15:45 〜 16:00
[SCG45-28] Pore fluid overpressures estimated from geometric data of crack-seal extension veins in subduction mélange
キーワード：鉱物脈、流体、地震、沈み込み帯、クラック
Pore fluid pressure (Pf) is of great importance to understand slow earthquake mechanics. In this study, we estimated the pore fluid pressure during the formation of foliation-parallel quartz veins filling mode I cracks. The foliation-parallel extension cracks can function as the fluid pathway in the mélange. In outcrops, multiple mineral veins can be observed in parallel. In the poro-elastic model, the distance (D) between parallel mineral vein groups is known to depend on the size of the pore fluid overpressure (dPf = Pf – σ3 – Ts; Ts: tensile strength) in the fracture, Young's modulus (E), and the aperture width (W) of the mineral veins (Price and Cosgrove, 1990). The formation of mineral veins requires elastic shortening of the host rock side by the thickness of the mineral vein precipitated in the fracture (Price and Cosgrove, 1990). Therefore, in the 2D poro-elastic model, the relationship between the spacing (D) between parallel mineral veins, the pore fluid overpressure dPf in the fracture, the Young's modulus E of the host rock, and the aperture width (W) of the mineral veins is dPf = E(W/D).
In this study, we estimated the pore fluid overpressure during the formation of foliation-parallel quartz veins filling mode I cracks in the Makimine mélange of Late Cretaceous Shimanto accretionary complex of SW Japan (temperature = 300–350°C, Palazzin et al., 2016). The mélange preserves quartz-filled shear veins, foliation-parallel extension veins and subvertical extension tension vein arrays. The coexistence of the crack-seal veins and viscously sheared veins (aperture width of a quartz vein: 10–80 of microns; Ujiie et al., 2018) may represent episodic tremor and slow slip (Ujiie et al., 2018). D of foliation-parallel quartz veins measured in the Makimine mélange is ~1.5 m. Estimated ΔPf in the case of the Makimine mélange are ~12–490 kPa (assuming depth = 10 km, tensile strength = 1 MPa and Young’s modulus = 3 GPa). The ΔPf are concordant with stress drop of a slow earthquake estimated from the shear veins in same outcrop (about 30–600 kPa; Ujiie et al., 2018).
Our results mean that as the pore fluid overpressure changes, the mineral vein spacing changes, i.e., the spacing of fractures through which fluid paths (the width of the fracture zone) changes. If mineral veins are related to slow earthquakes, the mineral vein spacing may provide information on the temporal variation of the excess pore fluid pressure within a slow earthquake cycle, in which case the pore fluid overpressure (pore fluid pressure greater than σ3) may provide a clue to the size of the slow earthquake (shear zone width, etc.).
Reference: Gudmundsson (1999) Geophys. Res. Lett., 26, 115–118; Palazzin et al. (2016) Tectonophysics, 687, 28–43; Price and Cosgrove (1990) Analysis of Geological Structures. Cambridge University Press, Cambridge, 502 p; Ujiie et al. (2018) Geophys. Res. Lett., 45, 5371–5379.
In this study, we estimated the pore fluid overpressure during the formation of foliation-parallel quartz veins filling mode I cracks in the Makimine mélange of Late Cretaceous Shimanto accretionary complex of SW Japan (temperature = 300–350°C, Palazzin et al., 2016). The mélange preserves quartz-filled shear veins, foliation-parallel extension veins and subvertical extension tension vein arrays. The coexistence of the crack-seal veins and viscously sheared veins (aperture width of a quartz vein: 10–80 of microns; Ujiie et al., 2018) may represent episodic tremor and slow slip (Ujiie et al., 2018). D of foliation-parallel quartz veins measured in the Makimine mélange is ~1.5 m. Estimated ΔPf in the case of the Makimine mélange are ~12–490 kPa (assuming depth = 10 km, tensile strength = 1 MPa and Young’s modulus = 3 GPa). The ΔPf are concordant with stress drop of a slow earthquake estimated from the shear veins in same outcrop (about 30–600 kPa; Ujiie et al., 2018).
Our results mean that as the pore fluid overpressure changes, the mineral vein spacing changes, i.e., the spacing of fractures through which fluid paths (the width of the fracture zone) changes. If mineral veins are related to slow earthquakes, the mineral vein spacing may provide information on the temporal variation of the excess pore fluid pressure within a slow earthquake cycle, in which case the pore fluid overpressure (pore fluid pressure greater than σ3) may provide a clue to the size of the slow earthquake (shear zone width, etc.).
Reference: Gudmundsson (1999) Geophys. Res. Lett., 26, 115–118; Palazzin et al. (2016) Tectonophysics, 687, 28–43; Price and Cosgrove (1990) Analysis of Geological Structures. Cambridge University Press, Cambridge, 502 p; Ujiie et al. (2018) Geophys. Res. Lett., 45, 5371–5379.