[S-CG46] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG46-P11] Characteristics in structural geology and geochemistry of the Mio Mélange in the Miyama Formation, Hidakagawa Group, Shimanto Belt

*Yoshiki Kinoshita¹, Tetsuro Hirono¹, Tsuyoshi Ishikawa², Tetsuya Sakuyama¹ (1.Osaka Metropolitan University, 2.JAMSTEC)

Keywords:Shimanto accretionary complex, mélange, thermal pressurization, fluid–rock interactions

To understand the structural and geochemical characteristics of slip zones within subduction-related mélange unit, we performed field investigation, microscopic observations, and mineralogical and chemical analyses, by using an electron-probe microanalyzer, of a primary slip zone and its surrounding host rocks in the Hidakagawa Group of the Shimanto accretionary complex, Kii Peninsula, Japan. The slip zone is characterized by the development of composite planar fabrics such as R₁ shear planes, flow structure, and micrometer-sized euhedral dolomite particles. In addition, previous research reported anormal concentrations of fluid-mobile elements such as Sr, Rb, and Cs in the slip zone, compared to those in the host rocks, and concluded that fluid–rock interactions in the slip zone occurred at 250–300 degree C. The presence of such high-temperature fluid in a fault could affect slip behavior of the fault, probably weakening shear strength on the fault through thermal pressurization, and existence of euhedral dolomite particles might be causing from fluid flow having high concentrations of Ca, Mg, and CO₂ from deeper portion along the subduction fault and/or from the surrounding basaltic rocks.