A microstructural study of the fault gouge in the granite, Yangbuk, Gyeongju, southeastern Korea, with implications for multiple faulting

Microstructures developed in the fault gouge zone, Yangbuk, Gyeongju city, Korea, were characterized with emphasis on distribution of remaining grains. Based on XRD analysis, the fault gouges consist of quartz, feldspar and clay minerals composed of abundant illite and chlorite, with vein minerals including laumontite and pyrite. As the fractal dimension (D-value) of grains gradually increases, the pore space in the fault gouge becomes smaller, causing the change in microstructures. Quartz and feldspar exerted a different influence on the development of microstructures in the fault gouge zone, derived from essential reactivity inherent between the two minerals during multiple faulting. Quartz grains intensively suffered grain size reduction while the content and size of coarse grains decreased. Feldspar grains tend to influence the formation of clay minerals made up for the matrix. Although large quartz grains highly fractured with abrasions caused the grain size reduction in the gouge zone during early fault activities, fine-grained quartz in the clay-rich matrix easily remained due to the reduction in friction resulted from clay minerals. On the other hand, feldspar grains underwent more abrasion and dissolution, leading to an increase in the amount of clay minerals newly formed during fault activities. Changes in microstructures were accompanied by precipitation of the secondary minerals during multiple fault activities in connection with fracturing of main rock-forming minerals such as quartz and feldspar derived from the host rock.