Neotectonics of the South Fossa Magna in Central Japan Deduced from Active Faults

Abstract

The South Fossa Magna in central Japan is thought to be a collisional zone between the Philippine Sea plate and the Asia plate. In this region, intense tectonic movements such as folding, faulting and other crustal movements have been proceeding since the early Quaternary. Several active faults which have an extremely high vertical slip rate are distributed along the north end of the Izu bar.
Three tectonic sub-regions can be distinguished on the basis of the sense of fault movement and other tectonic features. That is, the area of the southwestern foot of Mt. Fuji, the area of the south end of the Tanzawa Mts. and the Ashigara-Oiso area. The first area is located in the northern extension of the Suruga trough. This area is divided into three blocks by N-S trending active faults. The easternmost block shows distinct subsidence and the others upheave with a high rate. In the second area, an E-W trending high angle reverse fault bounds the south end of the Tanzawa Mts. The topographic feature of this fault is indistinct. The third area is located in the northwestern extension of the Sagami trough. The NW trending Kozu-Matsuda fault which has a sence of reverse and right lateral slip divides the area into the Oiso hills and the Ashigara plain. The former shows a distinct upheaval which occurred in recent geological ages and the latter suggests the decrease of its subsidence rate.
Studies of the crustal movements in each sub-region reveal the following common tectonic features. That is, each crustal block distant from the Izu bar has changed its movement style from the phase of intense subsidence to that of upheaval through the transitional phase. A newly created fault or a subsidence zone has always occurred in a block closer to the Izu bar. As a consequence, the latest tectonic zone in the South Fossa Magna seems to have migrated from a distant position closer to the Izu bar. Fig. 3 shows a concept of the tectonic process across the tectonic zone in the South Fossa Magna. This tectonic process is the process of accretion of the sediment to Honshu in the landward region of the Suruga and Sagami troughs. The active faults in this region are considered to be neither interplate faults nor branches from the main boundary fault but the imbricated thrust faults in the accretional sediments.