[S-CG57] New perspectives of subduction zone earthquake dynamics through experiments across-scales

[SCG57-04] Geochemical characteristics and slip behavior of ancient plate boundary fault in the Shimanto accretionary, Hidakagawa Formation, Kii Peninsula, Japan

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

Keywords:plate boundary fault, fluid rockfluid-rock interaction, albitization

In the accretionary complex exposed on land, information of ancient plate boundary faults is preserved. Therefore, the investigation is important for understanding the slip behavior of plate subduction boundary faults. It is important to understand the characteristics and the slip behavior of the faults along trench direction, because the minerals composing accretionary complex vary with the distance from the land. In this study, we investigated the mélange unit in the Shimanto belt, Hidakagawa Formation, Miyama Unit, which is exposed on the coast of Mio area in Gobo, Wakayama, Japan. The unit is composed of sandstone, mudstone, chert, and basalt show significant deformation structures. Some faults have good linearity and extend to east-west direction. We collected samples from the fault and surrounding host rocks, and conducted microstructure observation, major- and trace-elements concentration measurement and XRD analysis. As a result of microstructure observation, we confirmed that the primary slip zone had remarkable composite planar fabric and quartz fragments incorporated into the primary slip zone from cataclasite. Furthermore, as a result of major- and trace-elements analysis, the concentration of fluid mobile elements such as Li, Sr, Rb and Cs in the primary slip zone changed compared with host rock. Therefore, we carried out model calculation assuming that fluid-rock interactions happened in this fault. The result showed that high-temperature fluid (>350 °C) existed on the fault. Next, as a result of XRD analysis, the primary slip zone showed the decrease of quartz and increase of plagioclase compared to the host rock.
Therefore, we carried out model calculation assuming albitization which is caused between host rock and the high-temperature fluid. The result showed that the anomalous of elements concentrations in the primary slip zone can be explained by albitization at 300 °C or higher. These results indicated that the mineral composition of plate boundary faults changes due to the reaction with high-temperature fluids generated during and immediately after an earthquake. Such changes may affect slip behavior during earthquake.