Mechanisms of Lithospheric Dripping in Earth’s Convecting Mantle: Implications for Tectonic Switching

Soham Banerjee, Dip Ghosh, Nibir Mandal

Gravity-driven dripping is a key recycling process of lithospheric materials into the underlying mantle reservoir. Here, we use 2D computational fluid dynamic (CFD) thermo-chemical simulations to unveil how such dripping process occurs by modulating the thermal convection in Earth’s mantle. Our simulations incorporate the following variables: lithospheric buoyancy contrasts (B), metamorphic transformation of gabbroic crust into denser eclogite, mantle-to-lithosphere viscosity ratios, and mid-mantle phase transitions (olivine-wadsleyite, coesite-stishovite, and eclogitization). We recognize three distinct mechanisms; each of them gives rise to characteristic drip evolution that critically governs the layered versus whole-mantle convection dynamics. For buoyancy number B ≥ 0.29, the initial viscosity ratio M = 10 and the density change due to eclogitization at mantle transition zone ∆ρ_MM ≥ 150 kg/m3, lithospheric drips cluster into a train of superdrip structures (Mechanism 1), forming downwelling zones on a wavelength of 2800 km. The sinking superdrips eventually cross the 660 km barrier to penetrate lower mantle, fostering whole-mantle circulations. This drip mechanism undergoes a transition when ∆ρ_MM falls below 150 kg/m3 and also when 0.29 ≥ B ≥ 0.16 with M = 10, leading to stagnation of lithospheric drips at the 660 km boundary (Mechanism 2). We show that Mechanism 2 transforms the whole-mantle to layered convection styles, characterized by contrasting circulation patterns in the upper and lower mantle. In geodynamic conditions: B≤0.16 and 1000≥ M ≥10, drips grow into asymmetric keel-like structures at the lithospheric base (Mechanism 3), occurring as proto-subduction analogues. This mechanism restores whole-mantle convective circulations, but with their wavelengths approximately 4200 km, significantly larger than that produced by Mechanism 1. Finally, based on observational datasets, our drip models provide a unified framework of drip-mediated lithospheric recycling mechanisms and stagnant-lid to modern plate tectonic transitions.