Jurassic and Triassic paleomagnetism of South Korea

Middle to Late Jurassic samples have been collected from granitic rocks in three geological subdivisions of South Korea, from north to south: Kyonggi Massif, Okchon Folded Belt and Ryongnam Massif. Triassic samples (granites and sediments) were collected in the Ryongnam Massif and its border area with the Okchon Folded Belt. The Jurassic declinations of the two massifs are northeasterly (mean D/I = 39.6°/+57.6°, N = 7 sites, k = 31, α95 = 11.1°), whereas the intervening Okchon Folded Belt reveals northwesterly Jurassic declinations (D/I = 337.5°/+47°, N = 7 sites, k = 32, α95 = 10.8°). Reversed‐polarity directions are rare, but have been observed in three localities. The widespread occurrence of similar directions in a given province argues against major tectonic tilting of the granites; two sites in Jurassic sediments provide additional structural control. We interpret the direction difference between the Okchon belt and the adjacent massifs as due to left‐lateral shear in an actively deforming continental margin during the Late Jurassic‐Early Cretaceous Daebo Orogeny. The Triassic directions from the Ryongnam Massif and its border area are east–northeasterly (D/I = 62.9°/+35.7°, N = 5, k = 27, α95 = 15°) with the sediments showing normal and reversed polarities. Comparisons with Jurassic and Triassic paleomagnetic results from adjacent blocks in Asia (Eurasia, North Korea, North and South China) are hampered by a paucity of reliable paleopoles. What seems clear, however, is that North and South China and the major part of Korea yield Jurassic and younger paleopoles that are to first approximation similar; the exception is formed by the results from the Okchon Folded Belt. The aggregate of the China and Korea Blocks yields mean paleopoles for this interval that are different from those published for Eurasia, but this may be due to imperfections in the Eurasian reference poles. For the Triassic, the directions and paleopoles of the China Blocks and the Ryongnam Massif of South Korea are dissimilar, resulting in interpretations of the area in terms of relative motions (mainly rotations).