THE ORIGIN OF CIRCUMFERENTIAL FAULTING ON THE FLANK OF ALBA MONS, NORTHERN THARSIS REGION, MARS

The characteristics of circumferential faulting give us insight into how shield volcanoes, regional tectonics, and topographic relief interact to affect fault formation. Because Mars’ erosion and sediment deposition rates are significantly lower than Earth’s, fault morphologies are better preserved than on Earth. The Alba Fossae fault zone, on the western flank of the >500 km diameter shield volcano Alba Mons, is a circumferential system dominated by horst-graben structures. The fault network developed in the Middle to Late Amazonian, which post-dates Early to Middle Amazonian volcanic activity. With high-resolution image data from the CTX camera of the Mars Reconnaissance Orbiter (MRO) and a digital elevation model (DEM) from the Mars Orbiter Laser Altimeter (MOLA), we examine the geometric characteristics of the fault zone to assess its evolution and likely origin. We observed fault segmentation with both soft and hard segment linkage and en echelon geometries. Well-defined relay ramps are commonly intact but also display top- and/or base-breached forms. We subdivide the Alba Fossae fault system into northern, central, and southern zones for ease in description and analysis. The NE-trending northern region displays a ~100 km wide fault zone with 21 major faults (~4.9 km spacing) with a max. throw of ~500 m. The N-trending central region displays 13 major faults across a width of ~85 km (~6.5 km spacing) and a max. throw of ~300 m. The NW-trending southern region displays 11 major faults across a 63 km width (~6.2 km spacing) and a max. fault throw of ~300 m. The horst-graben structure of the Alba Fossae system accommodates extension on the flanks of the volcano, where crustal thickness decreases with distance from the volcanic peak. The fault system is also located along a topographic break between what appears to be a secondary, shield-like topographic high on the western margin of Alba Mons. Our data and analysis, when integrated with previous studies, suggest that this circumferential fault system may have magmatic origins with fault orientations controlled by the local and regional stress field on the flank of Alba Mons. We suggest that the horst-graben fault network may represent strain that helps accommodate some combination of shallow subsurface dike injection and topographic relaxation of the Alba Mons dome.