Abstract

Here we present the timings of reactivated events from a fault in the northern Pohang area, which should be located at the northern-end of Yangsan fault line, the major fault in the southeastern Korean Peninsula. Recently developed illite-age-analysis (IAA) approach was employed for determining the fault-activated timing, combined with illite-polytype quantification using the optimized full-pattern-fitting (FPF) method, and K-Ar age-dating for each size fraction(<0.1 μm, 0.1–0.4 μm, and 0.4–1.0 μm) of 4 fault clay samples. Two chronological records of brittle fault-activation events were recognized at 19.6±1.86 Ma and 26.1±2.55–27.9±3.46 Ma. The ages are much younger than those of fault clays from Sangcheon-ri area (41.5~43.5 and 50.7 Ma), the southern part of Yangsan fault line, and are close to the timing of East Sea-opening event. Further chronological analysis for additional sites of the Yangsan fault should be needed to reveal the time-scheme of the tectonic events and their spatial distributions along the fault line.

Keywords Yangsan fault, fault reactivation age, illite-age-analysis(IAA), K-Ar age-dating, fault clay