Redefinition of the boundary between the Taebaeksan Basin and the Ogcheon metamorphic belt, Korea: a remark on the South Korean Tectonic Line (SKTL)

Abstract

One of critical boundaries in Korea is the boundary between the Taebaeksan Basin and the Ogcheon metamorphic belt. We performed field mapping and geochronological/geochemical analyses in the Jecheon-Maepo areas. In the Bonghwajae section, the "Yamisan unit" (761 ± 3 Ma) belonging to the Ogcheon Supergroup occurs eastward beyond previous limit of Ogcheon metamorphic belt. Farther east, the post-Cambrian "Soyari unit" of uncertain affinity occurs. The Soyari unit borders a typical sequence of Taebaeksan Basin to the east in Maepo. We suggest that the boundary between the Ogcheon metamorphic belt and the Taebaeksan Basin is more than 20 km wide zone. Occurrence of the "Danri window" shows that the Ogcheon Supergroup sequence overlies the Joseon Supergroup unit along a decollement in the Jecheon-Maepo areas. The decollements and NW-to NNW-trending structures resulted from NE-or ENE-directed shortening probably during the Songrim Orogeny (Indosinian). We consider two alternative models for the origin/structural relationship between the Taebaeksan Basin and the Ogcheon metamorphic belt. In one plate model, the Taebaeksan Basin had formed close to the Ogcheon metamorphic belt. In two plate model, the Taebaeksan Basin had formed in the North China Block, and was transported hundreds kilometers southward along a continental transform fault.