Estimates of Martian crustal thickness from viscous relaxation of topography

Isosta.tica,lly compensated crusta,1 thickness variations and associated topographic contrasts a,t the surface of a planet result in lateral pressure gradients, which may cause the lower crust to flow and reduce the relief. Areas of thicker crust are genera, lly associated with more rapid relaxation of topography. On Mars, topographic features such as impa, ct basins a, nd the hemispheric dichotomy have survived for 4 Gyr. We use a finite difference representation of depth-dependent, non-Newtonian lower crustal flow to investigate how topography decays with time. For a dry diabase theology, total radiogenic concentrations 280% of terrestrial values. and crusta,1 radiogenic concentrations similar to terrestrial basalts, we find that an upper bound on the mea, n planeta,ry crustal thickness is •100 kin. In the probably unrealistic case where all the radiogenic elements are in the crust, this ma,ximum crustal thickness can be increased to •0115 kin. The main uncertainty in these results is the total radiogenic abundances on Mars. Compa, ring our results with the observed sha, pe of the crustal dichotomy provides no evidence that this slope is primarily the result of lower crustal flow. Both Hellas and the dichotomy are isostatically compensated; if the mecha, nism is Airy isostasy, then the lower bound on mean crustal thickness is •030 kin. Crustal thicknesses of 30-100 km on Mars can be produced by mid-ocean ridge spreading at potential temperatures of 1350 ø 1600øC. However, for such crustal thicknesses the lithosphere is likely to be positively buoyant, making subduction difficult.