Numerical modelling of post-orogenic evolution: isostasy, erosion and metamorphism

Post-orogenic evolution is controlled by a competition between erosion and subsequent isostatic uplift, which is dependent on both the rheology of the lithosphere and the density of the crustal root. In this latter, pressure P and temperature T changes induce significant variations of density by metamorphic reactions. Here we use a 2D finite element model, ADELI, which we modified to incorporate PT-dependent crustal densities, in order to study the effect of metamorphic reactions on the post-orogenic evolution of mountain belts. The density of each element is derived from petrogenetical grids yielding density ρ as a function of T and P for a given composition. To ensure local mass conservation, the exact density values are imposed in the model by volumetric variations, which are implemented using the tectonic stresses and the boundary conditions. The numerical model also takes into account erosion processes with river incision and hillslope landsliding. We have in particular tested different scenarios of post-orogenic phases depending on the mafic or intermediate composition of the lower crust. We suggest that post-orogenic crustal root can be preserved and stabilized by an increase of rock density due to metamorphism. Our results are consistent with previous seismic and geodetic observations, which indicate a progressive decrease of the ratio of surface relief to crustal root thickness after mountain building has ended.