Previous mechanical models of the western U.S. have concluded that plate boundary forces cannot generate far-field deformation. Such models have ignored preexisting large-scale lithospheric strength variations, an assumption that appears to be inconsistent with seismically determined variations in lithospheric structure. We have formulated a three-dimensional viscous flow model with imposed plate motions, but include lateral zones of low viscosity. These models show that strain rates are concentrated in weak zones with adjacent blocks experiencing little deformation. Deformation can extend far inboard of plate boundaries, contrary to the result of previous studies with rheologically homogeneous plates, and apparently compatible with the variation is seismic velocity and GPS determined deformations in western U.S. These results suggest that plate boundary forces cannot be neglected in the deformation of the western U.S., including the Cenozoic extension of the Basin and Range Province.