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

Abstract

The Bakjisan Syncline is located in the northwestern part of the Taebaeksan Basin, Korea. New paleomagnetic data for the Upper Carboniferous–Lower Triassic Pyeongan Supergroup from the Pyeongchang area on the west limb of the Bakjisan Syncline have been obtained, and synthesized and compared with previous data from the Jeongseon area on the east limb of the syncline. A total of 350 specimens were collected from 21 sites to clarify the relationship between the spatial distribution of remagnetized areas and the thrust system in the Taebaeksan Basin. The characteristic remanent magnetization (ChRM) isolated from all samples was a remagnetized component acquired after tilting of the strata and carried by various magnetic minerals (magnetite, hematite and pyrrhotite). From rock magnetic studies, electron microscope observations and XRD analyses, the pervasive remagnetization is interpreted to be associated mainly with a fluid-mediated chemical remanent magnetization (CRM). This is consistent with the results of previous work in adjacent areas. The paleomagnetic pole position (88.3°E, 83.9°N, A 95 = 4.9°) from the Pyeongan Supergroup in the Bakjisan Syncline indicates that the timing of the remagnetization event is Early Tertiary times (i.e. Paleocene to Eocene) by comparison with reliable paleopoles from the Korean Peninsula. Early Tertiary CRMs are also reported from previous studies of an adjacent region within the northwestern part of the Taebaeksan Basin. In contrast, a primary remanent magnetization was reported in the southeastern part of the Taebaeksan Basin. This implies that the major thrust system (the Gakdong thrust) which separates the two regions has caused them to experience substantially different geologic histories since deposition of the strata. Since many thrusts with NS trend are observed in the northwestern part of the Taebaeksan Basin compared with the southeastern region, it appears that the remagnetizing fluids pervasively penetrated the northwestern part of the basin by utilizing the already well-developed thrust system.