The thinning of subcontinental lithosphere: The roles of plume impact and metasomatic weakening

Geologically rapid (tens of Myr) partial removal of thick continental lithosphere is evident beneath Precambrian terranes, such as North China Craton, southern Africa, and the North Atlantic Craton, and has been linked with thermomechanical erosion by mantle plumes. We performed numerical experiments with realistic viscosities to test this hypothesis and constrain the most important parameters that influence cratonic lithosphere erosion. Our models indicate that the thermomechanical erosion by a plume impact on typical Archean lithospheric mantle is unlikely to be more effective than long-term erosion from normal plate-mantle interaction. Therefore, unmodified cratonic roots that have been stable for billions of years will not be significantly disrupted by the erosion of a plume event. However, the buoyancy and strength of highly depleted continental roots can be modified by fluid-melt metasomatism, and our models show that this is essential for the thinning of originally stable continental roots. The long-term but punctuated history of metasomatic enrichment beneath ancient continents makes this mode of weakening very likely. The effect of the plume impact is to speed up the erosion significantly and help the removal of the lithospheric root to occur within tens of Myr if affected by metasomatic weakening.