DIFFERENCE OF MICROSCOPIC TEXTURE BETWEEN FAULT GOUGES AND HYDROTHERMAL CLAY VEINS

Fault gouge in granitic rocks is commonly clay-rich, as a result of fracturing and fragment size reduction caused by localization of deformation and displacement, and subsequent retrograde hydration reaction. On the other hand, clay veins are often included in such granitic rocks due to hydrothermal alteration and weathering. Such clay veins show similar appearance to clay-rich fault gouge in field outcrops. Thus precise identification of earthquake-related fault gouge, including discrimination from aseismic clay veins, is important for a reliable seismic hazard assessment. In this study, in-depth description was carried out for samples of a fault gouge and a clay vein from the Kojaku Granite in the Tsuruga Peninsula, central Japan. Based on the field and optical microscopic observations, the fault gouge is composed of layer stacks showing different fragment size and shape in each layer, indicating an accumulation of repetitive shearing along the gouge, that is, shear localization, while internal structure of the clay vein seems homogeneous. X-ray diffraction analysis shows similar clay mineral composition between the fault gouge and clay vein, both of which include abundant smectite and a minor amount of illite and kaolinite. SEM observation of quartz particles sorted by sieving (approximately 20 to 70μm) shows most particles remain primary crystal structure in the fault gouge, while a lot of particles show irregular and concave-convex surface in the clay vein. Recent fault activities involved with fragmentation in the gouge could make new quartz particles preserving primary crystal structure, whereas irregular surface in the vein could be a result of solution due to hydrothermal alteration. In contrast, high magnification observation of very fine clay minerals (approximately 0.1 to 0.2 μm) using field-emission SEM and TEM shows primary crystal shapes are clearer in the clay vein than in the fault gouge. Clay minerals in the fault gouge experienced abrasion and solution due to repetitive fault activities, whereas those in the vein, most are smectite, could avoid secondary deformation and solution after they were formed by hydrothermal alteration.