EVOLUTION OF THOUGHT CONCERNING HIGH- AND ULTRAHIGH-PRESSURE METAMORPHIC BELTS: SUBDUCTION, RECRYSTALLIZATION, AND EXHUMATION

Alpine-type collisional belts are produced by subduction of an ocean basin and the underflow of sialic crust. Old, competent continental units typify these complexes; ophiolites are common in a few, but are rare in most such belts. Metamorphism of profoundly subducted parts of the orogen—60 to 125 km—ranges from high-pressure (HP) to ultrahigh-pressure (UHP). Coeval calcalkaline arc volcanics + plutonics are rare. In contrast, paired Pacific-type orogenic belts consist of an outboard, oceanic trench + accretionary prism deposited on oceanic crust, and an inboard continental margin or island arc, developed within and landward from long-lived subduction zones. The outboard graywacke + shaley mélanges, minor deep-water cherts ± carbonates, and ubiquitous disaggregated ophiolites recrystallized under HP conditions at depths of 15-35 km. A massive, coeval calcalkaline arc dominates the subparallel, inboard terrane; associated metamorphism is typified by high temperatures. In both Alpine- and outboard Pacific-terranes, structural vergence is seaward, reflecting similar convergent plate-tectonic settings. Because of their structural integrity, some downgoing Alpine-type microcontinents, island arcs, or continental promontories descend to great depths before decoupling from the sinking plate; in contrast, incompetent, ductile, Pacific-type graywacke + shale terranes break loose from the descending oceanic crust-capped lithosphere at shallow depths. In both, the rapid ascent (0.5-1.0 cm/yr) of portions of subducted material as thin aspect-ratio sheets (~1 km thick), combined with extension above and subduction below, promotes lateral heat conduction and cooling of decompressing HP and UHP metamorphic complexes. Exhumation to mid-crustal levels is due to buoyancy: worldwide, blueschist-eclogite terranes consist of small, high-density mafic and/or peridotitic lenses surrounded by voluminous, low-density quartzofeldspathic ± serpentinitic material, thus aggregate terrane density is less than that of unaltered mantle. Exposure of rising HP and UHP belts is a consequence of erosional decapitation and gravitational collapse of the subduction complex.