Pretectonic and posttectonic emplacements of the granitoids in the south central Okchon belt, South Korea: Implications for the timing of strike‐slip shearing and thrusting

Abstract

Structural analyses of three granitic plutons in the south central Okchon belt of the Korean peninsula reveal that the Baegnok and Cheongsan plutons are pretectonic with respect to right lateral strike‐slip ductile shearing along the Cheongsan shear zone and later top to the ESE thrusting, whereas the Boeun pluton is posttectonic with respect to these two deformation events. U‐Pb sphene age data from the three plutons indicate that the intrusion ages of the Baegnok, Cheongsan, and Boeun plutons are 222.7 ± 2.1 Ma, 216.9 ± 2.2 Ma, and 171.7 ± 1.4 Ma, respectively. Microstructural evidence preserved in deformed rocks from the NE striking Cheongsan shear zone and adjacent, NNE striking thrusts suggests that the right lateral ductile shearing event along the shear zone occurred before the ESE directed thrusting. Since these thrusts crosscut the late Triassic ‐ Early Jurassic Taedong Group, we estimate that the thrusts developed at ∼180 Ma during the Daebo tectonic event, just prior to the intrusion of the Boeun pluton. We also constrain the age of the Cheongsan shear zone at Middle ‐ Late Triassic time, corresponding to the Songrim orogeny, so that the shear zone is not related to deformation along the Middle Jurassic Honam shear zone (∼176 Ma). The Honam shear zone is not a simple tectonic boundary between the Okchon belt and the Yongnam massif but an anastomosing ductile shear zone not only affecting the Okchon belt and the Yongnam massif but also crosscutting the boundary between them at a low angle. We suggest that the right lateral Cheongsan shear zone is a major boundary between the Okchon zone and the Yongnam massif with Paleozoic cover rocks. The Kyonggi massif and the Okchon zone (South China block) were juxtaposed against the Yongnam massif and Taebaeksan basin (North China block) along the Cheongsan shear zone during a late stage of the Middle Triassic Songrim orogeny, probably related to the late Paleozoic ‐ early Mesozoic collision between the North and South China blocks.