Featured images for March 2011:
Deformation of Sedimentary Rocks in Valles Marineris, Mars
A large canyon system (up to 8 km deep) called Valles Marineris is located near the equator on Mars. The relative timing between the formation of the Valles Marineris canyon system and various light-toned stratified deposits observed within the different chasmata remains an outstanding question for the geologic history of Mars (Malin and Edgett 2000; Okubo et al. 2008) . Some of these stratified deposits have been deformed and understanding the mechanism(s) responsible for this deformation, both within and between chasmata, could provide insight into the relative timing of events within the Valles Marineris system (Metz et al. 2010).
Image 1: Example of detached slabs from Melas Chasma. Subscene of CTX image P05_002828_1711
Image 2: Kilometer‐scale convolute folds observed in a small basin in southwestern Melas chasma. Subscene of HiRISE image PSP_007087_1700.
Deformation of sedimentary rocks is widespread within Valles Marineris and observed styles of deformation include detached slabs which are rounded blocks of detached material that only locally shows evidence of layering. These slabs in Image 1 are inferred to be composed of sedimentary rocks, and they contain finely stratified deposits with the spectroscopic signature of evaporite minerals. Kilometer-scale convolute folds are similar to detached slabs, but they are composed of alternating dark and light toned strata that exhibit disharmonic folding (Image 2). Folded strata exhibit many geometries including similar and concentric folds, domes, basins, crescents, mushrooms and other interference patterns (Image 3). Pull-apart structures are areas that show evidence of possible brittle deformation and appear to be composed of fragments of strata that have broken off into small irregularly shaped pieces. In some areas the fragmented strata look as if they could be fit back together similar to puzzle pieces, whereas in other areas the fragmentation proceeded to a larger degree and only irregularly shaped fragments of strata remain (Image 4).
Image 3: Subscene of HiRISE image PSP_001918_1735 showing folded strata; this example is from Candor Chasma.
Image 4: Pull‐apart structures in Ius Chasma. Note how the strata look as if they have been pulled apart in the direction of the arrows. Subscene of CTX image P07_003606_1727
Several mechanisms have been proposed to explain this deformation and include subaerial or subaqueous gravitational slumping or sliding and soft sediment deformation (Lucchitta 2008; Weitz et al. 2003; Metz et al. 2010).
Lucchitta, B. K. (2008), HiRISE images of layered deposits in west Candor Chasma, Mars (II): Unconformities and possible gravity tectonics, Lunar Planet. Sci., XXXIX, Abstract 2317.
Malin, M. C., and K. S. Edgett (2000), Sedimentary rocks of early Mars, Science, 290, 1927–1937, doi:10.1126/science.290.5498.1927.
Metz et al. (2010), Thin‐skinned deformation of sedimentary rocks in Valles Marineris, Mars. J. Geophys. Res., 115, E11004, doi:10.1029/2010JE003593.
Okubo, C. H., K. W. Lewis, A. S. McEwen, and R. L. Kirk (2008), Relative age of interior layered deposits in southwest Candor Chasma based on high‐resolution structural mapping, J. Geophys. Res., 113, E12002,
Weitz, C. M., T. J. Parker, M. H. Bulmer, F. S. Anderson, and J. A. Grant (2003), Geology of the Melas Chasma landing site for the Mars Exploration Rover mission, J. Geophys. Res., 108(E12), 8082, doi:10.1029/2002JE002022.