Alpine and Pacific styles of Phanerozoic mountain building: subduction-zone petrogenesis of continental crust

A broad continuum exists between two distinct end‐member types of mountain building. Alpine‐type orogenic belts develop during subduction of an ocean basin between two continental blocks, resulting in collision. They are characterized by an imbricate sequence of oceanward verging nappes; some Alpine belts exhibit superimposed late‐stage backthrusting. Sediments are chiefly platform carbonates and siliciclastics, in some cases associated with minor amounts of bimodal volcanics; pre‐existing granitic gneisses and related continental rocks constitute an autochthonous–parautochthonous basement. Metamorphism of deeply subducted portions of the orogen ranges from relatively high‐pressure (HP) to ultrahigh‐pressure (UHP). Calcalkaline volcanic–plutonic rocks are rare, and have peraluminous, S‐type bulk compositions. In contrast, Pacific‐type orogens develop within and landward from long‐sustained oceanic subduction zones. They consist of an outboard oceanic trench–accretionary prism, and an inboard continental margin–island arc. The oceanic assemblage consists of first‐cycle, in‐part mélanged volcaniclastics, and minor but widespread cherts ± deep‐water carbonates, intimately mixed with disaggregated ophiolites. The section recrystallized under HP conditions. Recumbent fold vergence is oceanward. A massive, slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable, non‐subducted plate. It consists of abundant, dominantly intermediate, metaluminous, I‐type volcanics resting on old crust; both assemblages are thrown into open folds, intruded by comagmatic I‐type granitoids, and metamorphosed locally to regionally under high‐T, low‐P conditions. In the subduction channel of collisional and outboard Circumpacific terranes, combined extension above and subduction below allows buoyancy‐driven ascent of ductile, thin‐aspect ratio slices of HP–UHP complexes to midcrustal levels, where most closely approached neutral buoyancy; exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume. At massive sialic buildups associated with convergent plate cuSPS (syntaxes), tectonic aneurysms may help transport HP–UHP complexes from mid‐ to upper‐crustal levels. The closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts, so volcanic–plut