東北日本島弧‐海こう系の地形と地質

Abstract

The Japan Trench and the northeast Japan arc form a single arc-trench system. The topographic features and the geological history of this arc-trench system are briefly summarized. Distribution of the morphotectonic elements across the system east to west is as follows: An outer swell on the deep sea floor, a group of horsts and grabens bound by normal faults caused by the bending of the Pacific Plate, a deep, flat trench axis covered by thick sediments of both pelagic and hemipelagic nature, a trench inner slope partly exposing Cretaceous mudstone, a gradually uplifting and ponding bench, deep sea terraces covered by thick terrigenous sediments, a non-volcanic outer arc (Kitakami Massif and Abukuma plateau), a volcanic chain (volcanic front), an intra-arc depression, and a marginal sea (Japan Sea). These elements are distributed parallel along the arc. Geophysical parameters are also traceable along the arc. Every geological and geophysical feature around the Japan Trench and the Tohoku Arc show this polarity from the trench axis westward.In the Paleogene, around DSDP Sites 438 and 439 an ancient landmass is indicated by samples and seismic profilers where short-term calc-alkaline volcanic activity took place, boulders of the volcanic rocks then being transported to the site. The Japan Trench forearc area began to subside during Miocene times and diatomaceous hemipelagic mud and the volcanic ashes accumulated in the forearc basin of the Japan Trench area.Since Pliocene time, the Japan Trench area and the Tohoku Arc have gradually been uplifting together with the intrusion of the Tertiary granites.At about 5 Ma a drastic change of the depositional environment in this region resulted from the change of the mode of subduction of the Pacific Plate into the Japan Trench from Mariana-type since 15 Ma, to Chilean-type subduction from 5 Ma to the present.