Paleomagnetism and U-Pb zircon geochronology of Lower Cretaceous lava flows from the western Lhasa terrane: New constraints on the India-Asia collision process and intracontinental deformation within Asia

To better understand the India-Asia collisional process and intracontinental deformation within Asia, a paleomagnetic study has been conducted on the Qushenla Formation lava flows dated at similar to 132-120Ma from the Yanhu area in the western Lhasa terrane. Stepwise thermal demagnetization isolates stable characteristic remanent magnetizations, which include dual polarity and pass fold tests at a 99% confidence level, indicating primary magnetizations. The tilt-corrected site-mean direction for 51 sites is D=28.2 degrees, I=34.5 degrees, and k=74.3 degrees with (95)=2.3 degrees, corresponding to a paleopole at 61.4 degrees N, 192.9 degrees E (A(95)=2.1 degrees). Our new paleomagnetic data, combined with previous Cretaceous volcanic paleomagnetic data from the Lhasa terrane, show that the precollisional southern margin of Asia was at similar to 16.8 degrees N with a relatively E-W alignment and that the Lhasa terrane did not experience significantly discrepant north-south movement although local vertical axis rotations did take place after the Cretaceous. Comparison with the apparent polar wander paths (APWPs) of India and the Cretaceous-Paleocene paleopoles of the Tethyan Himalaya shows that the India-Asia collision was at similar to 54.3Ma and the Greater India during the Paleocene had a large northern extent of similar to 2000km (similar to 18.1 degrees) beyond its present northern margin. Comparison with the Cretaceous Eurasian APWP indicates that a latitudinal convergence of similar to 1000km has taken place between the Lhasa terrane and Eurasia since the India-Asia collision. The amount of north-south shortening deduced from Cretaceous paleomagnetic data is consistent with that accommodated by the Cenozoic fold and thrust belts between the Lhasa terrane and the Hexi corridor.