Paleomagnetic results from Cretaceous and Jurassic rocks of South and Southwest Yunnan: evidence for large clockwise rotations in the Indochina and Shan-Thai-Malay terranes

Cretaceous and Jurassic redbeds have been sampled from South and Southwest Yunnan, China, for paleomagnetic study. The paleomagnetic directions resolved from the Cretaceous rocks indicate that the Xiaguan area, which is located on the Red River fault, has not been rotated with respect to stable Eurasia since at least Late Cretaceous times and that the Jinggu-Mengla area 200–400 km farther south has been rotated by 46–65° clockwise. The Jurassic results reveal the same rotation pattern. At face value, the differences between the observed Cretaceous paleolatitudes and those predicted from the Eurasian apparent polar wander path (APWP) indicate a southward extrusion of the region, but the amount of extrusion is not large enough to be resolved by paleomagnetic methods. Comparison with Mesozoic paleomagnetic data from central Yunnan, northern Thailand and northwestern Borneo suggests that the Red River fault is not a demarcation between unrotated and significantly rotated regions, and that Sundaland did not respond to the India-Asia collision as a single coherent unit. The existing paleomagnetic data from East Asia appear to indicate that the crust in central Tibet and Tien Shan north of Tarim directly in front of the Indian indentor has been substantially shortened and thickened whereas the crust in the syntaxial areas has probably been largely squeezed out. To further evaluate the relative importance of crustal thickening and extrusion in the tectonic evolution of the Tibetan plateau, paleomagnetic investigation should be intensified in the bounding zones to the Indian indentor and be expanded into the Cenozoic sedimentary rocks in Tibet and surrounding regions.