北部フォッサ・マグナ地域の先第三系基盤岩について
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概要
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By the recent field survey it has been clarified that the Mesozoic strata of middle to upper Triassic, lower Jurassic and lower Cretaceous ages are more widely developed than ever known in the western side of the Katashina Tectonic Line (Zone). The Mesozoic formations are intimately associated with gabbroic or diabasic rocks. This fact suggests that the formations were the nepton formed in a eugeosynclinal basin. Similar gabbroic to ultrabasic rocks as pre-Tertiary basement have been found in the northern Fossa Magna Region through the investigation of volcanic ash erupted from the Yakeyama volcano in 1974 activity, deep drillings near Kashiwazaki City, and the studies of xenoliths included in the Pliocene volcanic lavas. The Joetsu-Ashio belts are displaced about 100 km against the corresponding belts developed in the west side of the Itoigawa-Shizuoka Tectonic Line, and display a distinct discontinuity against the zonal arrangement in the Kwanto Mountainland. Geometrically this discontinuity may be explained by a hypothesis that the Joetsu-Ashio belts were rotated counterclockwise about the point X, near the Nihonkoku gneiss area, from the original position. By the clockwise rotation about the point X by 20 degrees, 1) the Tonegawa Tectonic Line coincides with the northern part of the Kwanto Tectonic Line. It means that the both tectonic lines were originally the same; 2) the Joetsu metamorphic belt becomes continuous with the Sangun metamorphic belt and extends northward to Sado Island; 3) the Ashio belt coincides with the Mino belt; 4) the Tsukuba Ryoke-type metamorphics occupy the position near the northeastern side of the Kwanto Mountainland. Consequently the general zonal arrangement in the western side region of the Itoigawa-Shizuoka Tectonic Line as a whole extends to northeastern direction. The displacement of the Joetsu-Ashio belts by counterclockwise rotation from the original position might result in the opening of the upper crust in the Fossa Magna Region, and the opened districts might be filled by the basic igneous intrusions and the sediments of eugeosynclinal characters. The rotational displacement might have taken place first in the Jurassic age, and then accelerated during the late Cretaceous to Paleogene epochs.
- 日本地質学会の論文
- 1976-12-30
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