Fault type predictions from stress distributions on planetary surfaces: Importance of fault initiation depth

Abstract

Faults on planetary surfaces initiate at depth where stresses due to the weight of overburden are important. Consequently, the prediction of fault type from calculated stresses at depth can yield different faults than those predicted using the surface criteria commonly employed in geophysical models. For elastic plate flexure models of mascon loading on the moon, stresses calculated at the surface predict that the most likely faulting is strike slip (for which there is no direct evidence) at a distance from the basin center where grabens are found. Modifying these plate flexure calculations to account for the weak surface layer of ejecta (megaregolith), 1–3 km‐ thick that overlies the relatively strong lunar lithosphere, results in model stresses in the outer layers of the moon (tens of bars) that are an order of magnitude lower than in the lithosphere (hundreds of bars). Because faults bounding lunar grabens initiate at the mechanical discontinuity between the megaregolith and lithosphere and because the reverse faults likely responsible for wrinkle ridges probably initiate at the basalt‐basin floor contact (0.5–2 km deep), model stresses must be calculated at these depths. Superposing nonisotropic stresses due to the weight of the overburden at the depths of these mechanical discontinuities with plate flexure model stresses correctly predicts maximum stress differences in an interior zone of expected wrinkle ridges surrounded by a zone of concentric normal faults or grabens and effectively eliminates the strike‐slip zone, in agreement with observations. The absence of strike‐slip faults from almost all planetary and satellite surfaces may be explainable if lithospheric deformation model stresses are calculated at important shallow crustal mechanical discontinuities where the maximum compressive stress is vertical, thereby favoring the formation of normal faults. If true, mechanical discontinuities in the shallow crusts of planets and satellites play an extremely important and hitherto unrecognized role in the formation of structures observed on their surfaces.