北部北上山地東部の地質構造 : とくに地質構造帯区分に関する諸問題について

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The North Kitakami Mountains is characterized by geosynclinal sediments of Paleozoic and Mesozoic era with NW-SE trending zonal structures. During the past ten years, it has been divided into three tectonic belts ; "North Kitakami belt", "Iwaizumi belt" and "Taro belt". However, detailed geological study by Yamaguchi et al. (1979) revealed that the Taro fault was not so significant as it had been regarded as the boundary between two different tectonic belts, the "Iwaizumi belt" and "Taro belt". This paper presents the results of the investigation on the relationship between the "North Kitakami belt" and the "Iwaizumi belt" in connection with the geologic development of the North Kitakami Mountains. To make clear the stratigrphic correlation and the nature of hinterlands during deposition, the analysis of modal compositions of sandstones has been made. The sandstone of the Asanai Formation is composed mainly of finer grains of quartz, fresh plagioclase, inorganic calcite and volcanic fragments and they seem to have been derived from volcanic rocks. On the other hand, the sandstones of the other formations are composed mainly of coarser grains of potash feldspar and quartz of granitic or metamorphic rock origin and chert fragments. Stratigraphic sequence in the studied area is shown in Fig. 3. The "Kuzumaki tectonic line" which bounds the "North Kitakami belt" and the "Iwaizumi belt" was previously considered to have a significant tectonic meaning. However, the same stratigraphic unit called the Asanai Formation is distributed on both sides of the Omotogawa fault which has been regarded as a part of the "Kuzumaki tectonic line". This fact indicates that the "Kuzumaki tectonic line" is not a tectonically significant fault to separate two different tectonic belts. The geologic development of this area may be summarized as follows. In the Permian-Triassic-Jurassic(?) Period, geosynclinal sediments had been accumulated in the North Kitakami geosyncline. In the Early Cretaceous Period, volcanic activity which supplied the pyroclastics of the Harachiyama Formation (distributed in the eastern adjacent area) took place. The strata older than the Harachiyama were severely deformed by the Oshima orogenic movement in the Early Cretaceous Period. During the orogenic period, major structures such as overturned or isoclinal folds and longitudinal faults in this area were formed. Then granitic rocks intruded. The present zonal arrangement of geologic struc-tures in the North Kitakami Mountains was constructed as the result of intense deformation in this period, but are not necessarily the result of the zonal arrangement of the original basins. The Taro fault was formed in the process of the intrusion of NS trending granitic rocks in the zone II (Fig. 9) and was located on the eastern periphery of the zone II. In Kogawa-Iwaizumi district, the Omotogawa fault cuts the zonal geologic structures previously formed. From these evidence, the author comes to the conclusion that the North Kitakami Mountains is not to be divided into three tectonic belts. In the Late Cretaceous-Paleogene Period, an elongated sedimentary basin was developed along the Omotogawa fault. After the deposition of the Paleogene strata, the Omotogawa fault reactivated as a normal fault at which the southern side threw. The transverse faults with NE-SW trend were also formed after the intrusion of the granitic rocks.
東北大学の論文
1981-03-28

北部北上山地東部の地質構造 : とくに地質構造帯区分に関する諸問題について

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