Extended Data Fig. 5: Regime diagram showing the influence of metasomatized SCLM keel density and viscosity on instabilitymigration.

Results from 28 numerical experiments illustrate the state of the metasomatized keel (coloured zone) 50 Myr after rift onset, for different combinations of density and activation energy of dislocation creep of the keel. The activation energy is a key parameter in defining the viscosity of the metasomatized layer. The temperature-dependent density is shown in cold/warm colours overlain with viscosity in a transparent grey scale. Density and viscosity are changed only within the metasomatized layer; all other parameters match the reference model (demarcated with the dashed rectangle). The activation energy is varied within the uncertainty of experimental bounds⁷¹, a range that is notably included in the permissible spectrum based on independent numerical modelling⁷². The density of continental keels is not well known (for example, refs. ^85,86) and is dependent on the degree of metasomatism^87,88. Here, we varied metasomatized keel densities across a wide range that is arguably larger than inferred lithospheric excess densities of 0.2-0.4% (ref. ⁸⁹) and 0.5-1.2% (ref. ⁹⁰) Increasing density accelerates instability migration, consistent with analytical predictions⁹, while increasing viscosity (that is, increasing activation energy) slows down the migration.