Craton Formation in Early Earth Mantle Convection Regimes

How the geological record of cratons reconciles with the tectonic environments in which they formed has remained debated. We use 2D Cartesian geometry numerical models of mantle convection varying temperatures from present day to Archaean-inferred values, to address the formation of cratons, accounting for melt depletion-dependent rheological stiffening. For mantle temperatures comparable to present day, melting is negligible and the convective regime depends on the strength of the thermal lithosphere. For mantle potential temperatures higher than present day, high depletion degree and large depleted mantle volumes are formed at low lithospheric strength and high surface mobility, whereas these are negligible beneath a poorly mobile lithosphere. When compared to the models, the record of tectonics and large volumes of high-degree depleted mantle in Archaean cratons is best explained by a lithosphere initially prone to yielding and mobility. At high mobility, large depletion favors the progressive differentiation of the thermochemical lithosphere, which stiffens and thickens with increasing mantle temperatures. The ensuing reduced heat flow atop a hotter mantle is in agreement with the inferred Archaean thermal evolution, and may rule out the viability of a stagnant lid for the early Earth. Large-scale depletion stiffening resists plate margin formation and this wanes as heat production decreases, thus may hold the key for the establishment of plate tectonics during secular cooling.