伊豆弧衝突に伴う西南日本弧・東北日本弧の地殻変形と日本海拡大以前の両弧の連続性の復元

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Southwest Japan (SWJ) and Northeast Japan (NEJ) were separated from the Asian Continent and drifted oceanward to form the Japan Sea as a marginal basin in Miocene time. Since then, the Izu-Bonin Arc (IBA) collided with the eastern part of SWJ and the southern part of NEJ, and caused intra-arc deformations in both of them widely. We need to estimate these deformations and reconstruct the connection between SWJ and NEJ before discussing the pre-Miocene tectonics of Japanese Islands. This paper attempts this reconstruction semi-quantitatively based on the structural trend of basement rocks. First we discuss the basement structure in the boundary area between SWJ and NEJ. Some geologists consider that the Ashio Belt belongs to SWJ and that the Tanakura Tectonic Line (TTL) is the boundary between SWJ and NEJ. However, the Ashio Belt should be included in NEJ, because its general structural trend is same as that of the Abukuma and the Southern Kitakami Belts on the east of the TTL, except for its southern part bending westward. Data of borehole geology and geophysical explorations suggest that the Southern Kitakami, the Sambagawa and the Chichibu Composite Belts are juxtaposed from north to south under the Kanto Plains. The structural trend of these basement rocks is in ENE-WSW direction and parallel to that of the Ashio Belt in the Tsukuba and the Keisoku Mountain Blocks which is the nearest basement exposure. These basements also belong to NEJ. There are some gaps in structural trend and distribution of the Sambagawa and the Chichibu Composite Belts between the Kanto Mountains and the Kanto Plains and between the Kanto Mountains and the Akaishi Mountains. Hence the Kanto Mountains does neither belong to SWJ nor NEJ. A relatively wide zone from the northern Fossa Magna through the Kanto Mountains to the southern Kanto district forms the boundary part between SWJ and NEJ. Second we estimate intra-arc deformations of SWJ and NEJ and restore their original structures. The eastern part of SWJ bends northward. The Inner Zone of SWJ in the intrados of this bend forms megakinks as buckling. The eastern part of the Outer Zone of SWJ also shifted northward but in different manner. This shift is considered a combination of counterclockwise rotation with respect to the main part of SWJ and left-lateral strike-slip movements along the Gokasho-Arashima Fault, a shear zone between the Akaishi Fault and the Komyo Fault, and a shear zone along the Itoigawa-Shizuoka Tectonic Line. These deformations are removed, assuming that the structural trend was originally straight and parallel to the general trend of the main part of SWJ represented by the strike of Median Tectonic Line (MTL) in Shikoku and Kii Peninsula. The westward bending of structural trend of the Ashio, the Southern Kitakami, the Sambagawa and the Chichibu Composite Belts in the southern part of NEJ is attributed to the initial collision of rotating NEJ with IBA. Megakinks are also observed in the western part of the Ashio Belt. It is due to N-S shortening caused by progressive collision of IBA. The effects of these bending and megakinks are removed in the same way as SWJ. Then the structural trend of these belts are restored to almost parallel to TTL (or the Hatagawa Tectonic Line (HTL) ) and the Futaba Fault (FF). Finally, we reconstruct the connection between SWJ and NEJ in the following manner. They are rotated so that their structural trends become parallel to each other, and arranged in the positions where the pairs of counterparts (e.g. Mino-Tamba Belt and Ashio Belt; Sambagawa and Chichibu Composite Belts in the Akaishi Mountains, Kanto Mountains and Kanto Plains) connect smoothly. In this way, we successfully reconstruct the original connection of most geologic belts and a branching Cretaceous left-lateral fault system (MTL, TTL, HTL and FF).

伊豆弧衝突に伴う西南日本弧・東北日本弧の地殻変形と日本海拡大以前の両弧の連続性の復元

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