南・北大東島のサンゴ礁段丘からみた第四紀後期の地殻変動

Abstract

Daito Islands, ca. 150 km east of the Ryukyu Trench, are located on the northwestern part of the subducting Philippine Sea Plate, and are composed of two main islands, Kita (north) -Daito and Minami (sourth) -Daito. Both islands are well-known raised atolls and tectonic movements are expected to be different from the Ryukyu Islands on the Eurasian Plate.We investigated raised coastal morphology and geology at eight localities of these islands with special reference to the occurrence of fossil corals, and detailed maps and cross sections are drawn using Electric Distance Meter. Thirty three autochthonous corals are dated by 230Th/234U method and some corals are dated by ESR and non-destructive 226Ra/238U methods. Some allochthonous corals are also dated. 230Th/234U dates on autochthonous corals cluster in 113±6 to 133±6 ka in Kita-Daito and in 113±6 to 159±10 ka in Minami-Daito. Therefore, the dated autochthonous corals, except for one relatively older date, represent the sea level of the last interglacial maximum, corresponding to isotope stage 5e. The upper limit of occurrence of autochthonous corals is 8.1 m in Kita-Daito and 11 m in Minami-Daito. Elevation of the last interglacial shoreline judged by emerged coastal morphology is 10 m in Kita-Daito and 12 m in Minami Daito. No autochthonous corals younger than the last interglacial maximum exist in these islands. However, the occurrence of allochthonous corals dated at ca. 80-100 ka suggests the possible presence of corals corresponding to isotope stage 5c and 5a, below the present sea level.Average uplift rate in both islands is nearly the same, that is 0.03 m/ka at Kita Daito and 0.05 m/ka at Minami-Daito, however, it is significantly lower than 1.8 m/ka in Kikai Island and even 0.3 m/ka in Hateruma Island, both are located on the Eurasian Plate, being subducted by the Philippine Sea Plate. Sequence of vertical tectonic movement of Daito Islands can be estimated as follows : Daito Islands had been subsided resulting in the formation of atolls and lagoons underlain by more than 400 m thick coralline limestone (Sugiyama, 1934 and 1936), and then tectonic uplift, time of which is unknown because of dolomitization of calcareous sediments, has started in association of the northwestward migration of the Philippine Sea Plate and has resulted in the formation of upbulge which is represented by the emerged atolls, fringed by a series of fringing reefs. Uplift rate has been very slow at least since the last interglacial maximum, possibly implying that these islands are tending towards the new tectonic phase (stable to subsidence).