Quantitative estimation of fluid pressure ratio of shear vein in an on-land accretionary complex, the Yokonami mélange, the Cretaceous Shimanto Belt, Kochi, southwest Japan

Abstract

The fluid pressure ratio and effective friction coefficient of shear veins in an exhumed accretionary complex in the Cretaceous Shimanto Belt, southwest Japan, were constrained by fluid inclusion and stress inversion analyses. The shear veins are considered to be developed along the decollement. Temperature and pressure conditions for shear veins were found to be in the range of 175–225°C and 143–215 MPa, respectively. Fluid pressure ranged between the lines of 70–100% lithostatic pressure, when the geothermal gradient was assumed on the basis of the slab age at the trench. The effective friction coefficient of shear veins was estimated to be 0.1–0.2 by stress inversion analysis, which corresponds to a fluid pressure ratio of 0.67–0.83. Both pressure–temperature and friction coefficient results indicate that the fluid pressure ratio is higher than about 0.7. In this paper, we propose the procedure to constrain paleo-fluid pressure ratio from natural materials. Wedge states were examined using the weak decollement friction coefficient, with shallow and steep taper angles indicating that the wedge state is stable or compressively critical, and is controlled by the fluid pressure ratio within the wedge.