IMPLICATIONS FROM THE SEISMIC CRUSTAL STRUCTURE OF THE NORTHERN IZU-BONIN ARC

The results of a controlled source seismic reflection-refraction exper iment carried out in 1992 reveal the following characteristics of the northern Izu-Bonin (Ogasawara) oceanic island are-trench system. (1) T he crust rapidly thickens from the Shikoku back-are basin to the arc, is thickest beneath the active rifts, and then gradually thins to the forearc. The thickness of the crust beneath the are rift zone and the back-ape basin are similar to 20km and 8km, respectively. (2) The Moho vanishes beneath the forearc. Velocities rapidly decrease eastwards b eneath the inner trench wall. (3) The velocity of the lower crust of t he are and the back-are basin is 7.1-7.3 km/s. This velocity is higher than the typical oceanic lower crust whose velocity is similar to 6.7 km/s. (4) The velocity of the middle crust of the are is similar to 6 km/s. This layer does not exist beneath the back-are basin. (5) A sli ght difference in the velocity gradient of the middle crust exists bet ween the are rift zone and the forearc. Based on these findings and pr evious studies, it is inferred that: (i) the middle crust is probably granitic rock and formed in more than two episodes; (ii) the lower cru st formed by igneous underplating which may also have affected part of the back-arc basin; and (iii) the root of the serpentinite diapir on the inner trench wall is a low-velocity mantle wedge that was probably caused by large amounts of water released from the subducting Pacific plate at depths shallower than 30 km.