FRICTIONAL RHEOLOGY - HARDENING BY ROTATION OF ACTIVE NORMAL FAULTS

A simple model for work-hardening associated with rotation of active n ormal faults around horizontal axes mimics features of extended region s. We use Mohr-Coulomb envelopes to derive a frictional theology for t he upper crust. Sliding on block-bounding faults is assumed to constra in the stress state in the upper crust regardless of the deformation w ithin the blocks. The model predicts that the stress-strain relations at a constant depth will be saw-toothed in shape with stages of gradua l work-hardening following events of abrupt softening. The rupture of faults at the weakest zone in the loaded crust corresponds to the soft ening stage that is followed by work-hardening due to local block rota tion. Work-hardening can drive lateral distribution of tectonic extens ion 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 o f shallowing that decrease the frictional resistance. We use experimen tally derived parameters of strength and friction to quantify the mode l predictions and to analyze the distribution of extension in Basin an d Range and the frequency of tilt values in this region.