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Geological Society, London, Special Publications; 2006; v. 259; p. 269-292;
DOI: 10.1144/GSL.SP.2006.259.01.21
© 2006 Geological Society of London

Part 4: Rifting in the Afar volcanic province: Geophysical studies of crustal structure and processes

Crustal structure of the northern Main Ethiopian Rift from the EAGLE controlled-source survey; a snapshot of incipient lithospheric break-up

P.K.H. Maguire1, G.R. Keller2, S.L. Klemperer3, G.D. Mackenzie1,*, K. Keranen3, S. Harder2, B. O’Reilly4, H. Thybo5, L. Asfaw6, M.A. Khan1 & M. Amha7

1 Department of Geology, University of Leicester, Leicester LE1 7RH, UK pkm{at}le.ac.uk
2 Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, USA
3 Department of Geophysics, Stanford University, Stanford, CA94305-2215, USA
4 Dublin Institute of Advanced Studies, 5 Merrion Square, Dublin, Ireland
5 Geological Institute, University of Copenhagen, Oster Voldgade 10, DK-1350, Copenhagen, Denmark
6 Geophysical Observatory, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia
7 Ethiopian Science and Technology Commission, P.O. Box 2490, Addis Ababa, Ethiopia
* Compagnie Générale de Géophysique, Vantage West, Great West Rd., Brentford, Middlesex, TW8 9GG, UK

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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