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.

中文翻译：

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使用风化指数和 X 射线计算机断层扫描对主要断裂带中最年轻的活动域进行物理化学表征


脆性断层岩的物理化学特征对于理解断层演化和变形机制至关重要，也有助于为核电站、放射性废物处理场和储油基地等重要设施选择合适的场地。在这项研究中，我们确定了沿三个活动断层的脆性断层及其原生岩的化学风化指数 （W）：花岗岩中的 Shiraki-Nyu 断层、花岗岩和绿岩地质接触处的敦贺断层以及亚细岩中的山田断层，它们都位于日本西南部若狭湾的外围地区。我们根据计算机断层扫描数据与蚀变强度 （AI） 值之间的关系，研究了脆性断层岩最年轻的活动域的物理化学特征。断层岩石的计算机断层扫描值（表示密度）在断层凿孔中最低，对应于最新的活动断层带。断岩的W值主要受Na2O和CaO含量变化的影响，对应于花岗岩中斜长石和方解石、绿岩中斜辉石和角闪石、方解石中斜长石和斜长石的溶蚀或沉积。新鲜的斜长石片段存在于最新活动断层带的断层凿孔中。W 值表示热液蚀变的影响发生在高达 W = 50 %–60 % 的地方，风化发生在 W > 60 % 时。主断层平面 Y 周围 ∼10 cm 以内的脆性断层岩受密度降低、热液蚀变和风化作用的强烈影响。 将 W 值应用于脆性断层岩是理解与断层岩热液蚀变和风化相关的矿物学变化的有效方法，并且可以通过联合分析计算机断层扫描数据来提高识别主要断层带中最年轻的活动域的准确性。