Neogene and Quaternary Vulcanism in the Idu District. (Preliminary report)
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概要
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Owing to a tectonic disturbance that occurred during the Eogene, central Japan was subjected to dislocation, resulting in a great fracture -Naumann's fossa magna or Yabe's Itoigawa-Sunto Line-that severed the geological continuation of Japan between southwestern and northeastern Japan. It is highly probable that this disturbance was responsible for the vulcanism that operated at the boundary of these two large geologic units of Japan in the early Neogene. So great was the vulcanism that in many districts at the boundary, volcanic rocks and pyroclastic sediments are the chief representatives of the lower Neogene formations. Since that time vulcanism has operated in these districts repeatedly with alternate renewal and decay of activity. The Idu district occupies the Pacific side of the boundary between southwestern and northeastern Japan. The volcanic history, as inferred from the sequence of volcanic rocks of this district, is briefly summarized as follows : 1. Miocene vulcanism. The oldest vulcanism occurred during early Miocene or a little before it, with submarine eruptions of andesites. This oldest vulcanism was followed, probably some time later when the formations practically composed of the oldest andesites had more or less deformed, by submarine eruptions of liparites and dacites. This phase of vulcanism, was succeeded by a subordinate phase of activity, characterized by minor extrusions of andesites. All these Miocene volcanic rocks were subjected more or less to alterations-chloritization, silicification, sericitization, etc. The succession of these rocks implies considerable time, although its actual duration is a question we have no means of answering at present. The only precise datum is that afforded by faunal evidence, which assigns the dacitic volcanics lying some distance above the base of the complex to lower Miocene. 2. Pliocene vulcanism. Regarding vulcanism during late Miocene and early Pliocene, we are largely in the dark as only a few rocks have been identified with these ages. In the latest Pliocene age, eruptions of dacites and andesites took place. They were again largely submarine eruptions. 3. Pleistocene vulcanism. At the close of the Tertiary, or in the beginning of the Pleistocene, Idu peninsula was subjected to crustal movement, generally in the sense of uplift. It was not until some time after the movement that raised so much of the Tertiary sea-floor into land had taken place, that Pleistocene vulcanism became vigorous. It was chiefly concentrated in the northern part of the peninsula ; while in the southern part, Tertiary vulcanism was succeeded by Pleistocene and Recent relative quiescence. The andesitic volcanoes Amagi, Usami, Taga and Yugawara, together with other yet uriinvestigated volcanoes in the northwestern part of the peninsula, are, as also the dacitic masses in the vicinity of Atami, products of Pleistocene vulcanism. The basaltic Oomuroyama group represents the last vulcanism, which probably occurred some time later, at the time when the early Pleistocene volcanoes Usami, Taga, etc., were subjected to more or less dislocation and dissection. 4. Recent vulcanism. In the Idu peninsula, none of the latest volcanoes is known to have erupted in historic times ; while the numerous hot-springs there are displaying post-volcanic thermal activity. In the Idu Sitito, on the contrary, liparitic and basaltic vulcanism was active during historic times. An examination of the materials that were erupted during the whole course of the vulcanism and so fully recorded in the geology of the Idu district, reveals the fact that andesitic and liparitic or dacitic rocks have played their role in every age froin lower Miocene up to Recent. Thus, a discrimination of the two lineages-andesitic and liparitic or dacitic-is essential to a proper understanding of the vulcanism of the district. As to the genetical relationship between the two lineages, very little can be said now. If we assume in the meantime that the rocks of these two lineages were derived from a common stockmagma, then their differentiation from this magma must be explained by one or other of two alternatives : (1) Either the partial magmas that separated out during early Tertiary had remained throughout succeeding ages as available sources of the andesitic rocks on the one hand and of the liparitic or dacitic rocks on the other, and that further differentiation had gone on within them ; or (2) that differentiation to an advanced stage had proceeded repeatedly along definite lines at wide intervals of time. Of these two alternatives, the writer is inclined to accept the former, agreeing with S. Tsuboi in the opinion that the basaltic and liparitic rocks which erupted during Recent time in the Idu Sitito territory are descendants respectively of the andesitic and liparitic or dacitic rocks that erupted during the Tertiary and early Pleistocene in the Idu peninsula. Examples presenting close analogies with the volcanic sequence of the Idu district could be cited from adjoining districts : Sinano, Asigara, and Oiso. The distribution of the sites of vents of liparitic and dacitic rocks shows distinct zones parallel with each other in the direction from N. E. to S. W. The directions of these zones are very remarkable in contrast with the Huzi Volcanic Zone, which runs in the N. N. W.- S. S. E. direction, and in which direction the Quaternary andesitic and basaltic volcanoes of the district are aligned. The zones of these two directions-N. E.-S. W. and N. N. W.-S. S. E.-seem to represent the underground fracture lines connected with available sources, respectively, of the acidic volcanic rocks and the basic volcanic rocks.
- 東京帝国大学地震研究所,Earthquake Research Institute, Tokyo Imperial University,地震研究所の論文
- 1932-02-22
東京帝国大学地震研究所,Earthquake Research Institute, Tokyo Imperial University,地震研究所 | 論文
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