EVOLUTION OF THE CENTRAL OKCHEON FOLD-THRUST BELT WITH IMPLICATIONS FOR PHANEROZOIC TECTONIC EVOLUTION OF THE KOREAN PENINSULA ALONG THE EAST ASIAN CONTINENTAL MARGIN

The Okcheon FTB is a prominent mountain belt that preserves important clues on Korean orogenesis. The geological evolution of this belt has been debated until recently because of the general lack of understanding of the geology of the area in sufficient detail including sedimentation, deformation, metamorphism, and their ages during its evolution. Recent studies showed that sedimentation age of thick sedimentary package composed of clastic wedge is post-Devonian to pre-Triassic based on the presence of Permian plant fossils, post-Devonian biomarker analysis results as chemical fossils, and post-tectonic Triassic intrusions etc. The sedimentary wedge shows lateral-facies change from conglomerate to sandstone and shale, and these fresh water clastic deposits are intercalated with shallow marine sandstones/shales and limestones at the bottom, and with changing borderlands with the Cambro-Ordovician marine carbonate deposition through the Paleozoic Era. The clasts of quartzite, sandstone, calc-silicates, and basement gneisses in various size from the Hwanggangri conglomerate formation indicate sedimentary sources mostly from uplifted metamorphosed mountain belts in the Gyeonggi Massif during post-Devonian to Permian (similar to Allegany orogenic time span in Appalachian) possibly related to subduction associated with the Okcheon orogeny. The clastic wedge experienced deformation and metamorphism (viz. Songrim orogeny) related to the subsequent (Permo-)Triassic collision along the Korean collision belt (viz. Hongseong-Imjingang belt) forming regional fold-dominated mountains of the Okcheon FTB, which includes (1) regional folding of post-Devonian sequences intruded by Triassic mylonitized granitoids that are supported by SHRIMP and LA-MC-ICPMS U-Pb age-datings, (2) the younger sedimentary formation as part of the clastic wedge is three-dimensionally covered by the older Ordovician limestone along the folded thrust fault defining a tectonic window, (3) the Ordovician limestone in the hanging wall of the folded thrust shows the type-2 mushroom-like fold interference pattern, among other features. The tectonic evolution from subduction to collision in this belt is important in tracing the Phanerozoic tectonic evolution of the Korean Peninsula along the East Asian continental margin.