HR: 0800h
AN: T11C-1281    [Abstracts]
TI: Tectonic evolution of the Koshiki Islands at Kyushu in Japan: Extentinal tectonics of the eastern Asian margin at Cenozoic
AU: * Satoshi, T
EM: tohnai@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Science, University of Kyushu, Fukuoka, 812-8581, Fukuoka, Fuk 812-8581 Japan
AU: Shoichi, K
EM: kiyokawa@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Science, University of Kyushu, Fukuoka, 812-8581, Fukuoka, Fuk 812-8581 Japan
AU: Tetsumaru, I
AF: Research Institute of Natural Science, Okayama University of Science, Okayama, 700-0005, Okayama, Oka 700-0005 Japan
AU: Yoshitaka, H
AF: Department of Natural Environmental Science, Kochi University, Kochi, 780-8520, Kochi, Koc 780-8520 Japan
AB: The Koshiki islands contain one of the best-preserved exposures of upper Cretaceous to Paleogene sedimentary successions in western Kyushu of Japan. These stratified sedimentary sequences also contain well-preserved deformations, used to identify surface conditions and tectonics of the Eastern part of the Asian continental margin. The upper Cretaceous Himenoura Group is approximately 800m thick in this area. It consists of well-rounded conglomerate, cross-bedded sandstone, and black shale and forms coarsening-upward sequence with HCS (hummocky cross stratification) containing many upper cretaceous fossils. This group had been formed in an offshore environment. The Eocene Kamikoshikijima Group, approximately 1700m thick, unconformably overlies the Himenoura Group. It consists of quartz rich cross-bedded sandstone, fine laminated greenish siltstone, and red shale, which are identified as fluvial sediment. Two types of faults are identified; 1) type1: NW-SE trending normal faults composed of thick shear zone with brittle-ductile fabric, and 2) type2: NNE-SSW trending normal faults with sharp fault planes with dip-slip slickenside. The type1 faults have 10m to more than 100m of displacement. Kanoko Fault, which is the biggest type1 fault in this area, has a 5m wide shear zone containing highly foliated shales with dip-slip lineations and asymmetric fabrics. This fault was affected by top to the northeast movement. Based on the comparison between each group stratigraphy, approximately 500m displacements are estimated. We determined the two K-Ar clay mineral ages in the type1 fault gouges. One, which is the Kanoko Fault, dated 16.5-19.1 Ma in age, and another fault, which is deeper situation in Himenoura Group, is 20.6-24.1 Ma in age. The temperature estimation from the fluid inclusions within lineated calcite veins in Kanoko Fault shows that the veins were formed at more than 166?C. Type2 faults contain thin fault gouge with slickenside. These faults clearly truncate the type1 faults. The NNE orientation of the type2 faults is almost parallel to the Koshiki islands and the northeastern Okinawa trough. Three deformations and one intrusive event are identified in this area; 1) tilting of the Himenoura Group (D1), 2) NW-trending normal faults (24-16.5Ma)(D2), 3) intrusion of igneous rocks (14.0+-1.6Ma) and 4) NNE-trending normal faults (D3). During early to middle Miocene in southwest Japan, regional extended stress filed occurred, and D2 to D3 deformations in Koshiki islands might correspond to the changed regional stress filed.
DE: 9320 Asia
DE: 9604 Cenozoic
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8105 Continental margins and sedimentary basins
DE: 0905 Continental structures (8109, 8110)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting