Abstract
A four‐layer model of the upper 150 km of the Earth is used to calculate the viscous response of continental crust and the underlying mantle to tectonic denudation. The model comprises a strong upper crustal layer, a weak lower crustal layer, a very strong mantle lithosphere layer, and a weak mantle asthenosphere layer, which is in accord with experimental constraints on strength‐depth profiles for continental lithosphere. The strength of each layer is represented by its effective viscosity. Flow in the crust and mantle is driven by buoyancy forces, which arise from the unloading of an allochthon along a detachment fault by a series of instantaneous displacements (earthquakes or rapid creep events). Numerical solutions, obtained by using a finite element technique, predict footwall uplift, Moho deflection, and surface topography that are consistent with observations from the Basin and Range province of the western United States. The calculated curvature of the footwall uplift is also similar to that observed and is sensitive to the geometry of the detachment fault. Such bending need not be elastically controlled; hence the curvatures of footwall domes do not clearly place limits on the effective elastic thickness of the extending crust. The upward deflection of the Moho and the surface topography are sensitive to the viscosity structure and enable us to bound the range of the various viscosities. By matching observations from the Basin and Range province, which indicate no Moho deflection and low magnitude of surface topography (≤3–5 km), we estimate the upper crustal, lower crustal, and mantle lithospheric viscosities in the ranges 1021–1023 Pa s, 1019–1021 Pa s, and 1021–1023 Pa s, respectively.
