Paleogeography of the Ryukyu Islands.

Abstract

Recent submarine, geological, and geophysical investigations including diving surveys reveal the geo-history of the Ryukyu Islands and the East China Sea. Two stages are fundamental for formation processes of the Ryukyu Arc. The crustal thinning in the western part of the East China Sea and thus eastward drifting of the Arc may have occurred in the late Miocene to middle Pliocene at the first stage. Between 1.6 - 1.3 Ma, the East China Sea area, including most of the Okinawa Trough, may have been subaerial. At that time, the Ryukyu Arc region may have been a part of the Eurasian continent. Extensive subsidence may have occurred at the second stage, at about 1.3 Ma, in the early Pleistocene. The present Ryukyu Arc (Ryukyu Ridge) has been formed since then. The Ryukyu Arc may have been nearly connected to the Chinese continent, through Taiwan as a land bridge, sometime during the two major development periods (such as sometime during 1.6 - 1.0 Ma, and 0.2 - 0.025 Ma). The paleo-land may have been submerged step by step since 0.03 Ma by both crustal movement and sea-level rising after the last Ice Age. Submarine stalactite caves at 10 - 35 m deep off the Ryukyu Islands were discovered. The caves have subsided since the Wiirm Ice Age. Stone tools were also recovered inside one of them. Additionally, archeological evidence in the form of a stepped pyramid, estimated at greater than 6,000 years old, has been discovered beneath the sea off Yonaguni Island. Existence of such submarine ruins provide indicators of subsidence processes of the Ryukyu Arc.