**6. Discussions**

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

The illite crystallinity of the tectonic mélange and cataclasites samples is shown in Fig. 8. The host melange samples have an illite crystallinity ranging from about 0.3 to 0.46, with an average value of about 0.4. Illite crystallinity in the cataclasites with pseudotachylyte varies from 0.41 to 0.58, averaging around 0.46. The larger values of crystallinity suggest that the

The semi-quantification of illite and chlorite was conducted using the MIF method, as described above. The results are shown in Fig. 8. The illite ratio to chlorite for host rocks ranges from about 20 wt% to about 70 wt%, with an average value of 46 wt%. The same ratio in cataclasites with pseudotachylyte varies from 40 wt% to 90 wt%, with an average of 65 wt% (Fig. 8). The illite to chlorite ratio is larger in cataclasites than the host melanges, suggesting that the amount of illite increases in cataclasites if the amount of chlorite is

Fig. 7. A diagram of the Fe–Mg substitution of chlorite.

**5.4 Semi-quantification between illite and chlorite** 

illite in cataclasites is less crystallized than in the host melanges.

constant, or the amount of chlorite decreases relative to the amount of illite.

**5.3 lllite crystallinity** 

### **6.1 Iron consumption in chlorite in cataclasites with pseudotachylyte**

In chlorite from cataclasites with pseudotachylyte, the iron content is decreased in both the hydroxide and silicate layers, compared with host melanges, as described above (Fig. 7). The same trend has been reported in the Taiwan Chelung-pu fault (Hashimoto et al., 2007; Hashimoto et al., 2008); however, the initial state of the host rocks in Taiwan is different from those studied here. Although host rock samples from Chelung-pu fault displayed greater iron content than the Okitsu samples, fault rocks from both areas showed a decreased level of iron in their chlorite.

The decrease of iron in chlorite from fault rocks can be controlled by the temperature of the source fluid (Ohta and Yajima, 1988), the pH of the fluid (Malmstrom et al., 1996; Ross, 1969), or lithology (host rocks), as discussed in Hashimoto et al. (2008). Because the same trend of a decrease of iron levels in fault rocks was observed from different host rocks, lithology control is less significant. Ohta and Yajima (1988) reported that higher temperature is related to higher iron content in chlorite in hydrothermal environments, which is the opposite trend to that observed in our analysis, as fault rocks are expected to experience higher temperatures due to frictional heating. Therefore, temperature rise cannot explain the decrease in iron from chlorite in fault rocks. The pH of the source fluid is thus the most likely cause of the decreased iron content in chlorite from fault rocks. The change in pH of the fluid can be caused by radical reactions; that is, the reactions between water and newly 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, and is thus difficult to understand quantitatively outside the laboratory.

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

#### **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 pseudotachylyte formation.

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 related to the authigenic illite through pseudotachylyte formation.

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 earthquake occurred in 1999, and the Taiwan Chelung-pu fault Drilling Project (TCDP) was conducted in 2004. Within that 5-year interval, the alteration of glass to form smectite could have been progressed. Such alteration to form smectite from glass matrices in pseudotachylyte is also expected in the Okitsu examples. The formation of smectite might also be a significant rock-fluid interaction along a seismogenic fault.

The authigenic smectite is transformed to illite due to diagenetic processes. Smectite-illite transition proceeds with temperature, and the illitization is almost complete at 150°C (Moore and Vrolijk, 1992). Therefore, the smectite from pseudotachylyte glass could be transformed to illite. The illitization from authigenic smectite can also be related to a smaller size of illite, as identified by the larger value of illite crystallinity in cataclasites with pseudotachylyte. The linear relationship between the illite to chlorite ratio and illite crystallinity can be seen in Fig. 8, indicating that a larger illite ratio is associated with a smaller illite grain size. The relationship might also be explained by the illitization of authigenic smectite.

During illitization, the interlayer of smectite is dewatered. The water should migrate to the fault zone after a few years of seismogenesis.

Frictional behaviors of smectites and illites have been reported from laboratory experiments in a sliding velocity range from 0.1 to 200 µm (Saffer and Marone, 2003). While smectite indicates velocity weakening at low normal stress, illite represents only velocity strengthening behavior in the wide range of experiments. The velocity strengthening behavior in illite is not supported by the hypothesis that the smectite-illite transition is related to seismogenesis at the seismic front, as suggested by Hyndman et al (1999). Rather, illitization makes faults aseismic (Saffer and Marone, 2003). The illite concentration in cataclasites with pseudotachylyte in this study suggests that the rock-fluid interactions along a seismogenic fault, such as smectite formation from the glass of pseudotachylyte, and illitization from the authigenic smectite, are processes that modify the fault to an aseismic fault.
