Paleomagnetic Constraints From the Baoshan Area on the Deformation of the Qiangtang-Sibumasu Terrane Around the Eastern Himalayan Syntaxis

The Sibumasu Block in SE Asia represents the eastward continuation of the Qiangtang Block. Here we report a detailed rock magnetic and paleomagnetic study on the Middle Jurassic and Paleocene rocks from northern Sibumasu, to document the crustal deformation during the India-Asia collision since the Paleocene and reconstruct the overall strike of the Qiangtang/Sibumasu elements before the India-Asia collision. Although the fold test is inconclusive based solely on our data, a positive reversal test, a positive regional fold test with previous paleomagnetic results, and a detrital origin of hematite in the red beds as indicated by scanning electron microscopy suggest that the magnetizations obtained from the Jurassic and Paleocene rocks are most likely primary, showing an ~80° clockwise rotation since Paleocene. These results, together with previously published paleomagnetic data, suggest that the northern Sibumasu and northern Simao elements experienced a ~60-80° clockwise rotation since Paleocene. This large clockwise rotation is also consistent with the surface GPS velocity field and NE-SW fault networks, suggesting a rotational motion of crustal material from southeastern Tibet during late Cenozoic. We infer that the large clockwise rotation is a sum of rotation in the Eocene to Middle Miocene time associated with Indochina extrusion and rotation after the Middle Miocene associated with the E-W extension in central Tibet. This suggests that the eastward motion of Tibetan crustal material along the Xianshuihe-Xiaojiang fault after Middle Miocene is transmitted to the southwest toward Myanmar. Jurassic and Cretaceous paleomagnetic results suggest that the Qiangtang/northern Sibumasu was originally a curved structure with an orientation of N60°W in Tibet and changes to N10°W in southern Sibumasu