INDOSINIAN DEXTRAL DUCTILE FAULT SYSTEM AND SYNKINEMATIC PLUTONISM IN THE SOUTHWEST OF THE OGCHEON BELT (SOUTH-KOREA)

The structure of the mylonite belt in the southwestern part of the Ogcheon belt has been investigated and interpreted in the light of recent radiometric data. The shear belt is composed of several en-echelon dextral shear zones relayed by compressional or extensional tectonic bridges. The dextral displacement along the fault system, which may be extended to the Hida zone of Japan, is of the order of 200 km. It has brought into contact the Ryeongnam block and its Cambrian-middle Ordovician cover of North China affinity, with the southwestern margin of the Ogcheon belt characterized by Cambrian-middle Silurian rift facies and shelf deposits of South China affinity. Synkinematic plutons are emplaced within the shear belt, which is postdated by uppermost Triassic unfoliated granitoids. At the surface, the shear belt generated elongated intermontane coal basins initiated as transtensional half-grabens. Because of its post-Permian, pre-early Jurassic age and its dextral kinematics, we correlate the shear belt with the late stages of the Akyioshi (Indosinian) collisional orogeny of the east Asian margin. The synkinematic plutonism that accompanied the shear zone may be due to post-collisional crustal stretching and subsequent anatexis at the mantle-crust interface.