Subduction produces asymmetric foreland fold & thrust belt and an accretionary prism as well. 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 and when the subducting slab is dismembered and emplaced into the eugeosynclinal assemblage, it is called an ophiolite. These have a sedimentary component and a volcanic as well and usually their presence indicates in Precambrian or other older rocks the "fossilized" sutures.