[S-CG40] Science of slow-to-fast earthquakes

[SCG40-P33] Geochemical evidence for fluid-rock interactions around the thermally controlled base of the subduction seismogenic zone

*石川 剛志¹、氏家 恒太郎² (1.海洋研究開発機構高知コア研究所、2.筑波大学生命環境系)

キーワード：流体、沈み込み帯、スロー地震、交代作用、地球化学

Fluid-rock interactions are thought to play an essential role in various types of slow earthquakes. However, the source, composition, and transfer process of slow earthquake-related fluids, and how chemical reactions associated with such fluids can induce fault weakening, are not well understood. In this study, we performed major and trace element and isotopic analyses on paleo-megathrust rocks from the Makimine mélange of the Shimanto accretionary complex, Japan, for which aseismic megathrust slip is proposed to have been caused by albite metasomatism at 370°C and concomitant viscous creep along the metasediment-metabasite (metabasalt or metadolerite) boundaries (Ujiie et al., 2022). The composition of fluid-immobile elements and isotopes indicates that the metasomatized rocks are entirely of metasediment origin. Fluid-mobile elements show a clear systematic variation within the metasomatized reaction zone, with concentrations of elements such as Rb, Cs, and Ba decreasing dramatically toward contact with metabasite. This suggests that intense fluid-rock interactions occurred there. The estimated fluid-mobile element compositions of fluids equilibrated with metasomatized rocks near the contact resemble those of Arima-type saline fluids, which are considered to represent slab-derived fluids (e.g., Kusuda et al., 2014) and may exist in the megathrust of the Nankai Trough subduction zone at depths where deep slow earthquakes such as tectonic tremors occur (e.g., Umam et al., 2022; Ishikawa and Tanimizu, 2023). Geochemical model calculations indicate elevated fluid/rock ratios near the contact with metabasite, suggesting extensive lateral fluid flow along the metasediment-metabasite interface. It is conceivable that the observed metasediment-metabasite boundaries of the Makimine mélange represent pathways for Arima-like saline fluids derived from the deeper part of the slab, and that extensive fluid-rock interactions caused albite metasomatism and reduction of megathrust strength, promoting aseismic slip.

References: Ishikawa and Tanimizu, JpGU abstract, SCG45-30, 2023; Kusuda et al., EPS, 66, 119, 2014; Ujiie et al., Geochem. Geophys. Geosystems, 23, e2022GC010569, 2022; Umam et al., Geochem. J., 56, e8-e17, 2022.