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. Tivo stages are fundamental for formation processes of the Ryukyu Arc. The crustal thinning in the westem part of the East China Sea and thus eastward drifting of the Arc may have occuned 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 duiing 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 lce Age. Stone tools were also recovered inside one of them. Additionally, archeological evidence in the form of a stepped pyramid, estimated at greater than 6,0(X) years old, has been discovered beneath the sea off Yonaguni Island. Existence of such submarine ruins provide indicators of subsidence pro@sses of the Ryukyu Arc, Key words: Ryukyu paleoland / Okinawa Trough / paleogeographic map / land bridge / submarine stalactite cave / submarine ruins. In recent years submarine, geological, and geophysical investigations have been canied out in the Ryukyu Arc region by multiple international organizations (e.g., Kimura, 19854 b, 1996a, b; Japanese DELP Research Group on Back-Arc Basins, 1991; Sibuet et al.,1987,1998). The Ryukyu Arc (Fig. 1) lies along the eastern margin of East China Sea. This paper discusses geologic environmental change of the Ryukyu Arc and the East China Sea using all available data. Seismic reflection profiles of single and multi-channel systems, obtained from most organizations in the world, were compiled for the present study to clarify geological and stratigraphic sequenoe in the Ryukyu Arc. Dredging and drilling data were also incorporated to determine the stratigraphy in the studied area. Detailed topographic data collected with multi-nanow beam, manned anal unmenned submersible, and seismic refraction experiments were also available to study the geologic and geophysical features in the Ryukyu Islands (Nansei-Shoto Islands) areas. Adding to this, diving surveys around the Ryukyu Islands, using SCUBA, show detailed submarine topography and geolog5r.