Tectonic relationships between ductile shear zone and brittle fault zone in the upper crust

该项目的目的是研究含假玄武岩的地震断裂与前述韧性剪切带的构造关系，分析地震脆性断裂的糜棱岩和滑动面的面理、走向和运动意义，以及糜棱岩和地震断裂的剪切和滑动条件。研究区仅限于沿其出土下地壳变质岩的逆冲断裂，即大岛岛、琉球边缘糜棱岩带和日中变质带。在Yawatahama Oshima变质岩片中，除了含有地震断裂的假玄武岩外，还发现了四期糜棱岩化作用。部分假玄武岩以韧性方式变形。对假玄武岩断层的详细观察表明，地震破裂多次发生在韧性-脆性过渡带内。地震断裂和叠加的韧性变形与大岛变质杂岩--地壳深部变质岩层向三巴川片岩的逆冲有关，这是在地壳深部形成糜棱岩II的连续运动。这意味着内陆地震是沿着地壳块体构造折返的逆冲断裂发生的。在长野县南部和田地区的Ryoke边缘糜棱岩带，沿着平行于糜棱岩面理的脆性断层，在石英闪长岩糜棱岩中新发现了假千里岩脉。糜棱岩退变为低绿片岩相矿物，假玄武岩中含有少量球晶。这些结果表明，玄武岩形成于地壳相对较浅的区域，但滑动面的运动意义与前述糜棱岩的运动意义相同。这也可能表明地震断裂与地壳深部变质岩的逆冲作用有关。

The project was aimed to investigate the structural relation of pseudotachylite bearing scismic faults with respect to foregoing ductile shear zones, analysing the foliation of mylonites and slip planes of seismic brittle faults, trends and sense of movement, and shear and slip conditions of mylonite and seismic faults, respectively. The research areaa are limited to thrust faults along which lower crustal metamorphic rocks were exhumed ; namely, Yawatahama Oshima island, the Ryoke marginal mylonite zone, and the Hidaka metamorphic belt. In the Yawatahama Oshima metamorphic sheet, four stages of mylonitization are identified, in addition to the pseudotachylite bearing seismic faulting. A part of the pseudotachylite was deformed in a ductile manner. Detailed observation of the pseudotachylite fautls revailed that the seismic rupturing multiplly took place within a zone of ductile and brittle transition. The seismic faulting and overprinted ductile deformation are related to the thrusting of Oshima metamorphic complex, deep crustal metamorphic layers, onto the Sambagawa schists, which is a continuous movement to form the mylonite II in the deep crust. This is significant to mean that inland earthquakes takes place along thrust faults by which crustal blocks tectonically exhume.In the Ryoke marginal mylonite zone of Wada area, southern Nagano prefecture, pseudotachylite veins were newly found in quartz-diorite mylonite along brittle faults paralell to mylonite foliation. The mylonite is retrogessively altered to low greenschist facies minerals, and the pseudotachylite contains a small amount of spherulite. These show that the psudotachylite was formed in a relatively shallow zone of the crust, but the sense of movement of the slip planes is the same as that of foregoing mylonite. It may also indicate that the seismic faulting is related to thrust mevement of the deep crustal metamorphic rocks.