High-pressure metamorphic terranes form when slivers of continental crust are pulled into the mantle by previously subducted oceanic lithosphere. Old, cold lithosphere subducts into the mantle at steep angles, dragging sialic crust to deep levels, quickly followed by steep, rapid exhumation. Young thin lithosphere subducts at shallow angles, pulling crustal slabs slowly into the shallow mantle, followed by slow exhumation, retrogression, and melting. Terranes that follow inboard-dipping subduction zones will subduct once, and will be overprinted by subsequent subduction and collisions. Terranes above outboard-facing subduction zones will be underthrust by the continental margin. Subsequent collisions will result in the re-subduction of an increasingly complex continental margin characterized by previously accreted terranes.