THE FORMATION AND PRESERVATION OF ULTRA HIGH PRESSURE (UHP) TERRAINS: LESSONS FROM FINITE ELEMENT THERMAL MODELING

The thermal history of subducted/obducted crustal fragments is explored to depths of 125 km using a forward modeling approach. The finite element code uses an adaptive gridding technique that allows deformation and differential movement of subgrids to simulate tectonic mass flow. The geometry and velocity of plate convergence and the large-scale strain partitioning (e.g., subduction channel, mobile tectonic fragments) are described explicitly as functions of time. The evolution of metamorphic conditions during descent and subsequent exhumation of a terrane within the subduction channel is computed for different kinematic scenarios. These include a range of velocities, relative to a stable continental block, for the subducting slab, the obducting (UHP) fragment, and a subduction channel of given width. Boundary conditions and the geometric set-up were chosen according to three case studies of known UHP terranes (eastern China, Western Norway, Western Alps). Simulations indicate that the temperature reached at a given depth during the subduction stage is crucially dependent on the convergence rate, width of the subduction channel, location of the fragment within the subduction channel, as well as on rates of convection in the subcontinental mantle and shear heating. The P-T-t path experienced by a tectonic fragment during its exhumation from P(max) is determined primarily by its size and exhumation velocity; the rate of continued subduction is less important given the rapid exhumation required to preserve UHP assemblages. The depth at which T(max) is reached by an obducting UHP fragment and the dP/dT slope of its exhumation path may critically affect the extent of irreversible phase transitions during decompression. Comparing data for the three UHP case studies with corresponding simulation results indicates general agreement of the P-T paths. Predicted T-t evolution paths are also compatible with well-constrained mineral chronometry from these areas. This agreement between field data and simulation results leads to a number of observables that may be useful in relating UHP occurrences to their evolution. Though essentially generic in emphasis, our models are designed to help understand real UHP occurrences, especially their exhumation history.