DEEP SEISMIC CRUSTAL STRUCTURE BENEATH THE BONIN TROUGH

The Bonin Trough and the Bonin Ridge are located in the fore-arc region of the southern Bonin Arc System. Such a conspicuous trough-ridge formation along an arc cannot be recognized in other trench-island arc systems. The southern Bonin Arc is characterized by other several unique geological and geophysical features. The tectonics controlling their formation and evolution has attracted interest of many earth scientists but has not been understood well. In 1987, we made a seismic refraction experiment across the Bonin Trough to clarify the structure of the crust and the upper mantle of this area which has not been known well to date. High quality seismic data of airgun shooting and of explosive sources were acquired by ocean bottom seismograph and hydrophone (OBSH) systems. A detailed seismic velocity structure model was obtained by modeling of the observed travel times and amplitudes using a two-dimensional ray tracing technique. The structure is considerably variable across the Bonin Trough, especially in the sedimentary layers, in spite of its flat sea-floor topography. Thinning of the upper crust is recognized in the trough suggesting that the Bonin Trough was formed by rifting of the island arc crust. Uplifting of the crust and the uppermost mantle beneath the Bonin Ridge is required in our structure model. The depth to the Moho boundary decreases from 18 km beneath the Shichito Ridge to 14 km beneath the Bonin Ridge; this is in good agreement with the crustal structure expected from large positive gravity anomaly observed over the Bonin Ridge.