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

点击翻译按钮获取中文摘要

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.