India-Eurasia convergence speed-up by passive-margin sediment subduction.

The fast increase of convergence rate between India and Eurasia around 65 million years ago (Ma)-from approximately 8 cm yr to a peak rate of approximately 18 cm yr-remains a complex geological event to explain, given the inherent uncertainty surrounding the tectonic history and the intricate interplay of forces influencing plate speed. Here we use a combination of geochemical analysis and geodynamic modelling to propose that this rapid convergence can be explained by sediment subduction derived from the northern Indian passive margin. Through isotope and trace element analysis, we find an enhanced contribution of terrigenous sediment melt to the mantle source of the Gangdese magmatic rocks around 65 Ma, concurrent with the acceleration of India-Eurasia convergence. Numerical experiments suggest that subduction of sediments more than 1 km thick covering an approximately 1,000-km-wide ocean basin abutting the northern Indian passive margin starting from 65 Ma could have spurred the increased convergence rate and further led to significant crustal extension, consistent with empirical observations. Our study implies that the acceleration of India-Eurasia convergence marks the arrival of passive-margin-derived sediments, constraining the initial India-Eurasia collision to be around 60 Ma. It further suggests that temporary accelerations in subduction rates might be a common feature at the final stage of continental assembly.