NONLITHOSTATIC PRESSURE DURING SEDIMENT SUBDUCTION AND THE DEVELOPMENT AND EXHUMATION OF HIGH-PRESSURE METAMORPHIC ROCKS

A subduction shear zone can be modeled as a long narrow channel, with the flow of subducted metasedimentary rocks in the channel driven by t wo sets of forces: the downward shearing force exerted by the subducti ng slab and the gradient in the hydraulic potential, which combines th e effect of both pressure and buoyancy. If the channel walls are effec tively rigid, very slight narrowing or broadening of the channel (conv ergence angles < 1 degrees) can result in very dramatic changes in the (nonlithostatic) pressure distribution along the channel. The geometr y of the subducting plate, which is forced to bend under the overridin g plate, suggests that the channel should initially narrow downward an d then gradually broaden. A model assuming this geometry, with initial channel width 1500 m, minimum width similar to 600 m and width at 100 km depth of again similar to 1500 m, a maximum viscosity of 10(19) Pa s, and a convergence rate of 8 cm/yr reaches pressures > 2 GPa in the channel at only 40 km depth. The model is consistent with a horizonta l balance of forces across the plates and with a reasonable value for the thickness of subducted sediment (similar to 650 m). The practical limit for overpressures attainable in subduction zones is determined b y the strength and permeability of the channel walls. At 40 km depth t he channel is effectively confined on both sides by cold lithospheric mantle, which should be strong enough to support a significant tectoni c overpressure. Episodic failure of the upper plate to produce great e arthquakes at 30-40 km focal depth could vent overpressured fluid from the channel, allowing a cyclical buildup and release of both rock and fluid pressure. Topography on the subducting plate (e.g., seamounts a nd thinned continental crust) may lead to an anvil-like jamming of the channel and local high overpressures. Tectonic erosion by topography on the lower plate of slivers from overlying continental crust and the compression of these slivers between the topography and the narrowing channel walls could produce high overpressures in continental rocks. A decrease in the convergence rate or cessation of subduction, with a consequent general warming within the channel and associated viscosity decrease, promotes exhumation by buoyant reverse flow. The most rapid reverse flow occurs in the region of previously greatest overpressure . Since the exhumation distance is shorter than for a simple lithostat ic pressure distribution and any increase in temperature is coupled wi th a strong increase in the rate of exhumation, preservation of high-p ressure assemblages at the surface in fossil subduction zones is promo ted for such a model.