The Geounri Shear Zone in the Paleozoic Taebaeksan Basin of Korea and Its Tectonic Implication

The Paleozoic Taebaeksan Basin in Korea suffered two major tectonic events, the Late Permian - Triassic collisional Songrim orogeny and the Middle Jurassic Daebo tectonic event. The distinction between the “Songrim” and “Daebo” structures in the basin has been difficult since both structures are known to have a similar orientation and vergence. The NNE-striking, brittle Deokpori (Gakdong) thrust with an ESE vergence is the most prominent Daebo structure along which the Yeongwol and Taebaek groups of the early Paleozoic Joseon Supergroup are juxtaposed against each other. In the Yeongwol area, we have identified a major reverse ductile shear zone (“Geounri shear zone”) also with NNE strike and ESE vergence to about 300 m west of the Deokpori thrust. The Geounri shear zone with a thickness of at least 100 m consists of phyllonite and marble mylonite. The phyllonite is composed mainly of muscovite and quartz. Along some layers of the phyllonite, chloritoid occurs as porphyroblast (100-300 μm in diameter). The quartz grains (20-350 μm) with a bimodal size distribution exhibit undulose extinction, deformation band and subgrains. Their boundaries are lobate or wavy, suggesting dynamic grain boundary migration. All these microstructures indicate that the quartz grains deformed by dislocation creep. Most of the chloritoid porphyroblasts are pre- or syntectonic and some occur as posttectonic porphyroblasts. Recrystallized calcite grains (50-120 μm) of the marble mylonite are elongated to form a grain-shape foliation. The calcite grains carry both mechanical and growth twins. Occasionally, remnant calcite grains (up to 300 μm) are present in the marble mylonite. 40Ar-39Ar age dating of muscovites from the phyllonite yielded ages of 204-222 Ma, suggesting that the Geounri shear zone developed during a late stage of the Songrim orogeny representing the collision between the North and South China cratons. Our study implies that the “Songrim” structures have a higher temperature condition for deformation than the “Daebo” structures in the Taebaeksan basin.