VOLCANIC GEOLOGY OF THE WESTERN FLANK OF ALBA MONS, MARS

Geologic mapping is providing new constraints on the volcanic history of the western flank (230-245°E, 37.5-47.5°N) of Alba Mons. We are using imaging and topographic datasets to examine lava flow fields that extend from the volcano’s summit region to the Martian northern plains and to document the distribution and character of the preserved volcanic features. Alba Mons’ western flank is characterized by low flank slopes (typically < 2° over baselines down to 1 km) extending over an elevation range of ~7.5 km for 900+ km from a highly deformed zone that encircles the summit. Mapped volcanic features include lava flow margins and lava tube segments. Their distribution is consistent with the broad shape of the volcano and local slopes (at 50 km scale), although flow paths have been deflected by local obstacles including pre-existing impact craters and volcanic flows. Although local relationships are complex, lava flows generally post-date adjacent lava tube systems.

Individual lava flows are typically elongate with relatively constant widths (i.e., tabular morphology), although width variations, branching, and broader lobes are observed. Typical flow widths are ~2-10 km and numerous flow lobes extend for 100+ km. Distinct source vents have not been identified, presumably due to burial on the flanks and because numerous flows appear to originate outside the map area. The preservation of flow margins varies both along individual lobes and across the region. Some flow margins exhibit distinct lobate scarps with preserved fine-scale sinuosity that reflects differential lateral spreading during emplacement.

Lava tube systems occur across the western flank, are concentrated in some locations, and are generally radial to the summit. Lava tubes are typically discontinuous and delineated by sinuous chains of elongate depressions, which in many cases are located along the crests of prominent sinuous ridges. Lava tube systems occur as both these ridged forms with lateral flow textures and more subtle features within the flat-lying flow field surface. We are using topographic analyses to extend image-based geologic mapping to identify volcanic ridges and characterize their morphometry in order to fully document lava transport pathways. Preliminary results indicate the presence of a series of lava tube systems of 600+ km in length.