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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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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SUMMARY In August 1985 the crustal structure underlying the southern part of the Kenya Rift Valley was investigated by long‐range explosion seismology. the experiment (KRISP 85) consisted of two seismic lines in the central sector of the rift, one along the axis and the other across it. Interpretation of the data, including time‐term analysis and ray tracing has shown that the thickness of rift infill varies from about 6km below Lake Naivasha to about 2 and 1.5km below Lake Magadi and Lake Bogoria respectively. the underlying material has a P‐wave velocity of 6.05 ± 0.03 km s‐1 which suggests that the rift is underlain by Precambrian metamorphic basement. A localized high‐velocity zone identified to the east of Nakuru may be due to basic intrusive material. the P‐wave velocity increases discontinuously to 6.45 ± 0.2 km s‐1 at a depth of 12.5 ± 1.0 km below sea level. This depth is similar to that inferred for the brittle‐ductile transition zone from a study of local seismicity in the Lake Bogoria region. A high P‐wave velocity layer (7.1 ± 0.2 km s‐1) occurs at 22 ± 2 km depth below sea level which might be associated with a sill‐like basic intrusion in the lower crust. an upper mantle velocity of 7.5 ± 0.2 km s‐1 (unreversed) is reached at a depth of 34.0 ± 2.0 km below sea level. This implies that only moderate crustal thinning has occurred beneath the central sector of the rift. No evidence was obtained for the existence of a continuous‘axial intrusion’reaching to shallow levels below the rift and associated with crustal separation as suggested by previous studies.