Timing of the clockwise rotation of Southwest Japan: paleomagnetic evidence from Miocene sedimentary rocks

Hiroyuki HOSHI; Daiki KATO; Yoshikazu ANDO; Kazuo NAKASHIMA

Presentation information

Oral

Symbol S (Solid Earth Sciences) » S-EM Earth's Electromagnetism

[S-EM34] Geomagnetism, paleomagnetism and rock magnetism

Sun. May 24, 2015 2:15 PM - 4:00 PM 102A (1F)

Convener:*Nobutatsu Mochizuki(Priority Organization for Innovation and Excellence, Kumamoto University), Masaki Matsushima(Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology), Chair:Masaki Matsushima(Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology), Hisayoshi Shimizu(Earthquake Research Institute, University of Tokyo)

2:30 PM - 2:45 PM

[SEM34-12] Timing of the clockwise rotation of Southwest Japan: paleomagnetic evidence from Miocene sedimentary rocks

*Hiroyuki HOSHI¹, Daiki KATO¹, Yoshikazu ANDO¹, Kazuo NAKASHIMA² (1.Aichi University of Education, 2.Yamagata University)

Keywords:paleomagnetism, rock magnetism, Southwest Japan, tectonic rotation, Mizunami area, Oidawara Formation

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.