Massive Debris Flows at Pavonis and Arsia Montes, Mars

Extensive deposits indicative of late-stage debris flows (or lahars) are found off the NW flanks of Pavonis and Arsia Montes on Mars. These deposits extend more than 100 km from their source areas, which originate at elevations of 4.8 km on Arsia Mons, and 5.8 km on Pavonis Mons. Originally identified from Viking Orbiter images, MOC and MOLA data now provide stronger evidence that these are debris flows or lahars, as opposed to other types of mass movements or a local tectonic process. They extend downslope to elevations of \sim3.0 km, but the distal edge has a variable elevation that does not follow the local contours. Viking data indicate that these lobes are emplaced on top of pre-existing features (impact craters and lava flows), and MOC images suggest that material within the flows is unconsolidated debris that is easily reworked by eolian processes. The crescent-shaped fans are >200 km in length and the terminal deposits have a rampart comparable to that found in large terrestrial analogs and controlled experiments. Image data show that the flows were produced in multiple episodes, or possibly had a wave-like breaking of a single event comparable to large terrestrial examples. We propose that large volumes of water, comparable to the volumes of the residual deposits themselves, must have been involved in formation of these debris flows. This water may have accumulated over time below the surface of the volcanoes or it may have been rapidly mobilized by an eruption. In either case, the conversion of mechanical energy into heat during the flow could have also converted significant quantities of ice into water even after the cessation of flow. Our research provides measures of the degree of conformance of the flow path to the topography and compare bulk flow velocities, duration of emplacement, and rheologic parameters to terrestrial examples. We will present alternative hypotheses about geologic processes engendering such debris flows and possibilities for triggering mechanisms. The inference is that these were "wet events" of a large scale, triggered relatively high on the flanks of the volcanoes. This increases the role of volcanism in the dynamics of the water liberation and transport on Mars.