Extended Data Fig. 4: Lithospheric geotherms for different thicknesses, depletion fraction, density contrast, and rheology, for an early-Earth-like mantle potential temperature.

Left, the geotherms (half-space cooling) reproduce the effect of thinning of a thick (magenta) lithosphere into a thinner one (blue and indigo). Thin lines for the dry and water-saturated solidi. Centre, the depletion degree and volumes increase with thinning during rifting, and become increasingly shallow. Right, the viscosity of the lithosphere during rifting increases with thinning, as larger melting is produced and embedded in the mechanical boundary layer. Thin line are viscosities for η₀, 10η₀, 10²η₀ and 10³η₀ for the temperature-dependent viscosity η(T) and temperature- and depletion-dependent viscosity η(T, F). Plastic viscosities are η_Y for the lithospheric yielding and \({\eta }_{{\rm{Y}}}^{{\rm{C}}}\) for the crust. The viscosity is calculated using σ₀ = 50 MPa for the lithosphere and background strain rate \(\dot{\varepsilon }={10}^{-15}\,{{\rm{s}}}^{-1}\).