2015 年 124 巻 5 号 p. 847-876
Interpretation of seismic reflection surveys shot by Japan Oil, Gas and Metals National Corporation (JOGMEC), Japan National Oil Corporation (JNOC) and Geological Survey of Japan in the Philippine Sea controlled by Ocean Drilling Program (ODP) as well as Deep Sea Drilling Project (DSDP) cores and ocean bottom samplings reveal the basin framework of the Izu–Ogasawara Arc. Seven seismic horizons from the acoustic basement to the seabed are interpreted, and isochron maps of six seismic units from Eocene to Quaternary are mapped to study the history of basin evolution. The Izu–Ogasawara Forearc Basin is located east of the volcanic front, is about 50 km in width and 2 to 3.5 sec. in Two Way Time (TWT) thickness. It extends more than 1000 km from east of the Ioto to off the Boso Peninsula, and subducts along the Sagami Trough. Paleogene sediments develop only in the forearc side. The Paleogene basin is bounded by NNE–SSW and NNW–SSE faults, has a zigzag pattern, elongates throughout the Izu–Ogasawara Forearc, and consists of echelon horsts and half grabens. Horsts are composed of volcanic rocks of Eocene to Oligocene age, and grabens are filled with turbidite sediments of the same age. Divergent seismic reflection patterns indicate that the turbidite sediments were deposited synchronously with the tilting and subsiding of the half grabens during the syn-rift period of the basin that was initiated by the rifting of the Proto-Izu-Ogasawara Arc. The latest Oligocene to Early Miocene sediments are mainly composed of calcareous nannofossil rich marls, which were deposited both on the Paleogene forearc basin as post-rift sediments, and in the backarc basin that was initiated when the Shikoku Basin was opened by the rifting of the Proto Izu–Ogasawara Arc. Well-stratified and flat seismic reflection indicates that the marls were deposited in a calm environment when volcanic activity ceased. The Izu–Ogasawara Arc re-started volcanic activity in the Middle to Late Miocene at the Nishi-Shichito Ridge in the backarc side. Sediments of this period are thick around the Nishi-Shichito Ridge. Seismic reflection is strong, discontinuous, and wavy near volcanoes, and weak, continuous, and parallel away from the arc. The former facies is mainly composed of coarse volcaniclastics, and the latter of finer clastics. In the Pliocene and Quaternary, the depocenter shifted from west to east as the volcanic front shifted.