Spatio-temporal evolution of rift volcanism controlled top-down by a deepening graben

Volcanism in continental rifts is generally observed to shift over time from the inside of the basin to its flanks and vice versa, but the controls on these switches are still unclear. Here we use numerical simulations of dike propagation to test the hypothesis that the spatio-temporal evolution of rift volcanism is controlled by the crustal stresses produced during the development of the rift basin. We find that the progressive deepening of a rift rotates the direction of the principal stresses under the basin, deflecting ascending dikes. This causes an early shift of volcanism from the inside of the graben to its flanks. The intensification of this stress pattern, due to further deepening of the basin, promotes the formation of lower crustal sill-like intrusions that can stack under the rift, shallowing the depth at which dikes nucleate, eventually causing a late stage of in-rift axial volcanism. Given the agreement between our model results and observations, we conclude that the temporal shifts in the location of rift volcanism are controlled to first order by the elastic stresses developing in the crust as the rift matures. We thereby suggest that geodynamic models should account for elasticity and the redistribution of surface loads in order to effectively reproduce rift-related magmatism.