The Geounri shear zone in the Paleozoic Taebaeksan Basin of Korea: Tectonic implications

The Songrim and Daebo orogenies represent two major Phanerozoic tectonic events that are well-preserved across much of the present-day Korean Peninsula. The Songrim orogeny corresponds to the Late Permian-Triassic collision of the North and South China cratons whereas the Jurassic Daebo tectonic event represents a thin-skinned contractional deformation in a continental arc setting. It is well-established that the Songrim orogeny left a strong imprint on the geological record preserved in the middle and northern parts of the Korean Peninsula (e.g., the Pyeongnam Basin and the Imjingang belt) with only a minor impact on the geology of southern Korea (e.g., the Okcheon and Taebaeksan basins). It was the Daebo tectonic event, however, which generated most of the deformational structures observed within the Okcheon and Taebaeksan basins. The Deokpori thrust in the Taebaeksan Basin is a significant fault structure that formed during the Daebo tectonic event; no other regional structures related to the Songrim orogeny have been found in the Taebaeksan Basin. In the vicinity of the Deokpori thrust, we have identified in this study a previously undocumented reverse-slip shear zone, which is named the Geounri shear zone. Microfabrics observed in phyllonite and marble mylonite samples suggest that this shear zone was developed in a plastic deformation regime at ~400 °C, in contrast with the nearby Deokpori thrust, which formed in a brittle deformation regime. A geochronological analysis of muscovite isolated from phyllonite samples of the Geounri shear zone, yielded two ⁴⁰Ar/ ³⁹Ar age spectra with a combined weighted mean age of 209 ± 5 (2σ) Ma. Thermal modeling of the Ar data, combined with structural interpretations, suggests that this muscovite ⁴⁰Ar/ ³⁹Ar age represents a robust lower age limit for the timing of the Geounri shearing event, therefore linking formation of the shear zone with deformational events caused by the Songrim orogeny. These results imply that structural features caused by 'Songrim deformation' were developed at higher temperatures than structures caused by 'Daebo deformation' as found within the Taebaeksan Basin.