Semi-brittle flow

Fault zones limit the strength of the lithosphere and operate under a broad range of pressure and temperature conditions. Within a single fault zone, strain localization and partitioning occurs at different depths to accommodate the imposed plate tectonic movement. Depending on the pressure, temperature and strain rate conditions, the rocks will either flow viscously or dilate, fracture, and slide frictionally. Depending on how much strain is accommodated by the viscous component (diffusive mass transfer processes, dislocation motion and grain boundary sliding) and by the frictional component (fracturing, granular flow and frictional sliding) the rheological response will be either more viscous or more frictional. The rheological behavior of rocks under purely viscous and purely frictional conditions is relatively well understood. However, this is not the case when both sets of processes interact.