**Rock-Fluid Interaction Along Seismogenic Faults Infered from Clay Minerals in Okitsu Mélange, the Cretaceous Shimanto Belt, SW Japan**

Yoshitaka Hashimoto and Umihiko Kaji *Kochi Univeristy Japan* 

### **1. Introduction**

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

Singh, S.K., Rodriguez, M., and Esteva, L., 1983. Statistics of small earthquakes and

Solow, A.R., 2001. An empirical Bayes analysis of volcanic eruptions. Math. Geol. 33 (1), 95102. Sornette, A., Sornette, D., 1989. Self-organized criticality and earthquakes. Europhys. Lett. 9,

Stuart, W.D., Mavko, G.M., 1979. Earthquake instability on a strike-slip fault. J. Geophys.

Teich, M.C., 1989. Fractal character of the auditory neural spike train. IEEE Trans. Biomed.

Teich, M.C., Heneghan, C., Lowen, S.B., Turcott, R.G., 1996. Estimating the fractal exponent

Telesca, L., Cuomo, V., Lanfredi, M., Lapenna, V., Macchiato, M., 1999. Investigating

Telesca, L., Cuomo, V., Lapenna, V., Vallianatos, F., 2000a. Self-similarity properties of seismicity in the southern Aegean area. Tectonophysics 321, 179–188. Telesca, L., Cuomo, V., Lapenna, V., Macchiato, M., 2000b. Analysis of time-scaling

Telesca, L., Cuomo, V., Lapenna, V. and Macchiato, M., 2001, Statistical analysis of fractal

Telesca L, Colangelo G, Lapenna V, Macchiato M. 2004. On the scaling behavior of rain

Telesca, L. and Chen, C.-C., 2010, Nonextensive analysis of crustal seismicity in Taiwan, Nat.

Thurner, S., Lowen, S.B., Feurstein, M.C., Heneghan, C., Feichtinger, H.G., Teich, M.C., 1997.

Turner, M. B., Cronin, S. J., Bebbington, M. S., and Platz, T.: Developing probabilistic

Turcotte, D.L., 1990. Fractal and chaos in geology and geophysics. Cambridge University

Wickman, F.E., 1965. Repose period patterns of volcanoes, 5. General discussion and a

Wickman, F.E., 1976. Markov models of repose-period patterns of volcanoes. In: Merriam, DF (Ed.), Random Processes in Geology. Sprínger-Verlag, Berlín, pp. 135–161.

tentative stochastic model. Ark. Mineral. Geol. 4, 351–367.

the Irpinia-Basilicata region (southern Italy). Fractals 7, 221–234.

of point processes in bilogical systems using wavelet- and Fourier-transform methods. In: Aldroubi, A., Unser, M. (Eds.), Wavelets in Medicine and Biology.

clustering structures in time-occurrence sequences of seismic events observed in

behaviour in the sequence of the aftershocks of the Bovec (Slovenia) April 12, 1998

properties of point processes modelling seismic sequences, Phys. Earth Planet. Int.,

event sequence recorded in Basilicata region (Southern Italy). *Journal of Hydrology* 

Analysis, synthesis, and estimation of fractal-rate stochastic point processes.

eruption forecasts for dormant volcanoes: a case study from Mt Taranaki, New

Seism. Soc. Am. 77 (4), 1368–1381.

CRC Press, Boca Raton, FL, 1996, pp. 383–412.

earthquake. Phys. Earth Planet. Int. 120, 315–326.

Hazards Earth Syst. Sci., 10, 1293-1297.

Zealand, Bull. Volcanol., 70, 507–515, 2008.

Press, Cambridge, 1990, p. 221.

197–202.

125, 65-83.

296: 234-240.

Fractals 5, 565–596.

Res. 84, 2153–2164.

Eng. 36, 150–160.

frequency of occurrence of large earthquakes along the Mexican subduction zone. Bulletin of the Seismological society of America, vol. 73, No. 6, pp. 1779-1796. Smalley, R.F., Chatelain, J.-L., Turcotte, D.L., Prévot, R., 1987. A fractal approach to the

clustering of earthquakes: applications to the seismicity of the New Hebrides. Bull.

Rock-fluid interactions along seismogenic faults are significant issues, because they are strongly related to seismogenic mechanisms and also to modifications of the seismogenic fault itself. Various mechanisms for seismogenesis have been proposed, such as frictional melting (Sibson, 1975; Spray, 1992), thermal pressurization (Mase and Smith, 1987; Melosh, 1979; O'Hara et al., 2006; Sibson, 1977; Wibberley and Shimamoto, 2005), acoustic fluidization (Melosh, 1979; Otsuki et al., 2003), elastohydrodynamic lubrication (Brodsky and Kanamori, 2001), and silica gel lubrication (Di Toro et al., 2006). Some of these are related to frictional heating. Heating signatures from natural faults have been well-studied on the basis of the remaining grains in pseudotachylyte along faults (Ikesawa et al., 2003; Ujiie et al., 2007), the vitrinite reflectance anomaly (O'Hara et al., 2006), borehole logging (Kano et al., 2006; Mishima et al., 2006; Tanaka et al., 2006), the thermal decomposition of paramagnetic minerals (Mishima et al., 2006), and the distribution of minor elements (Ishikawa et al., 2008). In addition to the thermal effects, some of the seismogenic mechanisms are also strongly related to rock-fluid interactions. Studies focusing on rockfluid interactions along fossil seismogenic faults have been conducted at some major fault zones. These include the Nojima fault in Japan (an intra-crustal seismogenic fault), where bulk rock chemistry analysis was used (Tanaka et al., 2007), the Chi-Chi fault in Taiwan (an active subduction plate boundary fault), which was studied on the basis of its clay characteristics (Hashimoto et al., 2008; Hashimoto et al., 2007), fossil faults such as the Mugi mélange, in the Shimanto Belt, Japan, again using bulk rock chemistry (Hashimoto et al., 2009), and an out of sequence thrust in the Shimanto Belt, Japan, using minor element distributions (Honda et al., 2011; Yamaguchi et al., 2011).

In this study, we focused on clay minerals within the fossil seismogenic fault along the subduction interface, in order to understand rock-fluid interactions at the fault. Clay minerals are commonly produced along faults, possibly by alteration of fine-grained abraded host rock materials due to rock-fluid interaction. The characteristics of clay minerals along seismogenic faults, in comparison with those of host rocks, provide clues to help understand rock-fluid interactions at the time of seismogenesis or related phenomena.

The studied fault is a fossil seismogenic fault along a subduction interface, in the Okitsu melange, the Cretaceous Shimanto Belt, SW Japan. The Shimanto Belt is the most studied on-land accretionary complex in the world, with lithology, age, thermal structure, and deformation structures available. These studies have revealed that the Shimanto Belt includes a deformation along its subduction interface from underthrusting to underplating, and that the Shimanto Belt is experienced at the seismogenic depth on the basis of the thermal model for seismogenic zones (Hyndman and Wang, 1993; Oleskevich et al., 1999). At the northernmost boundary fault of the Okitsu melange, the first pseudotachylyte within the sedimentary rocks was reported (Ikesawa et al., 2003), indicating that the fault was formed by melt lubrication along the subduction interface.

We conducted an X-ray diffraction (XRD) analysis on the host and fault rocks along the fossil seismogenic fault, and examined mineralogy, iron and magnesium substitution in chlorite, illite crystallinity, and semi-quantification of illite and chlorite to determine the clay characteristics for seismogenic fault rocks, in comparison with those of the host rocks. Finally, characteristic rock-fluid interactions in seismogenic faults, due to melt lubrication along the subduction interface, are discussed.
