著者

小坂 共栄

出版者

信州大学理学部

雑誌

信州大学理学部紀要

巻号頁・発行日

vol.26, no.2, pp.75-140, 1992-02-25

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The fundamental and primary structural elements in the northern Fossa Magna region are the northeastward trending Shin-ethu Tectonic Trend. On the contrary to this, northward trending tectonic movements has occurred during late Miocene to early Pleistocene time. The Omine Belt is the representatives of this newly tectonic movements, so this trend is called the Omine Tectonic Trend. The Omine Belt is a narrow strip of land, five to seven kilometers wide and sixtyfive kilometers long, at the western margin of the northern Fossa Magna region. It is limited by the Itoigawa-Shizuoka Tectonic Line on the west, and by the Otari-Nakayama Fault on the east. Sedimentary and volcanic rocks of late Miocene to early Pleistocene age are widely distributed in this belt. They are characterized especially by a large amount of coarse clastic sediments with intercalated andesitic to rhyolitic volcanic rocks. Two larger volcano-sedimentary cycles are recognized, so they are subdivided into two formations, i.e. the Otari and Omine Formation in ascending order. The detailed sedimentological studies reveals that the main source area of a large amount of clasts is to the western and southwestern mountains of older rocks, i.e. the Japan Alps, and to the eastern area, i.e. the main part of the Fossa Magna region. Many large alluvial fans were undoubtedly formed in this belt.