FRICTIONAL RHEOLOGY - HARDENING BY ROTATION OF ACTIVE NORMAL FAULTS

A simple model for work-hardening associated with rotation of active normal faults around horizontal axes mimics features of extended regions. We use Mohr-Coulomb envelopes to derive a frictional theology for the upper crust. Sliding on block-bounding faults is assumed to constrain the stress state in the upper crust regardless of the deformation within the blocks. The model predicts that the stress-strain relations at a constant depth will be saw-toothed in shape with stages of gradual work-hardening following events of abrupt softening. The rupture of faults at the weakest zone in the loaded crust corresponds to the softening stage that is followed by work-hardening due to local block rotation. Work-hardening can drive lateral distribution of tectonic extension to form large extended regions with many domains of tilted blocks (Basin and Range type). In a thinning crust, however, material points undergo shallowing and the work-hardening is moderated by the effect of shallowing that decrease the frictional resistance. We use experimentally derived parameters of strength and friction to quantify the model predictions and to analyze the distribution of extension in Basin and Range and the frequency of tilt values in this region.