Intraplate continental deformation: Influence of a heat-producing layer in the lithospheric mantle
Résumé
Deformation of the continental lithosphere is traditionally modeled assuming that heat production in the subcontinental lithospheric mantle is negligible. Although this may be appropriate for highly depleted Archean lithosphere, studies of mantle xenoliths and lithospheric-derived mafic rocks suggest that heat productions of up to 0.4 ;μW/m3 may be attained in the mantle of Phanerozoic and Neoproterozoic regions due to modal metasomatism. To investigate the effect of a local enrichment in heat-producing elements within the lithospheric mantle on the continental deformation, we performed a series of 2D thermo-mechanical models simulating the deformation of a lithospheric plate 1000 km long, which contains in its central part a 200 km wide and 20 km-thick mantle layer with a heat production of 0.05;0.25 ;μW/m3 between 40 and 80 km depth, leading to a local enhancement of the surface heat flow by 1;5 mW/m2. Compression models show that a 20 km-thick layer with a heat production as low as 0.05 ;μW/m3 within the shallow lithospheric mantle leads to strain localization in both the enriched mantle and the overlying crust. Strain localization depends exponentially on the temperature increase in the lithosphere section, which is controlled by the volume of metasomatized mantle and the intensity of the metasomatic enrichment in heat-producing elements. A heat production of 0.25 μW/m3 between 60 and 80 km depth results, for instance, in strains within and above the metasomatized mantle 5 times higher than in the surrounding lithosphere. Strain localization also depends on the location of the heat-producing domain and on the plate rheological structure; deeper heat-producing domains and weaker plates leading to stronger localization. These results provide an explanation for strain localization in intraplate environments, leading to the formation of orogenic belts situated hundreds to thousands of kilometers away from known plate boundaries, as observed in the Neoproterozoic belts of Gondwana and in the present-day deformation of the Asian plate.