A back-arc basin is a submarine feature associated with subduction zones, formed behind a volcanic arc. These basins are created by the process of back-arc spreading, which occurs when the overriding tectonic plate is stretched due to the rollback of the subducting plate. Back-arc basins are important in understanding the dynamics of plate tectonics, particularly in how they relate to the formation of new oceanic crust and volcanic activity.
Source Stern, R. J., & Scholl, D. W. (2010). “Back-Arc Basins.” In: Encyclopedia of Ocean Sciences, Elsevier.