HR: 0800h
AN: T41A-0385    [Abstracts]
TI: Tectonic evolution of the northernmost Okinawa Trough
AU: * Oiwane, H
EM: oiwane@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, the University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, Japan, Tokyo, 1648639, Japan
AU: Tonai, S
EM: tohnai@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, the University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, Japan, Tokyo, 1648639, Japan
AU: Tonai, S
EM: tohnai@ori.u-tokyo.ac.jp
AF: Earth and Planetary Science, Kyushu University, 6-10-1, Hakozaki, higashi-ku, Fukuoka city Fukuoka, Japan, Fukuoka, 812-8581, Japan
AU: Nakamura, Y
EM: saru@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, the University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, Japan, Tokyo, 1648639, Japan
AU: Kiyokawa, S
EM: kiyokawa@geo.kyushu-u.ac.jp
AF: Earth and Planetary Science, Kyushu University, 6-10-1, Hakozaki, higashi-ku, Fukuoka city Fukuoka, Japan, Fukuoka, 812-8581, Japan
AU: Tokuyama, H
EM: tokuyama@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, the University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, Japan, Tokyo, 1648639, Japan
AB: Over the last decades, mechanism of backarc opening has been the subject of controversy. Okinawa Trough has been noticed as an incipient continental backarc basin. So far studies on the Okinawa Trough have been conducted mainly in its southern and middle part. Nevertheless, only a few studies have been conducted on the relationship between geologic structures and the tectonic evolution of northernmost Okinawa Trough. Our main object is to consider the tectonic evolution of the Okinawa Trough by means of structural geology, not only in ocean but also land area. We executed seismic reflection survey in northernmost Okinawa trough and investigated fault systems on Koshikijima islands located on northeastern edge of the Okinawa Trough. Then we compared geologic structures on land and sea floor. First of all, we interpreted seismic reflection data acquired by hydraulic department of Japan Coast Guard in northernmost Okinawa Trough in 1975. Furthermore, we executed multi-channel seismic reflection survey in this area (KT06-03, KT07-03 cruise). NNE and ENE trending fault systems were recognized. It is clear that Northernmost Okinawa Trough has been subsided by activation of NNE trending fault system at the east edge of trough basin with several hundred meters of displacement. The ENE trending fault system has displacement in the surface sediment, which means they are active faults. On the other hand, we held on-land field survey in northern part of the Koshikijima Islands. Fault system, which can be related to the Okinawa Trough, is reported in this area (Inoue et al., 1982). So we focused on that NNE trending normal fault system (F2 fault system). On the basis of crosscut relationship and K-Ar dating of fault rocks and dikes, F2 fault system was formed after late Miocene and likely to have been formed in a shallow underground. F2 fault system on Koshikijima islands and NNE trending fault systems in the Okinawa Trough have similar strike, displaced direction and lineaments. It is likely that they both had been formed in one event, which may be rifting of northernmost Okinawa Trough. As another noteworthy result of multi-channel seismic reflection data, strike slip fault and normal faults were observed almost same depth in shallow (approximately 750 m) sediments. It shows that they have been activated at the same time. Both Strike-slip and normal faults are accommodated by oblique rifting (Withjack and Jamison, 1986). We support that the Okinawa Trough rifting proceeded as an oblique rift system in recent time. This result is consistent with southward migration of Ryukyu arc derived from Earthquake slip vectors (Fournier et al., 2001), GPS survey (Nishimura et al., 2004).
DE: 0900 EXPLORATION GEOPHYSICS
DE: 0930 Oceanic structures
DE: 3001 Back-arc basin processes
DE: 8000 STRUCTURAL GEOLOGY
DE: 8169 Sedimentary basin processes
SC: Tectonophysics [T]
MN: 2007 Fall Meeting