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On the basis of the up-to-date knowledge of the geological history in and around Japan and other areas in the circum-Pacific region and also the classical areas in Eurasia, the writer has reached the ...following conclusions about the age of the geosynclinal volcanism: (1) A new geosynclinal belt appears, or is promoted to develop, when another, adjoining belt is subject to tectonic movements. (2) The initial magmatism in the new geosyncline is usually coeval with the orogenesis and granitic intrusion in the adjoining mobile belt. Even in the case when the initial magmatism is somewhat delayed from the birth of geosyncline, the intense submarine volcanism is coeval with the tectonic movements within and/or outside the geosyncline. (3) As a tectonic movement is intensified periodically, geosynclinal magmatism takes place periodically. The major period in a global scale is about 60-70 million years but may be shortened in the Cainozoic and expanded before the mid-Palaeozoic. (4) A large scale plateau basalt is flooded over the intracontinental area which is somewhat separated from the orogenic belt. This occurs almost simultaneously with or just after an orogenic period. The flood on the continent seems to take place when the submarine basaltic flows are not so great as the normal case in the neighboured geosynclinal area. In other words the flood of the plateau basalt and the geosynclinal basaltic volcanism seem to be complementary. Several problems are pointed out to be solved in the future.続きを見る
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