The Toyamagawa Fault in Central Japan: Meso- and Microstructure

The Toyamagawa Fault, which is located in Central Japan, is a right-lateral fault. It seems to be about 20 km in length on the basis of an interpretation of aerial photographs. The Matoze Outcrop is about 80 m in length, and is the largest known outcrop along the Toyamagawa Fault. We discuss here the distribution of cataclastic fault rocks on this outcrop and their microstructures. Results of our observations show that the distribution of fault rocks is asymmetric around a planar fault gouge layer several millimeters thick. The fault gouge layer is the only incohesive layer which can be observed anywhere on the outcrop, suggesting that it is the latest slip layer. An ultracataclasite layer about 10 cm in thickness, which consists of several thin planar layers several millimeters thick, was observed next to the fault gouge layer. Because the margins of those thin layers are sharp, each layer should be considered to have been formed by an older seismic slip event. We investigated the microstructures of specimens obtained from the outcrop, and obtained the grain size distribution (GSD) of specimens of fault gouge and ultracataclasite layers from SEM-micrographs. The GSD has been considered as a parameter which characterizes the fault rocks. It is considered that the GSD may have self-similarity and can be described by a power law relationship. Keulen et al. (2007), however, mentioned that two distinct slopes of a power-law distribution, a shallower slope at smaller grain sizes and a steeper slope at larger grain sizes, were obtained. The GSD of the thin fault gouge layer of the Matoze Outcrop shows a fractal pattern, though those of other ultracataclasite layers are not fractal: the slopes of a power-law distribution change at a certain grain size near 2-3 μm. This may suggest that the comminution mechanisms are different between the fault gouge and the ultracataclasite layers.