Study of crustal-scale bending structures in island arcs

1. 我研究了侏罗纪米诺增生复合体中发育的弯曲构造。主要研究地层为沿麦兹河和长原河的Kanayama单元，沿伊比河和新河的Funabuseyama单元，北部固州地区的samonake单元和约罗山的Kamiasou单元。本研究澄清了以下结果。金山单元和船边山单元的混杂岩主要是由于大洋板块的俯冲而形成的构造成因。提取包括地壳尺度弯曲在内的后期变形事件的影响，在混杂岩结构分析的基础上，估计出晚侏罗世和早白垩世板块的收敛方向，显示出左旋斜向俯冲。由于地层绕近垂直轴的旋转，samonake单元具有各种规模的弯曲构造。北西-东南向活动的Hachiman断裂位于区域尺度弯曲构造的枢纽区，表明该断裂与弯曲构造的形成有关，并在第四纪被重新激活。Yoro山Kamiasou单元整体北向南走向的巨大弯曲，几乎垂直于日本西南弧走向，是由半波长为bb10 ~ 10km的西倾褶皱所致。这个巨大弯曲的起源仍然未知，需要对这个问题进行进一步的研究。研究了米诺地区各增生单元泥质岩的伊利石结晶度（IC），以评价增生期间和增生后的物理状态。除samonake单元为中低成岩作用外，其余单元的IC值均为中高至中等成岩作用。这说明米诺带增生杂岩的古地热构造非常单调，没有受到后期变形事件的重新整理，其中逆冲和大规模弯曲等变形事件并不显著。为了下一步物理条件的研究，我们成功地分析了明石山脉石曼托带石英脉中的流体包裹体。该方法将在不久的将来用于分析米诺增生杂岩的形成和沿伊藤-静冈构造线（ISTL）的断裂条件。本文重新评价了赤石山脉基底岩向北弯曲与ISTL断裂系统的关系。对ISTL韧性变形构造的精确组构分析表明，在绿片岩相变质作用条件下，断裂的初始阶段主要为左旋走滑。包括地震反射研究在内的地球物理研究表明，赤石山脉的弯曲基底和原有的ISTL断裂带，沿着古ISTL以东形成的极低角度活动断裂系统，冲入了口福盆地第四纪沉积。少

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