Perceptions and issues in continental rifting

Abstract The present rifts of Africa are the most recent of a series of rift systems developed at various times since the Karroo. Modern rifts include the intracontinental East African rift system and the Red Sea and associated features, where sea floor spreading has already begun. The various intracontinental rift systems form a patchwork of rifted basins that have undergone little extension (generally less than 10%). Most of them appear to be similar to the currently active rifts, which consist of a series of half graben whose polarities alternate along the rift zones. Some rifts are volcanically active (such as the modern Gregory and Ethiopian rifts), and some are deficient in volcanic products. Much of the modern rifting has occurred across two broad domes (Kenya and Ethiopia). The Red Sea is associated with the Gulf of Aden in the south and with the Gulfs of Suez and Aqaba (including the Dead Sea transcurrent fault zone) in the north. The Red Sea has undergone separation of several hundred percent by some combination of lithospheric extension and sea floor spreading. The presence of continuous boundary structures along both sides of the Red Sea makes comparison with the intracontinental rifts difficult. None of the current rifting in, and on the margins of, Africa has developed broad intracontinental basins. Such large, nearly equilateral, extensional areas may develop only where adjacent continental margins undergo compression and/or subduction. The diversity of rift phenomena probably implies a variety of mantle processes that ultimately cause the extension. One major, unresolved, question is whether successful (oceanic-basin) rifts are a natural continuation of processes that form intracontinental rifts.