南琉球石垣島のジュラ紀付加コンプレックス富崎層と後期中生代のアジア東縁収束域

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Present study on the weakly metamorphosed pre-Tertiary rocks (Fusaki Formation) on Ishigaki Island, southern Ryukyus has brought new information concerning their tectono-sedimentary history as follows. 1) The weakly metamorphosed rocks form a sedimentary complex of olistostromal aspect, mostly dominated by pebbly mudstone. It contains abundant allochthonous blocks and lenses of sandstone, mudstone, chert, limestone, and so on. 2) Microfossils such as conodonts, radiolarians and smaller foraminifers were newly found out, providing age assignment for the exotic blocks and lenses. Namely, the olistostromal complex contains Permian limestone, Permian (+ pennsylvanian?) and Triassic bedded cherts and Early Jurassic siliceous mudstone. 3) Sandstone sills and dykes intruded into the allochthonous blocks, indicating that these chaotic sedimentary rocks were mixed before the consolidation of coarse-grained clastic rocks. Judging from these results, the weakly metamorphosed rocks are inferred to be a subduction-related sedimentary complex formed in trench environs during the Middle Jurassic, and are best compared with the Jurassic complex in Southwest Japan, which is interpreted as an ancient subduction-accretion complex along the eastern margin of Jurassic Asia. On the other hand, previous studies have revealed that high P/T metamorphic rocks with 240-160 Ma K-Ar ages (Tomuru Formation) tectonically overlie upon the Fusaki Formation, and that they are correlated to the Sangun metamorphic rocks in Southwest Japan. Thus the pair of the high P/T metamorphic rocks and the weakly metamorphosed olistostromal complex on Ishigaki Isiand, southern Ryukyu Arc, is compared with that of the Sangun metamorphic rocks and adjacent Jurassic olistostromal complex (Kuga Group) in the Inner Zone of Southwest Japan. In other words, the southern Ryukyu Arc is regarded as a southwestern extension of the Inner Zone of Southwest Japan, while the northern Ryukyu Arc is surely a southwestern extension of the Outer Zone. It is suggested, therefore, that the northern and southern Ryukyu Arcs are offset in left-lateral manner along the Kerama Gap, and that the width of the Outer Zone becomes considerably narrdwer southwestward on the south of Ishigaki Island. Concerning the Jurasic complex along the eastern margin of Asia, its lateral extension is probably traced from the Nadanhada area on China/USSR border to west Philippines (probably further to west Borneo) via Japanese Islands, and the Jurassic complex on Ishigaki Island gives a missing link in this orogenic chain between Southwest Japan and North Palawan, west Philippines. The boundary thrust between the Jurassic complex and the overlying pre-Jurassic orogenic complexes, newly designated as the Ishigaki-Kuga Tectonic Line in this paper, can be also traced along this trend in East Asia.
日本地質学会の論文
1989-04-26

南琉球石垣島のジュラ紀付加コンプレックス富崎層と後期中生代のアジア東縁収束域

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