Insights of fault activities through 3-D shape preferred orientation and illite age analysis

Combining the absolute age of illite in fault rocks with microstructure analysis plays a significant role in inferring the deformation processes of fault zones that have undergone multiple complex fault activities. We aim to propose a new approach to reconstruct fault activation history by conducting systematic fabric observations, X-ray diffraction analysis, 3-D shape preferred orientation (SPO) analysis, and illite age analysis (IAA) on fault gouge affected by brittle deformation in the Yeonghae area of the Yangsan fault in the southeastern part of the Korean Peninsula, where the tectonic environment has been continuously changing since the Cretaceous. We inferred the early fault activation process from large carbonate fragments, aggregates, and chlorite in the fault gouge, and the late fault activation process from the composition of matrix-filling clay minerals, the presence and foliation of clay-bearing layers, and the SPO of rigid clasts. The outcrop in the Yeonghae area of the Yangsan fault initially experienced fault activation at relatively high temperatures and depths, and finally underwent left-lateral strike-slip fault motion due to slip inferred as aseismic creep at relatively low temperatures and shallower depths. Comparing the K-Ar IAA results with previous studies analyzing paleostress fields, it is estimated that the multiple early fault activations in the Yeonghae area of the Yangsan fault occurred between 67 Ma and 38 Ma, and the late aseismic creep occurred at ca. 17−10 Ma. Integrating quantitative 3-D SPO analysis and IAA with in-depth fabric observations will provide evidence to elucidate the developmental history of fault rocks based on highly reliable data.