Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p11b1037b&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #P11B-1037
Physics
5464 Remote Sensing, 5475 Tectonics (8149), 5480 Volcanism (8450)
Scientific paper
A distinct competent layer is observed in the wall rock of southeast Coprates Chasma, part of the Valles Marineris canyon system on Mars. This layer is visible in Mars Orbital Camera (MOC) and Thermal Emission Imaging System (THEMIS) images. It outcrops in the northern wall of the chasma just below the elevation of the surrounding plateau surface. Also in this region of Coprates Chasma there is a massif that consists of wall rock material, and is characterized by flat-topped areas at its crest. THEMIS images of these areas indicate that the thermal inertia is similar to that of Ophir Planum surfaces, not Coprates Chasma floor surfaces. This and their level topography indicate that these areas are most likely remnants of the plains surface that was in this area before Coprates Chasma opened. This is a distinct characteristic in the Valles Marineris because it preserves pre-existing plains surface on a free-standing massif. The massif preserves this surface because the strong layer seen in the northern wall of the chasma is also seen to outcrop just below the level of the flat-topped surfaces of this massif. In fact, these flat-topped areas are preserved because this strong layer is helping the massif resist erosion. The Mars Orbital Laser Altimeter (MOLA) data can be used to measure the elevation of the layer outcrops in the northern wall of the chasma and along the crest of the massif. The elevations in the wall indicate that the layer dips down to the west only slightly at 0.04o. This is consistent with MOLA measurements of the dip of the floor of Coprates Chasma in this area. However, elevations of the layer on the massif show several different dip angles between outcrops ranging from 0.45 to 0.07o, with the dip from the westernmost outcrop to the easternmost outcrop being 0.13o. In some locations measurements can be made on both the north and south sides of the massif. In some locations the layer is flat in the north-south direction, and in one area there is a 5o dip down to the south. Furthermore, comparison of the layer elevations in the wall and the massif show a difference of 300~m. These measurements indicate a significant amount of subsidence of the massif block, as well as evidence of more complicated tilting motions. There are several models for how the Valles Marineris formed, including different combinations of rifting, collapse, and mass wasting. These observations indicate that this portion of Coprates Chasma formed primarily by extensional fault motion.
Beyer Ross A.
McEwen Alfred S.
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