GEOLOGIC MAPPING OF ASCRAEUS MONS VOLCANO, MARS AT 1:1M SCALE: AN ANALYSIS OF THE GEOMORPHOLOGY AND FORMATION OF ASCRAEUS CHASMATA

We are mapping the geology of the Ascraeus Mons volcano, one of four large-shield volcanoes in the Tharsis region on Mars, at a scale of 1:1,000,000. Four main geologic features are present in the mapping area: 1) the main shield, 2) the rift aprons, 3) plains lava flows, and 4) a fan-shaped debris apron. Along with the current version of the geologic map, we will present a focused analysis of the geomorphology and formation of chasmata associated with the rift zones. The rift aprons extend from the lower northeast and southwest flanks of the main shield. At the apex of the rift aprons are a dense concentration of intertwined chasms and chains of pit craters that together make up the rift zones. Morphologically, the chasms have theater-shaped heads and narrow downstream. The orientations of the chasms trend with the slope of the main shield. No levees or flow margins are apparent giving the individual chasms a morphologic appearance similar to lunar sinuous rilles. The chasms are typically 1-3 km wide, with some coalesced areas up to 8-km-wide, and are 100's of meters deep, based on MOLA gridded data (128 pixels/degree). High-resolution images from MOC (2.9-6.1 m/pixel) and High Resolution Imaging Science Experiment (HiRISE) (0.25 m/pixel) reveal layers of bedrock in the upper quarter to upper half of the chasm walls, with the lower sections of the walls covered by debris and dust. HiRISE image PSP_001642_1895 shows at least ten layers of bedrock cropping out in a staircase-like fashion in the upper quarter of a chasm wall on the southwest flank of Ascraeus Mons. We interpret the bedrock layers observed in this HiRISE image to be lava flows that were emplaced during construction of the main shield, which are now exposed as a result of the chasm formation. Formation of the chasmata is thought to be by various processes including volcanic, tectonic, fluvial, and glacial or a combination thereof. We agree with a volcanic origin for the chasmata, formed by high volume, long duration eruptions and the release of volatiles along a NE-SW trending rift zone. Chasms were modified later by other surface processes. Intruding magma surfaced along the lower flank of Ascraeus Mons feeding the flow field that comprises the rift apron. Understanding the origin of the chasmata will provide better insight into the eruption dynamics required to form the rift aprons.