Remagnetization of the Pyeongan Supergroup in the Yeongwol Area, Korea

Abstract

Paleomagnetic and rock magnetic investigations have been carried out for the late Carboniferous-Permian Pyeongan Supergroup, exposed in the Yeongwol area in eastern South Korea. A total of 228 independently oriented core samples was drilled from thirteen sites for the study. The mean direction after bedding correction (D/1 = 202.7°/-24.6°, k = 2.4, cm 5 = 36.9°) is more dispersed than the in-situ mean direction (D/I = 175.3°/-58.9°, k = 69.0, cm 5 = 5.3°), indicating that the fold test is negative at 95% confidence level. In addition, the stepwise unfolding of the characteristic remanent magnetization (ChRM) reveals a maximum value of at 0% unfolding. Furthermore, authigenic magnetite and hematite grains are identified by the electron microscope observations. These results collectively imply that the ChRM directions were acquired after tilting of the strata by a chemical remanent magnetization when the secondary authigenic magnetic minerals formed. The ChRM directions of the Supergroup, however, pass the reversal test at 95% confidence level suggesting that the remagnetization occurred during the period including both normal and reversed polarity intervals. Because the paleomagnetic pole position (39.4°E, 85.6°N, A 95 = 8.9°) of the Pyeongan Supergroup calculated from the mean site directions of the ChRMs is close to that of the Tertiary period of the Korean Peninsula, it is interpreted that the remagnetization in this area occurred during the Tertiary. However, it is also plausible that the study area might be rotated about 30° anticlockwise after the remagnetization at the end of the Daebo Orogeny, in the beginning of the Cretaceous period.