PALEOMAGNETIC STUDY OF PERMIAN AND MESOZOIC SEDIMENTARY-ROCKS FROM NORTHERN THAILAND SUPPORTS THE EXTRUSION MODEL FOR INDO-CHINA

The mode of deformation of southeast Asia resulting from the Indian collision is still controversial: is convergence betWeen Stable India and Siberia absorbed principally by crustal shortening and thickening and minor strike-slip faulting or by a mechanism of extrusion involving crustal thickening and lateral escape of blocks along major strike-slip zones? In order to test these models, we have carried out a paleomagnetic study of sedimentary rocks of Jurassic to Cretaceous age on the Khorat Plateau in Thailand. A large discrepancy between the poles of Indochina presented in this paper and a compilation of South China block pole positions both at the Triassic/Jurassic and the Jurassic/Cretaceous boundary imply some 1500 +/- 800 km of post-middle Cretaceous left-lateral slip along the Red River and Xian Shui He fault zones and approximately 14 +/- 7-degrees of clockwise rotation for the Indochina block relative to the South China block. Such relative motions are in agreement with a model of lateral extrusion of Indochina during the India-Asia collision. Additional data of Permian, Upper Triassic and Lower Jurassic age collected in Indochina indicate a post-late Permian remagnetization for the upper Permian limestones. A comparison of these data with those from Yunnan (South China), the North China block and the South China block shows that these blocks have probably been in contact at least since the late Triassic.