Study of crustal-scale bending structures in island arcs

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1. I examined the bending structures developed in the Jurassic Mino accretionary complex. The main research strata are the Kanayama unit along the Maze and Nagara Rivers, the Funabuseyama unit along the Ibi and Neo Rivers, the Samondake unit in the northern Gujo area and the Kamiasou unit in the Yoro Mountains. The following results are clarified in this study.The melanges of the Kanayama and Funabuseyama units are mainly of tectonic origin due to subduction of oceanic plate. Extracting effects by the later deformation events, which include the crustal-scale bending, we can estimate the convergent direction of plate during the late Jurassic and the earliest Cretaceous on the basis of melange fabric analysis that shows sinistral oblique subduction.The Samondake unit have a various scale bending structures due to the stratal rotation about subvertical axis. The NW-SE trending active Hachiman fault is situated on the hinge area of regional scale bending structure, suggesting that the faul … More t was produced in association with the formation of bending structure, and is re-activated during the Quaternary.The overall N-S trending giant bend in the Kamiasou unit in the Yoro Mountains, that directed almost perpendicular to the trend of Southwest Japan arc, are due to the westward-plunging folds with half-wavelength of > 10km. The origin of this giant bend is still unknown, and further studies on this problem are required.2. Illite crystallinity (IC) of argillaceous rocks of the accretionary units in the Mino area has been studied to evaluate the physical condition during and after the accretion. The IC values range in higher to middle grade diagenesis except for the Samondake unit which shows middle to lower grade diagenesis. This result suggests that the paleo-geothermal structure of the accretionary complex in the Mino Belt is very monotonous and is not rearranged by the later deformation events which include out-of-sequence thrusting and giant-scale bending are not significant. For the next step to study physical conditions, we successfully analyzed fluid inclusions in quartz veins in the Shimanto Belt of the Akaishi Mountains. This method will be applied in near future to the analysis of melange formation of the Mino accretionary complex and to the faulting conditions along the Itoigawa-Shizuoka Tectonic Line (ISTL).3. The relationships between the northward bend of the basement rocks in the Akaishi Mountains and the ISTL fault system have been re-evaluated in this study. The precise fabric analyses of ductile deformation structures along the ISTL have revealed that the initial stage of faulting was mainly left-lateral strike slip under the condition of greenschist facies metamorphism. The geophysical researches including seismic reflection study suggest that the bended basement of the Akaishi Mountains and the pre-existing ISTL fault zone thrust onto the Quaternary sediments of the Kofu Basin along the very-low angle active fault system which were formed to the east of the older ISTL. Less