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    Accretionary wedges are accumulations of sediment scraped off from the downgoing (underthrust) slab during subduction. They are landward of oceanic trenches formed by the subduction process. Subduction may be of 'oceanic lithosphere' or beneath a continent (continental arcs) or beneath other 'oceanic lithosphere' (island arcs).

Critical Wedge Taper in Accretionary Prisms (see this in this collection):

The thickness of the wedge depends on the amount of scraped sediment available. The critical taper of the wedge is based on strength of the wedge material and on the basal shear strength of the edge. Proper accommodation of the taper involves thrusting, slumping, and normal faulting. Offscraping and underplating of the sediment beneath the accretionary prism is accomplished by thrust faulting. The wedge is underlain by a detachment fault on the top of the descending slab. This fault ramps to progressively shallower levels toward the trench. See ramps and flats.  Subduction produces asymmetric foreland fold & thrust belt and an accretionary prism and progressive continental collision will produce symmetric foreland fold and thrust belts with opposite vergence. Volcanic arc rocks formed by subduction are complexly deformed during the eventual collision.