Paleomagnetism of the Pyeongan Supergroup in the Danyang Area and its tectonic significance

Paleomagnetic and rock-magnetic studies have been carried out for the Late Carboniferous Early Triassic Pyeongan Supergroup in the Danyang area. A total of 498 oriented core samples obtained from 42 sites were subjected to stepwise thermal and AF demagnetizations. Results of the conventional fold test and the stepwise untilting test suggest that the Manhang and Donggo formations retain a primary remanent magnetization, while the Jangseong Formation were remagnetized after tilting of the strata. In case of samples from the Geumcheon, Hambaeksan, Dosagok and Gohan formations, it failed to yield characteristic remanent magnetization components due to very weak intensities of magnetization. Based on results of 3-axes IRM demagnetization experiments and the microscopic observations, it is interpreted that detrital magnetite and hematite are the major magnetic carriers in the Manhang and Donggo formations, whereas authigenic pyrrhotite is the dominant carrier of remagnetized component in the Jangseong Formation. The paleomagnetic directions of the Manhang and Donggo formations yield paleomagnetic poles of the Late Carboniferous (41.7 degrees N/1.3 degrees E, A(95) = 4.5 degrees) and the Early Triassic (49.9 degrees N/15.2 degrees E, A(95) = 3.3 degrees), respectively. The new paleomagnetic poles obtained in this study were compared with previously reported paleopoles from the Taebaek area, the Yemi area, the North China Block (NCB), and the South China Block (SCB), which lead to some tectonic interpretations as follows. The Taebaeksan Basin including the Danyang, Yemi, and Taebaek areas have been located near the NCB since the Late Carboniferous and experienced at least two vertical axis rotations with respect to the stable NCB since the Late Permian. The Danyang area has experienced 28 degrees(R +/- Delta R = 27.7 degrees +/- 12.7 degrees) clockwise rotation which might be associated with the eastward and/or northeastward-stepping of thrusts during the Late Permian to Early Triassic, and then rotated counter-clockwise by about 17 degrees(R +/- Delta R= -16.9 degrees +/- 3.9 degrees). On the other hand, the Taebaek area has rotated clockwise by about 22 degrees(R +/- Delta R= 21.9 degrees +/- 9.9(degrees)), which might be due to the northwestward protrusion of the Yeongyang microblock in the Gyeongsang Basin during the Late Cretaceous. We hope that this study may induce further detailed structural studies to test our tectonic interpretation.