[S-SS30_29PM2] New perspective of great earthquakes along subduction zones

[SSS30-33] Generation of high-temperature fluid and its spatial distribution in an ancient megasplay fault

*Hirokazu MASUMOTO¹, Tetsuro HIRONO¹, Tsuyoshi ISHIKAWA², Wataru TANIKAWA², Hideki MUKOYOSHI³ (1.Department of Earth and Space Science, Graduate School of Science, Osaka University, 2.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 3.Faculty of Education and Integrated Arts and Sciences, Waseda University)

Keywords:Nankai trough, megasplay fault, Shimanto accretionary complex, fault rocks, fluid-rock interactions, X-ray diffraction

An ancient megasplay fault outcrop is identified within Kure region of the Shimanto accretionary complex and has been formed at 2.5–5.5 km depth. Recent works show pseudotachylyte produced by frictional melt, fluid-rock interactions at high temperatures (>350 ℃) and enrichment of incompatible element concentrations on the fault. However, spatial distribution of pseudotachylyte and high-temperature fluid is not investigated. These informations are important to understand an earthquake is able to produce extensively high-temperature fluid and thermal pressurization. Accordingly, we performed deformation structures analysis and obtained 46 rock samples from the outcrop and analyzed these samples by vitrinite reflectance measurement, powder X-ray diffraction-RockJock mineral composition analyses and trace elements compositions measurement. Therefore, analyses of black gouge samples from the slip zone indicate fluid-rock interactions at high temperatures, whereas footwall sandstone samples that are close to fault gouge indicate enrichment of quartz and decrease of rare-earth element concentrations. These contrasts may be regarded as mobilization of elements derived from seepage of yielded high temperatures fluid within the slip zone.