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. The lithosphere is quite weak below the crust for geothermal gradients greater than 20 ° C km −1 so attempts to treat the lithosphere as an elastic slab for time scales for which plastic flow contributes significantly to the deformation should be questioned. For a geothermal gradient of 10°C km −1, the entire continental crust and part of the upper mantle behave as an elastic-brittle slab. At higher geothermal gradients, the distribution of strength is variable vertically according to the magnitude of the geothermal gradient and the lithological make-up; large contrasts in strength occur across rheologically defined boundaries as well as across lithological boundaries. At these higher geothermal gradients, such contrasts in strength could act as nuclei for detachment zones within the continental crust and upper mantle. Detachment at or near the Moho is to be expected only for low geothermal gradients. Lateral variations in the geothermal gradient produce shallowly dipping zones of highly contrasting strength throughout the continental crust and uppermost mantle possibly resulting in flat lying thrusts and normal faults which may ramp upwards through the crust. Instabilities leading to seismic events which occur solely during plastic shearing should be common in thrust terranes where the stresses are relatively high (ca. 100–200 MPa), for geothermal gradients ranging from 10 to 50°C km −1 and for quartz-rich, felsic, mafic, and peridotitic rock types. Thrust terranes are therefore predicted to be characterized by coseismic plastic instabilities on all detachment zones throughout the crust and into the uppermost mantle but embryonic rift zones should be aseismic at depth ( > ca. 10 km).