India–Asia convergence driven by the subduction of the Greater Indian continent

The most spectacular example of a plate convergence event on Earth is the motion of the Indian plate towards Eurasia at speeds in excess of 18 cmyr−1 (ref. 1), and the subsequent collision. Continental buoyancy usually stalls subduction shortly after collision, as is seen in most sections of the Alpine–Himalayan chain. However, in the Indian section of this chain, plate velocities were merely reduced by a factor of about three when the Indian continental margin impinged on the Eurasian trench about 50million years ago. Plate convergence, accompanied by Eurasian indentation, persisted throughout the Cenozoic era1–3, suggesting that the driving forces of convergence did not vanish on continental collision. Here we estimate the density of the Greater Indian continent, after its upper crust is scraped off at the Himalayan front, and find that the continental plate is readily subductable. Using numerical models, we show that subduction of such a dense continent reduces convergence by a factor similar to that observed. In addition, an imbalance between ridge push and slab pull can develop and cause trench advance and indentation. We conclude that the subduction of the dense Indian continental slab provides a significant driving force for the current India– Asia convergence and explains the documented evolution of plate velocities following continental collision. Many authors have suggested that some Indian continental lithosphere has subducted, but owing to its buoyancy has underplated the Asian continent4,5. More recently, tomographic imaging of the mantle underneath India has indicated continental subduction to larger depths6,7. Reconstructions based on this evidence have proposed that 600–1,000 km of the Indian continental margin was pulled into the mantle, behind the sinking Tethyan oceanic lithosphere3,7,8, where it partly detached once it reached mid-mantle depths7,8. As the entrained continental lithosphere is generally considered too buoyant to actively drive further subduction, an external forcing at the plate’s boundaries, possibly provided by the Indian Ocean ridge push9 or by the pull of neighbouring slabs10, has been invoked to explain continued continental collision at current rates. We postulate here that the subducting Indian lithosphere, imaged in the upper mantle, has the negative buoyancy needed to sustain subduction regardless of its attachment to Tethys lithosphere. This explains continued collision without the need for forces external to the subducting Indian plate. The Indian lithosphere facing the Tethys Ocean was typical of thinned continental margins and extended 600–1,000 km north of the present location of India11. Quantitative backstripping of Zanskar Range units12, which represent the most complete transect through this ancient margin11, constrains the structure of the lithosphere, with a recovered crustal thickness of∼25 km overlying