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.
Highlights
Orogeny is a series of geological mechanisms at convergent plate margins where mountain ranges are created [1,2]
Once the continental crust is thickened by orogenic processes, its isostatically compensated elevation leads to a high gravitational potential energy and drives mechanical collapse and vertical and lateral crustal
In the Korean Peninsula, the western margin of the Gyeonggi Massif has been considered as a part of an orogenic belt in the East Asian continental margin, which had evolved via multiple tectonic events from the Proterozoic to the Mesozoic, in relation to the disruption and assemblage of the supercontinents
Summary
Orogeny is a series of geological mechanisms at convergent plate margins where mountain ranges are created [1,2]. In the Korean Peninsula, the western margin of the Gyeonggi Massif has been considered as a part of an orogenic belt in the East Asian continental margin, which had evolved via multiple tectonic events from the Proterozoic to the Mesozoic, in relation to the disruption and assemblage of the supercontinents. Since Early Mesozoic high-pressure metamorphic rock was discovered in the western Gyeonggi Massif [12], the lines of petrographic, geochemical, and geochronological studies have been conducted in the area and have given rise to several Korean collision zone models in the Permo–Triassic final amalgamation of the East Asia continent, including the Hongseong-Imjingang Belt [16], the Gyeonggi Marginal Belt [17], the Hongseong-Odaesan Belt [18], and the Crustal Detachment [19] models. Gyeonggi Massif, reflecting spatial and temporal changes in governing the tectonic process of the Mesozoic East Asian continental margin
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