A forearc or forarc, also called arc-trench gap[1] is a depression in the sea floor located between a subduction zone and an associated volcanic arc. It is typically filled with sediments from the adjacent landmass and the island arc in addition to trapped oceanic crustal material. The oceanic crustal fragments may be obducted as ophiolites onto the continent during terrane accretion.
The following features are recognized in a forearc. The steep inner trench wall (lower trench slope) flattens up into the upper trench slope, also known as the forearc basin. Usually a break in the slope is observed between the two areas, which can even be a small ridge, called the outer ridge, not to be confused with the outer trench swell on the subducting plate.[1]
The late Cretaceous - early Paleogene development of the Central Valley of California is an example of forearc development.