GEODYNAMICS OF THE OGCHEON BELT (SOUTH-KOREA)

The Ogcheon Belt is a stack of synmetamorphic southeastward verging nappes resulting from the deformation of a volcanosedimentary sequence deposited in an early Palaeozoic rift which was affected by polyphase tectonics. Its pre-Cretaceous structure results from three main orogenic events: the "Ogcheon" (late Silurian-Devonian (?)), Indosinian (late Permian-Triassic) and Yenshanian (middle Jurassic-early Cretaceous) tectonic phases. The external, autochthonous or para-autochthonous units represent platform zones with shallow-water deposits whereas rift facies characterize the internal domain. The Yeongweol sequence, forming an outer shelf on the southeastern margin of the rift, displays affinities with the Jiangnan basin of South China. It is in wrench fault contact with the easternmost Duwibong sequence correlated with the North China-North Korea platform. In the internal metamorphic units, post-Cambrian-early Ordovician pelitic and turbiditic rift metasediments overlie a thin Cambrian or Cambrian-Ordovician early platform sequence. Two subdomains are distinguished: the upper units (Chung Ju and Pibaryeong units) representing the deep "axial" part of the basin contain distal flyshoids and large olistoliths derived from the northwestern "narrow" margin; the lower units (Turung San, Poeun and Iwharyeong) are related to the "wide" margin located between the deep rift domain and the stable southeastern platform. The latter transitional domain is characterized by thick submarine debris flows, and volcanics of alkali or transitional tholeiitic affinity. The volcanic activity and mass gravity flow sedimentation are closely related to the crustal thinning and subsequent rifting and tilting. Lithostratigraphic evidence allows the correlation of the unfossiliferous rift metasediments with the Cambrian-Ordovician platform sequence in one single basin. The post-middle Silurian-pre-mid-Carboniferous "Ogcheon" ensialic orogeny resulted in the closure of the rift with relatively minor effects in the platform area. It gave rise to an intracontinental fold-thrust belt without ophiolites. This tectonic event may have been contemporaneous with early Palaeozoic tectonism in the Quinling Belt in Central China and in the Southeast China fold belt. The definitive constitution of the Sino-Korean craton took place before the mid-Carboniferous and "molasse" sedimentation was initiated on a single platform located on the western edge of the Yakuno oceanic basin of Southwest Japan. In late Permian and early Triassic times, the closure of the oceanic area and the subsequent westward collision of the Honshu Block are marked in Korea by the intracratonic "Indosinian" tectonism. Since late Triassic times, predominantly transcurrent movements have occurred along NNE-SSW faults associated with syntectonic intermontane troughs and important granitization. © 1990.