Mantle convection with a brittle lithosphere: thoughts on the global tectonic styles of the Earth and Venus

TLDR

Plates are integral to mantle convection, yet plate boundaries arise from brittle faulting that strongly influences plate motion. The study investigates the conditions for plate tectonics by incorporating brittle behavior into a mantle convection model. The model applies Byerlee’s law to limit maximum lithospheric stress and uses temperature‑dependent viscosity, showing that high yield stress confines convection beneath a thick stagnant lithosphere, low yield stress mobilizes the lithosphere into the circulation with localized surface deformation, and intermediate yield stress produces episodic cycling between these states. The mobile‑lid regime behaves like a fluid with temperature‑dependent viscosity, exhibiting a well‑defined Nusselt‑number–Rayleigh‑number scaling, surface velocities largely independent of yield stress, and Earth’s lithosphere mobilization possible with friction coefficients below 0.03–0.13, whereas Venus would experience episodic, catastrophic mobilization due to higher friction.

Abstract

Plates are an integral part of the convection system in the fluid mantle, but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults. The conditions for plate tectonics are studied by considering brittle behaviour, using Byerlee—s law to limit the maximum stress in the lithosphere, in a mantle convection model with temperature-dependent viscosity. When the yield stress is high, convection is confined below a thick, stagnant lithosphere. At low yield stress, brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation; surface deformation occurs in localized regions close to upwellings and downwellings in the system. At intermediate levels of the yield stress, there is a cycling between these two states: thick lithosphere episodically mobilizes and collapses into the interior before reforming. The mobile-lid regime resembles convection of a fluid with temperature-dependent viscosity and the boundary-layer scalings are found to be analogous. This regime has a well defined Nusselt number—Rayleigh number relationship which is in good agreement with scaling theory. The surface velocity is nearly independent of the yield stress, indicating that the 'plate' motion is resisted by viscous stresses in the mantle. Analysis suggests that mobilization of the Earth—s lithosphere can occur if the friction coefficient in the lithosphere is less than 0.03–0.13—lower than laboratory values but consistent with seismic field studies. On Venus, the friction coefficient may be high as a result of the dry conditions, and brittle mobilization of the lithosphere would then be episodic and catastrophic.