ON THE THERMOMECHANICAL EVOLUTION OF COMPRESSIONAL OROGENS

We present results from a newly developed fully coupled thermomechanic al model of the continental crust in which crustal shortening at a con vergent plate boundary is driven by a basal velocity discontinuity whi ch represents delamination and subduction in the underlying mantle. Th is new dynamical model incorporates complex rheologies (elasticity, th ermally 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 'dyn amic Lagrangian remeshing' (DLR) method, which uses Lagrangian spatial discretization of the crust and therefore; allows for an accurate tra cking of rock particles as they travel through the deforming orogen; t his information is, in turn, used to produce synthetic PTt paths. We h ave used this model to predict the distribution of apparent ages for a wide range of isotopic systems at the surface of an actively deformin g orogen. We have also predicted other geophysical and geological obse rvables, such as the metamorphic grade of exposed rocks (regarded here as a first-order approximation for total denudation), topography, sur face heat flux, and the thickness of sediment deposited in the adjacen t foreland basins. Results clearly demonstrate that the highest exhuma tion 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 vici nity of the retro-shear zone, a crustal-scale 'fault' which accommodat es most of the crustal shortening within the orogen. It is also in thi s region that the isotopic systems characterized by the greatest closu re temperatures display the youngest ages and the highest grade metamo rphic rocks are found. These conclusions are derived from the assumpti on 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 distribut ion of isotopic ages and metamorphic grades across the orogen.