Physicochemical characterization of the youngest active domain in major fault zones using the weathering index and X-ray computed tomography

The physicochemical characteristics of brittle fault rocks are essential for understanding fault evolution and deformation mechanisms, and also for selecting suitable sites for important facilities such as nuclear power plants, radioactive waste disposal sites, and oil storage bases. In this study, we determined the chemical weathering index (W) for brittle fault rocks and their protoliths along three active faults: the Shiraki-Nyu Fault in granite, the Tsuruga Fault at a geological contact between granite and greenstone, and the Yamada Fault in adamellite, which are all located in the peripheral area of Wakasa Bay, southwest Japan. We investigated the physicochemical characteristics of the youngest active domain of the brittle fault rocks based on the relationship between computed tomography data and alteration intensity (AI) values. The computed tomography values (which indicate density) for the fault rocks are the lowest in the fault gouge and correspond to the latest active fault zones. The W values of the fault rocks are mainly affected by changes in Na₂O and CaO contents, corresponding to the dissolution or deposition of plagioclase and calcite in the granite, clinopyroxene and hornblende in the greenstone, and plagioclase in the adamellite. Fresh plagioclase fragments are present in the fault gouge of the latest active fault zones. The W values indicate the effects of hydrothermal alteration occur at up to W = 50 %–60 %, and weathering occurs at W > 60 %. The brittle fault rocks within ∼10 cm of the main fault plane Y are strongly affected by density decrease, hydrothermal alteration, and weathering. The application of W values to brittle fault rocks is an effective method for understanding the mineralogical variations associated with hydrothermal alteration and weathering in fault rocks, and it is possible to improve the accuracy of identifying the youngest active domain in major fault zones by joint analyses of computed tomography data.