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Fig. 9 Schematic diagram of the tectonic history of northern and central segments of the Izu-
Ogasawara arc. Blue regions show arc crust. Red arrows show the volcanic front. In the central arc,
the crust beneath the volcanic front has remained thin because of repeated crustal rifting. ( a ) Tectonic
situation just after back-arc opening between volcanic front and rear-arc as described by Kodaira
et al. ( 2008 ). ( b ) Current situation in the northern part. ( c ) Current situation in the central part
shows that there has been more rifting in the central part than in the northern part.
Therefore, we suggest that the origin of the thin crust beneath the current volcanic
front is repeated crustal rifting.
6
Conclusion
We imaged the crustal structure of the central Izu-Ogasawara arc by tomographic
inversion and diffraction-stack migration techniques as well as MCS reflection images
processed from 204-channel streamer data. The Vp images show that this arc segment
has thinner crust (~15 km) than the northern Izu-Ogasawara arc (~25 km). Nevertheless,
the volume of arc crust is ~20% greater than that estimated for the northern segment
(Kodaira et al. 2007b ). Arc crust in the central arc segment lies beneath Ogasawara
Ridge, the fore-arc Ogasawara Trough, the volcanic front, the bathymetric high
between the volcanic front and the rear-arc region, and the rear-arc region. Thin rifted
crust that is approximately 10 km thick and high-velocity lower crust (>7.3 km/s) is
found beneath the Ogasawara Trough, a bathymetric low just behind the volcanic
front, the Sofu Trough, and the eastern part of the Shikoku Basin.
In reflection images along the two survey lines across the central arc segment, we
identified deformation related to extension, including normal faults and rotated
crustal blocks. The resulting rift zones occur in front of the volcanic front, in the
bathymetric low just behind the volcanic front, and in the Sofu Trough. In addition,
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