to expand radially toward the salt-free foreland (Davis and Engelder, 1985; Gaullier and Vendeville, 2005), resulting in margin-parallel, extensional grabens such as those at Claritas Fossae (Tanaka and Davis, 1988) and the Thaumasia Highlands (Grott et al., 2007).

The relative weakness of evaporites is due to creep-related recrystallization even at low differential stresses and low temperatures (20–200 °C) (Schreiber and Helman, 2005). Halite flows viscously at terrestrial surface temperatures, as shown by salt glaciers. In contrast, anhydrite deforms as a brittle material up to confining pressures of 15 MPa (Liang et al., 2007). Anhydrite begins to recrystallize at ~120 °C and begins to creep at temperatures >150 °C, and even lower temperatures at higher pressures (Müller and Briegel, 1978).

Could deformation of salt or salt-rich deposits on Mars explain the morphology of the Thaumasia Plateau despite the low topographic slope driving the deformation? Temperatures and confining pressures necessary for creep of salts would have existed within the upper 10 km of the martian crust during Hesperian times, assuming martian surface temperature of 218 K, crustal density of 3000 kg/m³, thermal conductivity of 1.7 W/mK, heat capacity of 840 J/kgK, and heat fluxes of 100 mW/m² for Hesperian conditions and 30 mW/m² for present conditions (Stevenson et al., 1983; Clifford and Parker, 2001) (Fig. 11). Moreover, below several km depth the martian crust could have hosted confined aquifers (i.e., reached temperatures >273 K) that, depending upon the material composition of the crust, could have been quite extensive. Water greatly weakens evaporites, even in mere traces (0.01%) (Urai et al., 1986), and wet gypsum aggregates creep more readily than dry gypsum aggregates (de Meer and Spiers, 1995). Creep rates for wet granular gypsum increase by up to 50 times if the pore fluid is saline, as is typical in mixed evaporites (de Meer and Spiers, 1999). Hence, Tharsis-related heating could destabilize salt deposits, triggering creep and flow down the flank of Syria Planum, even without injection of magma or hot water from below.

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.

From this range of terrestrial examples, we highlight two from the northern Gulf of Mexico as partial analogs for Thaumasia. Whiting Dome (Fig. 12) is one of hundreds of salt diapirs on the lower continental slope. Above a 24-km-long tablet of diapiric allochthonous salt, its Quaternary siliciclastic roof has moved downslope ~1 km by gravity spreading (Peel et al., 1995). A trailing graben at its updip end marks an extensional zone where the roof, detaching on salt, pulled away from the salt-free surrounding area. Conversely, a zone of uplift and probable overthrusting marks shortening at the downdip end of the system. These two zones are linked by a translational zone, whose lateral margins of strike-slip comprise either a principal fault or an en echelon array of transtensional normal faults. Multibeam bathymetry (Divins and Metzger, 2007) shows that apart from the minor trailing graben, the rest of this gravity-spreading system is elevated 200–400 m above the regional datum. The relief is strongly asymmetric, with high elevations concentrated in a frontal bulge caused by inflation of the underlying allochthonous salt. A much larger frontal bulge created by salt tectonics is the >750-km-long Sigsbee Escarpment in the Gulf of Mexico. This escarpment marks the seaward front of the allochthonous salt canopy, created by coalescence of hundreds of salt diapirs, which provides a detachment for gravitational spreading. Hence, the salt-cored Sigsbee Escarpment provides a terrestrial example of a massive frontal anticline and expression of uplift and shortening in the toe of the gravity-spreading system, which resembles the eastern and southern rim of the Thaumasia Plateau.

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).

Terrestrial orogenic belts also exhibit numerous examples of regionally extensive, thin-skinned deformation that serve as potential analogs for the shortening zone of the Thaumasia Plateau. Davis and Engelder (1985) identified 16 examples in which terrestrial fold-and-thrust belts occur at least in part on top of salt beds. In particular, the Appalachian Plateau in western New York and Pennsylvania provides a well-documented example of an extensive, outward-propagating fold-and-thrust belt detached on relatively thin (~75 m thick) salt beds where the limit of substantial folding corresponds to the extent of salt (Rogers, 1963; Davis and Engelder, 1985). At the termination of the salt beds, the largest anticline formed where the accumulated basal slip abruptly converged into splay faults as slip cut upward to the surface (Rogers, 1963; Davis and Engelder, 1985), much like at the base of the Thaumasia Highland and Coprates Rise. Moreover, the 300-km-long, 70-km-wide, fold-and-thrust belt of the Jura Mountains in central Europe formed above a ~100-m-thick, salt-rich décollement composed of gypsum, anhydrite, and halite, above which Mesozoic sediments are deformed into thrusts and boxfold anticlines, partly thrust-cored, separating broader synclines (Laubscher, 1975, 1992; Bitterli, 1990; Sommaruga, 1999).

