Microstructural Features and K-Ar Ages of Fault Gouges from Quaternary Faults along the Northern Yangsan Fault, SE Korea

초록

Microstructural characterization, identification of mineral assemblages, and K-Ar age dating of fault gouges from five Quaternary fault sites segmented along the northern Yangsan Fault, SE Korea were performed to understand formation condition and multiple activity of faults. The mean and median sizes of particles of bulk gouges vary among the studied faults: 1.75 µm and 1.43 µm for the Danguri Fault, 1.94 µm and 1.79 µm for the Yukjae Fault, 5.57 µm and 4.16 µm for the Yugye Fault, and 5.55 µm and 2.31 µm for the Bogyeongsa Fault. Fault gouges contain abundant secondary minerals, including smectite, chlorite, illite, kaolinite, laumontite, and mordenite, which are found in association with quartz and feldspar. K-Ar dating of the fault gouges (both bulk samples and separate size fractions) yields ages ranging from 59.1 to 18.8 Ma, with bulk ages of 47.6 Ma for the Yukjae Fault, 59.1 Ma for the Ansim Fault, 39.4 Ma for the Yugye Fault, and 22.6 Ma for the Bogyeongsa Fault. The finer fractions generally have younger K-Ar ages compared with the coarser fractions, and the finest fraction (<0.2 µm) is the youngest for each fault. Hydrothermal alteration of the gouges is considered to have occurred under low-temperature (100~200°C) conditions during faulting. Microstructural features and clay mineral assemblages of fault gouges and brecciated rocks should be considered when interpreting fault events and reactivation, in addition to age dating of faulting. © 2023 The Korean Society of Engineering Geology.

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