琉球弧の佛像構造線

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In Southern Kyushu, the zonal structure of Southwestern Japan (Chichibu, Sambosan and Shimanto Belts) is very crooked southwards, called the Hokusatsu bending by HASHIMOTO, I. (1962a), and reaching on the pre-Miocene rocks of the Ryukyu Islands. Some workers were elucidated to the problems of the origin and the mechanism for the Hokusatsu bending (MATSUMOTO, 1962, 1976; KIZAKI, 1978, 1979 and SHIMAZAKI etal., 1981). The tectonic contact between the Chichibu-Sambosan Belt and the Shimanto Belt has been named the Butsuzo tectonic line by KOBAYASHI (1931, 1941). The different lithological assemblage of the Sambosan Group (consists of limestone, chert, basic volcanic rocks, sandstone and slate) and the Shimanto Supergroup (consists mainly of sandstone and shale) has usually in formed about the position of the Butsuzo tectonic line. As pointed out by KONISHI (1963, 1965), and later confirmed by workers such as MATSUMOTO et al. (1966), NAKAGAWA (1967), ISHIBASHI (1968, 1969, 1974), ANMA (1976), HASHIMOTO, S. et al. (1976, 1978), KASHIMA et al. (1977, 1978), OSOZAWA et al. (1977, 1979), SAKAI et al. (1977), TAKAHASHI et al. (1979, 1980) and FUJITA (1980, 1981), there are abundant evidences of the correspondence about the zonal structure of Southwest Japan and Ryukyu Arc and of the Butsuzo tectonic line. In Ryukyu Arc, the pre-Miocene basement is exposed on many Islands, although the Butsuzo tectonic line at the boundary of the Motobu Belt and the Kunigami Belt is slightly exposed on Amami-Oshima, Kakeroma-jima and Okinawa-jima (Fig. 1). This paper represents the previous studies of the Butsuzo tectonic line in Ryukyu Arc, based on the fieled works of Kakeroma-jima, Amami-Oshima, Okinawa-jima and Kerama Retto. KASHIMA et al. (1977) introduced the important idea that, the pebbles and blocks in the Yuwan Formation (olistostrome) are fragmented into the northern marginal part of the Kunigami Belt from the Motobu Belt, resulting the tectono-sedimentary event owing to the original thrust movemment of the Butsuzo tectonic line. Recently, FUJITA (1980, 1981) reported a complex sedimentary body comprising various kinds of pebbles or blocks with in the Motobu Formation in Okinawa-jima, showing the same fashion of the Yuwan Formation. According the present knowledge, it is concluded that the boundary fault with igneous intrusion of the Motobu Formation and the Nakijin Formation may be equivalent to the Butsuzo tectonic line. Nevertheless, the detailed results of the origin and the tectonic evolution of the Butsuzo tectonic line are still not obtained. It is necessary to have a new approach to this problem.
日本地質学会の論文
1983-03-30

琉球弧の佛像構造線

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