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this region, not an optimal direction for the ENE-WSW seismic lines to reveal
high-angle faults, evidence for crustal extension should nevertheless be apparent in
the
Vp
models and reflection images. In this section, we also discuss our interpretation
of the relative timing of rift-related deformation.
Rifted regions of the Izu-Ogasawara arc that are underlain by high-velocity lower
crust show clear Moho reflections. High-velocity lower crust may form by under-
plating mafic materials or it may reflect the presence of serpentinized mantle (White
et al.
2003
). Mantle rocks undergo serpentinization by reaction with water at low
temperatures, and serpentinization is likely to be gradational with depth. If this were
the case, the Moho should not be a sharp reflector. In fact, continental margins where
serpentinization of the mantle is confirmed by drilling (e.g., ODP Leg 173 Shipboard
Scientific Party
1998
) have poorly defined Mohos (e.g., Dean et al.
2000
). High-
velocity lower crust in the Izu-Ogasawara arc is not likely to be serpentinite because
it is underlain by a sharp Moho. We conclude that high-velocity lower crust in the
Izu-Ogasawara arc has originated from underplating by basaltic magmas.
Crustal rifting often creates normal faults and causes tilting of shallow crustal
blocks. We confirmed such deformation in time-migrated reflection images of data
from the MCS surveys (Figs.
7
and
8
). On line IBr9, we identified deformed areas
in front of the volcanic front, immediately behind the volcanic front, and in the Sofu
Trough (Fig.
7
). The area in front of the volcanic front shows clear normal faults that
extend to the seafloor, indicating recent faulting (CDP 33000-34400). The area
behind the volcanic front is covered by thin sediments, but some of the normal faults
extend to the seafloor (CDP 29500-31700), again showing that the faulting is recent.
The Sofu Trough also has normal faults and tilted crustal blocks and is filled by thick
sediments. The eastern part has a thickness of ~0.5 s of sediments (~1 km, CDP
23000-26500). However, the western Sofu Trough has thicker sediments than the
eastern part (~1 s; CDP 21400-22700); thus, sedimentation may have started in the
west first. The oldest sediments in the deepest part of the western Sofu Trough show
a convex-upward shape, and the normal faults in the basin do not appear to cut the
old sediments. This may indicate that rifting in this region was coeval with deposi-
tion of the older sediments. These images suggest that rifting was activated in east-
ward order from the western Sofu Trough, the eastern Sofu Trough, the bathymetric
low area behind the volcanic front, and the fore-arc in front of the volcanic front.
The reflection images along line IBr10 have similar characteristics (Fig.
8
).
Normal faults are identified just in front of the volcanic front (CDP 22200-22700),
the bathymetric low corresponding to the northernmost Nishinoshima Trough
immediately behind the volcanic front (CDP 23000-23700; CDP 25000-26300),
and the Sofu Trough (CDP 29500-30200; CDP 31600-35100). The Sofu Trough is
filled by thick sediments, thickening toward the west. The bathymetric low along
the volcanic front is currently active on its eastern side in proximity to the Sofugan
tectonic line. These relationships are similar to those seen along line IBr9.
Kodaira et al. (
2008
) suggested that rifting occurred between the rear-arc and
current volcanic front in Oligocene time. In the central Izu-Ogasawara arc, we iden-
tified rifting in two places in the Sofu Trough. Another rift we identified behind the
volcanic front corresponds to the Sumisu rift to the north. Another rift we identified
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