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link subduction zone tectonics in the ocean and those recorded in on-land orogenic
belts. Serpentinite with HP metamorphic rocks in the Ohmachi Seamount still
remains in the intra-oceanic setting, and probably free from extensive structural
modification by subsequent accretionary or collision tectonics. They are thus
expected to show more primary structural and metamorphic features reflecting the
dynamics within a subduction channel. In addition, metamorphic rocks in the
Ohmachi Seamount contain relics of the eclogite facies metamorphism, which
occurred as deep as 70-80 km (600-700°C, 2.0-2.5 GPa: Ueda et al.
2004, 2005
),
as the only known locality of eclogite facies rocks of non-collisional subduction
setting in the modern ocean. The peak metamorphic depths are much greater than
the incipient blueschist facies clasts in the forearc serpentinite seamounts (~20 km:
Maekawa et al.
1992, 1993, 1995
). The serpentinite body and accompanied HP
metamorphic rocks in the Ohmachi Seamount will provide petrologic and structural
information of deeper portions of a subduction zone.
This paper aims to describe and summarize results of submersible dives
(JAMSTEC YK01-04 and YK08-05 cruises by R/V
Yokosuka
and submersible
Shinkai 6500
) and dredges (ORI/JAMSTEC KT04-28 cruise by R/V
Tansei-maru
)
on the serpentinite body of the Ohmachi Seamount, with reviews of earlier
researches, focusing on its geological structures. Accumulation of these data
enables us to construct an ocean-bottom geologic map, here newly presented, with
a resolution approaching to on-land geology. Based on these results, the signifi-
cance of the geologic structure related to subduction zone dynamics is discussed.
2
Geologic Setting and Bathymetry
The Ohmachi Seamount is located at 29°00-30'N and 140°35-55'E in the middle
part of the Izu-Bonin arc of the Philippine Sea plate (Fig.
1
) about 20 km east of
the Quaternary volcanic front and 180 km west of the trench. The seamount is part
of a structural high elongated along north-south direction (Fig.
2
) representing
the rear-side termination of the forearc basin (Ogasawara Trough) in the east, and
the trench-ward bank of the Quaternary intra-arc rift basin (Nishinoshima rift) in the
west. Moreover, the seamount is in prolongation of the NNE-SSW-trending
Sofugan tectonic line (STL) as the largest fracture zone separating the Miocene
back-arc basin into the Shikoku and Parece Vela basins (Fig.
1
). The Izu-Bonin arc
itself is also separated by the STL (Yuasa
1985, 1992
): the arc crust in the north is
more matured with 20-30 km thickness as opposed to the south with less than
20 km thickness (Kodaira et al.
2007
). The STL comprises more than 200 km-long
escarpment facing the southeast with vertical throws of 1,000-1,500 m, on which
basaltic lavas, dikes, and hyaloclastites are exposed (Sakamoto et al.
1997, 1998
).
Both the northwest and southeast blocks bounded by STL tilted northwestward,
resulting in the southeast-facing escarpment, and in a narrow depression
(Nishinoshima Trough) directly in the neighborhood to the southeast of the STL.
The crust in the Nishinoshima Trough, which terminates at the Ohmachi Seamount,
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