GEOLOGICAL MAPPING OF MARS' THARSIS VOLCANOES (Invited Presentation)

The Tharsis rise on Mars hosts the largest shield volcanoes in the Solar System. Over the last decade we have been funded by NASA’s Mars Data Analysis Program via three proposals to construct 1:1,000,000-scale geologic maps of the four largest Tharsis volcanoes: Olympus Mons, Arsia Mons, Pavonis Mons, and Ascraeus Mons. At present all four maps are in various stages of review or revision with the U.S. Geological Survey. Our maps use a combination of Mars Reconnaissance Orbiter Context Camera (CTX, ~6 m/pixel) and Mars Express High Resolution Stereo Camera (HRSC, ~12 m/pixel) images as basemaps, supplemented with Mars Odyssey Thermal Emission Imaging System (THEMIS, ~100 m/pixel) and Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA) digital elevation models (DEMs) for topography. Geologic mapping was done using ArcGIS™ software as now required for all NASA-funded planetary mapping. An initial goal for Olympus Mons mapping was to determine if rift zone-related processes had occurred here similar to that in the Tharsis Montes. Although nearly 100 small eruptions sites were identified across Olympus Mons flank, very few, possibly none, appear to be related to subsurface migration of magma along rifts, but are likely associated with breakouts along well established lava tubes. Lava tubes are common on the flank of Olympus Mons and suggest that many eruptive events were associated with long-lived, stable lava emplacement conditions. In contrast, Arsia Mons is the most advanced and oldest of the Tharsis Montes volcanoes; morphologic mapping shows the variety of eruption styles that formed the main shield, rift apron, and plains. The complexity of the glacial units suggests an extensive ice sheet that extended to the west and formed a series of deposits similar to terrestrial deposits. The main shield of Pavonis is similar to Arsia but the rift aprons are different. Pavonis has an undeveloped northeast apron and the southeast apron is comprised of low-shields and tube-fed flows. High-resolution mapping of Ascraeus Mons has confirmed that it was formed in several stages. Its western flank appears to have less visible volcanic features, possibly caused by atmospheric precipitation, erosion, or dust cover. Much of the younger flows from Ascraeus Mons are channel-fed on the flanks compared to tube-fed flows that preceded them.