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 degr ees N, 99.4 degrees E), western part of Yunnan province, China. After stepw ise thermal demagnetization, high-temperature component magnetization with unblocking temperature of about 675 degrees 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 degrees, I = 49.9 degrees w ith alpha(95) = 3.9 degrees, corresponding to a paleopole at 54.6 degrees N , 171.8 degrees E with A(95) = 4.4 degrees Comparison with previously repor ted Cretaceous paleomagnetic directions indicates that the Yunlong area has been subjected to southward displacement by 11.9 +/- 7.5 degrees (correspo nding to 1300 +/- 800 km) with respect to the stable Yangtze craton since C retaceous time as well as clockwise rotation through 36.3 +/- 13.6 degrees. This study elucidates that Indochina was squeezed out of the Asian contine nt due to collision of India accompanied with clockwise rotation. (C) 1999 Elsevier Science B.V. All rights reserved.