THE APOLLINARIS MONS FAN-SHAPED DEPOSIT: CHARACTERISTICS AND FORMATION CONSTRAINTS

Apollinaris Mons, Mars (174.3°E, 8.5°S), is a central-vent volcano that displays a striking fan-shaped deposit covering the volcano’s southeastern flank. The precise origin of this deposit remains unclear. The fan-shaped deposit has a maximum runout distance of ~150 km and a maximum width of ~170 km. It emanates from a single ~2-km-wide trough that cuts through the caldera rim. The fan-shaped deposit locally drapes the basal scarp that encircles the main edifice of Apollinaris Mons. Mars Orbiter Laser Altimeter data reveal that the topographic expression of the scarp can be observed beneath the fan-shaped deposit, supporting estimates of only 100 m - 250 m for the deposit’s thickness. These values give volumes ranging from 1.0 x 10³ km³ and 2.6 x 10³ km³ for the deposit. The deposit is modified by valley networks that are morphologically distinct from those found on the Apollinaris Mons main edifice: valleys incised in the fan-shaped deposit are ~2-3 times wider and appear to be more subdued and eroded than those elsewhere on the volcano. In addition, the valleys contained within the fan-shaped deposit locally exhibit tributaries; valleys on the main edifice flank tend to have neither tributaries nor distributaries. The fan-shaped deposit is clearly younger than the main edifice based on superposition relationships: the main source trough for the fan-shaped deposit cross-cuts the outermost caldera rim and slopes inward toward the center of the caldera complex (away from the volcano’s flanks). These observations indicate that the trough has been modified since its formation, probably due to one or more episodes of subsequent caldera collapse. The Apollinaris Mons main edifice is morphologically similar to a basaltic shield volcano, suggesting that it is composed primarily of mafic lavas. In contrast, the fan-shaped deposit has been interpreted to be formed of either fluid lavas or pyroclastic deposits. We will present preliminary results of our analyses of these origins for the fan-shaped deposit.