Nonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks

Abstract

A subduction shear zone can be modeled as a long narrow channel, with the flow of subducted metasedimentary rocks in the channel driven by two sets of forces: the downward shearing force exerted by the subducting slab and the gradient in the hydraulic potential, which combines the effect of both pressure and buoyancy. If the channel walls are effectively rigid, very slight narrowing or broadening of the channel (convergence angles <1°) can result in very dramatic changes in the (nonlithostatic) pressure distribution along the channel. The geometry of the subducting plate, which is forced to bend under the overriding plate, suggests that the channel should initially narrow downward and then gradually broaden. A model assuming this geometry, with initial channel width 1500 m, minimum width ~600 m and width at 100 km depth of again ~1500 m, a maximum viscosity of 1019 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 horizontal balance of forces across the plates and with a reasonable value for the thickness of subducted sediment (~650 m). -from Author