Estimates of Martian crustal thickness from viscous relaxation of topography

Isostatically compensated crustal thickness variations and associated topog raphic contrasts at the surface of a planet result in lateral pressure grad ients, which may cause the lower crust to flow and reduce the relief. Areas of thicker crust are generally associated with more rapid relaxation of to pography. On Mars, topographic features such as impact basins and the hemis pheric dichotomy have survived for 4 Gyr. We use a finite difference repres entation of depth-dependent, non-Newtonian lower crustal flow to investigat e how topography decays with time. For a dry diabase rheology, total radiog enic concentrations greater than or equal to 80% of terrestrial values, and crustal radiogenic concentrations similar to terrestrial basalts, we find that an upper bound on the mean planetary crustal thickness is similar to 1 00 km. In the probably unrealistic case where all the radiogenic elements a re in the crust; this maximum crustal thickness can be increased to similar to 115 km, The main uncertainty in these results is the total radiogenic a bundances on Mars. Comparing our results with the observed shape of the cru stal dichotomy provides no evidence that this slope is primarily the result of lower crustal flow. Both Hellas and the dichotomy are isostatically com pensated; if the mechanism is Airy isostasy, then the lower bound on mean c rustal thickness is similar to 30 km. Crustal thicknesses of 30-100 km on M ars can be produced by mid-ocean ridge spreading at potential temperatures of 1350 degrees - 1600 degreesC. However, for such crustal thicknesses the lithosphere is likely to be positively buoyant, making subduction difficult .