Dyke swarm emplacement in the Ethiopian Large Igneous Province: not only a matter of stress

In the Tana–Belaya area, western Ethiopia, field data and satellite imagery reveal the existence of two dyke swarms, the NE–SW Serpent-God dyke swarm, and the NW–SE Dinder dyke swarm. Both swarms are thought to have the same age, 30 Ma, and are likely to have contributed to feeding the traps. After a description of the swarms, this paper examines their relationships with the basement structures. The two dyke swarms follow major lithospheric weakness zones. The Serpent-God dyke swarm follows the Pan-African Tulu Dimtu ductile shear zone, and the Dinder dyke swarm follows a large NW–SE-trending Precambrian fracture zone already reactivated during the Mesozoic and Cenozoic as the northern boundary of the Blue Nile Rift. Because the dyke swarms are adjacent but their orientation differs, the stress trajectory patterns during their emplacement were spatially variable at local scale. Therefore, rather than plate-boundary processes, the origin of stress is thought to be primarily related to the Ethiopian plume. Postulating (in the absence of more data relating to the magma chambers that fed the traps) that dyke orientation is the result of an axisymmetric stress field, the location of the stress source can be placed close to Lake Tana, which is the centre of the Ethiopian broad negative regional Bouguer anomaly. The dykes in the Tana–Belaya area provide the first clues to the orientation of the stress field that prevailed in the early history of the Ethiopian mantle plume, and to some of the factors that guided the distribution of the trap feeders.