Accretionary Prism

An accretionary prism is a wedge of sediment and oceanic crust scraped off the downgoing plate at a convergent margin and plastered against the overriding plate. As the subducting slab descends into the trench, its sediment cover is offscraped along a basal decollement and stacked into imbricate thrust slices, the wedge thickening landward toward the volcanic arc. Pore pressure inside the prism is high because rapid burial traps fluids, which weakens the thrusts and sets the wedge taper. The Nankai and Cascadia prisms are drilling targets for this reason. It records subduction history and hosts the seismogenic plate-boundary fault.