In addition, several studies have envisaged anticlines in Washington State's Columbia River basalts as terrestrial analogs for martian wrinkle ridges (e.g., Plescia and Golombek, 1986; Watters, 1988; Watters and Robinson, 1997). In particular, Watters and Robinson (1997) found no evidence that martian wrinkle ridges accommodate vertical offsets between adjacent structural blocks so are unlikely to be the surface expression of deep-penetrating faults. Instead, they favored the anticlinal ridges of the Columbia Plateau in eastern Washington as an analog in which horizontal compression occurs along weak sedimentary interbeds within lava flows. Seismic and gravity data show no evidence of deeply rooted thrust faults beneath the anticlinal ridges of the Columbia Plateau, indicating that the thrust faults responsible for the anticlines occur on weak beds within the deforming sequence of basalt flows (Saltus, 1993; Jarchow et al., 1994), grossly analogous to what we hypothesize for the wrinkle ridges of the Thaumasia Plateau.

	HYDROLOGIC POTENTIAL

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

 
The base of the upper crust of Mars separates massive rock from a weaker, overlying megaregolith made of layered ash, impact breccia, lava flows, ice, and salts. Although this contact is not clearly exposed, it must involve a major lithologic and strength contrast. Thickness estimates of this megaregolith vary from 1 to 10 km (Clifford, 1993). Consequently, we envision deformation within a regolith consisting of an intermixed—and likely variably stratified and brecciated—pile of lava flows, ash, salts, and ice. Heating from Tharsis and/or crustal thickening could not only trigger flow of buried salts, but could further enhance deformation by dewatering hydrous salts and melting ground ice. Once deformation began along a basal décollement, even modest shear heating (Scholz, 1980) would further dewater evaporites or melt cryospheric ice. Hence, heating of a regolith containing salt beds or salt-ice mixtures would create sulfate-rich brines and soften the salt. At the same time, topographic loading by volcanic effusia from Tharsis and/or Syria Planum would increase the slope, add overburden, and possibly induce hydraulic fracturing. Such fractures could penetrate the cryosphere and provide hydrologic connection from a confined aquifer at the base of the regolith up to the surface.

The calculated martian geotherm discussed above indicates that a liquid-water aquifer would be stable above the inferred 8- to 10-km-deep décollement. Thus, fracturing in the toe of the deforming region would link a confined aquifer to outlets at the surface. Indeed, models of hydrofracturing by overpressured fluids predict growth of integrated fracture networks in a self-organizing process that can rapidly transmit large fluid fluxes (Bons and van Milligen, 2001). Were fractures to penetrate the cryosphere and link a confined aquifer with the martian atmosphere, the connection of a potentially overpressured system with a near vacuum would rapidly expel fluids at the surface. In a different context, Rodríguez et al. (2007) proposed that thrust faulting disrupted precipitation-generated aquifers in southwestern Chryse Planitia, leading to outflows and subsidence in this region.

Curiously, the major outflow channels around Valles Marineris (Echus, Juventae, and Coprates chasmata) are all sourced at similar elevations of ~3.5 km above the martian datum (Fig. 9). This unlikely coincidence could be explained by these outlets representing locations where fractures from within or the sole of the "mega-slide" reached close enough to the surface to focus discharge at the margin of the deforming area. Fractures penetrating through the cryosphere to the surface would allow fluids to vent under elevated hydraulic head within the deforming zone, due in part to the topographic head defined by the 5 km elevation difference within the slide mass from Syria Planum to the outflow sources. Given the substantial topographic head within the aquifer, overpressured fluids reaching the surface could discharge as fountains. If the internal plumbing of the "mega-slide" was the source for the outburst floods, then the headward opening of the main trough of Valles Marineris from Coprates Chasma along a preexisting radial fracture zone would have captured the source area for the Echus and Juventae outflows, routing residual discharge out through Coprates Chasma and shutting off the other surface vents.

