The Tertiary age East African Rift System (EARS) is predominantly located in zones of Precambrian orogenic belts avoiding stable Archean cratonic areas. The most profound influence of preexisting fabrics is exerted by Precambrian ductile fabrics. However, later predominantly extensional events of Permo- Triassic (Karroo), Jurassic, Cretaceous, and Paleogene age have also variably influenced the location and orientation of the Tertiary rift systems.

The two branches of the East African Rift System have undergone different tectonic histories. In general, the western branch can be regarded as a good model of a young continental rift while the eastern branch is representative of a “failed” mature continental rift system. Both are characterized by large half graben systems filled by fluvio-deltaic and lacustrine sediments, and/or by volcanics and volcaniclastics. The basin fills can be up to 7-8 km thick. In comparison with the eastern branch, the western branch is younger (late Miocene-Recent), less volcanic-rich but more seismically active, with deeper earthquakes (down to about 30-40 km). Extension estimates for the eastern branch depend upon location, but range up to 40 km (Turkana area), maximum extension in the western branch is about 10-12 km.

The eastern branch was probably initiated in the Eocene. In the Kenya and Ethiopian Rifts, older half grabens were abandoned in the Pliocene in favor of a narrow rift zone characterized by minor fault swarms, dike intrusions, and volcanic centers. There is evidence for an important active rift component (very thin mantle lithosphere, large topographic domes, anomalously low velocities in the mantle) as well as passive rifting. Deep refraction profiles indicate extension ranges between 10-40 km in the Kenya Rift (increasing northwards). However, this may be an underestimate if the crust has been inflated by intrusion of magma during extension.