Interplay between isostasy, erosion and lithospheric rheology inferred from numerical modeling of post-orogenic deformation

It is well accepted that during the post-orogenic phase the average elevation of mountain ranges declines in response to the combined effects of erosion and isostasy. The removal of mass at the surface by regional erosion causes isostatic uplift as the lithosphere acts to re-establish buoyant equilibrium. However the ratio between the rate of isostatic uplift and the rate of denudation is still unconstrained. Recently, both seismic and geodetic observations have indicated that isostasy is not respected in most post-orogenic settings, which rather exhibit a decrease in the ratio of surface relief to crustal root thickness during the post-orogenic phase. Variations of the buoyancy of the lower crust as well as a control by the strength of the continental lithosphere are commonly proposed to explain this imbalance. Here to test this last assumption, we use a 2D thermo-mechanical numerical modeling, which includes mechanical layering as well as thermal properties of continental lithosphere. To model the surface evolution we use an erosion law that accounts for river incision and hillslope landsliding. A Winkler spring foundation is also used to simulate isostatic restoring forces induced by erosional unloading. In our approach we assume a fully coupled thermo-mechanical modeling, in which lithospheric thermal conditions and surface erosion rates control the effective viscosity of the lithophere. Our preliminary results suggest that the effective viscosity of the lithosphere has a key control on post-orogenic decay that can vary from an erosion-dominated regime to a collapse-dominated regime. Furthermore our simulations support the existence of a decrease with time of the ratio of surface relief to crustal root thickness. The rate of decrease being dependent on the erosion rate, the rheological parameters, and the thermal conditions of the lithosphere.