New constraints to the onset of the India-Asia collision: Paleomagnetic reconnaissance on the Linzizong Group in the Lhasa Block, China

To better understand the onset of the India-Asia collision, a paleomagnetic study has been conducted on Paleogene volcanic rocks and sediments of the Linzizong Group (∼ 64-44 Ma) from the Linzhou and Namling basins of the Lhasa Block at the southern margin of the Asian continent. Following detailed rock magnetic investigations and progressive thermal or hybrid thermal and alternating field demagnetization, stable characteristic remanent magnetizations (ChRMs) were successfully defined from the Dianzhong (∼ 64-60 Ma), Nianbo and Pana (∼ 60-44 Ma) formations. These ChRMs are of dual polarity, and pass a fold test with some of them in volcanic rocks carried by oxidation-induced hematite; we thus interpret them as primary remanences. Paleomagnetic results from the bulk of the collection imply that the southern leading edge of Asia, i.e. the Lhasa Block, remained essentially stationary in low latitudes at ∼ 10°N without paleomagnetically-detectable tectonic rotation and latitudinal variation during the formation of the Linzizong Group. Consequently, a large-scale post-collisional northward convergence in the Tibetan plateau and central Asia is very likely to have occurred after formation of the Linzizong Group, and the definitive paleoposition of the Lhasa Block during the period embracing the interval between ∼ 64 and 44 Ma provides further constraints on the latitudinal position of the initial contact between India and Asia. This evidence rules out the model for a younger (∼ 35 Ma) India-Asia collision and implies that the initial India-Asia collision occurred between ∼ 65 and 50 Ma. Given that Greater India comprised the present Indian continent together with a ∼ 950 km extension beyond its northern margin prior to the rifting of Gondwana in the Middle Jurassic, we conclude that the initial India-Asia collision most probably occurred between ∼ 55 and 60 Ma.