Constraints on magnitudes of extension on Venus from slope measurements

Abstract

Listen

Translate article

Slopes related to normal fault scarps on Venus appear in Magellan synthetic aperture radar (SAR) images as tonal bands of increased or decreased radar backscatter relative to adjacent regions. Calculations, through measurements on Magellan stereo SAR, of 170 of these surfaces in plains regions show that their mean topographic slope is 36.4°±1.2°, regardless of height. Because there is little to no erosion on Venus, we infer from this mean slope that most faults have collapsed to talus slopes, approximately at an angle of repose, and thus the effective cohesive strength of the crust near most fault surfaces is low, probably because of secondary faulting and jointing. In areas well imaged by Magellan stereo SAR we show that simple, balanced cross sections through individual grabens on Venus can be made, assuming a Coulomb shear angle for fault orientation, thus constraining slip on causitive faults and extension across the grabens. In rifted areas imaged by one Magellan SAR viewing geometry, constraints on extension across rift zones can be made by measuring the tonal bands representing slopes related to individual normal faults and using the mean topographic slope calculated for similar slopes imaged in stereo SAR. With this technique we show that crustal extension due to faulting and folding for the most recent deformation in the Beta rift zone is <20 km. These modest strains, across one of the best developed rift zones on Venus, put severe limits on tectonic models of Venus that invoke significant lateral displacements of the Venusian crust.