日本のアルプス造山とグリーンタフ時代の諸問題(<特集>日本列島の構造発達史-とくにグリン・タフ時代を中心として(その1)-)
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概要
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The Alpine orogeny in the Shimanto Province, that has never been the matter of concern among Japanese geologists, is discussed. First, an attempt is made to define the term orogeny. An orogeny, when used in a strict sense, means a qualitative change of a series of geosynclinal sediments. General and complete folding represents a change in internal morphology, and is the most essential and necessary transformation. Regional metamorphism is a change in the state of crystallinity, while an acidic plutonism, even though it would not be a change of deposits themselves, means a change in chemical composition of the area as a whole. The three phenomena, very important and essential for an orogeny, are called the "three elements of orogeny" by the writer. This definition offers a reliable criterion for discrimination of various "orogenic movements." In the second place, the orogeny in the Shimanto Province is examined. The Shimanto Province, occupying the Pacific coastal terrain of Southwest Japan, extends for more than 1000 km, and is no less than 100 km in width. It is defined as the area made up almost exclusively of the Shimantogawa Group, a series of geosynclinal sediments of about 10,000 m in thickness. Several fossils hitherto found show an age ranging from late Jurassic to Oligocene. Late Triassic and early Jurassic deposits may underlie the upper Jurassic. The Shimantogawa Group is folded tightly, i.e. the folding is complete and general. The zone of regional metamorphic rocks, 10 to 30 km wide, never been attended before, can be traced along the northern periphery of the Province. Chlorite, and epidote in some areas, are the characteristics among the metamorphic minerals. Acidic plutonism is poorly developed, and is represented by several types of small, discordant, postkinematic masses. No essential synkinematic intrusive is known. From the above lines, it is obvious that the Shimantogawa Group went through an orogeny. The concrete phenomena of the orogeny are the general and complete folding and the regional metamorphism of the geosynclinal deposits and the acidic plutonism in this Province, of which the latter two are rather poorly developed. Such an orogeny, in which one or two of the three elements are meagre, should be called an incomplete orogeny, while that with full development of them a complete orogeny. The orogeny in the Shimanto Province corresponds with the Hiroshima Movement in the Honshu Province to the north, which was discussed and designated by the writer previously (YAMASHITA, 1957). The age of the orogeny, at least that of the folding, is late Oligocene or early Miocene, and the developmental stages of the Shimantogawa Group or of the Shimanto geosyncline are roughly contemporaneous with those of the Hidaka geosyncline of the Hokkaido Province (HUNAHASHI, 1957) as well as those of the Alpine geosyncline in Europe. Lastly, the Tertiary formations in Japan are reviewed, on the basis of the above considerations. They are, for the most part, represented by the Neogene rocks, and are developed rather extensively in the Honshu Province, i.e. outside the Shimanto Province. The mode of development of the Neogene formations reminds one of the Tertiary rocks in the Alpine region of Europe. But the resemblances should not be exaggerated, for there are also several important differences. Among the differences, the most significant is the fact that they contain a large amount of volcanic rocks. In the Alpine region, the accumulation of Molasse and the various crustal movements that followed seem to be considered as necessary results of the Alpine orogeny, or they are thought to represent indispensable parts of the magnificient Alpine orogenic cycle. But in Japan, Neogene formations can not be regarded merely as byproducts of the preceding orogeny. The Neogene formations of Japan are best developed in the socalled "Green Tuff Region" along the inner side of the Honshu Province, where the most vigorous volcanisms are recorded. The fact is taken seriously that the subsidence of the Neogene basins occured in a form of block movement. The violent submarine volcanism and the subsidence of the area, that followed the outbreak of deep fissures to conduct basic magama upward suggest that the formation of Neogene deposits is more likened to that in the orthogeosyncline of older ages. These characteristics of the Neogene deposits of Japan present a crucial problem whether they represent the final stage of the Alpine orogeny mentioned above or the initial stage of a forthcoming orogeny. An attempt is made, in the latter part of this paper, to classify the Neogene history both in space and time, so as to pursue the problem along the line just noted.
- 地学団体研究会の論文
- 1960-09-25