Thick shell tectonics on one‐plate planets: Applications to Mars

Abstract

A theory for stress distributions in thick lithospheric shells on one‐plate planets is developed based on the zero frequency equations of a self‐gravitating elastic spherical shell overlying a strengthless fluid. Stress distributions in lithospheres are reviewed for both the compensated and flexural modes. In the former case, surface stresses only depend on surface topography, while for the latter case, it is shown for long wavelengths that stress trajectories are mainly dependent on the lithospheric lateral density distribution and not on elastic properties. Computational analyses are carried out for Mars, and it is found that isostatically compensated models correctly predict the graben structure in the immediate Tharsis region and a flexural loading model is satisfactory in explaining the graben in the regions surrounding Tharsis. A three‐stage model is hypothesized for the evolution of Tharsis: isostasy with north‐south graben formation on Tharsis, followed by flexural loading and radial graben formation on the perimeter of Tharsis, followed by a last stage of loading with little or no regional deformation. This model is consistent with the Martian lithosphere monotonically thickening over geologic time.