17:15 〜 19:15
[SCG45-P17] Differential stress and fluid pressure ratio during shear vein formation from geological constraints
キーワード：応力逆解析、流体圧、鉱物脈、沈み込み帯
Fluids play key roles in deformation and mass transfer along subduction plate interfaces. In particular, fluid pressure, along with differential stress, is a controlling factor for brittle failure modes in the seismogenic zone at subduction plate interfaces. Recent studies have quantified of changes in fluid pressure during a part of seismic cycles in seismogenic zones by employing a combined method of paleo-stress inversion, rock fracture theory, and fluid inclusion analysis, focusing only on tensile failure within brittle failure modes (e.g., Hosokawa and Hashimoto, 2022, Sci. Rep). The method can be adaptable on the development of shear failure. Therefore, the purpose of this study focused on the shear failures to quantify the fluid pressure ratio and differential stress in the seismogenic zone at subduction plate interfaces by a combined methods in the Yokonami mélange, the Cretaceous Shimanto Belt, SW Japan.
The Yokonami mélange is composed mainly of sandstone and black shale. Shear veins are well developed in the mélange, mostly parallel and sub-parallel to mélange foliation. The shear veins include thin clay films within the quartz and calcite minerals, which indicate that the shear veins have activated repeatedly. Fluid inclusion microthermometry for shear veins revealed fluid temperature and pressure of about 175-225 °C and about 143-215 MPa (Hashimoto et al., 2012, Island Arc). Stress inversion analysis also showed that shear veins in the mélange formed under reverse and normal fault stress regimes (Hashimoto et al., 2014, Tectonics).
Based on rock failure theory, the fluid pressure ratio during shear failure is proposed to be a function of θr (the angle between the fault plane and the maximum principal stress) and the differential stress (e.g., Cox, 2010, Geofluids). Therefore, we classify sampled shear veins into two stress regimes using orientation data (strike, dip, rake) and calculates θr for each. Then, fluid inclusion analysis determines fluid pressure ratio for each shear vein with θr. Finally, differential stress is calculated for each sample using rock fracture theory with determined θr and fluid pressure ratio.
Our classification resulted in three samples for the reverse fault stress regime and four samples for the normal fault stress regime. For both normal and reverse fault stress regimes, θr ranges from 18-53°, fluid temperatures from 165-244°C, and fluid pressures from 136-265 MPa. Using a geothermal gradient of 22°C/km (Hosokawa et al., 2023, JpGU meeting), we estimated the formation depth of each shear vein and calculated the fluid pressure ratio to be 0.57-0.96. Based on the estimated θr and fluid pressure ratio from this study, and assuming a friction coefficient of 0.4, the differential stress was estimated to be 80-167 MPa for reverse fault stress regime and 5.5-59.9 MPa for normal fault stress regime.
Our results show that shear veins form in the mélange under a wide range of differential stresses and fluid pressure ratio, which may reflect the influence of complex factors such as the topography of the subduction plate boundary and the mixed rheology of the mélange.
The Yokonami mélange is composed mainly of sandstone and black shale. Shear veins are well developed in the mélange, mostly parallel and sub-parallel to mélange foliation. The shear veins include thin clay films within the quartz and calcite minerals, which indicate that the shear veins have activated repeatedly. Fluid inclusion microthermometry for shear veins revealed fluid temperature and pressure of about 175-225 °C and about 143-215 MPa (Hashimoto et al., 2012, Island Arc). Stress inversion analysis also showed that shear veins in the mélange formed under reverse and normal fault stress regimes (Hashimoto et al., 2014, Tectonics).
Based on rock failure theory, the fluid pressure ratio during shear failure is proposed to be a function of θr (the angle between the fault plane and the maximum principal stress) and the differential stress (e.g., Cox, 2010, Geofluids). Therefore, we classify sampled shear veins into two stress regimes using orientation data (strike, dip, rake) and calculates θr for each. Then, fluid inclusion analysis determines fluid pressure ratio for each shear vein with θr. Finally, differential stress is calculated for each sample using rock fracture theory with determined θr and fluid pressure ratio.
Our classification resulted in three samples for the reverse fault stress regime and four samples for the normal fault stress regime. For both normal and reverse fault stress regimes, θr ranges from 18-53°, fluid temperatures from 165-244°C, and fluid pressures from 136-265 MPa. Using a geothermal gradient of 22°C/km (Hosokawa et al., 2023, JpGU meeting), we estimated the formation depth of each shear vein and calculated the fluid pressure ratio to be 0.57-0.96. Based on the estimated θr and fluid pressure ratio from this study, and assuming a friction coefficient of 0.4, the differential stress was estimated to be 80-167 MPa for reverse fault stress regime and 5.5-59.9 MPa for normal fault stress regime.
Our results show that shear veins form in the mélange under a wide range of differential stresses and fluid pressure ratio, which may reflect the influence of complex factors such as the topography of the subduction plate boundary and the mixed rheology of the mélange.