Abstract

Structures analogous to oceanic spreading ridges, transform faults, and “leaky” fracture zones have been identified in Afar. They were formed during the past 3 to 4 m.y. and permit identification of the present plate boundaries within Afar and determination of the amount of spreading in this interval. At least two microplates are required in the zone of the junction of the African, Arabian, and Somalian plates. Variations of the spreading rate along single ridges or from one ridge to another, ridge jumping, migration of spreading with time, and counterclockwise rotation of the microplates are inferred by comparing volcanological, chronological, and geochemical data with aeromagnetic data. Deformation affects both accreting and transform plate boundaries within zones similar in width to the microplates, whose interiors are also systematically affected by regional faulting and locally by volcanism and transverse faulting. This indicates that although plate tectonics can explain most of the Afar features it fails when applied at such a small scale.

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