On the thermomechanical evolution of compressional orogens

We present results from a newly developed fully coupled thermomechanical model of the continental crust in which crustal shortening at a convergent plate boundary is driven by a basal velocity discontinuity which represents delamination and subduction in the underlying mantle. This new dynamical model incorporates complex rheologies (elasticity, thermally activated creep and brittle frictional behaviour), allows for extremely large deformation, and is coupled along its top boundary to a complex erosion/ sedimentation model. The model is based on the 'dynamic Lagrangian remeshing' (DLR) method, which uses Lagrangian spatial discretization of the crust and therefore; allows for an accurate tracking of rock particles as they travel through the deforming orogen; this information is, in turn, used to produce synthetic PTt paths. We have used this model to predict the distribution of apparent ages for a wide range of isotopic systems at the surface of an actively deforming orogen. We have also predicted other geophysical and geological observables, such as the metamorphic grade of exposed rocks (regarded here as a first-order approximation for total denudation), topography, surface heat flux, and the thickness of sediment deposited in the adjacent foreland basins. Results clearly demonstrate that the highest exhumation rates (and thus the youngest isotopic ages) are found in regions of maximum topography near the centre of the orogen, but that the most deeply exhumed rocks are found on the side of the orogen, in the vicinity of the retro-shear zone, a crustal-scale 'fault' which accommodates most of the crustal shortening within the orogen. It is also in this region that the isotopic systems characterized by the greatest closure temperatures display the youngest ages and the highest grade metamorphic rocks are found. These conclusions are derived from the assumption of uniform erosion across the orogen; in cases where rainfall (and thus erosion) is orographically controlled, the tectonic style of the orogen is different from the uniform erosion case, as is the distribution of isotopic ages and metamorphic grades across the orogen.