The initiation of orogenic margin reverse faulting

[1] Laboratory values of rock friction coefficients suggest that reverse faulting should be very difficult to initiate by simple horizontal compression of the crust. Values of stresses required by Andersonian faulting may be an order of magnitude higher than those actually present in orogenic margins. A simple stress balance calculation shows that the effect of the excess lithostatic pressure under an elevated orogen, if transmitted laterally through a crustal ductile layer to the orogenic margin, is to provide sufficient hydraulic lift under the orogen flanks to initiate reverse faulting by direct lift, even with rock friction coefficients of order 0.8. The required orogenic elevation above surrounding "normal'' lithosphere is about one fifth of the thickness of the brittle crust of the orogen. This elevation may be as small as 2 km in tectonically active regions. The mechanism works even in the absence of regional lithospheric compressive stresses.