Rheology of granite under middle and lower crustal conditions: tectonic implications

The rheology of granite under geological conditions cannot be determined directly by experiment, because of the different flow parameters for the constituent minerals, and changes in grain-size and microstructure during deformation. I use a recent geologically calibrated dislocation creep flow law for quartz, experimentally determined flow laws for feldspars, and a grain-size sensitive pressure-solution creep flow law for quartz-feldspar-mica ultramylonite utilizing a new stress/grain-size relationship for feldspar derived from subgrain piezometry. These flow laws are combined using various rheological mixing laws depending on the evolving grain-size and fabric anisotropy to give bulk rheological parameters.  This allows prediction of the effective viscosity for granite as a function of stress, temperature, and strain as the rock evolves from a load-bearing framework to an interconnected weak layer microstructure. The results have implications for tectonic processes such as rates of crustal thickening during continental collision, crustal thinning during rifting, channel flow, and diapirism.