Active versus passive continental rifting: Evidence from the West African rift system

Abstract

The West African rift system comprises the Cretaceous Benue trough and the Tertiary to Recent volcanic Cameroon line. The two features are remarkably similar in shape and size and may be superimposed perfectly by rotating one with respect to the other by 7° about a pole in Sudan. Three stages in the geological history of the system are postulated. 1. (1) The Benue trough was produced by lithosphere stretching as one arm of an RRR triple junction during the early stages of the opening of the South Atlantic. 2. (2) At about 80 Ma (Santonian) a short-lived period of clockwise rotation interrupted the otherwise anticlockwise rotation of Africa and decoupled the lithosphere from the asthenosphere. The hot zone in the asthenosphere beneath the Benue trough thus became displaced relative to the lithosphere and moved to a new position beneath Cameroon and the Gulf of Guinea. 3. (3) Anticlockwise rotation and asthenosphere-lithosphere coupling were restored allowing the hot zone to manifest itself as the Cameroon line. The Cameroon line is possibly a unique example of what a “rift system” produced actively by a thermal anomaly in the asthenosphere would look like. It is significant that, despite a long history of volcanism and uplift, the Cameroon line has not developed a graben structure. The implication is that rift systems with graben structures are produced passively by lithosphere stretching.