10:15 〜 10:30
[SCG49-12] Origin of overpressured fluids in subduction plate boundary near the downdip limit of the seismogenic zone
★Invited Papers
キーワード：マントル起源流体、希ガス同位体、剪断脈、脆性塑性遷移領域
Fluids play an important role in the seismicity in subduction zones. Overpressured fluids have been recognized as a key factor for occurrence of slow earthquakes near the down dip of the seismogenic zone. However, the origin of overpressured fluids in the source region of slow earthquakes remains poorly understood. The subduction mélange in the Shimanto accretionary complex, southwest Japan, preserves the quartz-calcite-filled shear veins and sigmoidal extension veins formed under fluid overpressure near the downdip limit of the thermally-controlled seismogenic zone. To examine the origin of overpressured fluids, we conducted fluid inclusion and noble gas analyses on the shear and extension veins.
The shear veins are characterized by the repetition of low-angle brittle fracturing and subsequent fast crack sealing by quartz precipitation associated with a large fluid pressure change of ~220-280 MPa (Ujiie et al., 2018; Nishiyama et al., 2021). The extension veins constitute the Riedel shear zones developed during subduction under vertical maximum principle stress. Helium isotope ratios 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 in the vein-forming fluids. Elemental ratios of heavy noble gases (84Kr/36Ar and 130Xe/36Ar) of the veins suggest that >64-94% of vein-forming fluids are supplied 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 that the infiltration of fluids 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
Ujiie K., Saishu H., Fagereng A., Nishiyama N., Otsubo M., Masuyama H., & Kagi H. (2018), Geophys. Res. Lett. doi: 10.1029/2018GL078374.
Nishiyama, N., Sumino. H., & Ujiie, K. (2020), Earth Planet. Sci. Lett. doi: 10.1016/j.epsl.2020.116199.
Nishiyama, N., Ujiie, K., & Kano, M. (2021), Earth Planets Space. doi: 10.1186/s40623-021-01448-7.
The shear veins are characterized by the repetition of low-angle brittle fracturing and subsequent fast crack sealing by quartz precipitation associated with a large fluid pressure change of ~220-280 MPa (Ujiie et al., 2018; Nishiyama et al., 2021). The extension veins constitute the Riedel shear zones developed during subduction under vertical maximum principle stress. Helium isotope ratios 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 in the vein-forming fluids. Elemental ratios of heavy noble gases (84Kr/36Ar and 130Xe/36Ar) of the veins suggest that >64-94% of vein-forming fluids are supplied 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 that the infiltration of fluids 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
Ujiie K., Saishu H., Fagereng A., Nishiyama N., Otsubo M., Masuyama H., & Kagi H. (2018), Geophys. Res. Lett. doi: 10.1029/2018GL078374.
Nishiyama, N., Sumino. H., & Ujiie, K. (2020), Earth Planet. Sci. Lett. doi: 10.1016/j.epsl.2020.116199.
Nishiyama, N., Ujiie, K., & Kano, M. (2021), Earth Planets Space. doi: 10.1186/s40623-021-01448-7.