Chemical remagnetization of the Upper Carboniferous-Lower Triassic Pyeongan Supergroup in the Jeongseon area, Korea: fluid migration through the Ogcheon Fold Belt

Abstract

SUMMARY Palaeomagnetic and rock magnetic investigations have been carried out for the Upper Carboniferous‐Lower Triassic Pyeongan Supergroup, exposed in the Jeongseon area in eastern South Korea. A total of 302 independently oriented core samples were drilled from 24 sites for the study. The mean direction in stratigraphic coordinates (D/I = 345.3 ◦ /38.4 ◦ , k = 4.8, α95 = 18.2 ◦ ) is more dispersed than the mean direction in geographic coordinates (D/I = 354.9 ◦ /61.5 ◦ , k = 58.0, α95 = 4.7 ◦ ), and the stepwise untilting of the characteristic remanent magnetization (ChRM) reveals a maximum value of k at 0 per cent untilting. Furthermore, authigenic magnetic mineral grains accompanied with altered clays are identified by electron microscope observations. These results collectively imply that the ChRMs of this study were acquired by the formation of authigenic magnetic minerals after tilting of the strata. The palaeomagnetic pole position (97.3 ◦ E, 82.4 ◦ N, A95 = 6.8 ◦ ) of the Pyeongan Supergroup calculated from the site mean directions of the ChRMs in geographic coordinates is close to those of the Late Cretaceous and Tertiary periods of the Korean Peninsula. The presence of many hydrothermal vein deposits, reportedly formed between 94 and 73 Ma, near the study area and the evidence for extensive alteration of clay minerals under the influence of fluids shown in the X-ray diffraction study strongly suggest that the chemical remanent magnetization was acquired by a fluid-mediated process during the Late Cretaceous‐Early Tertiary. Considering the different remagnetization timing of several areas in the Ogcheon Fold Belt, which extends from NE to SW diagonally across the Korean Peninsula, it is interpreted that the fluids and magmatism associated with this remagnetization originated from the subduction of the Kula/Pacific plates under the Eurasian Plate during the Cretaceous period. The fluids might have migrated northward or northeastward through the fault systems within the Ogcheon Fold Belt.