[S-CG46] Uncovering stress accumulation and fault strengthening of megathrust earthquakes

[SCG46-08] Hydrogeology of the frontal prism near the Japan Trench revealed by geochemical analyses of IODP Expedition 405

*石川 剛志¹、井尻 暁²、Brown Cameron³、諸野 祐樹¹、Rasbury Troy⁴、Robertson Rebecca⁵、Sun Chijun⁶、Zhang Junli⁷、Conin Marianne⁸、Fulton Patrick⁹、Kirkpatrick Jamie¹⁰、Regalla Christine¹¹、氏家 恒太郎¹²、小平 秀一¹、前田 玲奈¹、奥津 なつみ¹、the Expedition 405 Scientists (1.JAMSTEC、2.Kobe University、3.University of California Los Angeles、4.Stony Brook University、5.Durham University、6.University of California Davis、7.University of Bremen、8.University of Lorraine、9.Cornell University、10.University of Nevada, Reno、11.Northern Arizona University、12.University of Tsukuba)

キーワード：IODP、日本海溝、地球化学、流体、沈み込み帯、地震

Analyses of interstitial water (IW), headspace gas and rock samples from IODP Expedition 405 (JTRACK) provide high-resolution geochemical data over the entire 860 m-thick section consisting of the frontal prism, plate boundary fault zone (PBFZ), and lower plate sediment (Site C0019), as well as the entire 280 m-thick section of incoming sediment (Site C0026). These data allow us to define and characterize the pore fluid geochemistry of the frontal prism near the Japan Trench, and how it varies with depth below the seafloor. Depth profiles of IW and headspace gas geochemistry observed at Site C0019 reveal the presence of notable excursions above a major fault identified at 610 mbsf and within the PBFZ. Relatively sharp gradients in the concentration profiles observed in the IW at these depth intervals are unlikely to be in a steady state, but are controlled by dynamic fluid processes. The excursion above the 610 mbsf fault appears to correspond to a zone of relatively high porosity, suggesting a close relationship between the excursion fluids and the evolutionary history of the frontal prism architecture. The IW geochemistry of the PBFZ is well constrained from samples recovered from Holes C0019J and C0019K and also JFAST Hole C0019E. The observed IW geochemistry profiles are essentially consistent with each other between these three holes at this depth interval, and are characterized by intense IW excursions. Comparison with the signatures observed at Site C0026 suggests that fluids derived from the lower plate contribute to the PBFZ fluid geochemistry. IW signatures at Site C0019 also suggest that relatively low-temperature processes may dominate the hydrogeology of the frontal prism and plate boundary near the Japan Trench, in contrast to observations in warm subduction zones such as the Nankai Trough. These data from JTRACK will be fundamental for characterizing the hydrogeology of the PBFZ and the frontal prism and for understanding the architectural history of the shallow Japan Trench subduction zone.