The Early Tertiary chemical remagnetization in the Bakjisan Syncline, Korea: Its geotectonic implications

Bakjisan Syncline is located in the northwestern part of the Taebaeksan Basin, Korea. A new palaeomagnetic data for the Upper Carboniferous-Lower Triassic Pyeongan Supergroup from the Pyeongchang area, the west limb of the Bakjisan syncline, were obtained and synthesized with previous data from the Jeongseon area, the east limb of the syncline. The aim of this study is to clarify the relationship between spatial distribution of remagnetized areas and thrust system in the Taebaeksan Basin. A total of 350 independently oriented specimens were collected from 21 sites for the study. The characteristic remanent magnetization (ChRM) isolated from all the samples was a remagnetized component carried by various magnetic minerals (magnetite, haematite and pyrrhotite). Process of the pervasive remagnetization is regarded as a chemical remanent magnetization (CRM) based on the rock magnetic studies, electron microscope observations and XRD analyses, which is consistent with the results of previous works in adjacent areas. The palaeomagnetic pole position (88.3E, 83.9N, A95=4.9) of the Pyeongan Supergroup in the Bakjisan Syncline is close to the Late Cretaceous and Tertiary poles of the Korean Peninsula. The Late Cretaceous to Tertiary CRMs are also reported by previous studies of an adjacent region within the northwestern part of the Taebaeksan Basin. In contrast, the primary remanent magnetization was reported in the southeastern part of the Taebaeksan Basin. This aspect implies that the two regions were separated by a major thrust system (i.e. the Gakdong thrust), and have experienced different geologic events since the deposition of the strata. It is observed that there are many thrusts with NS trend in the northwestern part of the Taebaeksan Basin in comparison with those in the southeastern part, indicating that the remagnetizing fluids, migrated along the fault system within the Okcheon Belt, might penetrate more easily and pervasively into the northwestern part by utilizing the already well-developed thrust system.A package of computer programs used in this study was kindly provided by Dr. Randolph J. Enkin. This manuscript was greatly improved by careful and constructive reviews from Dr. John D.A. Piper and Prof. Graham J. Borradaile. We thank Prof. Keke Zhang for editing the manuscript. This study was supported by Korea Research Foundation Grant (KRF-2003-015-C00660)