Miocene-Pleistocene volcanism and tectonics in southern Korea and their relationship to the opening of the Japan Sea

Abstract

Miocene-Pleistocene volcanism and tectonics in southern Korea are discussed. The Miocene volcanism and related tectonics are represented by the Eoil Orogeny, which occurred in the Yangnam and Pohang basins in the southeastern corner of the Korean Peninsula. The Yangnam and Pohang basins represent the uplifted central part of the Pohang-Ulsan Graben which was rifted between the Proto-Korean Peninsula and Paleo-Ulleung Land, probably in Late Oligocene-Early Miocene times. The Eoil Orogeny is correlated with the Green Tuff Orogeny in Japan. Evidence of Pliocene volcanism is observed on Ulleung Island located in the western part of the central Japan Sea. Volcanic and tectonic events represented on Ulleung Island are, in chronological order: (1) volcanic eruption; (2) subsidence and marine transgression; (3) dome-like uplift; and (4) volcanic eruption. Evidence of Pleistocene volcanism is present on Cheju Island located at the southwestern entrance to the Korea-Tsushima Strait. Volcanic and tectonic events represented on Cheju Island are, in chronological order: (1) volcanic eruption; (2) subsidence and marine transgression; (3) uplift and volcanic eruption; and (4) doming and volcanic eruption. Three grabens, namely the Pohang-Ulsan Graben, the Ulleung Basin and the Yamato Basin, were formed in the southern half of the Japan Sea, which was above sea level in Early Miocene times. Paleo-Ulleung Land was present between the Pohang-Ulsan Graben and the Ulleung Basin in Miocene-Pliocene times. These three grabens are active rifts, and the tectonic events that occurred in this region originated from the action of rising magma. The extension and deepening of the southern half of the Japan Sea resulted from the tectonic movements of these active rifts and their surrounding regions. Thus, it is suggested that the opening of the Japan Sea progressed by diapiric tectogenesis.