Convergent Plate Boundaries and Accretionary Wedges

Abstract

Convergent boundaries represent a setting of tectonic boundaries where one plate subducts beneath the other plate, yielding a wide range of geological processes such as earthquakes, deformation, metamorphism, magmatism and mineralization. According to the nature of convergent plates, convergent boundaries are classified into oceanic-oceanic, oceanic-continental and continental-continental. In view of the temporal relationship between convergence and tectonism, these boundaries are categorized into ongoing convergent (converging) and fossil convergent (converged). A change in the thermal state of convergent plate boundaries results in different types of regional metamorphism for crustal rocks at lithospheric depths. Metamorphic products are categorized into Alpine, Barrovian and Buchan facies series, respectively, corresponding to metamorphism at low, intermediate and high thermal gradients. Mafic arc magmatism is prominent above ongoing oceanic subduction zones, but absent above ongoing continental subduction zones. Nevertheless, felsic magmatism is common along fossil convergent boundaries, marking crustal reworking in the post-subduction stage. Therefore, orogenic belts along convergent boundaries are divided into accretionary, collisional and intracontinental. As such, mountain building proceeds through subduction, collision and rifting processes along convergent plate boundaries. During plate subduction, supracrustal materials are often off-scraped from the subducting lithosphere and added to the leading edge of the overriding plate, forming accretionary (orogenic) wedges along ongoing convergent boundaries. Such wedges often show lower metamorphic grades in oceanic-continental convergent boundaries than in continental-continental convergent boundaries. The extent of deformation and metamorphism in them is largest in the internal belt and decreases toward the two sides, with the smallest ones at the external margins.