"早池峰構造帯" の地質学的研究
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概要
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The "Hayachine Tectonic Belt" borders the North Kitakami and South Kitakami Belts of the Kitakami Massif, and is characterized by basic and ultrabasic rocks extending from Morioka to Kamaishi. The purpose of this study is to elucidate the geological development of the Hayachine Tectonic Belt by stratigraphic and petrological investigation in the Kawai and Kamaishi areas, Iwate Prefecture, Northeast Honshu, Japan. The Shinden Fault divides the Kawai area into two parts. The Paleozoic formations including the Gabbro-Ultrabasic Rock Complex, Kogura and Yakushigawa Formations are distributed in the western part. The Mesozoic Tassobeguchi and Kirinai Formations, which are exposed only in the eastern part, represent the so-called North Kitakami type sediments characterized by shale and chert. The Kamaishi area is divided into three parts by the Sawahigawa-Omatsu and Obama-Matsukura Faults. The central part is a horst consisting of the Paleozoic formations which are in fault contact with the Mesozoic formations of the eastern and western parts. The Paleozoic formations of the central part comprise the Gab-bro-Ultrabasic Rock Complex, Senjogataki, Kogawa and Kuribayashi Formations. Rocks of these formations except the Gabbro-Ultrabasic Rock Complex are regarded as the "South Kitakami type" epicontinental sediments, which are well exposed also on the west of the horst. On the east of the horst, the North Kitakami type sediments composing the Kamaishi and Irinai Formation are distributed widely. The Shinden, Shiromiyama and Kozuchigawa Faults in the Kawai area extend into the Kamaishi area probably to the Sawahigawa-Omatsu, Obama-Matsukura and Sarusawa-Mizu-umigawa Faults, respectively. Thus connected Shinden-Sawahigawa-Omatsu Fault separates the South Kitakami type sediments on the west from the North Kitakami type sediments on the east. The sedimentary rocks in both areas are underlain by the Gabbro-Ultrabasic Rock Complex, which is a chaotic mixture of basic and ultrabasic rocks. Complicated structure of the complex indicates that tectonic disturbance occurred repeatedly in the magma chamber during and after crystallization. Detailed geological mapping revealed that the original rock sequence in the Gabbro-Ultrabasic Rock Complex was ultrabasic rocks, coarse-grained gabbro, dolerite and basalt, in ascending order, which is identifiable as ophiolite sequence. Petrologically, all constituents are cogenetic and chemical composition of dolerite and basalt corresponds to that of abyssal tholeiite. On these facts, the author concludes that the Gabbro-Ultrabasic Rock Complex was formed in a rift zone in thin continental or oceanic crust region. During the Devonian and Early Carboniferous, the Hayachine Tectonic Belts was a site of rift zone and emplacement of the Gabbro-Ultrabasic Rock Complex occurred. Subsequent subsidence of the zone allowed deposition of the Carboniferous and Lower Permian sedimentary rocks. Before deposition of the Upper Permian, this area was subjected to tectonic movement together with the South Kitakami Belt. During the Late Permian, Triassic and Jurassic, the northern and eastern parts of the Hayachine Tectonic Belt were submerged and thick sediments were deposited. Intense volcanism began in the Early Cretaceous and andesitic and basaltic rocks were accumulated there. All formations ranging in age from the Carboniferous of Early Cretaceous were intensely folded by the Oshima Orogenic Movement in the later stage of the Early Cretaceous. Differential subsidence of the Hayachine Tectonic Belt was accelerated by reactivation of the Shinden-Sawahigawa-Omatsu Fault owing to the orogeny.
- 東北大学の論文
- 1983-02-28