八溝山地における重力すべり構造とその意義

抄録

Yamizo Mountains at the northeastern corner of Kanto Plain are known to be composed of Jurassic olistostromes. Contradictory ages have been assigned to the sediments on the basis of the fossils discovered from the exotic blocks and sheets of various ages, which have been transported into the pelitic basin plain sediments of Jurassic age.
Study on the mode of occurrence of the exotic blocks and sheets revealed that they were transported mostly by the process of gravity sliding. Small scale structures ascribed to sliding can be observed throughout the area. Successive piling of exotic sheets are not uncommon. The direction of sliding can be determined by the small scale structures with directional features. Sliding was directed to West in the northern part of the mountains, and Northwest in the southern part.
High temperature metamorphism and felsic plutonism occurred at the outer (present Pacific side) margin of the mountains, while the gravity sliding was going on in Jurassic time. The area is therefore judged to be compared to a back-arc basin (or foreland basin) which has been created behind the subduction boundary.