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Pseudotachylytes are found along the Shimotsuburai fault, the southern segment of the Itoigawa-Shizuoka Tectonic Line active fault system, central Japan. They occur along the shallowly-dipping fault surface between the quartz-dioritic cataclasite of the hanging wall and the Upper Pleistocene alluvial deposits of the footwall, and also in the cataclasite zone of the hanging wall about -m wide. Some of them in the cataclasite zone are fault veins associated with small faults and others are injection veins forming complex networks into extension fractures. These veins vary from a few mm to cm in thickness, and are black to dark-brown in color and are aphanitic in appearance. They consist mostly of fine-grained angular fragments of quartz and feldspar, and some biotite crystals derived from the cataclasite. X-ray diffraction analyses suggest that the veins have the same mineral assemblage as that of the quartz-dioritic cataclasite. These textural and X-ray analytical results suggest that the pseudotachylytes were mainly formed by crushing but not by melting. On the basis of structural relationships between the boundary faults and veins, we interpret that some veins along and near the boundary fault were produced by the last faulting event occurred between and y. B.P. with dip-slip displacement of about . m at a depth of less than a few tens of meters. AbstractCThe Geological Society of Japan 2004 779

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