How much flow could the aquifer support assuming reasonable hydrologic conductivities and the >5 km of topographic head within the deforming region? Could such an aquifer account for the discharges estimated previously from Echus, Juventae, and Coprates chasmata? Although details of the subsurface materials and plumbing are speculative, we can broadly estimate a reasonable range of potential discharges based on the cross-sectional areas for the outflow sources and estimated values for the permeability and head gradient. The discharge from an aquifer supplying outflow sources can be related to aquifer properties through

where Q is the discharge (m³ s^–1); k is the horizontal permeability of the aquifer (m²); is the density of water (1000 kg m^–3); µ is the dynamic viscosity of water (10^–3 Pa s); dh/dx is the head gradient in the aquifer; and A is the cross-sectional area contributing flow (m²) (e.g., Carr, 1979; Head et al., 2003; Manga, 2004). Potential cross-sectional areas for the aquifers supplying the outflow channels from Echus, Juventae, and Coprates chasmata range from the roughly 10³ km² cross-sectional area of Juventae Chasma (10 km deep by 100 km across) to the >10⁴ km² cross-sectional area for the walls of Melas and Coprates chasmata (10 km deep by >1000 km long). We assume head gradients from 0.01 to 0.1 and a range of permeabilities from 10^–9 m², as adopted by Manga (2004) for a regional-scale permeability of lava flows, to 10^–7 m², used by Head et al. (2003) to define an upper limit for highly permeable deposits such as breccia or gravel. Incorporating this range of parameter values into Equation 3 predicts discharges of from 10⁴ to >10⁸ m³ s^–1, a range that overlaps with discharge estimates of 10⁷ to 10⁹ m³ s^–1 for martian floods that formed Kasei Valles and Ares Vallis (Robinson and Tanaka, 1990; Carr, 1996; Komatsu and Baker, 1997). Hence, estimated discharges are large enough to potentially supply estimated outflow volumes, although requiring permeabilities and head gradients at the high end of the reasonable range of aquifer properties.

Additional evidence for substantial discharge from a confined aquifer is given by evidence for "hydrothermal" springs along the margin of the Thaumasia Plateau in the Thaumasia Highlands and Coprates Rise (Tanaka et al., 1998). Valleys here have low drainage densities and cluster near evident geologic features, which Tanaka et al. (1998) interpreted to suggest a subsurface origin, such as flow from within a tectonically overpresssured aquifer. Moreover, warm fluid moving up through permafrost would enlarge conduits. Thus initial fracturing, venting, and seepage could lead to the growth of larger conduits through thermal feedback. The fastest growing conduits would eventually capture most of the discharge and potentially drain the aquifer, curtailing discharges from smaller outlets.

Regional groundwater flow down the pr-tharsis subsurface gradient defined by the base of the mega-regolith would favor discharge outlets on the down-gradient, northeastern side of the aquifer—as observed—and may explain why the NE side of the "mega-slide" produced major outflow channels. Alternatively, two-phase deformation, with a late Noachian event in the south and a later concentration of further deformation to the north could account for aquifer breaching and venting to the north.

	SOURCE AND TIMING OF DEFORMATION

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

 
The circular shape and elevation of Syria Planum, the highest part of the gravity-spreading system and presumed source of the spreading, are consistent with an underlying igneous plume, which later collapsed after extrusion. Tanaka and Davis (1988) attributed a dense array of annular fractures around the rim of Syria Planum to collapse following magma withdrawal during extrusion of lava flows forming Syria Planum and adjoining regions. Uplift and heating before collapse could have provided the heat and topographic head to initiate deformation.

But what of the nature of the décollement? Structural analyses support the inference of multiple detachments—a deeper surface for deformation of the Coprates Rise and Thaumasia Highlands and shallower detachments for the wrinkle ridges. Either ice or salts could detach and flow along what need not be a single, originally continuous layer. Ice exhibits viscoplastic behavior in which creep becomes increasingly power-law as grain size increases (Duval et al., 1983). Ice-rock mixtures in the martian megaregolith would also be viscoplastic, although with a higher viscosity than clean ice (Durham et al., 1992). As in salts, creep of ice-rich layers can therefore localize strain as ductile shear zones within layered strata. Hence, the basal detachments for gravity spreading could be impure salts or ice, or some combination thereof, as well as the base of the regolith for a deeper detachment.

