COLLISION OF THE IZU-BONIN ARC WITH CENTRAL HONSHU: CENOZOIC TECTONICS OF THE FOSSA MAGNA, JAPAN

Abstract

The Izu-Bonin arc joins with Honshu at the Fossa Magna where pre-Miocene terrains bend in a cusp form. The Miocene terrains in the region also have a northward-convex structure north of the Izu Peninsula. Moreover, highly compressive deformation, Quaternary strong uplift and anomalous trajectories of crustal stress axes also characterize this region.These features of central Honshu at the junction are explained well by assuming that a north-south trending plate boundary has been located off central Honshu since the late Cretaceous. The bend of the terrains was formed for the most part in the early Tertiary by buoyant subduction of aseismic ridges lying along the north-south trending transform fault.The Izu-Bonin arc, which was developed along this transform fault, has been dragged northward by oblique subduction of the Pacific plate and underwent subduction beneath central Honshu during the late Tertiary. In the early Quaternary, the Izu Block (the Izu Peninsula) of the Izu-Bonin arc collided with central Honshu and is pushing it north-northwestward. It is very likely that the triple junction off central Honshu has been located at its present position relative to Honshu since the late Mesozoic.