Spatio-temporal evolution of rift volcanism driven by progressive crustal unloading

Continental rifting can be accompanied by a large amount of volcanism, which is often observed to shift from the inside of the rift basin to its flanks and conversely, but the controls on this variability are still unclear. Maccaferri et al. (2014) proposed that elastic stresses under rifts are dominated by gravitational unloading due to the excavation of the graben. According to this model, off-rift volcanism follows the creation of a stress barrier below the rift that drives dikes diagonally away from the rift axis, or stops their ascent altogether so that they get stuck as lower crustal sills. The Maccaferri et al. (2014) model is however based on simplyfied assumptions that need to be relaxed to further test its validity. In particular, the model neglects the effect of the accumulating crustal intrusions on ascending dikes. Here we build on this model to explain the spatio-temporal evolution of rift volcanism in terms of the reorientation of principal stresses in the crust due to the progressive unloading of a rift basin with time. To do so, we extend the dike propagation boundary element code used by Maccaferri et al. (2014) to account for the stresses generated by previously ascended dikes. We find that volcanism in rift zones starts inside the rift depression for small values of basin depth. The deepening of the rift is accompanied by the development of a stress barrier under the basin which deflects ascending dikes, causing a shift of surface volcanism from the inside to the flanks. The intensification of the barrier due to further deepening of the basin promotes the formation of lower crustal sill-like structures that pile up under the rift, shallowing the depth at which magma is injected. This eventually leads to dikes being injected from above the stress barrier, moving surface volcanism back to the axial part of the rift. We compare the results of our model to observations of evolving volcanism and crustal structure for rifts of different graben width and depth.