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immediately in front of the volcanic front with normal faults exposed on the seafloor
is the most recent, and is not identified to the north. Together, our reflection images
suggest that rifting occurred four times after Oligocene time, although the details of
timing are poorly constrained without drilling data.
5.4
Reason for the Thin Crust in the Central Izu-Ogasawara Arc
Our study has clarified that arc crustal structure extends E-W from the Ogasawara
Ridge to the rear-arc region for 390 km and 430 km along lines IBr9 and IBr10,
respectively. This is significantly greater than the 300 km width of the northern
Izu-Ogasawara arc (Suyehiro et al.
1996
). Although it is known that arc materials
and high-velocity lower crust are widely distributed underneath the eastern half of
the Shikoku Basin (Ishizuka et al.
2006
; Takahashi et al.
2010
), we calculated the
crustal volumes of areas between the rear-arc and trench along both seismic lines
by using a procedure similar to that used in the northern Izu-Ogasawara arc by
Takahashi et al. (
2007a, b
). The crustal volumes we estimated along lines IBr9 and
IBr10 are 5,563 km
3
/km and 6,068 km
3
/km, respectively, notably greater than the
4,646 km
3
/km estimated for the northern Izu-Ogasawara arc by Takahashi et al.
(
2007a, b
). Thus, the crust is much thinner in the central Izu-Ogasawara arc than it
is to the north, but the crustal volume is much larger. These results probably reflect
greater extension in the central region. In this study, we identified four episodes of
rifting based on seismic-reflection profiles. This contrasts with two known episodes
of post-Oligocene rifting in the north and is an indication of more rifting in the
central region than in the arc to the north (Fig.
9
). Thus the volumetric and seismic-
reflection evidence are mutually consistent.
Kodaira et al. (
2008
) suggested that differences in rear-arc crustal volume
between the northern and central Izu-Ogasawara arc segments correlate to that
beneath the volcanic front, and they suggested that NNE-SSW rifting occurred
between the two segments. Such rifting is consistent with formation of the Sofu and
Nishinoshima Troughs. The earliest post-Oligocene rift corresponds to the two-stage
rifting identified by this study in the Sofu Trough. There is no evidence for similar
early rifting in the northern Izu-Ogasawara arc. Recent rifting in the north, like the
Sumisu Rift (post 3 Ma), corresponds to the bathymetric low just behind the volcanic
front documented in this study. The latest rifting identified here in front of the vol-
canic front has no correlative in the northern Izu-Ogasawara arc. Comparing the
northern and central Izu-Ogasawara arc segments, the central part exhibits more rift-
ing episodes, which is consistent with the greater width of this arc segment.
We infer from the results of our study that the total volume of crust (~5,000-
6,000 km
3
/km) and rate of crustal growth (~100-120 km
3
/km-m.y.) in the central
Izu-Ogasawara arc is ~20% greater than that of the northern part. This estimate is
a minimum because it does not take into account arc crust now preserved in the
eastern half of the Shikoku Basin and the Kyushu-Palau Ridge. The evidence also
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