The layered structure of the lithosphere: From the Archean to the present day

Mid-lithospheric discontinuities (MLDs) mark layering observed across cratons and younger lithospheres, yet their link to mantle dynamics and secular cooling remains unexplored. Using numerical simulations of mantle convection with melting and dehydration-dependent rheology, we show that convective thinning drives decompression melting, generating contrasting density and viscosity, strain localization, and porosity gradients within the lithosphere. Subsequent deformation (with imbrication and tilting) and cooling freeze the lithosphere into a three-layer structure: a paleo-thermal boundary layer, a compositionally modified layer, and a thermal sublayer. These features explain observed geophysical properties and reproduce seismic velocity gradients across MLD interfaces, though matching the full contrasts likely requires additional fluids/melt retention. This mechanism operates across the full range of mantle temperatures, from Archean to present-day conditions, providing a unified explanation for the abundance of MLDs in cratons, decreasing prevalence with secular cooling, and persistent occurrence in modern lithosphere on Earth and other planets.