OBSERVATIONS, MODELS, AND MECHANISMS OF FAILURE OF SURFACE ROCKS SURROUNDING PLANETARY SURFACE LOADS

Geophysical models of flexural stresses in an elastic lithosphere due to an axisymmetric surface load typically predict a transition with in creased distance from the center of the load of radial thrust faults t o strike-slip faults to concentric normal faults. These model predicti ons are in conflict with the absence of annular zones of strike-slip f aults around prominent loads such as lunar maria, Martian volcanoes, a nd the Martian Tharsis rise. We suggest that this paradox arises from difficulties in relating failure criteria for brittle rocks to the str ess models. Indications that model stresses are inappropriate for use in fault-type prediction include (1) tensile principal stresses larger than realistic values of rock tensile strength, and/or (2) stress dif ferences significantly larger than those allowed by rock-strength crit eria. Predictions of surface faulting that are consistent with observa tions can be obtained instead by using tensile and shear failure crite ria, along with calculated stress differences and trajectories, with m odel stress states not greatly in excess of the maximum allowed by roc k fracture criteria.