Faulting Processes and K-Ar Ages of Fault Gouges in the Yangsan Fault Zone

Microstructural research, XRD analysis and K-Ar age dating were performed with fault gouge samples taken from the Yangsan fault zone. Most of thin sections in microstructural study show that clay minerals have well grown under the influence of hydrothermal alteration in the matrix of the fault gouges and define clay foliations with parallel alignments. The foliations in the gouge zones could be interpreted as the latest products of faulting, since the preexisting features are not easy to be preserved through extensive reworking during faulting. Deformation mechanism in the gouge zones operated with the cataclasis of brittle deformation in the early stage of deformation and transferred subsequently to cataclastic flow in the regions of maximum localization of fault slip. Fluid-assisted processes were also occurred significantly in the gouge zones with every faulting. Clay minerals formed in the gouge zones are composed predominantly of smectite and illite/smectite (I/S) mixed-layers, with accessory illite, chlorite and kaolinite. Smectite is characterized by a dioctahedral type and I/S mixed-layers contain 70% smectite layers in the structure with R1 type in which smectite and illite layers are regularly alternated with each other. Illite is found to have the 1Mc1 polytype that is stable at low temperature, and its crystallinity ranges from 0.41 to 0.43. Illitization of smectite that was initially formed in gouges is considered to proceed through I/S mixed-layers in which the percentage of illite interlayers typically increases with reaction processes. Considering clay mineral characters analyzed, they were formed at relatively low temperatures during relatively a short period. K-Ar ages of the gouges derived from Bulguksa granites (Tdo and A₂) show 58.7 Ma and 50.3 Ma, and volcanic rocks of the Yuchon Group (Bhwa) 37.3 Ma. The ages of fault gouges are interpreted to represent those of hydrothermal alteration following the fault movements, corresponding to the early or old events of the Yangsan fault activities. The ages of 58.7 Ma (Tdo) and 50.3 Ma (A₂) which were gotten from the gouges taken along Quaternary faults mean that these faults reactivated along the preexisting faults in Quaternary.