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Rifting Structure of Central Izu-Ogasawara
(Bonin) Arc Crust: Results of Seismic
Crustal Imaging
Narumi Takahashi, Mikiya Yamashita, Shuichi Kodaira, Seiichi Miura,
Takeshi Sato, Tetsuo No, Kaoru Takizawa, Yoshiyuki Tatsumi,
and Yoshiyuki Kaneda
Abstract We obtained seismic velocity structures for two crustal profiles across
the central Izu-Ogasawara (Bonin) intra-oceanic arc using a multichannel reflection
system and ocean-bottom seismographs. The crust beneath the volcanic front in
this region is less than 20 km thick. A middle crustal layer (P-wave velocity Vp
~6 km/s), representing andesitic proto-continental crust, is found beneath the
volcanic front, the rear-arc, the fore-arc basin and Ogasawara Ridge. Crust with
15-20 km thick beneath the fore-arc basin may have formed in Paleogene time
because it is covered with thick sediments. Crustal thinning by rifting is seen
between the volcanic front and the rear-arc, where the crustal thickness is ~15 km,
the lower crust has high Vp (>7.3 km/s) and there are normal faults in the shallow
crust. Rifted crust also occupies a broad area in the eastern Shikoku Basin. The
central Izu-Ogasawara arc has been affected by four rifting episodes compared to
two for the northern Izu-Ogasawara arc. The volume of the entire arc crust in the
central region is ~20% greater than in the northern region, even though the crust
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