EARLY PLEISTOCENE BIRTH OF THE OKINAWA TROUGH AND RYUKYU-ISLAND ARC AT THE NORTHWESTERN MARGIN OF THE PACIFIC - EVIDENCE FROM LATE CENOZOIC PLANKTONIC FORAMINIFERAL ZONATION

Widespread development of the upper Pleistocene Ryukyu Limestone suggests the occurrence of prototypes of the Okinawa Trough and Ryukyu Island Arc, backarc-basin and forearc at the northwestern margin of the Pacific, before its deposition. Prior to the deposition, however, bathyal sediments of the Shimajiri Group were distributed over the arc region. On southem Okinawa Island, this group is divided into 11 planktonic foraminiferal zones, some of which may be not globally valuable but regionally useful, ranging in age from late Pliocene to earliest Pleistocene. This zonal scheme is applied to zoning 421 outcrop samples and a geologic map is successfully obtained. The map indicates the development of many faults that cut only the Shimajiri Group but not the overlying Ryukyu Limestone and majority of these faults run in perpendicular to the arc extension. Similar faults are also recognized in the lower Pliocene to lowermost Pleistocene Shimajiri Group on Miyako Island (southern Ryukyu Island Arc), in a submarine seismic profile, which was correlated with well stratigraphy, at the east of Miyako Island, and in the upper Pliocene to lowest Pleistocene Somachi Formation on Kikai Island (northernmost central arc). The structures observed in the Shimajiri Group and its equivalent may have been formed by an upward warping of the arc region associated with subsidence of the trough region. Considering the pre-Shimajiri basement geology as a whole, therefore, it is concluded that the initial appearance of this backarc basin and forearc occurred after the deposition of the Shimajiri and before that of the Ryukyu Limestone during the late Pleistocene (approximately 1.7-0.5 Ma).