Development and timing of two orthogonal fold systems in the western gyeonggi massif, korea

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).