Afar Depression, Ethiopia

Afar Depression, Ethiopia

One of the world’s largest, deepest regions below sea level that is subaerially exposed on the continents, home to some of the earliest known hominid fossils. It is a hot, arid region, where the Awash River drains
northward out of the East African rift system, and is evaporated in Lake Abhe before it reaches the sea. It is located in eastern Africa in Ethiopia and Eritrea, between Sudan and Somalia, and across the Red Sea and Gulf of Aden from Yemen. The reason the region is so topographically low is that it is located at a tectonic triple junction, where three main plates are spreading apart, causing regional subsidence. The Arabian plate is moving northeast away from the African plate, and the Somali plate is moving, at a much slower rate, to the southeast away from Africa. The southern Red Sea and north-central Afar Depression form two parallel north-northwest-trending rift basins, separated by the Danakil Horst, related to the separation of
Arabia from Africa. Of the two rifts, the Afar depression is exposed at the surface, whereas the Red Sea rift floor is submerged below the sea. The north-central Afar rift is complex, consisting of many grabens and horsts. The Afar Depression merges southward with the northeast-striking Main Ethiopian Rift, and eastward with the east-northeast-striking Gulf of Aden. The Ethiopian Plateau bounds it on the west. Pliocene volcanic rocks of the Afar stratoid series and the Pleistocene to Recent volcanics of the Axial Ranges occupy the floor of the Afar Depression. Miocene to recent detrital and chemical sediments are intercalated with the volcanics in the basins. The Main Ethiopian and North-Central Afar rifts are part of the continental East African Rift System. These two kinematically distinct rift systems, typical of intracontinental
rifting, are at different stages of evolution. In the north and east, the continental rifts meet the oceanic rifts of the Red Sea and the Gulf of Aden, respectively, both of which have propagated into the continent. Seismic refraction and gravity studies indicate that the thickness of the crust in the Main Ethiopian Rift is less than or equal to 18.5 miles (30 km). In Afar the thickness varies from 14 to 16 miles (23–26 km) in the south
to 8.5 miles (14 km) in the north. The plateau on both sides of the rift has a crustal thickness of 21.5–27 miles (35–44 km). Rates of separation obtained from geologic and geodetic studies indicate 0.1–0.2 inches (3–6 mm) per year across the northern sector of the Main Ethiopian Rift between the African and Somali plates. The rate of spreading between Africa and Arabia across the North-Central Afar rift is relatively faster, about 0.8 inches (20 mm) per year. Paleomagnetic directions from Cenozoic basalts on the Arabian side of the Gulf of Aden indicate 7 degrees of counterclockwise rotation of the Arabian plate relative to Africa, and clockwise rotations of up to 11 degrees for blocks in eastern Afar. The initiation of extension on both sides of the southernmost Red Sea Rift, Ethiopia, and Yemen appear coeval, with extension starting between 22 million and 29 million years ago.