[S-EM34] 地磁気・古地磁気・岩石磁気

[SEM34-12] 西南日本の時計回り回転運動のタイミング：中新世堆積岩から得られた証拠

*星 博幸¹、加藤 大貴¹、安藤 慶和¹、中島 和夫² (1.愛知教育大学、2.山形大学)

キーワード：古地磁気, 岩石磁気, 西南日本, 回転運動, 瑞浪地域, 生俵層

The clockwise rotation of Southwest Japan is a textbook example of near-pivot arc rotation associated with back-arc opening. However, its timing is still a matter of debate; earlier studies suggested rapid rotation at about 15 Ma, but this does not seem to be supported by recent paleomagnetic data. To address this problem, we have carried out a paleomagnetic study of biostratigraphically well dated (15.8-15.7 Ma) Miocene sedimentary rocks in the eastern part of Southwest Japan. A total of 288 rock samples of siltstone and felsic fine tuff were collected from a ~90 m sedimentary sequence. Of these, 142 yielded reverse polarity characteristic remanence directions, resulting in a formation-mean direction that can be used for tectonic discussion. We conclude that about 80% of the entire ~45^o rotation occurred in a period between 17.5 Ma and 15.8 Ma at a rotation rate of ~21^o/Myr, and the remaining ~20% by 15 Ma. This clockwise rotation happened in the latest stage of the late Paleogene to early Neogene opening of the Japan Sea.