UC Davis

Back-arc spreading centers initiate as fore-arc or arc rifting events when extensional forces localize within lithosphere weakened by hydrous fluids or melting. Two models have been proposed for triggering fore-arc/arc rifting: rollback of the subducting plate causing trench retreat or motion of the overriding plate away from the subduction zone. This paper demonstrates that there is a third mechanism caused by an in situ instability that occurs when the thin high-viscosity boundary, which separates the weak fore arc from the hot buoyant mantle wedge, is removed. Buoyant upwelling mantle causes arc rifting, drives the overriding plate away from the subducting plate, and there is sufficient heating of the subducting plate crust and overriding plate lithosphere to form adakite or boninite volcanism. For spontaneous fore-arc/arc rifting to occur a broad region of weak material must be present and one of the plates must be free to respond to the upwelling forces.