Study of Megabreccia in Ritchey Crater Central Uplift, Mars

Ritchey Crater (28.8°S, 309°E) is a 79 km diameter complex crater, which is near the boundary between Hesperian ridged plains and Noachian highland terrain units on Mars [1]. The central uplift of Ritchey crater is ~20 km wide. High Resolution Imaging Science Experiment (HiRISE) images of the peak reveal fractured massive bedrock and megabreccia [c.f. 2] with large clasts that appear to have been raised from depth. The stratigraphic uplift in a crater of this size is ~7.7 km [3], comparable to the depth of exposures in Coprates Chasm. Compact Reconnaissance imaging Spectrometer for Mars (CRISM) spectral parameter maps reveal low-calcium pyroxene, olivine, and hydrated silicate clays such as illite. We mapped the Ritchey crater central uplift into ten units[4]. Three types of megabreccia units were identified in this area named by (1)Megabreccia Clast-rich(MC), (2) Buried MegaBreccia(BM) and (3) Megabreccia with Relief(MR). There may be a fourth category with very large blocks(300m) of bedrock. MC is draped over the central uplift and we interpret it as a clast-rich and perhaps melt-rich, impactite produced by the Ritchey impact event. BM is found in small well-consolidated and exposures beneath low calcium pyroxene and olivine-rich massive bedrock. We detect MR distributed within or close to the central cavity of the uplift. MR clasts are more resisitant to weathering than the matrix. Its central location suggests that it could be the deepest and oldest material exposed in the central uplift. We are analyzing the clast size, composition, and stratagraphic arrangement of MR to determine the source of the megabreccia. Ritchey crater is superposed on an older crater of about the same size and its central uplift lies over the destroyed rim of the older crater. This juxtaposition may contribute to the large amount of megabreccia exposure in Ritchey. MR could also represent the exposure of a regionally widespread megabreccia layer due to the Argyre basin to the southeast, or perhaps the late heavy bombardment itself. Alternatively MR might be another class of megabreccia formed during the Ritchey crater formation from collapse of the central peak. References: [1] Scott D. H. and Tanaka K. L. (1986) USGS Map I-1802-A. [2] Tornabene L. L. et al., (2012) 3rd Early Mars abstract. [3] Grieve R. and Pilkington M. (1996) AGSO Journal of Australian Geology and Geophysics 16, 399. [4] Ding N. et al.,(2013) LMI V, #3021