Early continental breakup boundary and migration of the Afar triple junction, Ethiopia

Megascale accommodation zones mark the early continental breakup boundary of the Afro-Arabian plateau in Ethiopia. Motion along these megascale accommodation zones enabled the different arms of the Afar RRR (rift-rift-rift) triple junction to join together. An arcuate accommodation zone approximately 45 km wide and approximately 60 km long marks the transition from the N25 degrees W-striking Southern Red Sea Rift trend in the western Afar margin to the N25 degrees E-striking Main Ethiopian Rift. Another accommodation zone approximately 80 km long marks the transition between the northeast- to north-northeast-trending Main Ethiopian Rift and the nearly east-trending Gulf of Aden. The positions of accommodation zones where rift orientations change by approximately 50 degrees suggest that they served as soft linking points between the kinematically "misaligned" arms of the triple junction during the initial breakup of the Afro-Arabian dome. We use structural features to infer the present location of the triple junction in the Lake Abhe area. On the basis of the location of the paleo-triple junction, deduced from the position of accommodation zones and geomorphic considerations, it is estimated that the Afar triple junction has migrated approximately 1.5 degrees ( approximately 160 km) in a north- northeast direction with respect to the African (Nubian) plate. The estimated amount of migration of the Afar triple junction is less than the 200 km migration expected from plate-kinematic analysis. This discrepancy suggests either a slower rate of spreading than the current approximately 1.6 cm/yr (Africa-Arabia) during the early phase of rifting or a later early Miocene rather than Oligocene-Miocene age for the initiation of tectonic activity in the triple junction.