Rheology of the lithosphere in space and time

Abstract: The rheology of the lithosphere is afactor of primary importance inthe kinematics and dynamics of mountain belts. This paper attempts to clarify the role of rheology in orogenesis, by applying simple physical principles to the analysis of tectonic processes. The emphasis i on broad generalizations leading to order-of-magnitude estimates. Since the rheology of lithospheric materials is strongly dependent on temperature, the discussion opens with a review of continental and oceanic geotherms and an assessment of their reliability. Then the brittle (frictional) and ductile (high-temperature c ep) properties of the lithosphere are considered. In the brittle field, particular attention ispaid to the problem of fault reactivation, which is shown to be more likely in extensional than in compressional regimes. In the ductile field, a summary of creep parameters for the most common lithospheric materials is presented. The central concept of rheological profiles (strength envelopes), essential to the estimation of the depth variations of lithospheric rheology, is discussed with reference not only to its applicability but also to its limitations (a two-dimensional example from the Canadian Cordillera is given). Processes related to the rheological properties and layering of the lithosphere- gravitational collapse in thickene