沖縄本島西方沿岸海域の地質と琉球弧形成史

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A geological map of the Okinawa Island region and its western offshore area is preliminarily complied depending upon the analyses of numerous seismic reflection profiles (80 east-west traverses and 20 crossing ones) and the re-evaluation of land-based in formations. The results can be summarized in relation to the geological developmment of the South and southernmost Central Ryukyu Arcs as follows: 1) Paralic sediments represented by the Lower Miocene Yaeyama Group seem to develop throughout the South Ryukyu Arc, because the Group was most recently found in a petroleum exploration well drilled just northeast off the Miyako Island as its northern most distribution limit at the present knowledge. The drilling, moreover, shows the Yaeyama Group sequence quite similar to the equivalent on the west side of Taiwan. While the Taiwan sequence has been vigorously deform ed accompanying with thrust-faults, the Yaeyama Group of the South Ryukyu Arc gently inclines southeastwards. Therefore, the Arc has necessarily been shifted toward the Pacific Ocean by a dextral strike-slip fault against the Taiwan Arc in the Middle Miocene age, during which an orogenic movement seemingly started in Taiwan. 2) The northern end of the South Ryukyu Arc is sharply bounded also by a dextral strikeslip fault (Kerama Gap). Beyond the fault, any southwestern extension of the zonal structure of pre-Neogene basement in the Central Ryukyu Arc can not be traced. At the same time, the fault is accompanied with so great vetical throw as attained to 1,700 min cutting through the Middle to Upper Pleistocene Ryukyu Limestone. In the Central Ryukyu Arc, therefore, there is not recognized any northern extension of the ca. 360 km long and ca. 70 km wide "500 m deep island shelf", which characterizes the South Ryukyu Arc and is throughly covered with about 40 to 70 m thick Ryukyu Limestone. 3) An east to west running sinistral Strike-Slip fault may presumably be traced along the northern slop of the Kerama Islands shelf, with in the southernmost part of the Central Ryukyu Arc. Among the basement complex of the Okinawa Island, the Nago Group has been considerably sheared and overturned with northwestward dip and northeast-southwest strike in general along the northern two-thirds of the Island-axis, whereas the rather weakly sheared Nago Group in the Kerama Islands inclines southwestwards with general dip less than 30 degrees. The fault estimated between these two blocks may have been accomplished before the Late Mioceneage, i.e., before the deposition of the Shimajiri Group, because the fault not affected the volcanic to pyroclastic facies of the Group, which forms the basement of the Kume Island located on its eastern extension. The western extension seems to have made an easterly protruded to pography for the northern side basement so that the Shimajiri Group might have not been deposited at the north beyond the presumed extension so far as the land area of the Okinawa Island region is concerned. 4) Many kolls are topographically recognized in the studied region,. Against some previous opinions, however, Recent volcanic front is not traceable into the region, since the knolls are covered with probably Pleistocene sediments excluding some smallest ones near the Okinawa Trough. Particularly it is noticed that each west side of four knolls, i.e. Izena-tai, Ryukyu-sone, Tori-shima, and Aguni-jima, is bounded by the northeast to southwest running fault line-cliffs with about 1,600 m vertical throw and that the east side is remarkably tilted toward the east in being covered by the westerly thinning sediments of the Pleistocene to probably Holocene age.
日本地質学会の論文
1983-03-30

著者

氏家宏
琉球大学理学部

沖縄本島西方沿岸海域の地質と琉球弧形成史

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