LATE PALEOCENE MIDDLE MIOCENE PELAGIC SEQUENCES IN THE BOSO PENINSULA, JAPAN - NEW LIGHT ON NORTHWEST PACIFIC TECTONICS

Late Paleocene-middle Miocene pelagic limestone/chert sequences from t he Mineoka Tectonic Belt, Bose Peninsula, central Japan,were biostrati graphically studied for planktic foraminifer fossils for the first tim e. The rock units are included as several isolated blocks tectonically within the ophiolitic melange together with the Mio-Pliocene Honshu a rc-derived terrigenous and Izu Arc-derived volcaniclastic materials. T he pelagic sequences are grouped into the newly proposed Kamogawa Grou p which is subdivided into the Paleocene Nishi Formation, Eocene-Oligo cene Heguri-Naka Limestone and early-middle Miocene Shirataki and Hegu ri Formations. This study of Kamogawa Group pelagic sequences throws n ew light on tectonic modeling of plate accretion to the unique trench- trench-trench (TTT)-type triple junction area off the Bose Peninsula. Different formations of the Kamogawa Group have different tectonic and paleogeographic significances for the oceanic plate with a seamount t hat was approaching the Izu and Honshu arcs during Pacific plate subdu ction, and that was accreted to the Honshu Are during the middle Mioce ne.