HR: 1340h
AN: T13C-0487    [Abstracts]
TI: Tectonic Evolution of Okinawa Trough and Koshikijima-Islands, Southwestern Kyushu, Japan
AU: * Ooiwane, H
EM: Ohiwane@geo.kyushu-u.ac.jp
AF: Kyushu university, 6-10-1, Hakozaki, Fukuoka, 8120053 Japan
AU: Tohnai, S
EM: tohnai@ori.u-tokyo.ac.jp
AF: Ocean research institute, Tokyo university, 1-15-1, minamidai, nakano-word, Tokyo, 1648639 Japan
AU: Kiyokawa, S
EM: kiyokawa@geo.kyushu-u.ac.jp
AF: Kyushu university, 6-10-1, Hakozaki, Fukuoka, 8120053 Japan
AB: Okinawa Trough is a incipient continental backarc basin (Letouzey and Kimura, 1986). However, there are poor evidence to identify tectonic evolution. Our study area, which is located on the northern end and northeastern edge of Okinawa Trough, are excellent place to recognize tectonic evolution of Okinawa Trough. We revealed the tectonic evolution of northernmost Okinawa Trough. Observing seafloor by seismic profiles in the northern Okinawa Trough, we can see NNE-SSW trending normal faults. These faults were activated since 1.6Ma, and some are identified as active faults (Tokuyama et al., 2001). The displacement of a fault at the eastern edge of the trough reaches about 600m. Detailed observation of seismic profiles shows that there are some smaller (30km across at most) basins, which has longer axis of ENE-WSW, oblique to trough axis. Geological structure of this area is restricted by NNE-SSW trending faults and shows ESE dipping half-glaben structure. On the other hand, Koshikijima-Islands are composed of the Tertiary and the Cretaceous sedimentary sequences. NW-SE(F2) and NNE-SSW(F3) trending normal fault systems are identified (Tohnai et al., 2004). Three types of intrusive rocks are distinguished(I1, I2, and I3 in chronological order). Especially, I3 has NNE-SSW strike and shows linear distribution in map scale. I3 and F3 seem to have formed in the same stress field (WNW-ESE extension) and similar age. Rifting of northern Okinawa Trough is also happened in the same stress field, WNW-ESE extenion. In Conclusion, I3, D3, and northern Okinawa Trough are all formed in the same stress field, which had been formed at least about 7Ma, earlier than formerly expected 2Ma(Kimura, 1996). On the basis of field observation and K-Ar dating, following tectonic evolution in northern Okinawa Trough and Koshikijima-Islands has been revealed. 1) Deformation before the Tertiary sedimentation :D1 :65-40Ma, 2) Intrusion of andesitic intrusive rocks :I1:30-20Ma, 3) Activation of NW-SE trending normal fault system (F2): D2:22-14Ma, 4) Intrusion of Granodiorite :I2 :14.7_}0.4Ma, 5) Intrusion of porphiry :I3 :7.0_}0.6Ma, 6) Activation of NNE-SSW trending normal faults (F3):D3 :about 7.0Ma-0Ma.
DE: 8104 Continental margins: convergent
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005