TECTONIC EVOLUTION OF THE CENTRAL OGCHEON BELT DURING PERMIAN~MIDDLE TRIASSIC SONGNIM AND EARLY~LATE JURASSIC DAEBO OROGENICS, KOREA

The Ogcheon sedimentary basin was constructed at least by 4 times rifting of the original Gyeonggi Massif with bimodal-type magmatism during Neoproterozoic~Middle Permian, and at least 5 times of tectonic events were recognized in the pre-Cenozoic constituent rocks of the Central Ogcheon Belt, Korea. The D1 (Late Permian~Middle Triassic Songnim orogeny) is marked by the closing of Ogcheon rift basin in its earlier phase (D1a), and by the coupling of South and North China blocks in its later phase (D1b). At the earlier stage of D1a occurred the M1 medium-pressure type metamorphism with growth of coarse biotites, garnets, staurolites in the Ogcheon Supergroup (OSG). At its later stage, the metamorphic rocks were exhumed as nappes with SE-vergence, and sheath or "A" type folds, regional foliation, stretching lineation were formed. At the D1b, thrusts with NNE- or/and SSW-vergence were formed in the front and rear parts of couple, and NNE-trending upright open folds of the Joseon and Pyeongan Supergroups (JPSG) and Cheongsan dextral strike-slip shear zone and Dandong basin were formed in its flank part. After the depositon of Late Triassic~Early Jurassic Daedong Group (DG), the D2 (Early~Late Jurassic Daebo orogeny) occurred by NNE-trending Honam dextral strike-slip shearing, forming asymmetric crenulated folds in the OSG and NNE-trending recumbent folds in the JPSG and upright folds in the DG and thrust faults with SE-vergence in which JPSG override DG. Its earlier phase occurred during Early~Middle Jurassic, forming syn-foliated sheet-type granite and the early crenulation included within old andalusite porphyroblasts. Middle Jurassic stock-type granite is permissively emplaced during the inter-tectonic sub-phases, occurring the M2 andalusite-sillimanite type contact metamorphism in the OSG. Its later phase occurred during Middle~Late Jurassic, forming the late crenulation which wraps around the old andalusites and the granitic gneissosity in the stock-type granite.