Backarc

A backarc is a geological region on the overriding plate of a subduction zone that lies behind the volcanic arc and trench. It contrasts with the forearc, which lies between the trench and the volcanic arc, and with the volcanic arc itself.

Backarc regions commonly experience extensional tectonics. A prevalent mechanism is the rollback of the subducting slab, which pulls the overriding plate and creates tensional forces that can cause crustal thinning and, in some cases, seafloor spreading behind the arc, forming a backarc basin. This process can also influence magmatic activity in the region.

Magmatism in backarc settings arises from dehydration of the subducting slab and hydration of the mantle wedge, producing basaltic magmas that may feed volcanoes in the backarc region or spreading centers. The crust in backarc basins is often thinner and more buoyant than in the forearc, and hydrothermal systems are common around spreading centers and vents.

Notable examples include the Mariana Trough behind the Izu-Bonin-Mariana arc, the Lau Basin behind the Lau arc in the southwest Pacific, and the Guaymas Basin in the Gulf of California behind the subduction of the Cocos Plate. The Sea of Japan has also been described as a backarc basin associated with the subduction of the Pacific Plate beneath Asia.

Backarc processes influence regional tectonics, magmatism, and hydrothermal activity and can affect basinal sedimentation, seafloor spreading, and resource potential, including hydrocarbons and mineral deposits in older backarc basins.