Crust and upper mantle structure in East Africa, together with the tectonic history of the region, is used to evaluate geodynamic processes commonly associated with the formation of magmatic rifted margins. Cenozoic rifting and volcanism in East Africa represent the earliest stage in the development of a rifted continental margin, and East Africa is one of the few places where geodynamic processes may be active that could lead to the development of a magmatic rifted margin. The Precambrian tectonic framework of East Africa is characterized by an Archean craton (Tanzania Craton) surrounded by Proterozoic mobile belts. An extensive nonmagmatic rift system associated with the separation of Madagascar from Africa developed in the mobile belts during the Permian-Cretaceous. In the Cenozoic, two rift branches (Western Rift and Eastern Rift) formed in the mobile belts. Volcanism is present in both branches, but most of it is concentrated in the Eastern Rift. Crustal structure away from the Cenozoic rifts is typical for Precambrian crust. Similarly, uppermost mantle structure across East Africa does not appear to have been altered, except beneath the rift valleys. Deeper in the upper mantle a thick (∼200 km) lithospheric keel is found under the Tanzania Craton, and a broad (200-400 km wide) thermal anomaly extending to a depth of at least 400 km is beneath the Eastern Rift. Several models cannot fully account for the tectonic history of East Africa and/or the structure of the crust and upper mantle, including edge flow in the convecting mantle around the keels of Archean cratons, broad (>500 km wide) thermal upwellings originating in the lower mantle, a relatively stationary plume head, and a plume head that flows outward along topography on the underside of the lithosphere. Consequently, these models are not strong candidates for the origin of volcanism and rifting in East Africa. Models with two or more plume heads, however, can explain the relevant observations, and therefore a multiple plume head explanation for the rifting and volcanism in East Africa is favored. These findings further call into question the viability of nonplume models for the formation of magmatic rifted margins, particularly models invoking edge flow in the convecting mantle around cratonic keels.
|Original language||English (US)|
|Number of pages||12|
|Journal||Special Paper of the Geological Society of America|
|State||Published - Jan 1 2002|
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