Rheology of the crust and its role in tectonic reactivation

Reactivation and tectonic inversion are the result of the interactions betw een changes in the stress field and the rheological properties of the litho sphere. The rheology varies in time and space as a function of variations i n several parameters, foremost among which are composition, temperature, an d pore fluid pressure. The predicted depth of the brittle/ductile transitio n depends strongly on composition and geotherm, varying from < 10 km for ho t felsic crust to > 30 km for cold mafic crust. In the brittle (frictional) field, reactivation of existing planes of weakness depends on their orient ation with respect to the stress field. The likelihood of reactivation as n ormal faults is higher than reactivation as thrust faults, and increases wi th increasing pore fluid pressure. In some cases, a decrease in the frictio n coefficient with increasing pressure appears to be required. In the ducti le field, the strength ratio at the Moho between the (harder) uppermost man tle and the (softer) lowermost crust is also a strong function of compositi on and temperature. It is highest for intermediate geotherms, and decreases away from this central range both for increasing and for decreasing temper ature. (C) 2000 Elsevier Science Ltd. All rights reserved.