A study on sea-level changes and tectonic movements during the late Quaternary deduced from the uranium-series method of dating for emerged coral reefs

本研究以珊瑚礁阶地为研究对象，比较琉球群岛至菲律宾群岛的相对海平面变化与构造运动。结果表明，“西太平洋区”在约100年前后经历了明显不同的构造运动。1.利用α-能谱法~（13）<230>Th/~（14）<234>U测年，证实了纪海岛更新世最低的礁阶是在海洋同位素第5 b阶段（MIS-5 b）至MIS-3阶段的不同沉积环境中形成的，并伴随着相对海平面的降低，也就是说，从岛屿大陆架上，礁沉积物沉积到浅水环境，在那里形成了礁。沿沿着荒木海岸的Kikai全新世礁阶地覆盖着薄的珊瑚灰岩，可细分为六个阶段，分别出现在7.4、6.6、4.0、3.0 ky或更晚。<230>对<234>采自亚国岛、多良间岛和石垣岛最低礁阶地的造礁珊瑚（MIS-7、MIS-5c和MIS-5e）进行Th/^ U测年，3.从几个阶地采集的造礁石珊瑚中获得了0.93±0.04 ~ 8.57± 0.13ky的U系年龄（最高海拔45米）。4. MIS-5e的礁阶地分布在保霍尔岛西南端的邦劳岛，海拔13-14 m，而在帕米拉干岛，同样年龄的礁阶地达到29 m。这意味着构造运动可能发生在小规模块体之间。在吕宋岛西北部沿着的全新世礁阶地和其他古海平面指示物，如潮汐缺口和一组微型环礁中，可以观察到这种构造运动。

This study was performed on coral reef terraces to compare regionally the relative sea level changes and tectonic movements from the Ryukyu through the Philippine Islands. The results revealed that the "Western Pacific Region" has experienced significantly different tectonic movement during ca. 210 ky, as follows :1.Based on the α-spectrometric ^<230>Th/^<234>U method of dating, the lowest Pleistocene reef terrace in Kikai Island was proved to have been formed in different depositional environment during Marine Isotope Stage 5b (MIS-5b) through MIS-3, with lowering the relative sea level, namely from the insular shelf where fore-reef sediments were deposited to shallow water environment where reefs were formed.Holocene reef terrace along Araki Coast of Kikai overlies thin coral limestone and can be sub-divided into six steps, have been emerged at 〜7.4, 6.6, 4.0, 3.0 ky or later, respectively.2.^<230>Th/^<234>U method of dating was successively applied to reef-building corals collected from the lowest reef terrace of Aguni, Tarama and Ishigaki Islands, which were assigned to MIS-7, MIS-5c and MIS-5e, respectively.3.U-series dates from 0.93±0.04 to 8.57±0.13 ky were obtained from hermatypic corals collected from several marine terraces (at a maximum altitude of 45m) along the west coast of Taiwan. This fact indicates that the upheaval rate of Taiwan during Holocene is highest in the world.4.The reef terrace assigned to MIS-5e is distributed at the altitude of 13-14 m in the Panglao Island adjacent to the southwestern end of Bohol Island, whereas the reef terrace of the same age is attained to 29 m at the Pamilacan Island. It means that the tectonic movement may occur among small-scaled blocks. Such a tectonic movement can be observed in Holocene reef terraces and the other paleo-sea level indicators such as tidal notches and a cluster of micro-atoll along northwest coasts of the Luzon Island.