The role of lithospheric stress in the support of the Tharsis Rise

Abstract

The Tharsis rise on Mars is a broad topographic high covered by volcanic plains and capped by large volcanoes. The geoid high associated with Tharsis shows that shallow isostatic compensation is not complete. We hypothesize that the Tharsis rise can be approximated as an axisymmetrical igneous construct. Linear theory for the deflection of planetary lithospheres is used to show that the lithospheric stresses required partially to support the construct are reasonable and consistent with the observed radial grabens around Tharsis. The computed thickness of the elastic lithosphere is between 110 and 260 km depending on the assumed values for crustal thickness and crustal density. The computed thickness of the Tharsis load is between 40 and 70 km. In this model the height of the geoid is not specified a priori. Therefore, the agreement between the observed and computed geoid is evidence for the validity of our model. The contribution of Tharsis to the gravitational flattening of Mars is computed as ΔJ2 = −8.8 × 10−5. After a correction is applied for the flattening, the geoid low around Tharsis is more nearly axisymmetrical. The tectonics of the Tharsis region are briefly reviewed, and it is argued that all observations are consistent with the loading model.