Deformation of the Thaumasia Plateau involves Noachian to Late Hesperian units, beginning with Syria-centered volcanism and major deformation phases of the Thaumasia Plateau, which overlap in age (Dohm and Tanaka, 1999). Detailed mapping by Dohm and Tanaka (1999) indicates a sharp decline of normal faulting in the early to Late Hesperian, generation of wrinkle ridges in the Late Noachian to Early Hesperian, extension of Noctis Labyrinthus, and opening of the chasmata of Valles Marineris in the Late Hesperian. Thus, the Coprates Rise and Thaumasia Highlands could have deformed first, followed by Thaumasia Minor and Valles Marineris in a second phase of deformation. Far less clear is the time separating these events, especially because of the overlapping spread in potential ages inverted from crater counts (e.g., Dohm and Tanaka, 1999). Nonetheless, deformation soon after volcanic extrusion from Syria Planum could also account for deformation of the older Noachian surface, because crater counts constrain the time of surface formation—not its deformation. Many craters on the Noachian age surfaces are deformed, whereas craters on the Late Hesperian "last flows" from Syria Planum are undeformed. Hence, it is possible that much of the deformation of the Thaumasia Plateau arose from a single extended event or multiple events in Late Hesperian time. In addition, whereas the estimated Noachian heat flow would produce substantial salt softening and a thick crustal aquifer below a several-km-thick cryosphere, the reduced modern heat flow would result in a thin deeper crustal aquifer below a thicker cryosphere and therefore substantially less potential for mobility under present conditions.

	SUMMARY AND CONCLUSIONS

TOP ABSTRACT INTRODUCTION BACKGROUND OBSERVATIONS INTERPRETATION OF COMPOSITE... ANALYSES SALT TECTONICS HYDROLOGIC POTENTIAL SOURCE AND TIMING OF... SUMMARY AND CONCLUSIONS REFERENCES CITED

 
Analysis of structural and geomorphologic data indicates that deformation of the Thaumasia Plateau is consistent with a "mega-slide" driven by continental-scale salt tectonics. Our proposed hypothesis involves several episodes (Fig. 13).

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.

Second, the rise of Tharsis and subsequently Syria Planum increased both the regional heat flux and local topographic slope, loading the region with ash and lava flows and producing radial extension cracks that may have penetrated through the regional cryosphere. Such heating would have melted ground ice and dewatered hydrous salts, and potentially increased subsurface hydrologic connectivity, and topographic loading would have contributed to producing overpressured fluids.

Third, layers of salts, ice, and basaltic debris within the regolith provided multiple detachments for the gravity spreading southeastward, possibly in response to initial development of Tharsis and subsequent intrusion under Syria Planum.

Finally, fractures from the basal décollement cut through an aquifer at the base of the cryosphere (Carr, 1979). Where these deep fractures intersected radial extension cracks projecting from Tharsis, the aquifer found ready outlets to the surface along Valles Marineris and environs. These connections rapidly drained at least a portion of the extensive confined aquifer, which had substantial topographic head within the deforming zone. The resulting drainage carved the outflow channels.

This single hypothesis provides a unifying explanation for: (1) forming thrust faults and the massive frontal anticline along the Coprates Rise and Thaumasia Highlands; (2) strike-slip and extension on the western edge of Solis Planum; (3) the location, linearity, and depth of the Valles Marineris chasmata; (4) the regional concentration of rampart craters on the Thaumasia Plateau; and (5) the similar elevations for the origin of the Echus, Juventae, and Coprates outflow channels.

	ACKNOWLEDGMENTS

 
We thank Darrel Cowan for discussions of balanced cross sections and Harvey Greenberg and Charles Kiblinger for help in preparing the figures. M. Jackson was supported by the Applied Geodynamics Laboratory consortium. This paper is published with permission of the Director, Bureau of Economic Geology. A. Gillepsie's participation was partly supported by National Aeronautics and Space Administration's Mars Data Analysis Program, contract NNX07AV77G.

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