Afar Triple Junction
The Afar Triple Junction (also called the Afro-Arabian Rift System) is located along a divergent plate boundary dividing the Nubian, Somalian, and Arabian plates. This area is considered a present-day example of continental rifting leading to seafloor spreading and producing an oceanic basin. Here, the Red Sea Rift meets the Aden Ridge and the East African Rift. It extends a total of 6500 km in three arms from the Afar Depression to Mozambique.
The connecting three arms form a triple junction. The northern most branching arm extends North through the Red Sea and into the Dead Sea, while the eastern arm extends through the Gulf of Aden and connects to the Mid-Indian Ocean ridge further to the east. Both of these rifting arms are below sea level and are similar to a mid-ocean ridge.
The third rifting arm runs south extending around 4000 km through the countries of Kenya, Uganda, the Democratic Republic of Congo, Rwanda, Burundi, Tanzania, Zambia, Malawi, and finally Mozambique. This southern rifting arm is better known as the East African Rift or the East African Rift System (EARS), when it includes the Afar Triangle.
Doming and rifting
A rift is the result of pulling apart or extension of both the lithosphere and crust (note that the crust is a part of the lithosphere). This is a product of what is referred to as mantle upwelling where hotter asthenosphere rises up into colder lithosphere. This rise is associated with thinning and stretching of the lithosphere.
Rifting is said to have begun in the Late Cretaceous epoch to Paleogene period. At that time the African plate was experiencing far-field stresses caused by portions of the northern boundary of the African plate subducting under the Eurasian plate. Today, the Arabian plate is experiencing a crustal down pull, or slab pull, that has separated from the African plate. At the same time of the subduction in the north there was mantle upwelling causing the crust to down warp and swell into domes. There are many domes throughout the East African Rift System. The plume is thought to have begun under Lake Tana in Ethiopia. The Kenyan dome has been studied extensively.
Gani 2007 proposes that episodic increase of incision of the Ethiopian Plateau suggests episodic growth rates within the plateau. This is proposed since the incision rates have no correlation to the past climate events. Using Archimedes’ principle of isostatic rebound, 2.05 km uplift has occurred within the last 30 Ma.
Using the environmental correlations and current topographic locations of the Jurassic Upper Limestone and Cretaceous Upper Sandstone, the net rock uplift of the Ethiopian Plateau would be 2.2 km since ca. 150 Ma. The thinned Ethiopian lithosphere could have resulted in ponding from mantle plume and subsequent uplift.
Baker, B.H. 1972 also suggests that the uplift of this area is sporadic and divided by long periods of stability and erosion. Some periods of uplift are recorded at the end of the Cretaceous that resulted in 400 m uplift and the end of the Neogene with a staggering 1,500 m in magnitude. The Ethiopian dome experienced its largest uplift coinciding with the end of the Neogene uplift associated with the Kenyan dome. It has been argued that the current Ethiopian plateau is a result of the most recent uplift of 500 m estimated to be an Oligocene-early Miocene event. But the most accepted argument of the plateau is the result of the Paleogene flood-basalts. The uplift associated with both domes has resulted in major structural features due to the swelling and warped crustal extension. The two areas of swelling resulted in a large depression between the two domes and subsidence along the coastal regions. The uplift caused by the Ethiopian dome resulted in a massive faulting area of 1000 m in the Afar region.
East African Rift
The East African Rift is an active rift between the Nubian and Somali protoplates. This rift is caused by elevated heat flow from the mantle under Kenya and the Afar region. Trending NNE to SSW, the East African Rift is composed of a western and an eastern branch. The eastern branch (sometimes called the Gregory Rift) is characterized by high volcanic activity and the western branch (sometimes called the Albertine Rift) is characterized by deeper basins, which contain lakes and sediments. The lakes in this area (e.g. Lake Tanganyika and Lake Rukwa) are located in highly rifted basins and have an inter-fingering relationship with faults. Many of the lakes are bounded by normal or strike-slip faults. The extension rate for this rift starts at about 6 millimeters per year in the north, and declines to the south.
Red Sea Rift
The Red Sea Rift is between the African (or Nubian) and Arabian Plates. The rift runs along the length of the Red Sea, starting from the Dead Sea to the Afar triple junction. Within the rift, in the Red Sea, there are many volcanoes, including the Jabal al-Tair. The extension rate for this rift varies from about 7 to 17 millimeters per year.
