FIRST FIELD EXAMINATION OF THE NEAR-SURFACE RIM STRUCTURE ON A LARGE IMPACT CRATER, OPPORTUNITY ROVER, ENDEAVOUR CRATER, MARS

The near-surface geologic and structural characteristics of large impact craters are poorly known. On Earth, there are no topographically-preserved large impact craters for field study, and remote observations of craters on other bodies lack sufficient resolution. Using the Opportunity rover we have examined the bedrock outcrops along the rim of a large impact crater on Mars, the 22 km-diameter Endeavour crater, during a 15 km traverse field geologic reconnaissance traverse. Two principal unexpected characteristics of note are: (1) The attitudes of sheets, foliations (distinct from those produced by degradation), and layers in rim breccias dip inward as well as outward, broken only by inboard terrace faults where inward-dipping attitudes frequently steepen further, and (2) coherent blocks of crust along the crater rim divide the rim into topographically and structurally distinct segments bounded by radial, vertical fractures and faults. The inward dipping foliations are unanticipated based on existing simple models of complex impact craters with monotonically outward-dipping structure. Working hypotheses for observed monoclinal inward dips along the rim include (1) drag folding along slumps into the initial excavation cavity during crater formation and at the time of rim uplift, (2) drag folding during post-impact slumping of the rim and walls, or (3) cantilevering of the outcrops into the crater during much later degradation and backwasting of the inner crater walls. Differential uplift of crustal blocks along the rim by scissor faults accounts for observed radial vertical fault boundaries between the rim segments. In situ observations of outcrops in the vicinity of the segment-bounding fractures contain elemental, mineralogic and spectral evidence for alteration, possibly associated with former subsurface aqueous fluids. Our observations along the rim and in valleys cutting it are evidence that fault-line brecciation along radial crustal faults and associated joints serve as major vertical discontinuities of high fluid permeability. This supports the idea that rim-crossing fractures can have high fluid permeability and are pathways for localized aqueous alteration and fluid transmission that could easily support spring discharge sites on the inner walls of Martian impact craters.