K-Ar illite dating to constrain multiple events in shallow crustal rocks: Implications for the Late Phanerozoic evolution of NE Asia

A recently developed illite-age-analysis (IAA) approach was applied to determine the multiple events for the Chugaryeong fault belt, Korea. Each event was determined by a combined approach of the optimized illite-polytype quantification and the K-Ar age-dating of clay fractions separated from the fault clays. The Late Cretaceous to Paleogene events (76.5±0.8, 69.1±0.6, 59.3±0.7, and 48.2±0.7Ma) were recognized by calculating the authigenic 1M/1M_d illite ages on the IAA plots of the fault clays. The Early Cretaceous ages (121.7-124.7 and 112.4±1.5Ma) were also obtained from the convergent intercepts of 100% 2M₁ illite on the IAA plots. The absence of the 2M₁ illites in the host-rock indicates that the Early Cretaceous ages represent the timings of high-T hydrothermal events of >280°C. The 2M₁ illites in the fault clays should be pre-formed by a fluid-rock interaction under a relatively high-T subsurface condition, and be mechanically reworked into the near surface along the fault by post-tectonic events. This is the first report determining the absolute age constraints of multi-activated tectonic events from a fault. These geochronological determinations of the multiple events recorded in the Chugaryeong fault belt are crucial to establish the tectonic evolution of the Korean Peninsula since the Late Cretaceous.