隆起サンゴ礁からみた最終間氷期以降のアジア・フィリピン海プレート境界付近の地史

抄録

Variation at three islands of Nansei-Shoto (-Islands) in the presently occupying elevations of the Last Interglacial (Riss/Würm) emerged reefs, which have been identified stratigraphically with geochemical control of both uranium-series radiometry of hermatypic corals and oxygen isotopic measurment of associated molluscs, are best explained in terms of the neotectonic interplay at plate convergence of the active Ryukyu island are system, of which frontal arc is divided morphotectonically into three major tectonic blocks, Northeast, Central and Southwest Ryukyus.
The islands closest to the trench axis (e.g. Kikai) in the Central Ryukyu block are being uplifted rapidly and tilted towards the Asian continent (“reverse dipping”) through compression between the relatively resistive continental lithosphere and gently subsiding (25°-35°in dipping angle of the Wadati-Benioff zone) West Philippine Sea lithoshere in contact (“Chilean-type” of plate convergence by Uyeda and Kanamori).
Atolls of at least 50-60m. y. old (e.g. Minami-daito) on the two remnant arcs (Daito and Okidaito) resting on the West Philippine Sea lithosphere to the south of the trench began to uplift to be bulge since Early Pleistocene as the result of lithospheric flexure, when the lithosphere traveled northward and lately approached close to the trench.
In Southwest Ryukyu block south of Miyako Depression, the most trenchward island of the frontal arc (i.e. Hateruma) indicates far slow rate of uplift, if ever, in the magnitude of practically almost none, if compared with Kikai, as deduced from the altitude of, besides the referred Last Interglacial, the preceding “Interglacial” (correlative to Deep Sea Core Oxygen Isotope Stage 7: ca. 220k. a. B. P.), which represents the first radiometric confirmation of the Middle Pleistocene in the raised coral reef stratigraphy in the Northwest Pacific. This morphotectonic block sits next to a steeply dipping (55-65° in average and more at the lower tip) Wadati-Benioff zone and tesile field behind the frontal arc where Okinawa Trough (backarc basin) is actively spreading, like the case of the “Mariana-type” of plate convergence.
The present study verifies that neotectonic rate of uplift within a frontal arc of the same island arc system varies systematically not only in transverse as “reverse dipping” from trench toward backarc basin or continent, but along its axis (longitudinally), depending upon its morphotectonic blocks which may differ in mode of subduction of convergence underneath.