Discontinuous and diachronous evolution of the Main Ethiopian Rift: Implications for development of continental rifts

Abstract

The Main Ethiopian Rift (MER) is commonly considered the archetypal magma-assisted rift. Tomographic images of upper-mantle upwellings beneath the rift, aligned anisotropy beneath magmatic segments, and pervasive magmatic modification of the crust all indicate the importance of magmatic processes in present-day rift evolution. It has been suggested that this magmatic development is responsible for the straight and continuous path the rift cuts across the Ethiopian Plateau. We compile new evidence indicating that the MER is not as continuous and its development not as simple as previously believed. Significant lithospheric heterogeneities are evident in our compilation of recently acquired seismic, gravity, and geologic data. Numerical models of rift propagation in such heterogeneous lithosphere show that rift propagation may stall at rheological boundaries. We propose that the heterogeneities in the MER caused irregular rift propagation, resulting in a distinct discontinuity visible within the rift lithosphere. This discontinuity in structure spatially correlates to an apparent discontinuity in the age of extension between the northern MER and the central MER, lending support to our hypothesis. Our interpretation leads to a two-phase model of rift propagation in the MER, with initial rift development primarily controlled by lithospheric structure and a later phase during which magmatic processes are dominant. During the initial phase, rift propagation was irregular and at times stalled or was diverted away from the modern rift trend along pre-existing structures. Our model, while acknowledging the importance of magmatic processes in volcanic extensional regions, shows that even in this classic example of magma-assisted rifting, inherited lithospheric structure localized initial extension and controlled rift propagation. This early phase formed the template for future rift development and continental break-up.