Palaeomagnetism of the (Upper Palaeozoic-Lower Mesozoic) Pyongan Supergroup, Korea: a Phanerozoic Link with the North China Block

Abstract

SUMMARY The Upper Carboniferous-Lower Triassic Pyongan Supergroup is exposed in an E-W trending syncline comprising the Samcheok Coalfield in eastern South Korea; it borders the southern margin of a NE-SW trending (Okcheon) zone of Upper Jurassic-Lower Cretaceous deformation (Daebo Orogeny). Although a Recent field overprint widely recorded in Korean rocks is represented here, an ancient field component structure is recovered from the red bed facies in this Supergroup by detailed thermal demagnetization. A prominent Upper Jurassic-Cretaceous overprint is widespread (DIZ = 56.1/54.8, agS = 9.6, palaeopole at 200.4E, 46.7N) and records the magnetic effect of the Daebo Orogeny within the Okcheon Belt. It is post-deformational in origin and appears to have been confined mainly to the Cretaceous Normal Superchron. It has been rotated clockwise since these times, in common with the main synclinal axis, as a consequence of dextral movements along the Tan Lu Fault System. Components predating this deformation are recovered from the Lower Triassic (DIZ = 1.1/19.4, agS = 18.2, palaeopole at 306.1°E, 63.2N), Permian (DIZ = 358.3/11.5, c~~~ = 6.3, palaeopole at 311.9E, 58.7N) and Upper Carboniferous (DIZ = 34l.U-9.2, mgS = 7.2, palaeopole at 335.7E, 44.6N) sediments. Second-order small circle dispersions of site palaeopoles record unaccommodated tectonic rotations and are largest in the oldest beds below an Upper CarboniferousLower Permian unconformity. However, the presence of reversals, probably acquired before and after the Carboniferous-Permian Reversed Superchron, defines ancient field axes and identifies an Upper Carboniferous-Lower Triassic APW path. This path correlates with the contemporaneous path from the North China Block and is removed from the path from the South China Block; agreement with North China is enhanced when these results are corrected for the rotation indicated by the Cretaceous overprint. We therefore identify a first-order correlation between the Korean Peninsula and North China at least since Upper Palaeozoic times. Establishment of, and movements along, the Tan Lu Fault System since Mesozoic times have not obscured this palaeomagnetic correlation.