Continental versus oceanic subduction zones

TLDR

Subduction zones are tectonic expressions of convergent plate margins where oceanic and continental crust descend into the mantle wedge, producing mafic arc volcanics above oceanic subduction and high‑to‑ultrahigh‑pressure metamorphic rocks in continental subduction, with mantle wedge origins differing between asthenospheric and lithospheric settings and dehydration of descending crust generating fluid‑mobile incompatible elements. When the subducting slab decouples from the mantle wedge, lateral incursion of asthenospheric mantle heats the slab–mantle interface, triggering dehydration melting of the slab surface and metasomatized mantle wedge base. The tectonic regime of subduction zones evolves in time and space, as evidenced by ophiolites documenting the transition from seafloor spreading to oceanic subduction beneath continental margins and ultrahigh‑temperature metamorphic events marking the shift from compression to extension in orogens.

Abstract

Abstract Subduction zones are tectonic expressions of convergent plate margins, where crustal rocks descend into and interact with the overlying mantle wedge. They are the geodynamic system that produces mafic arc volcanics above oceanic subduction zones but high- to ultrahigh-pressure metamorphic rocks in continental subduction zones. While the metamorphic rocks provide petrological records of orogenic processes when descending crustal rocks undergo dehydration and anataxis at forearc to subarc depths beneath the mantle wedge, the arc volcanics provide geochemical records of the mass transfer from the subducting slab to the mantle wedge in this period though the mantle wedge becomes partially melted at a later time. Whereas the mantle wedge overlying the subducting oceanic slab is of asthenospheric origin, that overlying the descending continental slab is of lithospheric origin, being ancient beneath cratons but juvenile beneath marginal arcs. In either case, the mantle wedge base is cooled down during the slab–wedge coupled subduction. Metamorphic dehydration is prominent during subduction of crustal rocks, giving rise to aqueous solutions that are enriched in fluid-mobile incompatible elements. Once the subducting slab is decoupled from the mantle wedge, the slab–mantle interface is heated by lateral incursion of the asthenospheric mantle to allow dehydration melting of rocks in the descending slab surface and the metasomatized mantle wedge base, respectively. Therefore, the tectonic regime of subduction zones changes in both time and space with respect to their structures, inputs, processes and products. Ophiolites record the tectonic conversion from seafloor spreading to oceanic subduction beneath continental margin, whereas ultrahigh-temperature metamorphic events mark the tectonic conversion from compression to extension in orogens.