[S-SS08] Fault Rheology and Earthquake Physics

[SSS08-15] Slip trace along an ancient plate boundary fault in the Shimanto accretionary, Hidakagawa Formation, Miyama Unit, Kii Peninsula, Japan: Organic geochemical and elemental approaches

*Ayana Shimamoto¹, Tetsuro Hirono¹, Tsuyoshi Ishikawa², Takeaki Ogawa¹ (1.Department of earth and space science, Graduate School of Science, Osaka University, 2.Kochi Institute for Core Sample Research, JAMSTEC)

Keywords:Carbonaceous Materials, Spectroscopic analysis, trace elements

In recent years, ancient plate boundary faults preserved in onland accretionary complexes have been investigated in order to understand the slip behavior earthquake along during plate subduction. In order to elucidate the characteristics and slip behavior of faults, we investigated the mélange unit in the Miyama Unit of the Hidakagawa Formation, which is exposed on the coast of the Mio area in Gobo, Wakayama, Japan. The unit is composed of sandstone, mudstone, chert, and altered basalt showing remarkable shear deformations, and some linear faults were developed. We collected samples from the main fault and its surrounding host rocks in the unit, and carried out microstructure observation, thermal background analysis using carbonaceous materials, and major- and trace-element concentration measurements. First, as a result of microstructure observation, we confirmed remarkable comminution in the primary slip zone compared to the surrounding cataclasites and host rocks. Next, as a result of Raman spectroscopic analysis of carbonaceous materials, the maximum temperature experienced host rock is estimated to 200 ℃ while primary slip zone is 400-900 ℃. Furthermore, as a result of major- and trace-element concentration analysis, the primary slip zone showed the decrease of quartz and the increase of plagioclase clearly compared to the host rock. Therefore, we carried out model calculation assuming albitization which is caused by metasomatism between host rock and high temperature fluid in the greenschist facies. The concentration change of elements in primary slip zone could be explained by albitization mediated by high temperature of 300 ℃ or higher. Accordingly, these results indicated that the mineral composition in the fault changed significantly due to the high temperature fluid generated during or immediately after an earthquake along plate boundary fault.