The Aden Ridge is a divergent plate boundary that divides the African (or Somali) and Arabian Plate. It extends from the triple junction eastward to the Owen Fracture Zone. The Aden Ridge is also a part of another triple junction in the Indian Ocean to the east, called the Aden-Owen-Carlsberg triple junction, which include the African, Arabian, and Indo-Australian plates. The spreading rate for Aden Ridge is about 17 millimeters per year near the Afar Triple Junction.
Before the initial rifting began, Africa was one plate but as rifting proceeded the plate began to tear in three directions. The rifting propagated along three branches that have now formed three separate plates: the Arabian, Somalian, and the Nubian (also mentioned as the African plate). In 1969 McKenzie and Morgan published a paper and systematically explained types of triple junctions and their stability.
The Afar Triple Junction is known as a ridge-ridge-ridge or RRR triple junction. This describes the movement of the three plates with respect to each other. The Arabian, Somalian, and Nubian plates are all divergent margins, or ridges, with respect to the adjacent plates. Following Mackenzie and Morgan's triple junction stability model, RRR geometry is stable and will continue through time until there is a change in the tectonic movement.
The Afar Depression is a geological depression that ranges in heights from 1000 m to -120 m. This area, as mentioned above, experienced many domal uplifts. One of these uplifts was called the Afar dome. It began rising 40 Ma. This uplift caused massive crustal extension leading to horst and graben structures associated with normal, extensional, faults. The uplift of the Afar dome ultimately led to its collapse around 25 Ma. The Afar depression encompasses an area of more than 200,000 km2 and is spreading at a rate of 6 to 17 mm/yr.
Implications of volcanism
There are many active volcanic areas centralized in the East African Rift System in comparison to the other areas in the Afro-Arabian rift system. Many protruding horsts show many layers of flood basalts. Using 40Ar/39Ar-isotope dating an age constraint can be implied on this basaltic series. It is found to be approximately 30 Ma years old. The trap series is dated to a time soon before the major rifting events began. Chorowicz, J. 2005 illustrated the trap series surrounding the newer Neogene volcanics. This helps quantify the amount of crustal extension and gives a model of pre-rifting crustal connection.
Seismic tomography compiles P-wave and S-wave data from movements within the earth to create a 3D velocity model of the Earth's subsurface. The models distinguish between fast velocity, high anomaly, and slow velocity, slow anomaly, time measurements.
Multiple tomography models show a slow anomaly structure beneath southern Africa. Grand, S. (1997) models the large anomaly to extend from the base of the mantle to approximately 1000 km depth. This slow anomaly is considered to be a plume upwelling.
Opening of a basin
Horsts and grabens are very well documented throughout this region. Although horsts and grabens do show and produce crustal extension, for a sufficient ocean basin to form, there needs to be extension that can accommodate for the extensive down fall of the grabens. Listric faults produce the correct model for this sufficient crustal extension. These faults have been documented by Chorowicz, J. 2005 and aid in further verification of the future of this region and the potential for continued extension and subsidence.
Past rifting events have been recorded in the geologic record and major rifting events have been seen to have an aulacogen with two successful rifting arms. Some geologists have proposed that the East African Rift System will be the aulocogen in the future but as of present-day there seems to be no aulocogen and the rifting in the EARS does not show any evidence of slowing its motion.
There is also the possibility to a subduction zone forming along the eastern most side of the continental Somalian plate. This could be associated by the spreading of the Mid-Indian Oceanic ridge and the East African rift. To accommodate the compression of the Somalian plate due to two extensional edges of the plate the oceanic plate might begin to subduct below the continental plate.
Summary and problem
Evidence is showing that the East African Rift System is a classic continental-continental rifting event but the extent of research due to its age and continuing formation is diverse and filled with many hypothetical models that support and contrast each other. The rifting began in the Paleogene due to the far-field stress from the subduction of the Arabian plate under the Eurasian plate and the mantle upwelling that has been seen to move over time because of the multiple area of hot spots around the EARS.
This crustal uplift has created extension and horst and grabens and even listric faults which indicate a pre-oceanic basin structure. The future of this area is unknown. If current tectonics continue without change it is thought that an ocean basin with a mid-oceanic ridge will eventually separate the Nubian, Somalian, and Arabian plates.
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