Paleomagnetic study of middle Cretaceous rocks from Yunlong, western Yunnan, China: evidence of southward displacement of Indochina

Middle Cretaceous red sandstones and siltstones were collected at 20 sites for paleomagnetic study from the Nanxin Formation around Yunlong (25.8°N, 99.4°E), western part of Yunnan province, China. After stepwise thermal demagnetization, high-temperature component magnetization with unblocking temperature of about 675°C is isolated. A positive fold test at the 99% confidence limit shows that the high-temperature component magnetization is primary for the middle Cretaceous Nanxin Formation. A tilt-corrected formation mean direction is D = 40.2°, I = 49.9° with α95 = 3.9°, corresponding to a paleopole at 54.6°N, 171.8°E with A95 = 4.4°. Comparison with previously reported Cretaceous paleomagnetic directions indicates that the Yunlong area has been subjected to southward displacement by 11.9±7.5° (corresponding to 1300±800 km) with respect to the stable Yangtze craton since Cretaceous time as well as clockwise rotation through 36.3±13.6°. This study elucidates that Indochina was squeezed out of the Asian continent due to collision of India accompanied with clockwise rotation.