S U M M A R YWe present the results of velocity modelling of a recently acquired wide-angle seismic reflection/refraction profile across the Main Ethiopian Rift. The models show a continental type of crust with significant asymmetry between the two sides of the rift. A 2-to 5-km-thick layer of sedimentary and volcanic sequences is modelled across the entire region. This is underlain by a 40-to 45-km-thick crust with a c. 15-km-thick high-velocity lowest crustal layer beneath the western plateau. This layer is absent from the eastern side, where the crust is 35 km thick beneath the sediments. We interpret this layer as underplated material associated with the Oligocene flood basalts of the region with possible subsequent addition by recent magmatic events. Slight crustal thinning is observed beneath the rift, where Pn velocities indicate the presence of hot mantle rocks containing partial melt. Beneath the rift axis, the velocities of the upper crustal layers are 5-10 per cent higher than outside the rift, which we interpret as resulting from mafic intrusions that can be associated with magmatic centres observed in the rift valley. Variations in seismic reflectivity suggest the presence of layering in the lower crust beneath the rift, possibly indicating the presence of sills, as well as some layering in the proposed underplated body.

The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) was undertaken to provide a snapshot of lithospheric break-up above a mantle upwelling at the transition between continental and oceanic rifting. The focus of the project was the northern Main Ethiopian Rift (NMER) cutting across the uplifted Ethiopian plateau comprising the Eocene-Oligocene Afar flood basalt province. A major component of EAGLE was a controlled-source seismic survey involving one rift-axial and one cross-rift c. 400 km profile, and a c. 100 km diameter 2D array to provide a 3D subsurface image beneath the profiles’ intersection. The resulting seismic data are interpreted in terms of a crustal and sub-Moho P-wave seismic velocity model. We identify four main results: (1) the velocity within the mid- and upper crust varies from 6.1 km s−1 beneath the rift flanks to 6.6 km s−1 beneath overlying Quaternary axial magmatic segments, interpreted in terms of the presence of cooled gabbroic bodies arranged en echelon along the axis of the rift; (2) the existence of a high-velocity body (Vp 7.4 km s−1) in the lower crust beneath the northwestern rift flank, interpreted in terms of about 15 km-thick, mafic under-plated/intruded layer at the base of the crust (we suggest this was emplaced during the eruption of Oligocene flood basalts and modified by more recent mafic melt during rifting); (3) the variation in crustal thickness along the NMER axis from c. 40 km in the SW to c. 26 km in the NE beneath Afar. This variation is interpreted in terms of the transition from near-continental rifting in the south to a crust in the north that could be almost entirely composed of mantle-derived mafic melt; and (4) the presence of a possibly continuous mantle reflector at a depth of about 15–25 km below the base of the crust beneath both linear profiles. We suggest this results from a compositional or structural boundary, its depth apparently correlated with the amount of extension.

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We present analysis of new gravity data to produce a 2D crustal and upper mantle density model across the northern Main Ethiopian Rift (NMER). The magmatic NMER is believed to represent the transitional stage between continental and oceanic tiffing. We conclude that beneath our profile, magma emplacement into the upper crust occurs in the form of a 20 kmwide body beneath the axis of the rift, and a 12 km-wide off-axis body beneath the NW margin of the rift. These are coincident with Quaternary volcanic chains, anomalies in seismic velocity and conductivity identified by the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) along the same profile. We also identify a shallow, high-density body beneath the axial Boset volcano interpreted as either a dyke zone or a magma reservoir that may have fed Quaternary felsic volcanism. Our results provide supporting evidence for a c. 15 km-thick mafic underplate layer beneath the northwestern rift flank, imaged by the EAGLE controlledand passive-source seismic data. A relatively low-density upper mantle is required beneath the underplate and the rift to produce the long wavelength features of the gravity anomaly. The resulting model suggests that the lithosphere to the SE of the rift is unaffected by rifting processes. Our results combined with those from other EAGLE studies show that magmatic processes dominate rifting in the NMER.

As continental rift zones evolve to sea floor spreading, they do so through progressive episodes of lithospheric stretching, heating, and magmatism, yet the actual process of continental breakup is poorly understood. The East African Rift system in northeastern Ethiopia is central to our understanding of this process, as it lies at the transition between continental and oceanic rifting [Ebinger and Casey, 2001]. We are exploring the kinematics and dynamics of continental breakup through the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE), which aims to probe the crust and upper mantle structure between the Main Ethiopian (continental) and Afar (ocean spreading) rifts, a region providing an ideal laboratory to examine the process of breakup as it is occurring. EAGLE is a multidisciplinary study centered around the most advanced seismic project yet undertaken in Africa (Figure l). Our study follows the Kenya Rift International Seismic Project [e.g., KRISP Working Group, 1995],and capitalizes on the IRIS/PASSCAL broadband seismic array [Nyblade and Langston, 2002], providing a telescoping view of the East African Rift within this suspected plume province.

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Duringthe past decade, asuiteofwide-angleseismic reflection/refraction profileshasbeens hot in the Porcupine, Rockall andH atton basins,a sw ell asacross the Hatton continentalm argin. Integration ofthe wide-angleseismic datawithnormal-incidence reflection profilesandwithgravity andmagnetic datareveals a clearpictureofthe regionalcrustalanduppermantlestructure, andofthe large-scalesedimentary geometry ofthe LatePalaeozoic-Cenozoic basins inthe area.The region contains asetoflarge sedimentary basins restingupon variably thinned continentalcrust. The crust beneaththe basin-boundingstructuralhighsist ypically around 25kmthick, withthe crust beneaththe basins beingasthinas6km. The PorcupineBasincontains up to 10 kmof sediments. Upt o7kmo fsedimentary strataa rei ndicated int he Rockall Basin,with4kmo fsediment int he Hatton Basin. Severalseismicallydistinctlayers,withP-wavevelocitiesof1.8-5.3 kms 2 1 ,wereidentified above the basement inthe sedimentary basins. Asignificant variability inthickness ofthe sedimentary layers isseenon the wide-angledata.Thesereflectthe presence ofanumberofolderMesozoic riftsedimentary basins. The wideangledatarevealmarked topographicirregularity atthe baseofthe sedimentary succession inthe Rockall Basin, interpreted asthe result ofextensivemulti-phaseMesozoic rifting.The Mohobeneaththe Rockall Basinshows a clearasymmetry,withasteepergradient alongthe eastern marginofthe basin. The continent-oceanboundary west ofthe Hatton High hasasignificant underplated andvolcanic component,whilethatatthe southern margin ofthe Rockall Basinshows evidence ofagreatertranspressivecomponent. The natureofthe lowercrust appears to exert acontrollinginfluence on the location ofcrustalseparation. Inareaswherethinningisaccommodated by upperandmiddlecrustalattenuation,the strengthofthe uppercrust (andunderlyingbrittlemantle)hasserved to preservethe integrity ofthe continentalcrust andinhibitseafloor spreading.However,wherethe lowercrust is thinned significantly,asbeneaththe Hatton continentalmargin,crustalseparation hastakenplace eventhough the upperandmiddlecrust remains relatively thick.

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