Evidence for past Kilometer-scale Overturn(s) in Deformed, Layered Terrain near the Deepest Point on Mars

Deep in the Hellas basin on Mars, wind has exhumed layered terrain showing ductile deformation. A kilometer-scale cellular pattern is identified, consistent with thermal and/or compositional (diapiric) convection. ‘Frozen-in’ convection within an impact melt sheet is consistent with observations, but overturn(s) within an evaporitic layer cannot be excluded. Spectrometer follow-up will be complicated by dust. Background: Kilometer-scale cellular and ‘honeycomb’ terrain in the NW Hellas basin has been interpreted as formed by ice blocks sinking into soft sediment [1], or as the result of salt diapirism [2]. Previous study [3] relied largely on Viking images. CTX and HiRISE permit a closer look, and confirm pervasive hectometer-scale deformation, most intense in shear zones at cell boundaries (Figure 1). The deformed, layered terrain is basal to the stratigraphy and is overlain unconformably by an erosionally resistant, relatively dark-toned, ridge-forming unit.