The geologic map shows the flanks and summit caldera of Arsia Mons, the southernmost of three large shield volcanoes (Arsia, Pavonis, and Ascraeus Montes) that form the Tharsis Montes volcanic chain (fig. 1). The volcanoes lie along the crest of a regional northeast-trending rise that extends more than 3,000 km along or near the planet-wide boundary that separates the southern highlands from the northern lowlands. Previous geologic mapping of this region (Scott and others, 1981; Scott and Tanaka, 1986) shows six major lava flows that were extruded from the Tharsis volcanoes during the Early Hesperian to Late Amazonian Epochs; the flows cover and area exceeding 7x106 km2 around the volcanic centers. All of these flows – mapped as members of the Tharsis Montes Formation – are present in the map area. Broad, fan-shaped surficial deposits that resemble landslides extend down the northwest sides of each volcano and of Olympus Mons, about 1,500 km northwest of Arsia Mons (fig. 1). The largest of these gently sloping, lobate deposits is on Arsia Mons, where ambiguous relations between one facies of the fan-shaped accumulations and underlying topographic features have provoked questions as to the origin of the deposits.
