New insights into the India–Asia collision process from Cretaceous paleomagnetic and geochronologic results in the Lhasa terrane

Abstract

To further constrain the India–Asia collisional process, a combined paleomagnetic and geochronologic study has been carried out on the Upper Cretaceous Jingzhushan Formation redbeds and the Lower Cretaceous Dianzhong Formation volcanic rocks dated at ~121–117Ma from the Cuoqin area in the central Lhasa terrane. Stepwise thermal demagnetization successfully isolated reliable characteristic remanent magnetization (ChRM) directions that include dual polarity and pass positive fold tests at 95% and 99% confidence levels, indicating prefolding primary magnetizations. The tilt-corrected site-mean direction for 33 redbed sites is D=316.8°, I=30.2° with α95=5.4°, corresponding to a paleopole at 49.0°N, 344.3°E with A95=5.3°, and the other for 12 volcanic sites is D=350.5°, I=25.5° with α95=7.7°, corresponding to a paleopole at 70.5°N, 292.9°E with A95=7.4°. Our new paleomagnetic results, together with reliable Cretaceous paleomagnetic data obtained from the Lhasa terrane, demonstrate that the southern margin of Asia was located at ~15.1°N during the Cretaceous. Comparison with the apparent polar wander paths (APWP) of India and the Cretaceous–Paleocene paleopoles of the Himalayan terrane suggests that the India–Asia collision was likely a complex process, which consists of the collision of the Lhasa and Himalayan terranes at 54.0±2.1Ma, the longtime subduction of an intra-oceanic basin between the Himalayan terrane and the Indian craton from ~54.0 to ~40.4Ma, and the collision of the Himalayan terrane and the Indian craton at 40.4±4.1Ma. Comparing with the Late Cretaceous average pole of the East Asia APWP reveals that a latitudinal convergence of 780±240km has taken place between the Lhasa terrane and East Asia (the Hexi corridor) since the India–Asia collision; the amount of latitudinal shortening deduced from paleomagnetic data is very consistent with the 600–1000km accommodated by the Cenozoic fold and thrust belts between the Lhasa terrane and Hexi corridor.