**7. Conclusion**

266 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

created surfaces of minerals due to the breakage of mineral grains. Laboratory experiments on radical reactions showed that the pH of a fluid could be decreased or increased by the breakage of quartz, feldspar, and micas (Kameda et al., 2003; Saruwatari et al., 2004). The change in pH of fluid depends on a complex series of interactions with the broken minerals,

The decrease of iron content in ultracataclasites with pseudotachylyte has also been observed in isocon diagrams from a bulk chemical analysis at another fault zone (the Mugi melange, Cretaceous Shimanto Belt, Shikoku, SW Japan; see Hashimoto et al., 2009). This suggests that the characteristic pH of the source fluid can be related to iron consumption from host rocks. The consumed iron from host rocks is observed within minerals precipitated from related fluid. Ankerite veins (Fe–Mg carbonates) are concentrated in

**6.2 Higher illite crystallinity and illite content in cataclasites with pseudotachylyte**  The illite crystallinity of cataclasites with pseudotachylyte had a higher value than that of host melanges. Illite crystallinity is commonly used as an index of paleo-maximum Temperature (e.g., Guithrie et al., 1986; Awan and Kimura, 1996), with higher illite crystallinity indicating a lower paleo-maximum temperature. The cataclasites with pseudotachylyte are expected to have been subjected to a higher paleo-maximum temperature than the host rocks, due to frictional heating. Ikesawa et al. (2003) estimated the minimum temperature from frictional heating to be about 450°C on the basis of the composition of pseudotachylytes. Therefore, the higher illite crystallinity in cataclasites cannot be interpreted by the paleo-maximum temperature. Theoretically, illite crystallinity is controlled by X-ray-scattering–domain size and percentage of expandable layers (Srodon and Eberl, 1984; Eberl and Velde, 1989). As the illite in this study did not include expandable layers within it, the effect of the percentage of expandable layers on the crystallinity is negligible. A wider peak (higher illite crystallinity) indicates a smaller scattering domain. Therefore, the higher value of illite crystallinity for cataclasites with pseudotachylyte indicates that the particle size of illite is smaller than that of host melanges. Possible mechanisms for making smaller illite particles include comminution during cataclastic deformation, and a smaller size of authigenic illite formation related to

The semi-quantitative analysis to examine the ratio of illite and chlorite indicates that the proportion of illite increases in cataclasites with pseudotachylyte, compared with that in host mélanges. This result suggests either an increase of illite or a decrease of chlorite in cataclasites. The chemical analysis and mineralogical observations of pseudotachylyte in sedimentary rocks indicates that the melt mainly originates from clay minerals, and quartz and feldspar grains that are resistant to melting (Ujiie et al., 2007). This resistance is due to differences in melting temperatures between clay and other minerals in sedimentary rocks. As it might be difficult to melt chlorite selectively, the increase in the illite ratio might be

Smectite concentrations have been reported from the seismogenic fault zone in the Chelungpu fault, Taiwan (Kuo et al., 2009). It was found that the smectite in the fault zone does not include an illite-smectite mixed layer, and this was interpreted as meaning that the smectite can be formed by alteration of glass (pseudotachylyte) (Kuo et al., 2009). The Chi-Chi

and is thus difficult to understand quantitatively outside the laboratory.

seismogenic faults, such as the Okitsu fault described in this study.

related to the authigenic illite through pseudotachylyte formation.

pseudotachylyte formation.

The iron content in chlorite in cataclasites with pseudotachylyte is smaller than that of tectonic melanges (host rocks), as observed in other seismogenic fault rocks. The decrease of iron in chlorite suggests that a specific pH fluid reacted with fault rocks due to radical reactions.

Illite crystallinity in cataclasites is higher than in host rocks. In addition, the relative amount of illite is increased in cataclasites compared with host rocks. These results imply that the smectite altered from glass in pseudotachylyte transformed into illite along a seismogenic fault.
