Anelasticity explains topography associated with Basin and Range normal faulting

Abstract

Topography associated with normal faulting in the Basin and Range (western United States) is usually modeled as a flexure of a broken elastic plate. However, modeled effective rigidities are usually 100 times lower than the rigidity deduced from upper-crustal thickness. This discrepancy may be related to a significant anelastic deformation, which we explore through numerical modeling. Because experimental rock rheology evidences a pressure-dependent yield stress beyond the elastic limit in crustal rocks, we made a finite element model that accounts for such a crustal rheology and also for the frictional behavior of an embedded high-angle fault. Resulting topography after extension is similar to that obtained from a thin elastic plate model; however, the corresponding strain pattern differs. First, the footwall rotates in a rigid fashion over a width of 15 km that matches the typical size of uplifted Basin and Range blocks. Second, hanging-wall subsidence results from a significant horizontal extension and rotation of the crust. We suggest that plastic deformation in the deep part of the footwall could trigger the development of a new high-angle fault with a fault spacing that matches Basin and Range structure.