Post-folding magnetization of the Triassic rocks from western Guizhou and southern Yunnan provinces: New evidence for large clockwise rotations in the Simao Terrane

Abstract

Thermal demagnetization of the redbed samples collected from the Lower Triassic Feixianguan Formation at two localities in western Guizhou Province reveals that the natural remanent magnetization was dominated by a postfolding component of probably recent origin and the prefolding, possibly primary, component was barely preserved. This confirms that the Feixianguan Fm in western Guizhou and northeastern Yunnan provinces was severely remagnetized. Thermal demagnetization also resolves two postfolding components from the limestone samples of the Mid-Triassic Dashuijingshan Formation from southern Yunnan Province: a low temperature component acquired in recent times and a high temperature component probably acquired shortly after the India–Asia collision. The mean in-situ direction for the high temperature component based on eighteen sites is D=95.4°, I=28.4° with α95=6.2°, which indicates a clockwise rotation of 86.3±7.3° with respect to stable Eurasia. This result together with paleomagnetic results previously reported from the Jurassic–Eocene rocks in the same area demonstrates beyond reasonable doubt that the central Simao Terrane has experienced large clockwise rotations due to the India's collision with, and continuing penetration into, Asia.