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

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.