The Making of Early Continents and the Initiation of Plate Tectonics on Earth
Abstract:We propose a simple, self-consistent model to explain a range of puzzling observation made in many Archean cratons including 1/ the marked petrological stratification of the lithospheric mantle; 2/ the multimodal polybaric volcanism of greenstone covers; 3/ the regional and temporal overlap between komatiitic-tholeiitic basalts and arc-volcanism; and 4/ the mid-lithospheric seismic discontinuity mapped across several cratons. This model also explains the possible transition from a stagnant-lid regime to a transient then steady-state plate tectonic regime.
We performed several series of 2D thermo-mechanical numerical experiments to investigate the effect of an early continent on mantle convection. We considered a composite lid including 1/ a continent consisting of 55 km thick crust - made of TTG and continental flood basalts – above a 170-km-thick strongly depleted, strong and buoyant lithospheric mantle, and 2/ an oceanic lid including a 15-km-thick basaltic crust.
These experiments show that the continent slowly spreads laterally toward the adjacent oceanic lid. The spreading and thinning of the continent drives exhumation of the fertile sub-continental mantle, which in turn promotes polybaric decompression melting producing komatiitic and tholeiitic basalts. Continental boudinage and rifting accompanying the spreading drives further upwelling and decompression melting to even shallower depths. This partial melting produces a moderately depleted mantle layer, progressively incorporated through cooling to the base of the continent. Our numerical experiments also show that spreading continents force the adjacent oceanic lid into the convective mantle, promoting subduction of the oceanic lid and temporal overlap between Archean komatiitic-tholeiitic basalts with arc-volcanism. Spreading and thinning of the continent lead to a sub-horizontal litho-tectonic fabric in the mantle, with a possible major discontinuity between the older strongly depleted mantle, and the younger moderately depleted accreted mantle.
Our experiments show that early continents acted as kick-starters of subduction until plate tectonics became self-sustaining through the increasingly negative buoyancy of the oceanic lithosphere.