Geodynamics of Long-Term Indian Continental Subduction and Indentation at the India-Eurasia Collision Zone

The India-Eurasia convergence is a long-term, ongoing process despite the continental collision and associated velocity reduction from ∼15 to 20 cm/yr in the Late Cretaceous-earliest Eocene to ∼4–5 cm/yr (since ∼40 Ma). Previous studies that impose Indian plate motion imply an external force driving long-term convergence, continental subduction and Indian indentation, and the onset of India-Eurasia collision as the trigger for the abrupt deceleration. In this study, we investigate the mechanism(s) of this deceleration, continued convergence, long-term continental subduction and long-term Indian indentation using buoyancy-driven geodynamic models. We conduct three large-scale analog experiments to simulate the subduction and collision processes at the convergent boundary with different boundary conditions at the 660-km discontinuity, including a lower-upper mantle viscosity ratio (η_LM/η_UM) that is infinitely high, no viscosity step (η_LM/η_UM = 1) and an intermediate viscosity step (η_LM/η_UM = ∼8.6). The experiment with infinite η_LM/η_UM shows deceleration when the slab tip reaches the 660-km discontinuity, while the other two experiments show a deceleration at the onset of continental subduction. Our experiments show that a higher η_LM/η_UM favors a lower velocity drop at the onset of continental subduction, lower convergence velocities, reduced continental subduction and a higher indentation amount. Furthermore, our models suggest that the negative buoyancy force of both upper and lower mantle slab segments is the main driver of long-term convergence and continental subduction, while the combination of rollback-induced mantle flow of the Sunda slab and slab negative and positive buoyancy forces of the Sunda and India slabs, respectively, drives northward Indian indentation.