Modelling constraints on rifting in the Afar region: the birth of a triple junction

Rifting in the Afar region is considered to be the only known example of the formation of an incipient divergent triple junction. Taking the Afar region as an example, we use three-dimensional (3D) laboratory experiments to test hypotheses for the formation and evolution of divergent triple junctions. We systematically evaluate the role of mechanical weakening due to plume impingement versus inherited weak linear structures in lithospheric mantle under both far-field orthogonal and rotational extensional boundary conditions. The interaction between far-field boundary forces and inherited rheological heterogeneities results in a range of complex rift propagation geometries and structural features, such as rift segmentation and ridge jumps, which are comparable to those observed in the Afar region. The combination of rotational boundary conditions and inherited linear heterogeneities favours the formation of rifts that connect at high-angles. Lithospheric weakening associated with a mantle plume triggers different rifting styles but has little influence on large-scale continental breakup. When compared to the Afar region, our results suggest that the rotation of the Arabian plate since the Oligocene led to rifting of the Red Sea and the Gulf of Aden, which are distinct from the formation of the Main Ethiopian Rift. We suggest that rifting in the Afar region is not consistent with the incipient divergent triple junction hypothesis. Rather, the Afar triple junction formed as a result of complex multi-phase rifting events driven by far-field tectonic forces.