東北本州弧における新生代火山活動域の変遷 [in Japanese] Historical change of the Neogene and Quaternary volcanic field in the Northeast Honshu Arc, Japan [in Japanese]
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Time and spatial variation of the volcanic field and the migration of eastern border of the field (volcanic front) through the past 20 m.y. are investigated in every two or three million years. An extent of the field in each period is determined by the distribution of intrusive rocks and volcanic central facies (lava flow and volcanic breccia), described in 30 Northeast Honshu Arc geotraverses by KITAMURA ed. (1986). The historical change of volcanic field determined by the well-correlated volcanic central facies found in land area at present tells that the volcanic front, since about 16.5 Ma when the widest Neogene marine transgression initiated, tends to shift westwards, from the Pacific coast to the volcanic front today. The volcanic fronts at 16 and 14 Ma extended from the east of Hachinohe to the Joban area through the westem part of Kitakami Mountains and Matsushima Bay. The Hachinohe-oki Volcanic Rock on the shelf and continental slope off Hachinohe is probably an eastern extension of 16 to 14 Ma volcanic rocks exposing in mainland, so the volcanic front in this period extended from off Hachinohe to the Joban area. The distance between the estimated volcanic front and recent one is about 90 km, At around 10 Ma, the front lay along the eastern margin of the Ou backbone Mountain area at present, about 10 km east from the front today. About 20 km eastwards migration of the front took place during the period between 10 and 6 Ma, and then returned to almost the sarne position in 10 Ma, until 3 Ma. Reconstruction of volcanic field before the marine transgression is difficult because of local geological phenomena, such as differential subsidence, uplift and erosion, and insufficient evidence in correlation. Based on the radiometric ages for volcanic rocks, the authors concluded that the initial green tuff volcanism took place in the coast and mountain areas along the Japan Sea at around 30 Ma, and that during the period from about 30 to 20 Ma, the field spread to the east, DSDP site 439, off Hachinohe, about 200 km apart from the volcanic front today, and then began to recede westwards. It appears from the above that the westward shifting rates of the volcanic front since 20 Ma are higher in the former half (about 2 cm/year) than in the latter.
地質学論集 (32), 431-455, 1989