Geological Structure and Tectonics of the Ogcheon Zone in the Chungju-Jangseonri Area, South Korea

The Ogcheon Zone of the Chungju-Jangseonri area consists of some nappes, in descending order of structural level Kyemeongsan nappe and Chungju nappe which overlie the Busan nappe I composed of the Precambrian Busan gneiss complex with its cover rocks (A-unit), Busan nappe II whose basal part contains a small-scale slice of Precambrian gneiss complex, and Sanjeoteo nappe which overlies the Precambrian Bakdalryeong gneiss complex with its cover rocks (A-unit). The Kyemyeongsan nappe, Chungju nappe, Busan nappe II and Sanjeoteo nappe consist of two units, upper unit in which acidic rocks are predominant among original igneous constituents of metamorphic rocks with subordi-nate amount of basic rocks and lower unit in which basic rocks are predominant, showing that the magmatism was of bimodal type but that the predominant magmatism was of basic type during the earlier phase of the development of the Ogcheon sedimentary basin and of acidic type during the later phase. The acidic rocks of the Kyemeongsan nappe are plutonic rocks with volcanic rocks, but these of the Chungju nappe, Busan nappe II and Sanjeoteo nappe are only of volcanic type, showing that the Kyemeongsan nappe appears to have been derived from an activity center of acidic volcano-plutonism in the Ogcheon sedimentary basin as an intracontinental rift zone. The metamorphic rocks of the Ogcheon Zone of the Chungju-Jangseonri area were produced by a medium pressure type metamorphism during the first phase and by an andalusite type metamorphism during the second phase. The first phase metamorphism occurred in the metamorphic field which showed a downward increase of temperature from the upper unit to the lower unit, giving rise to kyanite, staurolite and garnet in the lower unit, though in the Kyemeongsan nappe such metamorphic minerals crystallized also in its upper unit. The exhumation of the medium pressure type metamorphic rocks occurred forming the above-mentioned nappes. It was followed by the folding with westward vergence throughout the nappes. The andalusite type metamorphism occurred throughout the nappes after this folding.