STRUCTURAL MAPPING OF GRABEN SYSTEM EMANATING SW FROM ARSIA MONS, THARSIS, MARS

Graben topography produced under the influence of a dike exhibits a characteristic concave-down, flat-bottomed geometry. The topographic signature of dike-related graben can be identified from cross-sectional profiles, yet it is unclear whether this characteristic geometry necessitates the presence of a dike in the subsurface. To ascertain attributes diagnostic of dike-related graben we select a fault system, for detailed structural mapping, that earlier studies concluded to have formed in association with diking. Sirenum Fossae is an extensive system of narrow closely spaced graben that emanate from the plains at the base of Arsia Mons, the southernmost Tharsis volcano. Mapping is performed at 1:100,000 scale, using an ~6 m/pixel digital mosaic composed of ~200 images from the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX). We mapped over 2800 normal faults with lengths ranging from ~150 m to ~275 km. High-resolution CTX imagery reveals full-graben and extensional fractures, segmented and continuous, arranged in linear, curvilinear, intersecting, and en echelon patterns. On a regional scale, the Sirenum Fossae graben strike NE-SW, extending >3000 km, radial to Arsia Mons. Dike-induced faulting should result in the formation of full-graben; the symmetry of the dike itself dictates that local stresses above the dike will lead to symmetrical, i.e. full-graben, geometries. On the other hand, if the faults observed radial to Arsia Mons are solely extensional and lack dike control we should expect to see extensional strain accommodated on a variety of structures including tensile cracks and fissures, and half-graben geometries as well as occasional full-graben. Stress variation with distance from the volcano should be distinguishable in the displacement profiles of purely extensional faults and we should expect to see greater displacement on fault segments that formed closer to the volcanic center. The geometric properties predicted by dike-induced faulting include anomalously long faults, relative to displacement, because the extent of dike propagation in the subsurface will control the fault length, symmetry, and narrow spacing between faults.