Geologic Mapping of the Summit Region of Alba Mons, Mars

Geologic mapping of the summit region of Alba Mons, the northernmost volcano in the Tharsis volcanic province of Mars, employs imaging and topographic datasets to examine eruptive activity, tectonism, and degradation in the caldera region and on the surrounding flanks. A 1:1M-scale USGS geologic map is being produced using Geographic Information Systems software and analysis tools. The map area (245-255°E, 32.5-47.5°N) includes six 1:500,000-scale Mars Transverse Mercator quadrangles (45112, 45107, 40112, 40107, 35112, and 35107) centered on the summit caldera complex and includes the main Alba Mons edifice, circumferential grabens encircling the volcano (Alba, Ceraunius, and Tantalus Fossae), and parts of the mid-flank and lower flank regions containing lava flows and dendritic valley networks. Populations of small, superposed impact craters are being used along with stratigraphic and cross-cutting relationships to derive relative and absolute age constraints. Initial mapping has been done for the caldera region to develop a preliminary unit and symbol scheme, focusing on the intra-caldera flows and flows on the upper flanks of the volcano extending from the caldera complex. Based on flow lobe patterns, multiple sequences of lava flow emplacement have occurred from a series of vents within the summit region. Geologic features have been mapped within and surrounding the caldera complex using THEMIS and CTX images. Based on cross-cutting relationships, a 65 km-across collapse depression on the southeast edge of the caldera complex represents the last stage of collapse. The rim of this depression shows diverse morphologic expression. To the south, scalloped, steep-walled areas presumably represent discrete, late-stage collapse events. Graben and scarps aligned with the eastern and western margins show the larger extent of deformation in summit materials related to this depression. Analyses of the morphology and topography of the depression floor indicate: a) the most recent eruptive activity formed small constructs to the southeast; b) flows extended from southeast to northwest across the depression floor; and c) floor materials are obscured by mid-latitude mantling deposits and the effects of aeolian activity.