Control of rift obliquity on the evolution and segmentation of the main Ethiopian rift

Abstract

The main Ethiopian rift has evolved in two stages, with successive activation of differently oriented fault systems. Analogue modelling of the Earth’s lithosphere demonstrates that such a rift evolution requires neither magma weakening nor a change in plate kinematics. The main Ethiopian rift is an active rift in the break-up stage, and it marks the incipient boundary between the Nubia and Somalia plates1. Rifting started with the activation of large boundary faults and diffuse volcanism, followed by focused magmatism and faulting in the rift floor, along step-like segments oblique to the rift axis that now act as a protoridge for future seafloor spreading2. This concentration of volcano-tectonic activity has been thought to be either magma assisted2,3 or controlled by a change in rift kinematics, with a late oblique rifting phase that would have caused the development of the step-like fault segments that focused magma upwelling4,5. Geodetic6,7, seismic8 and stress-field9 data confirm current oblique rifting kinematics, but plate kinematics models do not predict a change in Nubia–Somalia motion in the past 11 million years10. Here, I use lithospheric-scale analogue models of oblique rifting to analyse the development of the main Ethiopian rift. I find that neither magma weakening nor a change in plate kinematics are required to simulate a two-phase evolution with successive activation of differently oriented fault systems. I conclude that rift evolution and segmentation are controlled by rift obliquity, independent of magmatic processes.