MAPPING LAVA FLOW ORIENTATIONS IN THE THARSIS REGION, MARS: INDICATORS OF LATE AMAZONIAN VOLCANIC LOADING AND SUBSIDENCE

The Tharsis region of Mars is an uplifted region near the equator that has been the primary focus of volcanic and tectonic activity on the planet over the past several hundred million years. Tharsis spans over a thousand km and rises nearly 10 km above the mean elevation of the planet. Olympus Mons, the tallest volcano in the Solar System, is located on the NW edge of the Tharsis region. The Tharsis Montes are three somewhat smaller but still enormous shield volcanoes that lie along the crest of the Tharsis uplift, named Arsia, Pavonis, and Ascraeus. Their heights range from 12 to 15 km and basal diameters range from 350 to 475 km, and their volumes are over ten times that of the largest volcano on Earth, Mauna Kea in Hawaii. Such large volcanoes can act as flexural loads on the lithosphere and produce downward-subsided moats in the surrounding topography, as they do around Mauna Kea and Olympus Mons.

The purpose of this study is to identify and measure such flexural effects caused by the Tharsis Montes, using nearby lava flow orientations as indicators of subsidence. Lava flows on the plains near the volcanoes that originally flowed directly downhill when they were emplaced may become misaligned with modern topography as moat-forming progresses. These flows are thus indicators of “paleo-slopes” and their orientations relative to modern-day topography can be used to measure the magnitude of subsidence and to model flexural loading and the required magmatic volumes. In this study, we measured the azimuthal orientations of over 200 relatively long (>20 km) and straight lava flows on the plains surrounding the Tharsis Montes on a Mars Reconnaissance Orbiter Context Camera (CTX) image mosaic. Each flow records the downhill topographic direction at the time it was erupted, and these orientations, when compared with current downhill directions derived from blended Mars Orbiter Laser Altimeter (MOLA) and High-Resolution Stereo Camera (HRSC) topographic data collected from buffer regions around each flow, yield evidence of post-flow slope reorientation. The results show late-Amazonian deformation in areas near the Tharsis shields and are being used with geophysical modeling software to constrain the responsible volcanic loads, and crater counts on the deformed plains constrain the timing of the identified deformation and magmatic activity.