<国立科博専報>北海道中軸部の地史解明のための諸資料と問題点
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In this parper the author discusses about the plutonism and metamorphism of the Hidaka metamorphic belt and the period of these activities, the stratigraphic division and geologic age of the strata older than the Yezo group in the axial zone of Hokkaido, the green schists in the upper reaches of the Tokachi River, and the Mesozoic to Paleozoic formations in southern Hokkaido. The axial zone of Hokkaido is marked with two large tectonic zones running parallel in the north-south direction. The one on the east is called the Hidaka belt, centering on the Hidaka metamorphic belt which is characterized by volcanic and plutonic activities. The one on the west is known as the Kamuikotan belt, being composed of the Kamuikotan metamorphic rocks of chiefly dynamo-metamorphic origin. It is often difficult to define the boundary between the two belts. As to the structural significance of the Toyokoro-Kitami belt, the views are divergent. The age of the green schists in the upper reaches of the Tokachi River remains undetermined. Some geologists hold a view that the Hidaka and Kamuikotan metamorphic belts were formed as a pair, but these belts in Saghalien Island are apparently independent of each other. So, further investigations are needed for the belts in Hokkaido. Formation of the Hidaka metamorphic belt began with the forerunning intrusive activity of diabase which gave rise to the deposition of the Sorachi group. However, opinion is divided on the process of formation of fissures that provided a stage for intrusion and effusion of the diabase. Little is known about the status of the basement upon which the Hidaka geosyncline, or the Yezo geosyncline in a broad sense, was developed. Difficulties are experienced in dividing, either lithologically or stratigraphically, the strata developed in the Hidaka belt. Divisions hitherto proposed by various investigators are more or less problematical in some points. In some places the diabase, whose activity is supposed to have an intimate relation with the Sorachi group, is found to cut across the plane of exfoliation of slate that was formed in the portion penetrated by the diabase. Therefore, whether the intrusion of the diabase was contemporaneous with the phyllitization is questionable. The age of the Intra-Sorachi Orogeny which took place during the deposition of the Sorachi group may be late Jurassic or early Cretaceous, although the scale and the character of the movement are not very clear. The metamorphism in the Kamuikotan metamorphic belt may have begun simultaneously with that of the Hidaka metamorphic belt, but it was completed in the period of the Intra-Yezo Orogeny or in the Epi-Sorachi Orogeny at the earliest. In the Hidaka metamorphic belt, however, activity of gabbro and subsequent migmatization took place under a condition of stress which changed free afterwards, followed by intrusion of gabbro-amphibolite and granite. The uprise of the Hidaka metamorphic belt involves many problems. The thrust fault, which marks the western border of the Hidaka metamorphic belt, pushed and folded the Cretaceous system between the Hidaka and the Kamuikotan metamorphic belts. The gabbro-amphibolite exposed along the Cretaceous system is believed to be closely related to the thrust fault. This is the latest gabbroic activity in the Hidaka metamorphic belt, and pebbles of the gabbro-amphibolite are found in the Tertiary formation of the Urahoro coal field. Accordingly, the thrust fault must have been most active in the Epi-Cretaceous orogenic time.
- 国立科学博物館の論文
- 1971-11-30