長野県北部,糸魚川-静岡構造線の鮮新世以降の活動様式: 大峰-SK110テフラの古地磁気方位に基づいて

概要

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This study aims to reveal Late Pliocene to Early Pleistocene geomorphic and structural evolution of the Omine Belt, North Fossa Magna, central Japan, associated with tectonic movement of the Itoigawa-Shizuoka tectonic line. A comparison of the paleomagnetic directions of a distinct wide-spread tephra, which is exposed in different areas, is useful to detect tectonic rotation because it gives instantaneous paleomagnetic directions from proximal to distal areas. The Omine-SK110 tephra with an estimated age of 1.65－1.60Ma is distributed in the Omine Belt as Omine Welded Tuff and in the west Tama Hills of the Kanto Plain as a co-ignimbrite of the Horinouchi 2 Tuff. Pro-gressive alternating field and thermal demagnetization experiments reveal that both tephras have reversed paleomagnetic directions that are almost identical to each other. As the west Tama Hills had been in a tectonically stable area since Late Pliocene, there might be no vertical-axis rotation in the Omine Belt. Thus, the Omine Belt would not be a pull-apart basin with left-lateral strike-slip faulting between the Otari-Nakayama fault and the active fault system of the Itoigawa-Shizuoka tectonic line.
2008-06-23