Late Cretaceous-Tertiary chemical remagnetization event recorded in the Paleozoic rocks in the Bagjisan Syncline, Korea

Abstract

Ogcheon Folded Belt, running NE to SW diagonally across the Korean Peninsula, has been regarded as a promising candidate of an eastern extension of the Quingling-Dabie Suture Zone, a boundary between the North China and Yangtze Blocks. The paleomagnetic study has been carried out for the Upper Carboniferous-Lower Triassic Pyeongan Supergroup, exposed in the Bagjisan Syncline in the northeastern part of the Ogcheon Belt (Taebaeksan basin), to decipher the tectonic history of the Korean Peninsula as well as the Ogcheon Belt. A total of 652 independently oriented core samples was drilled from 45 sites for the study. The characteristic remanent magnetization (ChRM) isolated from all the samples was a remagnetized component carried by various magnetic minerals (magnetite, hematite, and pyrrhotite). Secondary authigenic magnetic mineral grains accompanied with altered clays are identified by the electron microscope observations. Furthermore, presence of many hydrothermal vein deposits near the study area and extensive alteration of the rocks by fluids (e.g., illitization, chloritization) evidenced by the XRD studies suggest that the rocks in the study area was remagnetized due to the formation of the secondary authigenic magnetic minerals related to the fluid-mediated process. The paleomagnetic pole position (88.3°E, 83.7°N, A95=4.9°) of the Pyeongan Supergroup calculated from the site mean directions of the ChRMs is close to those of late Cretaceous and Tertiary period of the Korean Peninsula. This late Cretaceous to Tertiary remagnetization event is also reported by a previous study of an adjacent region within the northwestern part of the Taebaeksan Basin. However, the primary remanent magnetization was reported by some studies in the southeastern part of the Taebaeksan basin. This aspect implies that the two regions were separated by the Kagdong thrust and have experienced different geologic events since the deposition of the strata. It is observed that there are more thrusts with NE-SW trend in the northwestern part of the Taebaeksan basin in comparison with those in the southeastern part, indicating that the migrating fluids along the fault system from southwest to northeast within the Ogcheon Belt might penetrate more easily and pervasively into the northwestern part by utilizing the already well-developed thrust system. This interpretation is in accordance with the report that the orogenic or basinal fluids were triggered by the subduction of the Kula/Pacific plates under the Eurasian plate during Cretaceous times, and migrated along the fault system within the Ogcheon Belt.