Mars: Thickness of the lithosphere from the tectonic response to volcanic loads

We determine the thickness (or flexural rigidity) of the elastic lithosphere on Mars in the vicinity of large volcanic loads, including six volcanoes and the Isidis basin mascon. Starting from the hypothesis that graben concentric to each volcanic feature formed as a result of stresses induced by flexure of the lithosphere in response to loading, we relate predicted surface stresses to the range of radial distances between the graben and the load centers in order to derive local estimates of the elastic lithosphere thickness at the time of loading. The basic method involves the selection of graben of likely flexural origin and the calculation of vertical displacement and surface horizontal stresses produced by the loading and flexure of a shell or plate. Bounds on thickness are obtained from a formal inversion of graben positions as well as from a consideration of important time-dependent effects, including viscoelastic relaxation, volcano growth, and lithospheric thickening. Assuming Young's modulus to be 1012 dyn/cm², we find preferred values of elastic lithosphere thickness in the range 20 to 50 km (or flexural rigidities from 1030 to 1031 dyn cm) for regions surrounding Ascraeus Mons, Pavonis Mons, Arsia Mons, Alba Patera, and Elysium Mons. For the Isidis basin region the elastic lithosphere thickness must have exceeded 120 km at the time of graben formation; for Olympus Mons the absence of circumferential graben requires the elastic lithosphere to have been at least 150 km at the time of loading (these thicknesses correspond to a flexural rigidity greater than approximately 1032 dyn cm). Because the derived lithospheric thicknesses do not show any relationship to the relative ages of the loads, we interpret the results as an indication of a pronounced local thinning of the lithosphere beneath the central regions of the Tharsis and Elysium volcanic provinces.