Cretaceous And Paleogene Paleomagnetic Results From The Southeastern Part Of Eastern Himalaya Syntaxis And Its Implication For Tectonic Evolution

In order to contribute the understanding of the history of India/Asia collision, and the intracontinental deformation of the southeastern Tibetan Plateau and adjacent blocks, we carried out a systematic paleomagnetic research in a Paleocene sedimentary basin in the Chuandian Fragment, the Cretaceous and Paleogene sedimentary rocks in the Simao terrane of the Shan-Thai block.Paleocene red sandstones were collected at 22 sites from the Ninglang formation at Ninglang area (26.9°N,100.8°E), the Northwestern part of the Chuan Dian Fragment (CDF) of the Yangtze block. After stepwise thermal demagnetization, a high temperature component with unblocking temperature of about 680℃ is isolated from 19 sites that passes the McElhinny/McFadden’s fold test at the 95% percent confidence level. The tilt-corrected mean paleomagnetic direction for the Ninglang formation is D=17.7°,I= 47.8° with α95=5.3. Combined with previous paleomagnetic data from the CDF, these results indicate an insignificant latitude translation for the whole fragment with respect to the South China Block (SCB) since the Early Cretaceous. Paleomagnetic data indicate that the CDF has been rotated 9.6° clockwise relative to the SCB since the initial movement of Xianshuihe-Xiaojiang Fault System. After a detailed analysis of available paleomagnetic data from the CDF and their relationship with the correlative faults, we conclude that in comparison with the central CDF, relative motion along the western part of the Luzhijiang fault resulted in further clockwise rotation. Except the strong deformation in the western part along the Luzhijiang fault, the deformation of CDF behaved with the non-continuum or discrete model.A detailed study of magnetic fabric was carried out on red beds of the Paleocene Ninglang Formation near the Chenghai fault in the Chuandian fragment.287 oriented samples were collected from 22 sampling sites along the Zhan river and Xibu river. Comprehensive analysis shows 4 types of magnetic fabrics, initial deformation magnetic fabric, pencil-shaped magnetic fabric and two types of transitional forms in between. The preferred orientation of K1 in the western part of the studying area (sampling site 1-9) was NNE-SSW, and changed to S-N in the east (sampling site 13-22). The abrupt changes of K1 direction may be related to the existence of a blind fault. In addition, we find the magnetic fabric can make a fair judgment of the block rotations that is bonded by a series of faults. After correcting the local rotation, the result indicates that the study area was suffered from an E-W maximum principal compressional paleostress during the Middle-Late Eocene, which may be caused by the India/Asia collision. Moreover, the E-W paleostress is significantly different from present approximately S-N stress direction, which was related to the changed deformation types of the Tibetan plateau from the initial stage of compression to the later E-W extension.Through the paleomagnetic research from the Cretaceous and Paleocene sedimentary rocks in Simao terrane, we further confirmed that the Lanping-simao Basin have experienced different tectonic rotations. Discrepancies of rotations in different places may be caused by the extrusion pressing from different time span and different areas. After summarizing the available geological evidences, such as the slipping age of the main faults, outcrops of the magmatic rocks, we make a further confirmation that tectonic evolution of the Shan-Thai Block and the Indochina Block is different from the beginning of the Oligocene. The discrepancies may be caused by the different lithosphere structures by one side and the distance from the eastern Himalaya syntaxis on the other side.