MODELING ULTRA HIGH PRESSURE TERRANE EVOLUTION: INITIAL RESULTS

Ultra High Pressure Metamorphism (UHPM) is a rarely preserved type of metamorphism that occurs at very high lithostatic pressures. One laterally extensive and well-documented outcropping of UHPM rocks occurs in the Western Gneiss Region of Norway, where Baltica basement was briefly subducted beneath the Laurentian margin during the Scandian Orogeny. The physical processes leading to the exhumation of UHPM rocks are poorly understood.

This study represents the first stage of an investigation into the UHPM of the Western Gneiss Region. Analog and 3-D numerical modeling techniques are utilized to explore the processes leading to the creation and ultimately, to the exhumation of deeply subducted materials. Particular attention is paid to the force balances that define the P-T path of subducting materials and relative sensitivity of the system to changes in the various forces.

Initial analog modeling results suggest that viscous drag within the asthenosphere and the characteristic length of the subducting plate provide primary control on the geometry of the subducting slab. Analog studies further suggest that coupling between the descending oceanic slab and continental lithosphere is necessary to transport the evolving terrane into UHP conditions. Decoupling of the dense slab from the buoyant continental lithosphere is therefore a necessary first step in the process of UHP terrane exhumation. Early numerical modeling results suggest that metamorphic weakening reactions thought to occur in the descending slab may initiate necking and eventual drop-off of the subducting slab. On-going numerical modeling serves to relate observed structures and mineral assemblages in natural rocks to orogen-scale dynamics.