Paleomagnetic constraints on the tectonics of the Eastern border of the Qiangtang terrane (Tibetan Plateau) during the India-Asia collision

Pierrick Roperch (1) , Guillaume Dupont-Nivet (1) , Stéphane Guillot (2) , Fanny Goussin (2) , Anne Replumaz (2) , Wentao Huang (3) , Yang Zhang , Loris Wacquier (1)

1 GR - Géosciences Rennes
2 ISTerre - Institut des Sciences de la Terre
3 Department of Geosciences [University of Arizona]

Paleomagnetism should be a useful tool to estimate northsouth convergence across the Himalaya and Tibetan Plateau. Unfortunately the large negative inclination anomaly for most of the Cenozoic rocks has been the subject of numerous controversies (non dipolar geomagnetic fields, inclination flattening in sediments or large scale continental deformation of Eurasia). We present new paleomagnetic results from two Cenozoic basins of the Eastern part of the Qiangtang terrane characterized by two short-lived volcanic fields at ~37-38 Ma (Nangqian area) and 49-51 Ma (Xialaxiu area). In the Nangqian basin, sites have been collected in red beds sediments, sills and dikes intruding the red beds and in extrusive volcanic rocks mainly found on top of the sedimentary sequence. A well-defined secondary component of magnetization with normal polarity was recovered in the red beds in the temperature range ~150-600C. 24 out of 25 sites in magmatic rocks, have a primary magnetization of normal polarity. The remagnetization in the red beds is thus clearly related to the volcanic event. The paleomagnetic data confirm previous field interpretations indicating that volcanic activity occurred at the end of a phase of deformation in the syntectonic Nangqian basin. There is no inclination anomaly in the mean paleomagnetic result (D=18.0, I=50.1 a95 =7.9) from 25 sites in volcanic rocks when compared to the Eurasian reference pole. This paleomagnetic result is confirmed by the remagnetization in red beds. In the Xialaxiu area, we sampled the volcanic field and red beds filling a basin 10 to 20 km farther north. Results from the red beds confirm the result (D=322.0, I=32.3, a95=9.5) previously obtained at this location by Cogné et al. (1999). However, the mean direction from 21 sites in volcanic rocks is different (D=11.9, I=41.6, a95=8.0) from that in the red beds suggesting different age of magnetization. In conclusion, the mean inclination in ~50 Ma volcanics at Xialaxiu is similar to that obtained by Dupont-Nivet et al. (2010) for the Lhasa terrane at the same age suggesting about 1100 km ± 500 N-S convergence with stable Eurasia. However, our results from the ~37 Ma Nangqian rocks yield a higher mean inclination implying a paleolatitude similar to expected for Eurasia at this time. This suggests that significant N-S convergence detectable by paleomagnetism did not occur north of the Qiangtang terrane after 37 Ma. Taken at face values, our results rather suggest that most of the ~1000 km convergence occurred north of the Qiangtang terrane before 37 Ma but we cannot exclude that this potential inclination anomaly affected both ~50 Ma Qiangtang and Lhasa terrane results. Finally, despite most regional structures being NW-SE oriented, there is no evidence for consistent clockwise rotations as expected with a model of deformation implying a large component of extrusion or dextral shear along the eastern margin of the Tibetan Plateau.

Mots clés: Tibet; tectonics; paleomagnetism

Domaines: Tectonique; Géophysique [physics.geo-ph]

Pierrick Roperch, Guillaume Dupont-Nivet, Stéphane Guillot, Fanny Goussin, Anne Replumaz, et al.. Paleomagnetic constraints on the tectonics of the Eastern border of the Qiangtang terrane (Tibetan Plateau) during the India-Asia collision. 25 ème Réunion des sciences de la Terre (RST 2016), Société Géologique de France, Oct 2016, Caen, France. pp.279. ⟨insu-01388673⟩