The strength of the continental crust, detachment zones and the development of plastic instabilities

Abstract

The maximum strength of the continental crust is constrained by the geothermal gradient, the lithological make-up of the crust, and whether or not the Byerlee relation holds to the base of the continental crust. If this linear (Byerlee) relation holds, then maximum shear stress levels sustained at low geothermal gradients (10–20°C km⁻¹) could be as high as 300 MPa in the upper part of the mantle for strike-slip regimes, and up to 700 MPa in thrust regimes towards the base of the crust. However, if Byerlee's relation breaks down at moderate temperatures and pressures, and if such breakdown is associated with the transition from unstable to stable sliding on pre-existing faults, then the maximum stress levels in the crust, being set by the shear stress at the breakdown depth, are much lower, ca. 300 MPa for low geothermal gradients and thrust regimes.