Episodes 2021; 44(2): 83-97
Published online June 1, 2021
Copyright © International Union of Geological Sciences.
by Hyeong Soo Kim1*, Hee-Cheol Kang2, Hyoun Soo Lim2, and Shinae Lee3
1 Department of Earth and Environmental Sciences, Korea University, Seoul 02841, Republic of Korea
2 Department of Geological Sciences, Pusan National University, Busan 46241, Republic of Korea
3 Division of Earth and Environmental Sciences, Korea Basic Science Institute, Cheongju, 28119, Republic of Korea
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We present the deformation history based on analysis of two orthogonal fold phases and U–Pb zircon geochronology of metasedimentary and igneous rocks from the western Gyeonggi Massif (Gukhwado and Ippado areas) in the Korean Peninsula. Multiple generations of folds are recognized in three lithological units: (1) a psammitic/pelitic unit consisting of quartz–mica schist and quartzite, (2) a calcareous unit consisting of calcareous schist and calcsilicate, and (3) a mafic unit consisting of amphibolite with and without garnet porphyroblast. These rocks are intruded by a Triassic syenite. First-generation folds (Fn) are tight to isoclinal, oriented N–S, and have both shallow and steep fold axes. The Fn folds include asymmetric and buckle folds that are refolded by vertical Fn+1 folds. The latter are oriented ~E–W, approximately orthogonal to the earlier folds. We infer from U–Pb zircon ages of the psammitic/ calcareous units and the syenite that the two superimposed folding phases associated with crustal-thickening and shearing in the western Gyeonggi Massif occurred between the Pennsylvanian and the Middle Triassic (~228 Ma).