Mesozoic transpressional structural inversion and synkinematic crustal fluid circulation around the northeastern Chungnam basin, southwestern Korean Peninsula: Tectonic implications

This study explores the coupled processes of structural inversion and fluid migration around the northeastern part of the Mesozoic Chungnam Basin in the southwestern Korean Peninsula, focusing on quartz vein systems genetically linked to an orogenic gold deposit. Our results show that the inversion structures reflect the coexistence of strike-slip simple shear and pure shear components caused by NW–SE crustal shortening, leading to a transpressional deformation model. The quartz veins, filling the high-angle faults with a reverse slip component and adjacent hydrofractured wall rocks, represent transient fluid flow related to seismic faulting and fault valving during the inversion. New K–Ar ages of illite polytypes in fault gouges, determined using the Illite-age-analysis (IAA) method, indicate Late Jurassic–Early Cretaceous fault reactivations. Notably, ca. 160 Ma 2M1 illite age from a fault zone filled with an orogenic gold–bearing quartz vein indicates that the inversion and auriferous hydrothermal fluid flow started with a magmatic quiescence in the southern Korean Peninsula likely owing to the flat subduction of the Paleo-Pacific Plate. Our findings suggest that the orogenic gold–bearing vein system was likely sourced from the subcrustal metamorphic fluid in the flat subduction zone of the Mesozoic East Asian Continental margin.