Geology of the Eastern Rift System of Africa
The eastern rift of Africa is a zone of normal faults separating the Horn of Africa from the remainder of the continent. The zone is typically troughlike, 40 to 65 km wide, and traverses two broad, elongated domal uplifts in Ethiopia and Kenya.
The foundation rocks of the region are metasediments and intrusives of the late Precambrian orogenic belt, which has a meridional trend. The Paleozoic was dominantly an era of denudation in eastern Africa, but late Paleozoic continental sediments (Karroo System) are locally preserved. Mesozoic marine sediments represent an epicontinental marine transgression and regression. Severe coastal warping occurred along the Indian Ocean margin, and in the early Tertiary such warping initiated the Red Sea, Gulf of Aden, and Afar depressions.
Uplift of the Ethiopian and Kenyan domes has been synchronous in three major pulses of late Eocene, mid-Miocene, and Plio-Pleistocene age. Volcanism of intermediate and silicic type shows some relation to uplift in time and space and to the onset of graben faulting, but major flood basalt extrusions in the early Tertiary in Ethiopia were related to massive crustal warping along the future rift margins. The volcanism associated with the eastern rift is overwhelmingly alkaline, and at some volcanoes a strongly alkaline fractionation series is distinguished from a more mildly alkaline series. The flood phonolites, trachytes, rhyolites, and ignimbrites of Kenya, and the pantelleritic ignimbrites of Ethiopia, could have resulted from anatexis of a mantle-derived accreted layer at the base of the crust.
The eastern rift began as a chain of marginally warped depressions which were accentuated as domal uplift proceeded, until, in mid-Miocene to early Pliocene times, faulting produced asymmetrical grabens. The final uplift phase in the early Pleistocene was accompanied by major graben faulting, and subsequent faulting has intensely fractured the floor of the rift along an axial zone marked by caldera volcanoes. The evolution and nature of the faulting, the evidence from the distribution and ages of volcanoes, and seismic and gravity data all indicate that the eastern rift lies along a zone of progressive crustal thinning with local crustal disruption.
The eastern rift can be considered as a plate boundary which meets the Red Sea and Gulf of Aden spreading axes at the Afar triple junction. Plate analysis suggests that the eastern rift marks a line of very slow crustal spreading, which helps account for many of the peculiar or unique features of this continental rift.
Figures & Tables
- areal geology
- geophysical methods
- geophysical surveys
- gravity methods
- igneous rocks
- metamorphic rocks
- rift zones
- sedimentary petrology
- sedimentary rocks
- structural geology