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Seafloor Geology of the Basement Serpentinite
Body in the Ohmachi Seamount (Izu-Bonin
Arc) as Exhumed Parts of a Subduction Zone
Within the Philippine Sea
Hayato Ueda, Kiyoaki Niida, Tadashi Usuki, Ken-ich Hirauchi,
Martin Meschede, Ryo Miura, Yujiro Ogawa, Makoto Yuasa,
Izumi Sakamoto, Tae Chiba, Takahiro Izumino, Yoshiyuki Kuramoto,
Toyoto Azuma, Toru Takeshita, Takeshi Imayama, Yuki Miyajima,
and Takahiro Saito
Abstract The Ohmachi Seamount in the Izu-Bonin frontal arc is one of the very
rare localities where rocks from a deep subduction zone are exposed on the modern
sea floor. Submersible and dredge results revealed that the basement serpentinite
body is accompanied by small amounts of amphibole schist (six float stones less
than 20 cm in diameter were collected) with relics of the blueschist to eclogite
facies minerals, and is covered by volcanic and sedimentary sequences of Eocene
to Miocene ages. In contrast to the occurrences of well-known serpentine mud
volcanoes in the Mariana forearc, the Ohmachi Seamount serpentinite body is a
coherent mass composed dominantly of massive serpentinite in upper horizons
and of schistose serpentinite with amphibole schist in lower horizons. Both types
of serpentinites consist mainly of antigorite ± olivine, and suffered greenschist to
amphibolite grade metamorphism. Geologic structures are truncated by the base
of the Paleogene, and the serpentinite body is interpreted as a basement complex
representing a set of the hanging-wall wedge mantle (massive serpentinite) and the
subduction channel (schistose serpentinite), which trapped pieces from the foot-
wall subducted slab. The complex was exhumed probably along with one of the
back-arc spreading in the Philippine Sea plate.
Keywords Subduction channel • Antigorite schist • Eclogite • Wedge mantle
Geological map • Submersible clinometer
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