Timing of Collision In the Western Himalayan Orogen

The Kohistan-Ladakh (K-L) block occupies the northwestern corner of the Himalayas and has long been recognized to represent an Island Arc constructed on ocean floor during Jurassic and Cretaceous times. Because the K-L block now lies within the Asian continent it is important to know how and when it became sandwiched between India and the rest of Asia. We have found from analysis of paleomagnetic data that in Late Cretaceous/Early Paleocene times the K-L island arc could not have been far from the equator. India was close to the equator but the southern margin of Asia was more than 3,000 km to the north. Our new U/Pb zircon ages from rocks of the K-L block show that calc-alkaline volcanic arc igneous activity ended in the K-L arc by 61 Ma. We interpret that cessation to date the collision of Kohistan with India. This new timing is confirmed by evidence that the southern hemisphere enriched DUPAL mantle source had been involved in the generation of the latest Cretaceous Teru Volcanic Formation rocks of the K-L arc. Final incorporation of India, now carrying the K-L block in its NW corner, into Asia took place at the Shyok suture. The best evidence of the timing of that suturing at ca.50 Ma comes from two post-collisional granites (ages 47 Ma and 41 Ma) in northern Kohistan which show in their zircon isotopic compositions evidence of the involvement of ancient Asian continental crust that did not exist under Kohistan before the suture formed. The 50 Ma age for Shyok suturing against the then active Karakorum Andean arc fits well with the extension of the suture beyond the eastern end of the K-L block to join the precisely dated at ca.51 Ma Yarlung Tsango Po suture between India and the southern margin of Asia.