NEW PALEOMAGNETIC RESULTS FROM LADAKH, WESTERN HIMALAYA SUPPORT MULTI-STAGE COLLISION SCENARIO BETWEEN INDIA AND EURASIA

The Kohistan-Ladakh arc (KLA) separates India from Eurasia in the western Himalaya. Two conflicting hypotheses have been developed concerning the collision between India, the KLA, and Eurasia. In the classical model, the KLA is thought to have collided with Eurasia in the Cretaceous (~85-110 Ma), and, subsequently, India collided with Eurasia and the KLA at ~50 Ma along the Indus-Tsangpo suture. Alternatively, it has been proposed that India first collided with the KLA at ~50 Ma, and, subsequently, India and the KLA collided with Eurasia at ~40 Ma along the Shyok-Tsangpo suture zone. These different collision scenarios make distinct predictions for the absolute timing of the India-Eurasia collision. In the classical hypothesis, the KLA should have been located at ~20°N after collision with Eurasia, whereas the alternative model predicts that the KLA remained far removed form Eurasia, therefore south of ~20°N in the Paleo-Tethys until 50 Ma. We conducted a paleomagnetic study to test these two conflicting hypotheses by determining the paleolatitude of formation for a ~67-52 Ma sequence of the Khardung volcanics--a unit located in the Shyok-Nubra valley and overlying the KLA.

Samples were collected at four sites. During stepwise thermal demagnetization, samples from all four sites contained co-directional high-temperature (HT) magnetization components persisting to the magnetite Curie temperature of 580°C or greater. A baked contact test at one site suggests that these HT magnetizations predate dike intrusion and bedding tilt, indicating that the HT components likely reflect primary magnetization. The average of the HT site mean directions implies a paleolatitude of 5°N.

Our results preclude the possibility that the KLA collided with Eurasia at ~ 20°N in the Cretaceous. Instead, they support the hypothesis that the KLA initially collided with India at ~ 50 Ma in the equatorial region of the Tethyan ocean.