Measuring Late Amazonian Topographic Deformation with Paleo-slope Indicators near Arsia Mons, Mars

Arsia Mons on Mars, 12 km tall with a basal diameter of 350 km, is the southernmost of the three enormous Tharsis Montes volcanoes. Crater size-frequency studies of Arsia and the other large Mars volcanoes reveal a long history of volcanism that extended into the Late Amazonian (<100 Ma). The volume and hence importance of this recent edifice-building activity, however, is not clear, as estimates of the extrusive/intrusive ratio and magmatic flux (rate of total magma production, both extrusive and intrusive) have proved elusive. Since only extrusive lavas are observable at Arsia, for instance, the young surface lavas could be just a thin carapace emplaced during the waning stages of growth or they could be part of a relatively voluminous surge of activity that contributed a substantial portion of the volcano's growth. In this study, we measured the azimuthal orientations of 77 relatively long (>20 km) and straight lava flows on Daedalia Planum to the south of Arsia Mons (Figure 1) 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 relict flow orientations, when compared with current downhill directions derived from Mars Orbiter Laser Altimeter (MOLA) topographic data collected from buffer regions around each flow, can yield evidence of post-flow slope reorientation. The magnitude and direction of the slope change is then used to model and infer the cause of the deformation, while crater counts on the deformed plains are used to constrain the deformation timing. Our preliminary results show that modern slope azimuths for a region of older (>400 Myr) terrain to the southeast of Arsia have been rotated clockwise >10° for 14 of 44 flows (orange) mapped there (ranging from 10.2° to 41.9°), and none were rotated >10° counterclockwise. This deformation is consistent with subsidence toward the west of these flows or uplift to the east. For the younger (<200 Myr) plains to the south and southwest of Arsia, the flows show less evidence of deformation, with 31 of 33 mapped flows (green) showing less than 10 degrees of azimuth difference from modern local slopes in that area. These observations suggest significant deformation of Daedalia Planum, perhaps related to Arsia Mons magmatism, took place in the interval between these two dates.