[S-SS08] Fault Rheology and Earthquake Physics

[SSS08-14] Overpressured subduction plate boundary caused by mantle-derived fluids: Evidence from noble gas isotope analysis on veins in subduction mélange

★Invited Papers

*Naoki Nishiyama¹, Hirochika Sumino², Kohtaro Ujiie¹ (1.University of Tsukuba, 2.The University of Tokyo)

Keywords:Subduction fluids, Subduction plate boundary, Noble gas isotopes, mineral-filled vein

Overpressured fluids cause a weak subduction plate boundary and affect the degree of locking on the plate boundaries. The generation of fluid overpressure has been invoked for compaction disequilibrium and mineral dehydration through diagenetic and metamorphic processes of subducting sediments and altered oceanic crust (Hyndman & Peacock, 2003). The subduction mélange in the Shimanto accretionary complex, southwest Japan, preserves the quartz-calcite-filled shear veins and sigmoidal extension veins under fluid overpressure near the downdip limit of the thermally-controlled seismogenic zone. The shear veins record the repeated low-angle brittle thrusting under fluid overpressure, while the extension veins constitute the Riedel shear zones developed during subduction under vertical maximum principle stress. To examine the origin of overpressured fluids, we conducted fluid inclusion and noble gas analyses on the shear and extension veins.

The result of fluid inclusion analysis shows various fluid pressures ranging from lithostatic to hydrostatic pressures during the vein formation, which were associated with warm fluid infiltration. Helium isotope ratios (³He/⁴He) of shear veins and extension veins range 1.6-2.5 Ra and 1.6-2.3 Ra, respectively, indicating the presence of mantle helium (Ra is the ³He/⁴He ratio of air). Argon isotope ratios (⁴⁰Ar/³⁶Ar) and elemental ratios of heavy noble gases (⁸⁴Kr/³⁶Ar and ¹³⁰Xe/³⁶Ar) of the veins are similar to those of serpentinized mantle, suggesting that mantle helium originated from serpentinized mantle. Because the mantle-derived rocks such as peridotite and serpentinite are absent in the mélange, the results of noble gas analyses represent the infiltration of fluids from the serpentinized mantle into the mélange shear zone. The ratios of serpentinized mantle-derived fluids to the vein-forming fluids estimated by the ⁸⁴Kr/³⁶Ar and ¹³⁰Xe/³⁶Ar values are >64-94%, showing that the fluid supplied from the serpentinized mantle contributed to fluid overpressure in subduction plate boundary near the downdip limit of the seismogenic zone (Nishiyama et al., 2020). The infiltration of serpentinized mantle-derived fluids could contribute to the generation of tensile cracking during subduction-related deformation and brittle thrusting under fluid overpressure.

References
Hyndman, R.D., & Peacock, S.M. (2003), Earth Planet. Sci. Lett. 212, 417-432. doi: 10.1016/S0012-821X(03)00263-2.
Nishiyama, N., Sumino, H., & Ujiie, K. (2020), Earth Planet. Sci. Lett. 538, 116199. doi: 10.1016/j.epsl.2020.116199.