Structural architecture and late Cenozoic tectonic evolution of the Ulsan Fault Zone, SE Korea: New insights from integration of geological and geophysical data

Abstract

Integration of geological and geophysical data is essential to elucidate the configuration and geometry of surface and subsurface structures, as well as their long-term evolution. The NNW–SSE-striking incised valley and parallel mountain range in the southeastern margin of the Korean Peninsula, extending 50 km from Gyeongju to Ulsan cities, are together regarded as one of the most prominent geographical features in South Korea. This paper presents an investigation into the structural architecture and deformation history of the valley and mountain range during the late Cenozoic based on combined data from field observations and gravity and electrical resistivity surveys. Our results based on integrated and reconciled geological, structural, and geophysical data are as follows. First, the incised fault valley can be divided into 1) the northern part, which comprises several distributed buried or exposed fault strands; and 2) the southern part, which comprises a concentrated deformation zone along the eastern margin of the valley. Different deformation features between the two parts are controlled by the lithology of host rocks and by the location and geometry of the neighboring major structures, that is, the Yeonil Tectonic Line (YTL) and the Yangsan Fault. Second, we defined the Ulsan Fault Zone as a NNW–SSE-to N–S-striking fault within the incised valley and along the eastern margin of the valley. In particular, the constituent strands located along the eastern margin of the valley have acted mainly as an imbricate thrust zone, characterized by an east-side-up geometry with moderate to low dip angles and reverse-dominant kinematics in the shallow subsurface during the Quaternary. Third, reactivated strands within the Ulsan Fault Zone during the Quaternary are interpreted as shortcut faults developed in the footwall of Miocene subvertical structures, predominantly the YTL. In addition, movements on the Ulsan Fault Zone and the YTL during the Miocene to Quaternary were arrested by the NNE–SSW-striking Yangsan Fault, which was a prominent and mature pre-existing structure. Our results highlight the spatiotemporal structural variation in SE Korea and emphasize the strong control of the configuration and geometry of pre-existing structures on the distribution and characteristics (i.e., geometry and kinematics) of the subsequent deformation under changing tectonic environments through the late Cenozoic.

Geomorphic indices for unveiling fault segmentation and tectono-geomorphic evolution with insights into the impact of inherited topography, Ulsan Fault Zone, Korea

“…However, subsequent studies have suggested that the UFZ is located either in and around the incised valley, or that it lies along the mountain front to the east of the incised valley, or possibly in both locations (Okada et al, 1998;Ryoo et al, 2002;Choi, 2003;Choi et al, 2006;Ryoo, 2009;Kee et al, 2019;Naik et al, 2022). A recent study attempted to comprehensively re-interpret previous studies along with adding new field observations and geophysical data to propose a new definition of the UFZ (Cheon et al, 2023). This definition includes some strands of exposed faults along the 100 mountain front and several strands of buried faults near the centre of the incised valley (Fig.…”

“…A recent (Cheon et al, 2023) also divided the incised valley containing the UFZ on the basis of: (1) differences in faulthosting rocks, and (2) width of the deformation zone. These authors divided the UFZ into only two northern and southern segments 655 at the boundary between our segments 3 and 4.…”

“…Channel lengths between the fault and the channel heads are longer in segment 1 than in segment 2. In segment 1, buried faults are developed 720 in the incised valley, far to the west from the mountain front (Cheon et al, 2023). The response time of a channel to tectonic events https://doi.org/10.5194/egusphere-2024-198 Preprint.…”

“…Therefore, 60 we postulate that geomorphic indices reflect the cumulative influence of past and present geomorphic processes and their drivers, and that drawing inferences from these indices without considering the influence of inherited topography can lead to incorrect interpretations of landscape evolution. the influence of inherited topography on the present landscape evolution (Park et al, 2006;Cheon et al, 2012;Son et al, 2015;Kim et al, 2016b;Cheon et al, 2023;Kim et al, 2023a).…”