Collision of the izu block with central Japan during the quaternary and geological evolution of the Ashigara area

Abstract

The Ashigara area is located in the northern part of the Izu collision zone and has been suspected as a possible location of the boundary between the Eurasian and the Philippine Sea plates. In order to analyse its evolution during the collision, we have determined the age of the Ashigara Group and studied its depositional environments and its deformation. The age of the Ashigara group is early to middle Pleistocene. The lower part was deposited in a 1000–2000-m deep trough which lay between the Izu peninsula and central Japan at that time. The sedimentary facies show subsequent upward coarsening and shallowing. The beginning of the collision resulted in the uplift of the Tanzawa mountains, which are the source of the voluminous conglomerates deposited in this area. Soon after the deposition of the upper part of the Ashigara Group, this area was over-thrust by the Tunzawa mountains along the Kannawa fault and strongly folded under a NW-SE compressional stress field. Then, about 0.3 Ma ago. the direction of compression drastically changed to N-S or NE-SW when the colliding Izu peninsula was finally locked against central Japan.