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that the resolved depth reaches from 15 km (beneath the Shikoku Basin) to 25 km
(beneath the Ogasawara Trough), covering all of the crust and uppermost mantle. The
recovery of the
Vp
perturbation was maximum 2-4% on the background
Vp
of 6.0-
7.5 km/s, and the
Vp
accuracies were 0.10-0.25 km/s, similar to those for line IBr9.
5
Discussion
In this section, we describe the structural commonalities and differences between
lines IBr9 and IBr10 of the central Izu-Ogasawara arc. The most important varia-
tions are in the distribution of middle crust with
Vp
of 6.0-6.5 km/s and lower
crust with
Vp
> 7.3 km/s, as described in
Sect. 4
. We also summarize the distribu-
tion of rifted lower crust with
Vp
> 7.3 km/s. We confirm from the reflection
images that the rifted crust is associated with deformation including normal faults
and rotation of crustal blocks. Finally, we construct a tectonic scenario for the
central Izu-Ogasawara arc that accounts for its structural differences from the
northern Izu-Ogasawara arc.
5.1
Structural Commonalities and Differences
Crustal models for the IBM arc have been constructed by many authors (e.g.,
Suyehiro et al.
1996
; Takahashi et al.
2007a, b, 2008, 2009
; Calvert et al.
2008
;
Kodaira et al.
2007a, 2007b, 2008
). Although the Eocene arc is slightly different
(Takahashi et al.
2009
;
Kodaira et al. 2010
), these models are similar, including a
middle crust with
Vp
of 6.0-6.5 km/s, thick and heterogeneous lower crust, and
slow uppermost mantle. The middle crust contributes most to crustal thickening,
and thick arc crust always has thick middle crust (Kodaira et al.
2007a
).
Our modeling identified common thick arc crust with
Vp
of 6.0-6.5 km/s beneath
the western Ogasawara Trough in the fore-arc region, the arc region between the
volcanic front and the rear-arc, and the rear-arc. Rifted crust with a high
Vp
lower
crust (
Vp
> 7.3 km/s) is found beneath the eastern Ogasawara Trough, the bathymet-
ric low just behind the volcanic front, the Sofu Trough, and the eastern Shikoku
Basin. These characteristics are similar to those described by previous studies.
Differences in crustal structure can be seen beneath the volcanic front and the
eastern Shikoku Basin. The crustal structure beneath the volcanic front along line
IBr9 is similar to those of previous studies. Although the crust here is thin
(15-17 km), it has a middle crust with
Vp
of 6.0-6.5 km/s and a proportion of
middle crust similar to that seen for the northern Izu-Ogasawara arc. Much of the
lower crust here has
Vp
of 7.0-7.3 km/s. However, the volcanic front along line
IBr10 overlies lower crust with
Vp
> 7.3 km/s, similar to lower crust found in rifted
regions. Because middle crust with
Vp
of 6.0-6.5 km/s exists along line IBr10 and
because the crustal thickness is approximately 12 km, the crust has arc rather than
oceanic character. This part of line IBr10 is at the northern tip of the Nishinoshima
Trough, and we infer that this crust was strongly affected by rifting to form the
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