Backarc Basin

A backarc basin is an extensional basin that opens behind a volcanic island arc, on the overriding-plate side of a subduction zone. Rollback of the subducting slab pulls the arc oceanward and stretches the crust behind it, sometimes far enough to start true seafloor spreading and generate new oceanic crust, as in the Mariana Trough and the Lau Basin. The basin floor carries high heat flow, active hydrothermal vents, and a spreading-related magnetic fabric. Sedimentation mixes arc-derived volcaniclastics with pelagic material. Backarc basins are marginal seas distinct from the trench and forearc that lie on the opposite side of the arc.