Accretionary wedge

An accretionary wedge is a wedge-shaped mass of sediment and rock scraped from a subducting plate and piled against the overriding plate.

Subduction: The descent of one tectonic plate beneath another at a convergent plate boundary. The descending plate supplies sediment and rock that the wedge incorporates.
Convergent boundary: A tectonic plate boundary where two plates move toward each other. Accretionary wedges form at convergent boundaries where subduction occurs.
Accretionary prism: A body of deformed sediment and rock assembled at a subduction zone, often used interchangeably with accretionary wedge. The terms overlap, though prism often emphasizes the three-dimensional body.
Nankai Trough: A subduction zone south of Japan where the Philippine Sea Plate descends beneath the Eurasian Plate. Its accretionary wedge is a major natural laboratory for faulting and fluid flow.
Décollement: A low-angle fault or shear zone along which a body of rock moves independently of underlying material. This basal detachment lets sediment separate from the downgoing plate.
Oceanic trench: A long, narrow depression on the seafloor formed where one tectonic plate bends downward beneath another. The wedge accumulates on the landward side of the trench.
Mélange: A chaotic rock assemblage containing blocks of varied sizes and origins in a finer-grained matrix. Mélanges can form within wedges where intense shearing mixes diverse rocks.
Cascadia subduction zone: A subduction zone off the Pacific Northwest where the Juan de Fuca Plate descends beneath North America. Its sediment-rich margin includes a broad accretionary wedge.
Thrust fault: A low-angle reverse fault along which older or deeper rocks move over younger or shallower rocks. Thrusts stack scraped-off layers and help the wedge grow outward.
Overriding plate: The tectonic plate that lies above another plate at a subduction zone. The wedge is attached to the edge of this plate.

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