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Geological Society, London, Special Publications; 2006; v. 259; p. 253-267;
DOI: 10.1144/GSL.SP.2006.259.01.20
© 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 receiver function studies

G.W. Stuart1, I.D. Bastow1 & C.J. Ebinger2

1 School of Earth and Environment, University of Leeds, Leeds, UK graham{at}earth.leeds.ac.uk
2 Department of Geology, Royal Holloway, University of London, Egham, UK

The northern Main Ethiopian Rift captures the crustal response to the transition from continental rifting in the East African rift to the south, to incipient seafloor spreading in the Afar depression to the north. The region has also undergone plume-related uplift and flood basalt volcanism. Receiver functions from the EAGLE broadband network have been used to determine crustal thickness and average Vp/Vs for the northern Main Ethiopian Rift and its flanking plateaus.

On the flanks of the rift, the crust on the Somalian plate to the east is 38 to 40 km thick. On the western plateau, there is thicker crust to the NW (41–43 km) than to the SW (<40 km); the thinning taking place over an off-rift upper mantle low-velocity structure previously imaged by travel-time tomography. The crust is slightly more mafic (Vp/Vs ~ 1.85) on the western plateau on the Nubian Plate than on the Somalian Plate (Vp/Vs ~ 1.80). This could either be due to magmatic activity or different pre-rift crustal compositions. The Quaternary Butajira and Bishoftu volcanic chains, on the side of the rift, are characterized by thinned crust and a Vp/Vs > 2.0, indicative of partial melt within the crust.

Within the rift, the Vp/Vs ratio increases to greater than 2.0 (Poisson’s ratio, {sigma} > 0.33) northwards towards the Afar depression. Such high values are indicative of partial melt in the crust and corroborate other geophysical evidence for increased magmatic activity as continental rifting evolves to oceanic spreading in Afar. Along the axis of the rift, crustal thickness varies from around 38 km in the south to 30 km in the north, with most of the change in Moho depth occurring just south of the Boset magmatic segment where the rift changes orientation. Segmentation of crustal structure both between the continental and transitional part of the rift and on the western plateau may be controlled by previous structural inheritances. Both the amount of crustal thinning and the mafic composition of the crust as shown by the observed Vp/Vs ratio suggest that the magma-assisted rifting hypothesis is an appropriate model for this transitional rift.





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