<p>Sedimentary strata in the northwestern margin of the Taebaeksan Basin, mid-eastern Korea, provide a key into regional tectonic evolution. These strata are long and narrowly distributed, divided into the Bangrim and Anmiri groups. This study reveals a major tectonic inversion, where the basement rocks are overthrust above the sedimentary sequences. This inversion is attributed to a Late Paleozoic collision process between the Gyeonggi and Yeongnam massifs and the subsequent thrusting mechanisms. Detailed geological surveys and U–Pb detrital zircon geochronology reveal that the Bangrim Group was deposited during the Early Paleozoic (from Cambrian to Ordovician), while the Anmiri Group records an Early-to-Middle Cambrian deposition. Crucially, this study identifies a previously unrecognized thrust fault, herein designated as the “Geumdang Thrust,” which not only juxtaposes these stratigraphically distinct groups but also induces a reversal of their stratigraphic order. The thrust fault is a newly defined tectonic boundary between the Gyeonggi and Yeongnam massifs in the northwestern margin of the Taebaeksan Basin, connecting with the South Korea Tectonic Line. The subsequent NNE-trending thrust associated deformation further modified the preceded thrust structures, resulting in the development of tectonic foldings in the direction perpendicular to the thrust deformation. This tectonic model reconciles the apparent structural contradiction of relationship between the basement rocks and overlying sedimentary strata, and provides new tectonic insights into the collision history between the Gyeonggi and Yeongnam massifs.</p>