A paleomagnetic age constraint on collision

In northwest India and Pakistan, the rocks of the Eurasian continent are separated from those of the Indian continent by the remains of an intra-oceanic arc system called the Kohistan-Ladakh arc. Constraining the age of collision between the arc and Eurasia is important to reconcile discrepancies between the amount of continental convergence and crustal shortening in the India-Eurasia tectonic system.

To determine the age of collision between the Kohistan-Ladakh arc and Eurasia we measured the magnetic record of sedimentary and volcanic rocks from the collision zone. We also fingerprinted the origin of the clastic material in the sedimentary layers by measuring the ages of zircon crystals that were transported in ancient rivers before being deposited in the ocean.

Our zircon age results show that the sedimentary rocks we sampled contained detritus transported from across the Eurasian continent and that they were deposited close to the Eurasian continental margin. The latitude at which these Eurasian-derived rocks formed was significantly further north than the latitude of rocks from the other side of the collision zone. This means the arc and Eurasia were still separated by an ocean basin approximately 1300 km wide in the Paleocene and the continental collision had not yet begun.