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trough. We also identified a structural difference in the Shikoku Basin as shown by
the two lines. Beneath line IBr9, high-velocity lower crust occupies a narrow region
between the rear-arc and the eastern Shikoku Basin, whereas high-velocity lower
crust along line IBr10 occupies a relatively broad region.
Origin of the Arc Crust
Crustal growth in the Izu-Ogasawara arc has occurred since 50 Ma. The first arc
crust formed in Eocene time (e.g., Taylor 1992 ) and included boninites. This ancient
arc crust is exposed on Ogasawara Ridge, where the boninitic arc shows a different
velocity structure (Takahashi et al. 2009 ). The arc beneath the current volcanic front
is composed of thick middle crust with Vp of 6.0-6.5 km/s, heterogeneous lower
crust, and low-velocity uppermost mantle. However, the ancient middle crust of
Ogasawara Ridge has Vp of approximately 6.4-6.5 km/s (e.g., Takahashi et al.
2009 ). The Aleutian arc also has high-Mg andesite materials (Kelemen et al. 1993 ),
and the 6.0 km/s layer there is thin (Holbrook et al. 1999 ). In addition, the Ogasawara
Ridge has been found to have a narrow (<10 km) region of thin crust (approximately
7 km), similar to the thickness of oceanic crust ( Kodaira et al. 2010 ). This implies
that an ophiolite-forming process is one candidate for the formation of boninitic
fore-arc crust, as suggested by Pearce et al. ( 1984 ).
After Eocene arc construction, crust continued to be constructed in Oligocene time.
Magnetic anomaly studies (Yamazaki and Yuasa 1998 ) suggest that the Oligocene arc
lies beneath the fore-arc basin between the current volcanic front and the Eocene arc.
Oligocene arc rocks are also found in the rear-arc region (Kodaira et al. 2008 ).
Although arc crustal formation appears to have ceased during opening of the Shikoku
and Parece Vela Basins, arc crust formation began again in the Miocene (15 Ma;
Straub et al. 2010 ). The region affected by Miocene arc igneous activity was broad;
volcanism moved from the current rear-arc region to the current volcanic front
(Ishizuka et al. 2006 ), where subsequent arc volcanism has occurred.
We interpreted the ages of arc construction according to this history of arc mag-
matic activity. Crust beneath the fore-arc basin corresponds to the Oligocene arc,
which is covered with sediments that have not been intruded by younger magmas.
The wide region of arc crust between the rear-arc and the volcanic front provides
evidence that arc igneous activity has occurred there since Oligocene time. The arc
structure with Vp of 6.0-6.5 km/s is consistent with the results of Kodaira et al.
( 2008 ), and the rear-arc construction probably occurred in Oligocene time.
History of the Rifted Crust
Here we discuss the consistency between the time-migrated MCS reflection sections
and the distribution of modeled high-velocity lower crust identified beneath the
Ogasawara Trough, the bathymetric low immediately behind the volcanic front, the
Sofu Trough, and the Shikoku Basin. Despite the NNE-SSW orientation of rifts in
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