沖大東島(隆起準卓礁)の第4紀地殻変動に関する一考察
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概要
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A short reconnaissance of Okino-Daito-Jima Island, especially of the southeastern part of the island, has been carried out to observe raised coral reef terraces associated with Quaternary crustal movements. This island (the highest elevation : 31.1 m) is a raised almost-table reef on the Okino-Daito Ridge in the northern Philippine Sea, composed of small-scale central depression (maximum depth : 15m), Pleistocene marine terraces (Surface I to Surface IV in the descending order), and Recent surf benches. The strontium isotope dating of the specimen (low magnesian calcite) obtained from the highest terrace in the southeastern area (about 25m above sea level) shows an age of 0.6+0.5 / -0.2 Ma B.P. (counting error : 2σ). The lowest terrace (5m above sea level in the southeastern area) may be correlated to the oxygen isotope stage 5e (about 125,000 yr B.P.). The island has tilted to the southeast with abundant fissures (dominant directions : southeast, north, northeast, and southwest) along the coasts.These characteristics suggest that about 0.6 to 0.7 million years ago Okino-Daito-Jima on the Philippine Sea plate approached the Marginal Swell along the seaward area of the Ryukyu Trench, where the flexure of the Philippine Sea plate has occurred due to the subduction of the plate beneath the Ryukyu Trench ; consequently the raised and tilted almost-table reef has been formed on account of movement of the island toward the highest Marginal Swell.On the other hand, Kita-and Minami-Daito-Jima Islands, about 160-170 km north of Okino-Daito-Jima, are assumed to have approached the Marginal Swell about 1.6 to 2 million years ago, resulting in the occurrence of the raised atolls at present.Okino-, Kita- and Minami-Daito-Jimas on the Philippine Sea plate are estimated to have moved to the northwest at the rate of several centimeters per year during the Quaternary period.
- 琉球大学教育学部の論文