A geochemical estimation of fluid flux and permeability for a fault zone in Mugi mélange, the Cretaceous Shimanto Belt, SW Japan

Abstract

The fluid flux and permeability along seismogenic subduction interfaces were estimated roughly based on geological evidences. A decrease in silica from the host rocks to the deformed rocks due to rock–fluid interaction within the fault–fluid system was qualitatively examined. The area under study was the northernmost part of the Mugi mélange in the Cretaceous Shimanto Belt, Shikoku, SW Japan. A fault zone with pseudotachylytes is located at the area as the roof thrust of the Mugi mélange. The pressure–temperature conditions of the fault zone correspond to the depth around the onset of the seismogenic zone along the subduction interface. On the basis of deformation texture, rocks are classified into three types—mélange, fault breccia, and ultracataclasite. The bulk chemical analysis of the abovementioned rocks was conducted by employing the X-ray fluorescence (XRF) analysis method. The average mass loss from the mélange host rocks to the fault rocks was estimated by the isocon method. In particular, a decrease in silica was observed clearly. Based on the decrease in silica and certain other geological constraints such as the pressure–temperature conditions of fluid, length of the fault, and the duration between deposition and exhumation, the amount of fluid reacting with rocks and the fluid flux were examined roughly. The estimated fluid volume was approximately thousand times the rock volume, and the fluid flux was about 10 −8–10 −6 m/s. Based on the fluid flux and some constraints in temperature pressure conditions of fluid and pressure gradient, the permeabilities of fault zone rocks were estimated, which was approximately of the order of about 10 −17–10 −16 m 2 and about 10 −16–10 −14 m 2 in the fault breccia and ultracataclasite, respectively. While the estimated value of the permeability of the fault breccia is similar to that obtained in laboratory experiments, the estimated value of the permeability of the ultracataclasite is considerably larger than that obtained from laboratory experiments. This probably indicates that the permeability of the ultracataclasite is not constant as observed in the laboratory experiments but can evolve during seismic cycle.