Microstructural and Geochronological Analyses of Mesozoic Ductile Shear Zones in the Western Gyeonggi Massif, Korea: Implications for an Orogenic Cycle in the East Asian Continental Margin

Abstract

In response to orogenic cycles, the ductile shear zone records a complex crustal deformation history. In this study, we conducted a microstructural analysis of two NW–SE trending ductile shear zones (Deokjeok Shear Zone (DSZ) and Soya Shear Zone (SSZ)) in the Late Triassic post-collisional granites along the western Gyeonggi Massif in the Korean Peninsula. The DSZ, overlain by the Late Triassic to the Early Jurassic post-collisional basin fill (Deokjeok Formation), has asymmetric microstructures indicative of a top-down-to-the-northeast shear. Depending on the structural position, the SSZ, which structurally overlies the Deokjeok Formation, exhibits two contrasting styles of deformation. The lower portion of the SSZ preserves evidence of top-up-to-the-southwest shearing after top-down-to-the-northeast shearing; on the other hand, the upper portion only indicates a top-up movement. Given the primary deformation mechanisms of both quartz and feldspar, the deformation temperatures of DSZ and SSZ were estimated at ~300–350 °C and ~350–400 °C, respectively, indicative of the mid-crustal condition. New zircon U-Pb isotopic ages from mylonitic granite in the SSZ and volcanic rocks in the Deokjeok Formation, combined with previously published geochronological data, indicate that the post-collisional granites and volcano-sedimentary sequence were nearly contemporaneous (ca. 223–217 Ma) and juxtaposed because of the Late Triassic orogenic collapse and subsequent new orogenic event. In this study, we highlight the role of the extensional DSZ as a detachment propagated into the middle crust during the Late Triassic orogenic collapse. Our results report a deformational response to a transition from the collisional Songrim Orogeny to the subduction-related Daebo Orogeny in the western Gyeonggi Massif. This, in turn, provides essential insight into cyclic mountain building/collapse in the East Asian continental margin during the Mesozoic time.

Structural Geometry of a Regional-scale Overturned Fold in the Daecheong Island, Central-western Korean Peninsula

“…연구 지역의 구조 영역 1-3에 광범위하게 발달하 는 비대칭 기생습곡(Figs. (Oh et al, 2005;Cho et al, 2007;Kim et al, 2000;2008;2011;Park et al, 2014;2020;de Jong et al, 2015;Lee et al, 2019)…”

Post‐collisional sedimentation of the Early Mesozoic Munamdong Formation in South Korea

“…The Permian–Triassic continental collision between the North China Block and South China Block during the closure of the Palaeo‐Tethys Ocean resulted in the formation of the Early Mesozoic orogenic belt (Songrim orogeny) in the central Korean Peninsula (e.g., Cho et al, 2017; Choi & Park, 2019; de Jong et al, 2015; Kim, Kwon, et al, 2011; Kim et al, 2021; Kim et al, 2006; Kim, Oh, & Kim, 2011; Kwon et al, 2009; Kwon et al, 2013; Oh, 2006, 2012; Oh et al, 2005; Oh et al, 2015; Park, Kim, et al, 2014; Park, Kwon, et al, 2014; Park & Park, 2020). Recent studies suggest that, at the end of the Songrim orogeny, gravitational collapse of the thickened crust produced nonmarine sedimentary basins within or in front of the orogenic belts (e.g., Lee, Park, Choi, & Sim, 2021; Park et al, 2018; Park et al, 2020). The Yujeon Basin is one of the non‐marine sedimentary basins, filled with fluvio–lacustrine sediments (the Munamdong Formation).…”

Editorial for Special Issue “Microtexture Characterization of Rocks and Minerals”

“…The articles published in the issue can be divided into several topics. The articles by Park et al [1], Mao et al [2] and Noh et al [3] are the case studies investigating some geological processes at the regional scale, based on microtextural characterization. Park et al [1] present new microstructural data from two ductile shear zones, constrain the kinematics and deformation conditions of ductile shearing, and provide an improved understanding of the tectonic process of the Mesozoic East Asian continental margin.…”

“…The articles by Park et al [1], Mao et al [2] and Noh et al [3] are the case studies investigating some geological processes at the regional scale, based on microtextural characterization. Park et al [1] present new microstructural data from two ductile shear zones, constrain the kinematics and deformation conditions of ductile shearing, and provide an improved understanding of the tectonic process of the Mesozoic East Asian continental margin. Mao et al [2] investigated the microstructures of rhyolitic lavas filled by solidified crude oil from a Basin in NE China using quantitative image analysis, and defined the type, morphology, pore-size distribution, and pore-volume contribution of different pores, which can be beneficial to the further understanding of the microstructure and pore system of the rhyolites as a potential hydrocarbon reservoir.…”