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fore-arc basin The part of the fore-arc (arc-trench gap) adjacent to the island arc which is characterized by flat-lying sediments, in contrast to the highly deformed accretionary wedge adjacent to the oceanic trench. Fore-arc basins lie behind the topographic high-point of the wedge, which in places forms an outer (sedimentary) island arc. Fore-arc sediments lie unconformably on accretionary wedge material and show progressive shoaling. The source of the material is the adjacent volcanoes and the erosion of uplifted plutonic— metamorphic basement.