日本列島の基盤問題--島孤前縁帯海底地質学の視点から (続・日本列島の基盤)

Abstract

(1) Generalized structural sections of the Japanese fore-arc regions by Shiki and Misawa (1979, 1980, 1981) offer a few important suggestions on the basement problems of the Japanese Islands. (2) At the foot of the inner trench slope of the regions, only the upper part of the trench-fill deposits are folded and thrust faulted. In some inner places, lower part of them and underlying pelagic sediments, which were once set beneath the thrusts ("substration"), are cut by low angle thrusts forming decollement to the beneath-set (foot wall) layers. Then, in much inner places, the basaltic layer (layer 2) and even some part of the layer 3 are cut from underlying oceanic slab. These cut off sediments and rocks are added to the overlying accretional body and the continental crust, but do not follow to the down-falling oceanic slab. (3) These oceanic layers including the basement rocks, thus added to the continental crust, can rise and crop out by two sorts of mechanism. One is climbing with acidic and basic magma accompanied with the thrust movement. The Tertiary fore-arc igneous rocks which crop out at the structural high including the Shionomisaki belt and the igneous and metamorphic complex in the Kurosegawa tectonic belt in the Permian and the Triassic age, seems to be a new and an old example showing such activitities. (4) Another type of uprising of the basement rocks is usual upheaval of the continental crust and suffering erosion. Thrusting up of the rocks of oceanic origin may play role to the uprising of them, also. It occurs in the continental crust. But, this process has been regarded as "obduction" by many geologists.