We present a set of self-consistent numerical experiments resulting in the development of local weak zones within a wide region of extending brittle lithosphere overlying viscous asthenosphere. In these two-dimensional models, the brittle yield strength is controlled by a Byerlee's Law friction coefficient and a value for cohesion. A portion of the brittle strength is reduced as a function of plastic strain (strain beyond yield). This strain weakening can result in concentration of strain on spontaneously formed weak zones accommodating dip slip, or model normal faults. The temperature-dependent viscous rheology is based on a laboratory-derived power-law creep flow law for dia-base. The initial temperature gradient is taken to be linear with depth, and controls the depth range over which the viscosity decreases beneath the brittle lithosphere. The viscous flow of the transition region below the lithosphere can result in a distributed set of model normal faults, in some eases with regular periodic spacing. The model pattern of deformation depends on a broad range of parameters, including the thickness of the brittle lithosphere, the depth range for the decrease of viscosity with depth, the strength reduction with brittle strain, the rate of strength loss, and the rate of regaining of strength through fault "healing." In this preliminary set of models, we show that the spacing of model basins and ranges can depend on the amount of strain weakening on faults, with wider spacing for larger amounts of strain weakening. For a temperature profile that gives a similar to10 km thick brittle layer and about 20 MPa of strength loss with strain on faults, the model results in a pattern of topographic relief that roughly resembles what is seen in profiles across the Basin and Range province of the western United States.
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