Magma-assisted continental rift margins : the Ethiopian rift - Volume 1

Continental rifting and incipient seafloor spreading are observed either side of the main Ethiopian rift (MER). EAGLE (the Ethiopia Afar Geoscientific Lithospheric Experiment) included a 400 km-long profile containing 97 passive seismometers to investigate the change from mechanical to magmatic extension by defining the lithospheric structure and extent of magmatism beneath the rifted northern MER. Changes in crustal structure along the cross-rift profile are imaged using forward modelling, H-kappa stacking and non-linear inversion analyses of receiver functions. The lithospheric structure is inherently different beneath the north-western rift flank, rift valley and south-eastern rift flank, with contrasting crustal thickness and composition, upper mantle velocity and lithospheric anisotropy. Magmatic addition is imaged in the form of an 6--18 km-thick underplate lens at the base of the crust beneath the high Ethiopian plateau and zones of intense dyking and partial melt beneath the rift valley. The underplate layer probably formed synchronous with an Oligocene flood basalt event and therefore pre-dates the rifting by ~20 Myr. A 20--30 km-wide magmatic system pervades the entire crust beneath volcanic chains that mark the locus of current rift extension. To the southeast of the rift, a lithospheric suture is inferred, which was created during the Precambrian collision of East and West Gondwana. Collision-related lithospheric fabric is proposed to be the main source of strong anisotropy observed along the entire profile, which is locally augmented by rift-related magmatism. An active followed by passive magma-assisted rifting model that is controlled by a combination of far-field plate stresses, pre-existing lithospheric framework and magmatism is preferred to explain the evolution of the northern MER.