Abstract
Fault-generated pseudotachylytes showing reddish, greenish and grayish color have recently been found from the mylonitized Ryoke Granites along the Median Tectonic Line (MTL). These pseudotachylytes are suggested to have a melt origin on the basis of petrographic observations of typical melt-quenched microstructures such as microlites and amygdales. Preferential melting of low melting point minerals is the most probable melting process. The absence of lithic fragments of hornblende and biotite, and systematic decrease of bulk SiO2 contents in the pseudotachylytes compared with their host rocks support this interpretation. These pseudotachylytes were formed after mylonitization and were post-dated by weak cataclasis resulting from movement of the MTL. Similar orientation of the MTL, fault veins of pseudotachylytes, and mylonitic foliation indicate that the deformation sequence of mylonite, pseudotachylyte, and cataclasite formation probably progressed under a single tectonic stress field. These pseudotachylytes are examples of the remnants of ancient faulting with frictional melting indicating seismic activity of the MTL.
