関東山地三波川帯・秩父帯の地質構造の研究

Study on the Geological Structure of the Sambagawa-Chichibu Belts in the Kanto Mountains.

TOKUDA Mitsuru

廣島大學地學研究報告

Geological report of the Hiroshima University

The geological and rock structures of the Sambagawa—Chichibu belt have been studied in three districts of the Kanto Mountains, Kanra-cho district (Gunma Prefecture), Kama-buse-Nagatoro district and Tokigawa district (Saitama prefecture), central Japan. The study has resulted in the clarification of fundamental characteristics of geological structure, history of structural development, relationship between geological structure and thermal structure which was produced during the highest temperature stage of the Sambagawa metamorphism, and time-relationshp between tectonism and metamorphism in the Sambagawa-Chichibu belt of the Kanto Mountains.
The Sambagawa-Chichibu belt of the Kanto Mountains is divided into three metamorphic zones, chlorite zone, garnet zone and biotite zone, with reference to appearance of garnet and biotite in the pelitic schists as metamorphic minerals which were produced by the Sambagawa metamorphism of the highest temperature stage. Albite appears as porphyroblast in biotite zone but not in garnet zone. The zone bundaries are generally parallel or subparallel to the lithologic layering.
From analysis of rock structures it has been clarified that the tectonic processes in the Sambagawa belt occurred at least in 5 stages, 1st stage for formation of S0 (S0')-schistosity, 2nd stage for formation of S1 (S1')-schistosity, 3rd stage for formation of S2 (S2')-cleavage, 4th stage for formation of S3 (S3')-cleavage and 5th stage for formatin of F4 (F4')-folds.
Generally, the Sambagawa schists of the Kanto Mountains have distinct schistosity of single set. The schistosity corresponds to S1 (S1')-schistosity. The major structure of the Sambagawa schists in the western half of Sambagawa belt of the Kanto Mountains is a northward vergence recumbent fold (Kanra recumbent fold) with nappes. The S1 (S1')-schistosity corresponds to the axial plane schistosity (= differentiated crenulation cleavage) of the Kanra recumbent fold. The core of Kanra recumbent fold consists of Sambagawa schists of higher grade than its mantle does. Therefore, it has been concluded that the major geological structure and rock structure [S1 (S1')-schistosity] of the Sambagawa belt in the Kanto Mountains were produced by the tectonism to collapse of the Sambagawa metamorphic field and to be related to uplifting of the Sambagawa schists, and, judging from northward vergence of the Kanra recumbent fold, that the uplifting occurred with northward movement sense as nappe. The S0 (S0')•schistosity is commonly found as form surfaces of rootless intrafolial folds in the S1 (S1')-schistosity, and it appears to have been produced by the tectonism during the highest temperature stage of the Sambagawa meta-morphism.
The S2 (S2')-cleavage is referred to the type of crenulation cleavage-differentiated crenulation cleavage. The cleavage develops as axial plane cleavage of folds with southward vergence, indicating that the uprifting of the Sambagawa schists occurred with southward movement sense during the 3rd stage of tectonism. The folds in the eastern half of the Sambagawa belt of the Kanto Mountains develop as recumbent folds of large-scale associating shear zone, while they in the western half develop only in micro-minor scales.
The S3 (S3')-cleavage is referred to the type of crenulation cleavage. The cleavage develops as axial plane cleavage of upright gentle-folds of large-scale with axes of E-W trend. The up-right folding occurred during the last stage of the Sambagawa metamorphism. The F4 (F4')-folds are referred to the type of conjugate kink fold. They appear to be associated with the formation of upright gentle-folds of large-scale with axes of N-S trend (e.g. Yasudo syncline) which were produced during Tertiary age.
In the Kanto Mountains, generally, the Mikabu greenstones and associated schists overlie as nappe the Sambagawa schists (Sambagawa schist proper). They are not involved in the Kanra recumbent fold. The pelitic schists of the Chichibu belt in the western half of the Kanto Mountains which just overlie the Mikabu greenstones are commonly of Jurassic age. While, in the pelitic schists of the Chichibu belt of the Tokigawa district which just overlie the Mikabu green-stones, have been found radioralian fossils of Permian age. The Permian pelitic schists appear to be correlated with the member of the Kurosegawa zone. In the Kanto Mountains it could be said that the Kurosegawa zone overlies as nappe the Sambagawa belt.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676