The Izu-Ogasawara (Bonin) arc is along the eastern boundary of the Philippine Sea plate, where the Pacific plate is subducted from east. It is one of the typical intra-oceanic island arcs, and consists of the following units from the oceanic side; the “forearc ophiolite”, forearc made of volcanic island arc of old age, active island arc volcanics, rift (“back-arc depression”), and back arc basin with several inactive volcanics. On land in the southern Kanto to the Southern Fossa Magna regions and surroundings, there are many lines of evidence for the ancient relicts of the Paleo-Izu materials and other oceanic materials, which have been accreted by various ways since Oligocene.
Before the accretion of island arc materials, ophiolitic rocks, composed chiefly of MORB with alkali basalt and harzburgite, were jammed within the terrigenous accretionary complex in the Shimanto belt. These rocks might be the obducted materials from the Pacific side. After that abundant volcanic island arc suites have been accreted to Honshu, sometimes mixed with Honshu-derived terriginous materials, sometimes island arc materials only. The Fossa Magna might be formed between the stages of ophiolite obduction and island arc com-mencement, around 17 MaBP. Several accretionary prisms composed of the Izu forearc mate-rials are developed in the present Miura-Boso Peninsulas. The volcanic arc materials, consisting remarkably of ash or lapilli fall deposits near the volcanic front, were offscraped and accreted to the Honshu arc side on the Sagami Trough. This reflecs a particular tectonics around the trench-trench-trench triple junction off the Boso Peninsula, which arrived at around the present position at about 17 MaBP.