LATE MIOCENE EXTENSIONAL DEFORMATION OF THE NORTHERN OKINAWA TROUGH: GEOLOGIC STRUCTURES AND PALEOMAGNETIC IMPLICATIONS IN THE KOSHIKIJIMA ISLANDS, JAPAN

The stress field evolution and paleomagnetic directions were investigated on the Koshikijima islands northeastern part of the Okinawa Trough. The area is located at the northernmost of the Ryukyu Arc. It is inferred that the tectonic history of this area have been controlled by the opening of the Okinawa Trough. However details of such relations are still unknown. The Koshikijima islands consist of Upper Cretaceous and Paleogene sedimentary rocks and several ages of Tertiary igneous rocks. The upper Cretaceous Himenoura Group is approximately 2,200 m thick in this area and mainly consists of marine sediments. The Paleogene Kamikoshikijima Group is approximately 1,700 m thick and unconformably overlies the Himenoura Group. The lower part of this group consists of fluvial sediments and the upper part of this group consists of marine sediments. Miocene granite and granodiorite bodies and several kinds of dikes are included in these sedimentary rocks. Structural features of faults on outcrop and minor fault analysis show that two extensional paleostress fields were developed in the Koshikijima islands and the change of the stress fields was occurred in middle to late Miocene. The younger extensional stress field is attributed to the opening event of the northern Okinawa Trough and the course of the older one is unsolved. There are two kinds of igneous dikes in the Koshikijima islands. One is the middle Miocene Northwest – Southeast trending andesite or trachyte (Type 1; 14.7 ± 0.4 Ma, K-Ar dating). The other is the late Miocene North-northeast – South-southwest trending porphyry (Type 2; 7.0 ± 0.6 Ma, K-Ar dating). More than 300 samples of igneous rock dikes from 28 sites were collected and measured paleomagnetic directions. As a result, Type 1 dikes show a counterclockwise deflection in declination (D=-38.8°, I=50.4°, α₉₅=10.5°, κ=41.6), whereas Type 2 dikes show no significant discrepancy in declination. On the basis of the results of structural features and paleomagnetisms, we suggest that the opening of the Okinawa Trough started from late Miocene and the eastern arc area of the trough including of the Koshikijima islands have rotated counterclockwise. Since late Miocene, although the opening of the Okinawa Trough has continued and southeastern part of Kyushu was rotated counterclockwise, the study area has not experienced rotation and tilting.