Tectpmoc History of the Ryukyu Island Arc, Dcduced from Uranium-Seried Dating of Raised Coral Reefs.

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Results of this study are summarized as follows :1. Pleistocene coral reef terraces on Ie Island and Hedo-misaki, Okinawa Island, are morpho-stratigraphically subdivided into several steps, by geomorphologic field survey, using 1 : 10,000 aerial photograph. U-series dating vindicated the existence of Holocene coral limestone at the littoral zone around Ie Island, although U-series dates correlative to late Pleistocene were not obtained there, because of diagenetic alteration of fossil corals.2. Results of U-series dating on Pleistocene corals from the Huon Peninsula, Papua New Guinea, and the Bohole Island, Philippines, suggest that age of coral reef terraces younger than 70ka nust be revised. An eustatic sea level change during the last 70,000 years could be proposed, based on those results of U-series dating and the depositional depth inferred from litho- and bio-facies analyzes of terrace deposits.3. The occurrence of raised coral reef formed during the last interglacial maximum was certified at an altitude of 1 to 2 m above the present sea level on the Bohole Island. This fact means that the Bohole Island has not uplifted during the last 130,000 years.4. Two sorts of "concordia", |^<230>Th/^<234>U|-|^<234>U/^<238>U| and |^<230>Th/^<234>U|-|^<231>Pa/^<235>U| curves, were considered to be one of the most valuable tools to verify the reliability of a ^<230>Th/^<234>U coral date.