forearc

Forearc refers to the region of the overriding tectonic plate that lies between a subduction-zone trench and the associated volcanic arc. It is the zone on the434 oceanic side of the volcanic front, lying landward of the trench and seaward of the arc. The forearc is distinct from both the trench itself and the volcanic arc that forms inland above the subducting slab.

Key structural features of the forearc include the accretionary wedge or prism at the trench toe, formed by sediments scraped from the downgoing plate and deformed as they are accreted to the overriding plate. Landward of the trench lies the forearc basin, a depression that commonly fills with thick sedimentary sequences produced by tectonic subsidence and ongoing deformation. Above the subducting slab exists the forearc mantle wedge, a region of the mantle above the slab that may become serpentinized by fluids released from the subducting plate, influencing thermal structure and seismic behavior.

Geologically, the forearc plays a central role in subduction dynamics. The arrangement controls patterns of seismicity, fluid release, and metamorphism, and it can host economically important forearc basins with hydrocarbon potential. The state of the mantle wedge and the degree of sediment accretion affect the style of plate boundary earthquakes and the initiation and evolution of arc volcanism inland from the forearc.

Notable forearcs are found around the Pacific Ocean, including regions near the Cascadia subduction zone, the Japan and Kuril arcs, and the Peru-Chile trench system.