北部フォッサ・マグナ地域の第四紀火山の時代とその配列

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Quaternary volcanoes and volcanic formations distributed in the North Fossa Magna region are classified into those of the following five stages by correlation with tephra beds in the Uonuma and Yachio-Minamisaku Groups and age determination; First stage (2.8-1.4 Ma), second stage (1.4-0.8 Ma), third stage (about 0.5-0.3 Ma), fourth stage (0.15-0.01 Ma) and fifth stage (0.01 Ma-) volcanic rocks of the first stage are the Sekita volcanic rocks erupted in the bottom of freshwater, or older lavas of the Kusatsu-shirane volcano and others. Volcanoes of the second stage such as the Torikabuto and Kirigamine ones are terrestrial volcanoes, whose volcanic topographies are poorly prserved. Voicanoes of the first and second stages (Pliocene-early Pleistocene) are distributed in direction of NNE-SSW. Volcanoes of the third stage (middle Pleistocene) are stratovolcanoes and are distributed in direction of EW in northern side of the South subzone of the Nasu volcanic zone. These volcanoes are Sumon, Iiji, Naeba, Kenashi and older Shiga ones. Stratovolcanoes of the fourthstage (late Pleistocene) are Akagi, Haruna, Asama, Kusatsu-shirane and Shigayama ones of the South subzone of the Nasu zone, and Iizuna, Kurohime and Myoko ones of the North subzone of the Fujivolcanic zone. Some of them have erupted also in the fifth stage. Volcanic activity of the Yatsugatake volcanic chains extends from the first stage to the fourth stage. Volcanic activities of the fifth stage (Holiocene) are those of the Yakeyama volcano and the youngest activities of Akagi, Asama, Kusatsu-shirane, Kurohime and Myoko volcanoes, Chemical compositions, especially total alkali and K_20 contents, of volcanic rocks were compared with those of the North subzone of the Nasu zone, the Chokai zone and the North subzone of the Fuji zone. From stage of volcanic activity, location and chemical composition, Quaternary volcanoes of the North Fossa Magna are located in main volcanic zones of the Northeast Japan respectively as follows: 1) Asama, Kusatsu-shirane and Shigayama volcanoes are western extension of the Northsub zone of the Nasu zone, 2) Sumon, Iiji, Naeba and Kenashi volcanoes are not extension of the Chokai zone, but are extension of a volcanic zone between the North subzone of the Nasu zone and the Chokai zone, 3) Kayagatake, Yatsugatake, Iizuna, Kurohime and Myoko volcanoes are located in the North subzone of the Fuji zone.
日本地質学会の論文
1984-11-30

北部フォッサ・マグナ地域の第四紀火山の時代とその配列

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