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back-arc basin The zone of thickened sedimentation and extensional tectonics which lies behind an island arc. Sedimentation in back-arc basins can be very varied, ranging from pelagic through turbidites to alluvial fans, although volcaniclastics (see CLAST) are common. Various convective systems have been postulated to explain extension, which is oriented parallel to the compression in a destructive margin. Back-arc basins have been classified into retro-arc basins and inter-arc basins. Examples include the Japan Sea (between Japan and Korea) and the basins west of the Marianas Trench.