Rheology of the crust and its role in tectonic reactivation

Reactivation and tectonic inversion are the result of the interactions between changes in the stress field and the rheological properties of the lithosphere. The rheology varies in time and space as a function of variations in several parameters, foremost among which are composition, temperature, and pore fluid pressure. The predicted depth of the brittle/ductile transition depends strongly on composition and geotherm, varying from <10 km for hot felsic crust to >30 km for cold mafic crust. In the brittle (frictional) field, reactivation of existing planes of weakness depends on their orientation with respect to the stress field. The likelihood of reactivation as normal faults is higher than reactivation as thrust faults, and increases with increasing pore fluid pressure. In some cases, a decrease in the friction coefficient with increasing pressure appears to be required. In the ductile field, the strength ratio at the Moho between the (harder) uppermost mantle and the (softer) lowermost crust is also a strong function of composition and temperature. It is highest for intermediate geotherms, and decreases away from this central range both for increasing and for decreasing temperature.