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1 Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK
2 Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
Mechanical processes largely control the along-axis segmentation of continental rifts; however, asthenospheric processes strongly influence the along-axis segmentation of mid- ocean ridges. We examine the distribution of strain and magmatism in the transitional active Main Ethiopian rift. Magmatic construction, diking, and faulting during the past 1.6 m.y. have created 
20-km-wide, 60-km-long magmatic segments with or without axial valleys. Magmatic segments are arranged en echelon within the 100-km-wide rift valley bounded by mid-Miocene border faults. Geodetic data show that magmatic segments accommodate >80% of the strain across the rift, indicating that border faults are no longer the locus of extension. Comparison with mid-ocean ridges suggests that magmatic segments, rather than detachment faults, mark the ocean-continent boundary in rifts with a ready magma supply. Magmatic margins, therefore, may contain detachments abandoned during continental breakup. The processes of localized dike intrusion with underplating would produce strips of mafic crust transitional to oceanic crust, but without coherent seafloor-spreading magnetic anomalies.
Key Words: extension tectonics • passive margins • dikes • magma • Ethiopian rift
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