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Geological Society, London, Special Publications; 2006; v. 259; p. 307-321;
DOI: 10.1144/GSL.SP.2006.259.01.23
© 2006 Geological Society of London

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

Northern Main Ethiopian Rift crustal structure from new high-precision gravity data

D.G. Cornwell1, G.D. Mackenzie1,2, R.W. England1, P.K.H. Maguire1, L.M. Asfaw3 & B. Oluma4

1 University of Leicester, University Road, Leicester, LE1 7RH, UK dgc2{at}le.ac.uk
2 Compagnie Générale de Géophysique, Vantage West, Great West Road, Brentford, Middlesex, TW8 9GG, UK
3 Geophysical Observatory, Addis Ababa University, PO Box 1176, Addis Ababa, Ethiopia
4 Geological Survey of Ethiopia, Geophysics Department, PO Box 2302, Addis Ababa, Ethiopia

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 rifting. We conclude that beneath our profile, magma emplacement into the upper crust occurs in the form of a 20 km-wide 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 controlled- and 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.





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