Reactivated Timings of Yangsan Fault in the Northern Pohang Area, Korea

Abstract

Here we present the timings of reactivated events from a fault in the northern Pohang area, which should be located at the northern-end of Yangsan fault line, the major fault in the southeastern Korean Peninsula. Recently developed illite-age-analysis (IAA) approach was employed for determining the fault-activated timing, combined with illite-polytype quantification using the optimized full-pattern-fitting (FPF) method, and K-Ar age-dating for each size fraction(<0.1 µm, 0.1-0.4 µm, and 0.4-1.0 µm) of 4 fault clay samples. Two chronological records of brittle fault-activation events were recognized at 19.6±1.86 Ma and 26.1±2.55-27.9±3.46 Ma. The ages are much younger than those of fault clays from Sangcheon-ri area (41.5~43.5 and 50.7 Ma), the southern part of Yangsan fault line, and are close to the timing of East Sea-opening event. Further chronological analysis for additional sites of the Yangsan fault should be needed to reveal the time-scheme of the tectonic events and their spatial distributions along the fault line.

Long‐Term Weakening Processes and Short‐Term Seismic Slip Behavior of an Intraplate Mature Fault Zone: A Case Study of the Yangsan Fault, SE Korea

“…Multiple stages of deformation of the Yangsan Fault have been suggested by various structural observations and radiometric age determinations of transected rocks (e.g., Chang, 2002;Chang & Chang, 1998;Cheon et al, 2019;Cho et al, 2007Cho et al, , 2016Kim, 1992), as well as by K-Ar illite ages (Sim et al, 2017;Song et al, 2016Song et al, , 2019 and electron spin resonance ages Lee & Schwarcz, 2001;Yang, 2006;Yang & Lee, 2012 of fault gouge. Recently, based on a compilation of previous results, Cheon et al (2019) proposed the tectonic evolution of the fault and age constraints, including the following: (a) Late Cretaceous sinistral movement with a component of extensional deformation (e.g., NW-SE-striking extensional structures, and the Dadaepo pull-apart basin between the Yangsan and Dongnae faults; Cheon et al, 2017;Cho et al, 2007Cho et al, , 2016Ha et al, 2016); (2) late Paleogene (∼43-23 Ma) dextral movement, which was the most intense phase of deformation on the fault, with 20-30 km of dextral offset being inferred from the distribution of several piercing points, such as A-type granite bodies (see Figure 1c; Chang et al, 1990;Hwang, Lee, et al, 2007;; and (c) subsequent reactivations along some segments during the late Miocene associated with sinistral kinematics (Chang, 2002;Chang & Chang, 1998;Choi et al, 2009).…”

“…Multiple stages of deformation of the Yangsan Fault have been suggested by various structural observations and radiometric age determinations of transected rocks (e.g., Chang, 2002; Chang & Chang, 1998; Cheon et al., 2019; Cho et al., 2007, 2016; Kim, 1992), as well as by K‐Ar illite ages (Sim et al., 2017; Song et al., 2016, 2019) and electron spin resonance ages (Kim & Lee, 2020; Lee & Schwarcz, 2001; Yang, 2006; Yang & Lee, 2012, 2014) of fault gouge. Recently, based on a compilation of previous results, Cheon et al.…”

Measurement of 3D-Shape Preferred Orientation (SPO) Using Synchrotron μ-CT: Applications for Estimation of Fault Motion Sense in a Fault Gouge

“…Most clay minerals contained in the matrix are illite and chlorite, and some smectite is also present [30]. Sim et al determined the fault activation ages were 20 and 26-28 Ma using the Illite age analysis (IAA) [31]. The fault plane observed in the outcrop stands vertically, and the direction of the fault plane is measured as N45• E 85• SE.…”

3D Fabric Analysis in Fault Rock Using Synchrotron μ-CT: A Statistical Approach to SPO (Shape Preferred Orientation) for Estimation of Fault Motion

“…Since the Cretaceous, the Yangsan fault has been activated at least twice due to large events, such as the collision of the Indian Plate and the opening of the East Sea (Japan Sea) [18]. From K-Ar illite dating for the area where the main fault line of Yangsan fault penetrates, the fault activation ages of 19.6 and 26.1-27.9 Ma for the Bogyeongsa area in the northern part, and 41.5-43.5 and 50.7 Ma for the Sangcheon-ri area in the middle part have been reported [19,20]. In addition, the tectonic environment affecting the Yangsan fault zone is different for each period.…”