Paleomagnetic constraints on the tectonicsof the Eastern border of the Qiangtangterrane (Tibetan Plateau) during theIndia-Asia collision

National audiencePaleomagnetism should be a useful tool to estimate northsouthconvergence across the Himalaya and Tibetan Plateau. Unfortunatelythe large negative inclination anomaly for most of theCenozoic rocks has been the subject of numerous controversies(non dipolar geomagnetic fields, inclination flattening in sedimentsor large scale continental deformation of Eurasia). We presentnew paleomagnetic results from two Cenozoic basins of the Easternpart of the Qiangtang terrane characterized by two short-livedvolcanic fields at ~37-38 Ma (Nangqian area) and 49-51 Ma (Xialaxiuarea). In the Nangqian basin, sites have been collected inred beds sediments, sills and dikes intruding the red beds and inextrusive volcanic rocks mainly found on top of the sedimentarysequence. A well-defined secondary component of magnetizationwith normal polarity was recovered in the red beds in the temperaturerange ~150-600C. 24 out of 25 sites in magmatic rocks,have a primary magnetization of normal polarity. The remagnetizationin the red beds is thus clearly related to the volcanic event.The paleomagnetic data confirm previous field interpretations indicatingthat volcanic activity occurred at the end of a phase ofdeformation in the syntectonic Nangqian basin. There is no inclinationanomaly in the mean paleomagnetic result (D=18.0,I=50.1 a95 =7.9) from 25 sites in volcanic rocks when comparedto the Eurasian reference pole. This paleomagnetic resultis confirmed by the remagnetization in red beds.In the Xialaxiu area, we sampled the volcanic field and red bedsfilling a basin 10 to 20 km farther north. Results from the red bedsconfirm the result (D=322.0, I=32.3, a95=9.5) previously obtainedat this location by Cogné et al. (1999). However, the meandirection from 21 sites in volcanic rocks is different (D=11.9,I=41.6, a95=8.0) from that in the red beds suggesting differentage of magnetization.In conclusion, the mean inclination in ~50 Ma volcanics at Xialaxiuis similar to that obtained by Dupont-Nivet et al. (2010) for theLhasa terrane at the same age suggesting about 1100 km ± 500N-S convergence with stable Eurasia. However, our results fromthe ~37 Ma Nangqian rocks yield a higher mean inclination implyinga paleolatitude similar to expected for Eurasia at this time.This suggests that significant N-S convergence detectable by paleomagnetismdid not occur north of the Qiangtang terrane after37 Ma. Taken at face values, our results rather suggest that mostof the ~1000 km convergence occurred north of the Qiangtangterrane before 37 Ma but we cannot exclude that this potentialinclination anomaly affected both ~50 Ma Qiangtang and Lhasaterrane results. Finally, despite most regional structures beingNW-SE oriented, there is no evidence for consistent clockwise rotationsas expected with a model of deformation implying a largecomponent of extrusion or dextral shear along the eastern marginof the Tibetan Plateau