Rifting Structure of Central Izu-Ogasawara (Bonin) Arc Crust: Results of Seismic Crustal Imaging - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 77

and fore-arc regions. Kodaira et al. ( 2008 ) showed that the crustal thickness
variations beneath the rear-arc side correlate with that beneath the volcanic front,
and they concluded that crustal rifting occurred along a NNE-SSW trend between
the volcanic front and the rear-arc. Based on their across-arc structural model of the
southern Izu-Ogasawara arc, Takahashi et al. ( 2009 ) showed that crustal thinning
occurred both between the volcanic front and the rear-arc, and between the volcanic
front and the fore-arc. They also found that such crustal thinning accompanying the
rifting process is associated with lower crust with high Vp (>7 km/s).
Kodaira et al. ( 2007b ) showed that the central Izu-Ogasawara arc has thinner crust
than the northern part, and that the proportions of upper, middle, and lower crustal
layers are similar. Two possible reasons for the thin crust in the central arc are that (1)
it has lower magma production rates than the northern arc and that (2) most crustal
growth occurred early in the history of both segments, but rifting in the central arc
caused thinner crust. To evaluate these possibilities, we carried out seismic studies in
the central part of the Izu-Ogasawara arc using ocean-bottom seismographs (OBSs)
and modeled the velocity structure across this area. In this paper, we show two
Vp models of the central Izu-Ogasawara arc and discuss crustal structure from the
Izu-Ogasawara Trench on the east to the Shikoku Basin on the west. We also show
multichannel seismic (MCS) reflection profiles along these lines. We use these pro-
files to better understand rifting of the central Izu-Ogasawara arc, and thence to
address the question of why the arc crust here is so thin.

2 Data Acquisition

Seismic data to construct crustal structures were collected during cruises of R/V
Kaiyo and R/V Kairei of the Japan Agency for Marine-Earth Science and Technology.
Cruise KY06-09 of R/V Kaiyo (Aug. 3 to Sept. 1, 2006) deployed 95 OBSs along
line IBr9 on Fig. 1 (Takahashi et al. 2007b ), and cruise KR07-13 of Kairei (Sept. 30
to Oct. 29, 2008) deployed 102 OBSs along line IBr10 (Miura et al. 2008 ).
Line IBr9 runs from the Izu-Ogasawara Trench to the eastern foot of the Kinan
Seamount chain in the Shikoku Basin. It traverses the northern tip of the Ogasawara
Trough, the northern foot of Nichiyo Seamount (volcanic front), the northern part
of the Sofu Trough, An'ei and Kan'ei Seamounts in the rear-arc region, and the
Kinan escarpment. Line IBr10 is parallel to and south of line IBr9 and runs from
the Izu-Ogasawara Trench to the Kinan Seamount chain through the northern part
of Ogasawara Ridge, the northern part of the Ogasawara Trough, the northern foot
of Suiyo Seamount (volcanic front), the northern tip of the Nishinoshima Trough,
the northern foot of Kyowa Seamount in the rear-arc region, and the Kinan escarp-
ment. Both lines cross the volcanic front where it has relatively thin crust (Kodaira
et al. 2007b ).
Both cruises used arrays of eight airguns with total capacity of 12,000 cu. in.
(1,500 cu. in. each). Shot intervals were 200 m along both lines. Air pressure sent
to the airguns was 2,000 PSI.