Keywords:Slow earthquake, Stress, Crack, Fluid, Subduction
Pore fluid pressure Pf is important for understanding slow earthquake mechanics. In this study, we estimated the pore fluid pressure during the formation of foliation-parallel quartz veins filling mode I cracks in the Makimine mélanges, eastern Kyushu, SW Japan. In the coastal region of the Makimine mélanges (Late Cretaceous Shimanto accretionary complex of SW Japan; temperature = 300–350°C, Palazzin et al., 2016), the mélange preserves quartz-filled shear, foliation-parallel veins and tension vein arrays. We applied the stress tensor inversion approach proposed by Sato et al. (2013) to estimate stress regimes by using foliation-parallel vein orientations. The estimated stress is a reverse faulting stress regime with a sub-horizontal σ1-axis trending NW–SE and a sub-vertical σ3-axis, and the driving pore fluid pressure ratio P* (P* = (Pf – σ3) / (σ1 – σ3)) is ~0.1. When the pore fluid pressure exceeds σ3, veins filling mode I cracks are constructed (Jolly and Sanderson, 1997). The pore fluid pressure that exceeds σ3 is the pore fluid overpressure ΔPf (ΔPf = Pf – σ3). To estimate the pore fluid overpressure, we used the poro-elastic model for extension quartz vein formation (Gudmundsson, 1999). Pf in the case of the Makimine mélanges are ~280 MPa (assuming depth = 10 km, density = 2750 kg/m3, tensile strength = 5 MPa and Young’s modulus = 7.5–15 GPa). The normalized pore pressure ratio λ* (λ* = (Pf – Ph) / (Pl – Ph), Pl: lithostatic pressure; Ph: hydrostatic pressure) is ~1.03 (Pf > Pl).