CONSTRAINING THE CHRONOLOGY OF FAULT MOVEMENTS BASED ON FLUID BEHAVIOR CHARACTERISTICS

A fault is a deformation structure characterized by the relative displacement of rocks on either side due to slip (shear), resulting in features formed by fracturing or crushing. Large fault zones exhibit complex internal structures that can either facilitate or impede fluid movement. Fluid influx and release in a fault zone can cause or result from repeated slipping during the seismic cycle. Thus, in geological timescale, fault movement and development can be inferred to coincide with fluid influx during specific deformation stages. The fluids entering the fault zones mostly contain significant amounts of common lead, which can be used to constrain the timing of fault activity by determining the period of common lead influx. In this study, U-Pb dating was conducted on zircon separated from the host rock and fault core at major fault sites in South Korea. Before U-Pb dating analysis, backscattered electron imaging and cathodoluminescence imaging were performed to select appropriate analysis spots and to observe internal structural changes in the zircon caused by fluid. Observations of the zircon's internal structure reveal that the fresh host rocks predominantly contained zircons with well-developed oscillatory zoning typical of igneous rocks. However, zircons separated from the fault core exhibit numerous microcracks developed due to fluid influx, along with micro-inclusions formed or introduced along these cracks. The U-Pb dating results yield that the zircons from the fault-unaffected host rocks were mostly aligned along the concordia line, indicating the crystallization age of the zircon. In contrast, the U-Pb dating results for zircons separated from the fault core were plotted as discordant ages due to common lead influx. Utilizing these discordant ages caused by common lead can be effectively used in future to constrain the timing of fault movements.