Anyone who doubts that Africa has undergone some serious rifting need only be reminded of the Red Sea (opening of a new ocean in NE Africa), the rifts in west Africa associated with oil-bearing sediments, and the volcanism of the east Afrian rift basin.
The above diagram based on the 1973 research of Burke and Whiteman illustrates a system of R-R-R triple junctions. Following the doming hypothesis, they saw evidence of rift rift rift triple junctions on many continents including Africa. In many cases Burke and Whiteman note that one of the arms of the junction might fail, leaving a R-R-r triple juntions, where the failed arm would eventually subside, and perhaps become part of a major depositional basin. It is believed that the Benue Trough in Nigeria is an example of this kind of feature on the African continent.
It is obvious that there ware various stages of development of all of these triple junctions, and that some are further along than others in the process. It is useful to divide the developement of the typical rifted passive margin into at least 2 steps:
1) The Rift Valley (early graben formation) <== ex. The East african Rift Valley
2) Sea Floor Spreading (thermal effects dominate) <== ex. The Red Sea
Above: Map of East Africa showing some of the historically active volcanoes(red triangles) and the Afar triple junction, where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian) splitting along the East African Rift Zone. [Graphic from USGS]
GPS has been used to estimate the relative plate motion at the Afar triple juntion (pictured above) using 8 tracking stations (pictured below). The measurements show a substantially higher spreading rate along the NE-SW line than along the NW-SE line.
Eight points previously measured by GPS methods in 1992 across the northern Ethiopian rift near 8°N were re-measured in July 1997. The remeasurements show that widening of the rift is 3.7±0.9 mm/year, and that most of this widening is concentrated in the deepest, most youthful part of the rift zone (2.9±1 mm/year). [http://cires.colorado.edu/%7Ebilham/ethiopia.html]
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Last Updated May 2, 2005.