Investigation of Lobate-Shaped Deposits in Eastern Acheron Fossae, Mars

Two lobate-shaped flow features of 20 km scale centered at 36° N, 229° E in Acheron Fossae pose intriguing questions about their nature and emplacement processes. The near absence of impact craters (>50m) on their surfaces suggests recent emplacement or recent resurfacing activities. MOLA topographic profiles show distinctive convex-upward termini of both structures, suggesting they were deposited as materials with yield strength. MOLA data also show the deposits to have an overall slope of about 1-2° with depth estimated to be around 500 m. Surface striae of longitudinal ridges and grooves are observed on both deposits from Viking images (70 m/p) and THEMIS visible images (19 m/p), suggesting down-valley flow. THEMIS images show that the two deposits embay lower Alba Patera flow unit Ahap2 [1] plains material and thus must be younger and have a different origin. High resolution topography (70 m/p) of our studying region is being derived from Viking images using a newly developed photoclinometry tool [2], with MOLA 128 p/deg DEM providing absolute elevation control. This topographic model can provide detailed dimensional information for numerical simulations of flow emplacement mechanisms. The over all planimetric shape and morphologic characteristics suggest that these deposits are either lava flows or water/ice lubricated mass flows in nature. Chezy-type models provide a first-order view into the rheology of flow deposits, by calculating an empirical flow resistance parameter, C, from topographic and geomorphic data. Initial estimation of C (0.02) falls above the flow resistance factor for basaltic lava flows (0.01) but is in accordance with that of water lubricated mass flows such as debris flows. Convex-up lobate debris deposits, which are mainly found in the 30-60° latitude range, have been suggested to have formed due to creep of ice-containing materials[3], indicating rich source of ground water or ice. Furthermore, results from Mars Odyssey suggest that there is abundant water ice in the near-surface at mid to high latitudes[4]. Thus, these lobate deposits seem to be water-lubricated features and suggest recent activity. Further modeling is underway to better distinguish between lahar, debris flow, or ice-rich debris deposit model and debris-covered glacier model.{[1]}Scott, D.H. et al., MAP I-1276 (MC-2 SE), USGS, 1981. {[2]}Kirk, R. L. et al., Advances in Planetary Mapping 2003, Houston, March 2003. {[3]}Squyres, S.W, J. Geophs. Res., 84(B14), 8087-8096, 1979. {[4]}Boynton, W.V. et al., Science, 297, 81-85, 2002.