Impact of crust–mantle mechanical coupling on the topographic and thermal evolutions during the necking phase of ‘magma-poor’ and ‘sediment-starved’ rift systems: A numerical modeling study

Abstract : We used high-resolution (500 × 250 m) two-dimensional lithospheric-scale thermo-mechanical numerical modeling to unravel the unexpected topographic and thermal evolutions recorded during the necking phase of several rift systems worldwide. Through a systematic analysis we studied how the lithosphere rheology impacts the topographic and thermal evolutions across the entire width of magma-poor and sediment-starved rift systems until their crust is locally thinned to 10 km. We quantified the evolution of topography, uplift and subsidence rates, accommodation and emerged space creation, temperature and surface heat flow for a wide panel of crustal and mantle rheologies to provide an overview of possible rifting behaviors. Extension of a lithosphere for which the crust and mantle are mechanically decoupled by a weak lower crust generates complex morphotectonic evolutions, with the formation of temporarily restricted sub-basins framed by uplifted parts of the future distal margin. Mechanical decoupling between the crust and mantle controls also largely the thermal evolution of rift systems during the necking phase since, for equivalent extension rates and initial geotherms: (i) weak/decoupled lithospheres have a higher geothermal gradient at the end of the necking phase than strong/coupled lithospheres; and (ii) weak/decoupled lithospheres show intense heating of the lower crust at the rift center and intense cooling of the crust on either side of the rift center, unlike strong/coupled lithospheres. These behaviors contrast with the continuous subsidence and cooling predicted by the commonly used depth-uniform thinning model. Accommodation space in the evolving basins is first generated by vertical crustal velocities and subsequently by horizontal velocities causing the widening of the earlier formed basin. Processes such as strain softening and mantle fertilization have a limited impact on the primary morphology and thermal state of rift systems before the crust is thinned to 10 km but may locally amplify relief and thermal heterogeneities.