著者

吉野 博厚

出版者

地学団体研究会

雑誌

地球科學 (ISSN:03666611)

巻号頁・発行日

vol.36, no.3, pp.128-149, 1982-05-25

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Neogene marine sediments and volcanics are widely distributed in the so called "North Fossa Magna" region. The Suwa District, located in the southwest corner of the region, contains the Median Tectonic Line and the Itoigawa-Shizuoka Tectonic Line, which have been active since the Neogene. In terms of geological structure, the Suwa District is divided into two geologically different areas by the Itoigawa-Shizuoka Tectonic Line. 1) The stratigraphy of the Miocene series in the Suwa District is as follows: The Miocene series in the Moriya Area, located on the south side of the Lake Suwa, is called the Moriya subgroup. This subgroup is divided into the lower Moriya formation, which consists of clastic sediments with basal conglomerate, and the upper Gongenzawa formation, which consists of volcanics. The Miocene series in the Yokokawagawa Area, located on the north side of the lake, is known as the Takabochi subgroup. This subgroup is divided into the lower Yokokawagawa formation, which consists of coarse clastic sediments with andesite, and the upper Takabocchi formation, which consists of fine clastic sediments with basalt. The Gongenzawa formation can be correlated with the Yokokawagawa formation, and both can be correlated with the Uchimura formation in the Uchimura region. The Takabocch formation can be correlated with the Kokuzo basalt in the Uchimura region. The intrusions of Tertiary Granitoids, which consist of granodiorite, quartzdiorite, porphyrite, dolerite and rhyolite, are widely developed in both areas. 2) On the basis of the assemblage of secondary minerals identified in the volcanic rocks, the alteration and/or metamorphic area can be divided into the following six zones: I: Mixed layer mineral-Saponite Zone II: Chlorite-Mixed lay V: Actinolite-Chlorite Zone VI: Biotite-Actinolite Zone The zonation and distribution indicate that the alteration and/or metamorphism resulted from thermal or hydrothermal effects on the Tertiary Granitoids. 3) The geologic development of the area, especilly in the Miocene is mentioned below: (1) The Miocene basin was formed by collapses and the creation of sedimentary basin along the Median Tectonic Line in the Moriya stage. (2) Widespread and strong volcanism took place with collapses again in the Uchimura stage. At that time the center of the basin shifted northwards. (3) Plutonism and related metamorphism took place after the subsidence of the basin stopped. (4) After the plutonism an upheaval movement generated the volcano-plutonism at the crest of the area of maximum upheaval. This was accompanied simultaneously by hydrothermal alteration along the faults.