PALEOMAGNETISM AND GEOCHRONOLOGY OF THE EURASIAN MARGIN IN THE SHYOK SUTURE ZONE

Two incompatible models persist for the India-Eurasia collision. In the classical model, the collision was a single-stage event that initiated in the Paleocene. Alternatively, it has been proposed to be a multi-stage process involving at least two subduction zones. Much of the uncertainty surrounding these conflicting interpretations rests with the structure and geometry of the major suture zones that transect the Himalayan belt. In the western Himalaya, Indian and Eurasian terranes are separated by the Kohistan-Ladakh arc, the remnant of a Cretaceous-Paleocene intra-oceanic subduction system in the Neotethys Ocean outboard of the Eurasian margin. Collision between this subduction system and India along the Indus suture zone is well constrained to 50 – 55 Ma, but the age and nature of the Shyok suture zone between the Kohistan-Ladakh arc and the Eurasian Karakoram terrane is debated. In multi-stage collision models, the Shyok suture zone is interpreted to have formed during the subduction of a major oceanic plate after the arc-continent collision occurred on the leading edge of India. In the single-stage model, the Shyok suture zone is interpreted to have formed due to the closure of a minor back-arc basin on the Eurasian margin long prior to the final continental collision. We present U-Pb zircon geochronology and paleomagnetic data from rocks that are exposed just 15 km apart on either side of the Shyok suture zone in Ladakh, NW India. Our results show that during the late-Cretaceous, the Karakoram terrane was situated on the southern edge of the Eurasian continent at a paleolatitude consistent with the Lhasa terrane further east in Tibet. This was a significant distance north of the Kohistan-Ladakh arc, which was at a near-equatorial latitude until it collided with India in the Paleocene. Our results demonstrate that the Shyok suture zone marks the closure of a major ocean basin and show that the subduction of oceanic lithosphere accounts for a significant part of the India-Eurasia convergence since the Paleocene. Final India-Eurasia continental collision occurred along the Shyok-Tsangpo suture zone, not the Indus-Tsangpo suture zone.