CONTINENTAL-CRUST, CRUSTAL UNDERPLATING, AND LOW-Q UPPER-MANTLE BENEATH AN OCEANIC ISLAND-ARC

A detailed structural model of the crust, subducting slab, and underly ing upper mantle across the northern Izu-Ogasawara (Bonin) island are system is derived from a marine seismic reflection and ocean bottom se ismographic refraction survey and subsequent forward modeling combined with tomographic inversion. The model indicates that the crust is thi ckest beneath the presently active rift zone and a granitic crust may have formed in the mid-crust. A highly attenuative mantle (that is, on e with low quality Q) seems to be confined mainly beneath the presentl y active rift zone. In contrast, high P-wave velocity persists in the lower crust between the forearc and eastern margin of the back are bas in, suggesting a large-scale magma input responsible for the are forma tion.