FAULT ACTIVATION TIMINGS TO CONSTRAIN THE EPISODES OF BRITTLE DEFORMATION IN NORTHEASTERN ASIA SINCE THE LATE CRETACEOUS PERIOD

We present the K–Ar illite 1M_d/1M ages of some major faults in the Korean Peninsula, which were 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 K–Ar illite 1M_d/1M ages indicate that the major fault events has been recognized many times with time-interval of 5~10 Ma from about 87 Ma to 18 Ma, and most of the major faults reactivated several times. These confirm that the shallow crustal emplacement by some igneous activities, which should be related to the namely Bulguksa Orogeny, were initiated at about 87 Ma, and were continued to Neogene. In addition, the 18~20 Ma fault events of the Neogene would be related to the opening of East Sea. The 2M₁ illite ages may provide the episodes of the host rock formation, deformation, metamorphism and hydrothermal effects related to the tectonic events before the Late Cretaceous period. These geochronological determinations of the multiple events recorded in the major faults are crucial to establish the tectonic evolution of the Korean Peninsula along the East Asian continental margin since the Late Cretaceous.