Postextensional uplift and tilting of Quaternary lake sediments in the Halloran Hills, eastern Mojave desert, California, suggest that regional-scale flow within the lower crust is occurring in response to tectonic loads created during upper crustal extension in late Miocene time. The concept that significant flow within the lower crust may occur for millions of years after the cessation of extension indicates that topographic gradients can be modified by regional-scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting. An analytic solution, developed in this paper, combines viscous flow in the lower crust, flexural bending within the midcrust, and isostasy into an invertible linear differential equation that describes the topographic response to upper crustal extension. Assuming that there was no topographic relief across the Halloran Hills region prior to extension, inversion of modern topographic data indicates that crustal thinning above the Halloran Hills detachment increases westward with a listric geometry and that little crustal thinning has occurred east of the western flank of Clark Mountain, in good agreement with the known geometry of the fault and the location of its breakaway zone. Incorporation of Quaternary tiling data indicates that the modern viscosity beneath the Halloran Hills (assuming a 10-km lower crustal channel) is a maximum of 1019 Pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 Ma. This corresponds to a temperature increase of at least 75¿ to 100 ¿C at the Moho. In our opinion, the most likely source of this temperature increase is a regional-scale thermal event within the underlying mantle and diffusion of heat upward into the lower crust. If correct, these results have important implications for the way in which crustal extension is linked to mantle heating within the Basin and Range Province. ¿ American Geophysical Union 1994