Role of crustal flexure in initiation of low‐angle normal faults and implications for structural evolution of the basin and range province

Abstract

The mechanics of low‐angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults. Gently dipping shear stress trajectories are necessary for nucleation and propagation of low‐angle faults in a structurally isotropic medium. Such trajectories are not produced by simple horizontal extension, but do arise in the presence of flexural stresses. We model stress conditions in a cross section through the strong upper crust using Airy stress functions for a two‐dimensional elastic medium. Numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low‐angle normal fault initiation at intermediate depths in an elastic medium. We propose that the gentle dip of Cenozoic low‐angle normal faults in the Basin and Range Province resulted from flexural stresses that were produced by isostatically uncompensated surface and, especially, Moho relief. Resurgent mid‐Cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust, causing or promoting initiation of low‐angle normal faults. Temporal changes in structural style, from dominantly low‐angle to dominantly high‐angle normal faulting, may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the Moho and cooling of the mantle lithosphere.