Structure and evolution of the active Ulsan Fault Zone, SE Korea: New insights from geophysical studies

Integration of geophysical and geological data is essential to illuminate the configuration and geometry of surface and subsurface structures as well as their long-term evolution history. The NNW–SSE-striking incised valley and parallel mountain range in southeastern margin of the Korean peninsula, extended from Gyeongju to Ulsan cities (~50-km-long on land), have been regarded as the most active geographical feature in Korea, which was named as the Ulsan Fault zone (or system). This study presents a new insight of the structural architecture and its deformation history during the Cenozoic based on a combined data of gravity and electronic survey results with previous field observations. Our major results based on integrated data are as follows. First, the incised fault valley is divided into (1) the northern part of several distributed, buried and exposed fault strands and (2) the southern part of a concentrated deformation zone. Different deformation features between the two parts are controlled by the distribution pattern of the pre-existing Miocene structures (i.e., Yeonil Tectonic Line, YTL). Second, the Ulsan Fault is only constrained as a NNW–SSE-striking Quaternary fault zone within the incised valley-mountain range. The fault zone is composed of several interconnected and disconnected strands forming an imbricate thrust zone located along the western front of the mountain range (or eastern margin of the >2-km-wide incised valley). The constituent fault strands mainly exhibit an east-side-up geometry with moderate to low dip angles and reverse-dominant kinematics in near-surface. These strands are interpreted as reactivated ones of the pre-existing subvertical structures, such as the YTL. In here, we newly designate ‘the Ulsan–Yeonil Fault system’, composed of all NNW–SSE to N–S-striking buried and exposed faults on the incised valley-mountain range, regardless of tectonically controlled sequence of movement stages. Third, movements of the NNW–SSE-striking fault system during the Miocene to Quaternary were arrested by the NNE–SSW-striking Yangsan Fault, which is pre-formed prominent mature structure. Our results highlight the spatiotemporal structural characteristics in SE Korea, emphasizing that the configuration of pre-formed structures have strongly controlled the distribution and characteristics (i.e., geometry and kinematics) of the subsequent deformation during the Cenozoic crustal deformation.