Ascraeus Mons, Mars: Characterisation and Interpretation of the Fan-shaped Deposit on Its Western Flank

Introduction: Ascraeus Mons is one of the three Tharsis Montes shield volcanoes that are aligned along a N40°E trend on the crest of the broad Tharsis Rise. Although largely constructed of volcanic deposits, each of the Tharsis Montes has a distinctive and unusual fan-shaped deposit extending approximately northwest on their western flanks. Three major facies have been identified within the fan-shaped deposits of Arsia and Pavonis Mons [1-6]: (1) A ridged facies consisting of numerous ridges that trace the distal margin of the lobe, often cutting across flow features and craters without any obvious deflection. (2) A knobby-terrain facies characterized by a chaotic assemblage of equidimensional hills. (3) A smooth facies containing concentric ridges superposed on broad lobes. For some years there has been a debate over the em-placement of these fan-shaped deposits. The problem is important because it contributes to an understanding of the environmental history of the Tharsis region and its place within a wider planetary context. Hypotheses for their origin have been primarily based on three major processes: mass move-The work presented here includes characterisation of the Ascraeus Mons fan-shaped deposit and an assessment of the plausibility of a cold-based glacial origin, which has been developed from studies of Arsia and Pavonis Mons [3,4]. Previous Modes of Origin: Early observations from Mariner and Viking images led to classification of these features as massive landslide deposits [1,8]. In this scenario, the knobby facies is thought to represent a chaotic mass of debris that detached from the western flank of the volcano, while the ridged facies is thought to form by deformation resulting from the drag of the slide over underlying terrain. Difficulties in reconciling a purely landslide origin with the great size and length of the deposits, as well as the lack of obvious source areas of sufficient volume has led to the proposal that pyro-clastic activity may have contributed to the form of the fan-shaped deposits. Such activity is thought to be triggered by the slide events responsible for the initial emplacement of the deposits [2]. Although the evidence for pyroclastic activity has since been disputed, an origin of these features in terms of multiple processes is still favored by several authors [5,6,7]. Cold-Based Glacial Model: Building on early comparisons of the fan-shaped deposits with features characteristic of glacial depositional environments [5,6,9], the integration of MOLA, THEMIS and MOC data with depositional frameworks of polar glaciers in …