A short account of what we think we know
Valles Marineris (Figure 1) formed in a bedrock thought to be a succession of lava flows and pyroclastites (McEwen et al., 1999; Beyer et al., 2005), which in some areas overly crystalline rocks that may represent an older basement or intrusives. Crystalline rocks are well observed on some Coprates Chasma walls (Williams et al., 2003), along which dykes have also been locally identified at 3 sites (Flahaut et al., 2011). Chasma depositional and erosional history has been the topic of many post-Viking Valles Marineris studies. Most of them were hypothesized as soon as Mariner 9 obtained the first images of Valles Marineris in 1972 (Sharp et al., 1973).
Figure 1. ESA/DLR/HRSC digital elevation model of Valles Marineris (scale in meters) with location of the main chasmata and other troughs.
There is ample evidence that extensional tectonics has played a significant role in forming the southern chasmata, from observation that the troughs are parallel to narrow grabens observed on the plateau in which Valles Marineris formed and fault-population statistics (Schultz, 2000; Vallianatos et al., 2011) from internal chasma structural segmentation and from neotectonic morphologies (Peulvast et al., 2001). Most researchers indeed share the view that the chasmata are dominantly of tectonic origin (Masson, 1977, 1980; Frey, 1979; Schultz, 1991, 1995, 2000; Mège and Masson, 1996a, 1996b; Peulvast et al., 2001; Vallianatos et al., 2011; Andrews-Hanna et al., 2012; Tanaka et al., 2014). The structural similarities between Valles Marineris and rifts in the terrestrial sense, advocated in the Viking Orbiter times (Masson, 1977; Frey, 1979), remain interesting to investigate but need to be nuanced (Lucchitta et al., 1992; Mège et al., 2003; 2014; Hauber et al., 2010). The absence of horizontal offset of lava flow margins, which are abundant on the plateau surrounding Valles Marineris, is evidence of purely normal fault slip. Strike-slip parallel to the Valles Marineris chasmata has been advocated on the basis of hypothetical plate tectonics-based scenarios (Anguita et al., 2001, 2006; Yin et al., 2012a) but the presented observational evidence of geologic marker displacement (Yin et al., 2012b) is not supported by observations at higher resolution. Geomorphological considerations suggest that the northern, oval-shaped chasmata, although in the same extensional tectonic setting, have a partly different history than the southern chasmata (Schultz, 1998; Schultz and Lin, 2001), perhaps involving enigmatic "ancestral basins" (Schultz, 1998; Fueten et al., 2008, 2010) formed either by erosion, by vertical collapse into huge underlying tension fractures (Tanaka and Golombek, 1989; Jackson et al., 2011) or following vertical shears (Andrews-Hanna et al., 2012; Fueten et al., 2008).
There is very little evidence of tectonic deformation related to a far-field source or body forces generated by the Tharsis lithospheric load that would postdate the initial chasma formation stage. Most fault scarps and other deformation have been interpreted or reinterpreted as due to diagenesis (Okubo, 2009) and gravity or paraglacial tectonics (Okubo et al., 2008; Mège and Bourgeois, 2011; Gourronc et al., 2014; Okubo, 2010, 2014).
Further evolution. A long history of deposition and erosion took place, in which the unstable rotation axis of Mars (Laskar et al., 2004) probably played a key role, although accurate reconstruction of the rotation parameters cannot be computed for ages older than 10 Ma (Laskar et al., 2002). The sedimentary history mainly includes deposition of stratified deposits1 up to kilometres thick (Figure 2), the morphology of which denotes significant variations in erosional susceptibility, that probably correlates with variations in composition.
Nevertheless, they have been globally ascribed the name of Interior Layered Deposits (ILD). Near-infrared orbital spectra from MEx/OMEGA and MRO/CRISM indicate that these rocks include a proportion of sulfates (Gendrin et al., 2005; Mangold et al., 2008; Wendt et al., 2011) and ferric oxides (Le Deit et al., 2008; Wendt et al., 2011; Fergason et al., 2014). ILD fast degradation and retreat suggest that ice is a major component of ILDs (Grindrod and Warner, 2014), in agreement with the low viscosity of some of them, which have been interpreted as glaciers (Mège and Bourgeois, 2011; Gourronc et al., 2014) eroding mafic rocks and altering them to sulfates (Niles and Michalski, 2009; Cull et al., 2014). The ILD show internal deformation (Fueten et al., 2005, 2008, 2010, 2011, 2014) that is best interpreted as local effects (Okubo et al., 2008) such as gravity adjustments or sedimentary unconformities. Other minerals identified in Valles Marineris include clays (Weitz et al., 2015), hematite (Roach et al., 2010a) and hydrated minerals (Roach et al., 2010b).
Gravity tectonics has been a major component of wallrock modification, probably throughout the Valles Marineris history (Quantin et al., 2004), for instance by rock glaciers (Mège and Bourgeois, 2011; Brunetti et al., 2014), deep-seated gravitational slope deformation features (Mège and Bourgeois, 2011; Makowska et al., 2016), landslides -Quantin et al., 2004; Brunetti et al., 2014; Lucas et al., 2011; Lajeunesse et al., 2006), and rolling boulders (Chojnacki et al., 2014). Fluvial erosion and lacustrine deposition have also been documented (Quantin et al., 2005).
Figure 2: Synthetic geologic cross-section of eastern Candor Chasma (Mège et al., 2016), showing the basement in brown and red, ILD in blue, and other deposits in green and yellow.
Valles Marineris origin
Quite early, simple elastic models of Tharsis formation (Banerdt et al., 1982, 1992; Sleep and Phillips, 1985) have generated discussions (Schultz and Zuber, 1994; Mège and Masson, 1996c) about the source of stress that could have generated extension in Valles Marineris, especially whether plume uplift stress or magmatic load stress provide the suitable stress trajectory patterns. These models were oversimplified, as elastic models cannot adequately describe crust or lithosphere rheology (Banerdt and Golobek, 1992), strain path (Schutlz and Zuber, 1994), relevant topography (Mège and Masson, 1996c), and eventually, the complexity of plume tectonics (Schultz and Zuber, 1994; Mège and Masson, 1996c). Later qualitative models of continental rifting above a mantle plume (Ernst et al., 2001; Mège et al., 2001) provide a reasonable and valuable framework for understanding the orientation and formation of Valles Marineris at the first order. Such a plume tectonics model (Mège and Masson, 1996b) predicts that chasma geometry follows the orientation of a huge mafic dyke swarm (Figures 3 and 4), similar to volcanic rifts on Earth (Mège and Masson, 1996b; Mège, 2001), and consistent with observational evidence that depressions parallel to the giant chasma may be related to magma withdrawal by fissure eruptions (Mège et al., 2003).
Figure 3: Geologic time scales of Mars and Earth in m.y., with simplified indication of events mentioned in this project (Mège and Massno, 1996b: Mège and Bourgeois, 2011; Gourronc et al., 2014; Tanaka et al., 2014). Martian time scale after Hartmann and Neukum (2001), Earth time scale after the International Commission on Stratigraphy.
Figure 4: The Tharsis plume tectonics model Mège and Masson, 1996b) predicts that Valles Marineris opened following the orientation of a dyke swarm centred on the Syria Planum magma centre, the inversion of which results in the displayed stress trajectories. Stress field analysis indicates a stress regime controlled by magmatic overpressure and a regional crustal stress source.
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