Geodynamics of long-term continental subduction and Indian indentation at the India-Eurasia collision zone

India-Eurasia convergence velocities have dropped significantly from ~18 cm/yr in the Late Cretaceous-earliest Eocene to ~4-5 cm/yr since ~50 Ma. The mechanisms of convergence deceleration, continued convergence since ~50 Ma, long-term continental subduction and long-term Indian indentation into Eurasia still remain controversial. Many previous studies consider an external driving force for the long-term convergence, continental subduction and Indian indentation, and the initial India-Eurasia collision as the trigger for the deceleration. In this study, we investigate the mechanism(s) of the abrupt deceleration, the continued convergence, the long-term continental subduction and long-term Indian indentation using buoyancy-driven analog experiments. We conduct three large-scale experiments to simulate the subduction and collision process at the convergent boundary with different boundary conditions at the 660-km discontinuity, including an infinite viscosity step (the lower-upper-mantle viscosity ratio (η_LM/η_UM) is infinitely high), no viscosity step (η_M/η_UM =1) and an intermediate viscosity step. The experiment with infinite η_LM/η_UM shows a 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, and vice versa. Furthermore, our models suggest that in nature, with an intermediate-high η_LM/η_UM, the negative buoyancy force of both upper and lower mantle slab segments is the main driver of long-term convergence, continental subduction and Indian indentation.