Back-arc spreading is initiated with rifting along the volcanic zone of an island arc under extensional tectonics. The width of the volcanic zone of the arc, which is closely related to the dip angle of the subducting slab, should cause a variation in the style of initial rifting and subsequent back-arc spreading. Two modes of back-arc spreading, single-rift type and multirift type, are introduced from the comparative study of the Bonin arc and the Japan Sea. Divergent tectonics in an island arc above a steeply dipping slab will cause a single-rift system in a narrow volcanic zone but will cause a multirift system in a broad volcanic zone, which is related to a shallow-dipping angle of subduction.

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