The East African Rift is the best present example of the process of continental rifting, by which continents rift and ultimately give rise to a new ocean basin. The Rift is a spreading center along which Africa is splitting into Nubian (West Africa) and Somalian (East Africa) plates. The extension rate is so slow, less than 10 mm/yr, that the two plates are often treated as one. However, the rift topography, active faulting, and seismicity (dots) show the presence of a boundary zone. A recent plate motion solution (Chu and Gordon, 1999) shows that the northern East Africa rift opens at about 6 mm/yr, whereas the southern part opens at about half that, because the Euler pole is to the south. [Po-Fei Chen and Seth Stein, Northwestern University]
Focal mechanisms of earthquakes with moment greater than $10 sup 24~dyne-cm$ between 1964 and 1998. The normal fault mechanisms show the ongoing extension. White arrows are the horizontal components of tension axes, and the black arrows are the relative motion predicted from plate motions. Most earthquakes in the western branch of the rift exhibit horizontal tension axes normal to the rift geometry. Normal-faulting earthquakes extend to depths of 25-30 km, considerably deeper than at midocean ridges. Hence the lower crust appears to be surprisingly stronger and colder than might be expected in an active rift. [Po-Fei Chen and Seth Stein, Northwestern University]
