by Fiona Clerc
The East African rift system is a continental active rift system spanning the boundary between the diverging Nubian and Somali plates. It is made up of a series of graben basins, which are connected by transform fault zones. It is composed of a Western and Eastern branch, which split off from the Afar Triple Junction in the north. The Eastern branch is made up of the Main Ethiopian Rift and the Kenyan Rift Valley further south, while the Western branch is made up of the Albertine Rift and a series of lake valleys, such as the Lake Malawi Valley. [2]
The East African rift system is a continental active rift system spanning the boundary between the diverging Nubian and Somali plates. It is made up of a series of graben basins, which are connected by transform fault zones. It is composed of a Western and Eastern branch, which split off from the Afar Triple Junction in the north. The Eastern branch is made up of the Main Ethiopian Rift and the Kenyan Rift Valley further south, while the Western branch is made up of the Albertine Rift and a series of lake valleys, such as the Lake Malawi Valley. [2]
Left: Embedded map of the East African rift system. 1. General Description Rock types Since the rift system covers a large area, rock types vary considerably. Rock types are greatly affected by the volcanic activity present. Initially, rifting fissure eruptions released basalt and siliceous ignimbrites, however later in the Miocene and Pliocene, rhyolites and phonolites were produced by shield volcanoes. The Plio-Pliocene would see rhyolites along the main rift axis and basalts on the plateaus lining the rift. In Kenya and Tanzania, many phonolites, trachytes, or peralkaline rhyolites were formed by volcanoes in the central rift zone. [1] |
Age of the system and of deformations
The rift was first formed by a hot spot around 30 Ma in the Afar and Ethopian plateau. The three converging graben structures formed the triple junction at Lake Tana. Rifting then occurred in the Gulf of Aden betweene 29.9-28.7 Ma, the Afar depression having formed later in the Miocene. The first volcanism occurred in 20 Ma at the site of the future triple junction (between the northern Kenyan, central Kenyana and Nyanza rifts). The main Ethipoian rift did not appear until 11 Ma. In Lake Albert, rifting began at 8 Ma, and in Lake Tanganyika, the central basin subsided around 12-9 Ma, while the northern basin subsided around 8-7 Ma, and the southern basin around 2-4 Ma. During the late Miocene (8-9 Ma), the Malawi rift began to subside, and subsequently extended south.
In general the EARS evolved to the south, nucleating at different sites and linking isolated basins, starting in the late Miocene, and is still actively extending today. [4]
See slideshow below for the evolution of the EARS through time. Darker areas indicate a higher altitude, the black lines indicate rifting. The red areas indicate volcanic activity.
The rift was first formed by a hot spot around 30 Ma in the Afar and Ethopian plateau. The three converging graben structures formed the triple junction at Lake Tana. Rifting then occurred in the Gulf of Aden betweene 29.9-28.7 Ma, the Afar depression having formed later in the Miocene. The first volcanism occurred in 20 Ma at the site of the future triple junction (between the northern Kenyan, central Kenyana and Nyanza rifts). The main Ethipoian rift did not appear until 11 Ma. In Lake Albert, rifting began at 8 Ma, and in Lake Tanganyika, the central basin subsided around 12-9 Ma, while the northern basin subsided around 8-7 Ma, and the southern basin around 2-4 Ma. During the late Miocene (8-9 Ma), the Malawi rift began to subside, and subsequently extended south.
In general the EARS evolved to the south, nucleating at different sites and linking isolated basins, starting in the late Miocene, and is still actively extending today. [4]
See slideshow below for the evolution of the EARS through time. Darker areas indicate a higher altitude, the black lines indicate rifting. The red areas indicate volcanic activity.
Deformation in the EARS through time. [4]
Overall tectonic setting
The tectonics of the EARS are affected by the diverging plates. The lithosphere undergoes extensional strain locally, as a result faulting and subsidence occurs in the crust, creating elongated rifts. The resulting ductile thinning in the lithosphere causes asthenospheric intrusion. [3]
Refer to the "Stress contributing to the formation of the EARS" section for more information and a map showing the location of the tectonic plates and their boundaries.
The tectonics of the EARS are affected by the diverging plates. The lithosphere undergoes extensional strain locally, as a result faulting and subsidence occurs in the crust, creating elongated rifts. The resulting ductile thinning in the lithosphere causes asthenospheric intrusion. [3]
Refer to the "Stress contributing to the formation of the EARS" section for more information and a map showing the location of the tectonic plates and their boundaries.
2. Detailed Description of Structures on Geological Map - A discussion of deformation in the EARS
Refer to the geologic map at the top of the page.
The EARS is made up of several types of basins. Most of these were caused by rift grabens, and are asymmetric, having a roll-over structure. They have one thermally-uplifted shoulder, a result of asthenospheric intrusion. A second kind of basin correspond with the transform zones linking different parts of the rift system, and are made up of strike-slip faults. These basins do not show shoulder uplift. A third kind of basin are the half-graben basins which cam be found in the Omo-Turkana relay zone. These basins are not a result of asthenospheric ascension, since no uplifted shoulders can be observed. Finally, there exists appended basins, which are not found along the main rift line. These are just crustal structures and have no uplifted shoulders [1].
The main structures are the normal faults, though the presence of strike-slip, oblique-slip, and reverse faults can also be noted. Most of the rift faults have three throw components: vertical, horizontal strike-slip and horizontal transverse. In general, the normal faults are interpreted as listric, and the largest faults are found on one side of the graben basins. They penetrate the middle-lower crust and connect with the low-angle ductile-fragile transition zone at around 20-30 km in depth. There is usually only one major normal and detachment fault per graben segment, resulting in an asymmetric roll-over structure. Since the normal faults have a notable oblique-slip component, rhomb box faulting and strike-ramps can be observed. The listric faults with an open gap near the surface are filled with sedimentary breccias. Transcurrent faults (which are dominantly strike-slip) run over 500 km distances, and folds can occur related to these faults. [1]
The rift basins are connected through large NW-striking transform fault zones. These zones have a major strike-slip component. The Tanganyika-Rukwa-Malawi fault zone is a right-lateral transform fault zone that connects two sections of the Western branch (see figure in section 4). Along the zone, both low plains (synclines), and high relief parts (anticlines) can be observed (instead of the usual high shoulders), since folds can develop along the narrow sections between the faults. The Aswa transform zone connects the Kenyan Rift Valley and the Eastern branch, and contains left-lateral strike-slip faults (resulting in strong earthquakes). The transform zone is roughly 200 km long, and has en echelon faulting and fissuring to make up for differences in crustal extension from one end to the other. This system of rifts and transform zones resembles oceanic ridges and ocean transform faults (respectively). In fact the EARS is expected to develop into an ocean. [1]
Within a rift basin, transfer faults and accommodation zones can be found linking faults of changing geometries. The transfer faults apply to changes in fault geometries of the same age, therefore the fault is in the direction of the extension in the basin, while the accommodation zones apply to changes in fault geometries of different ages, and its faults segment are not parallel to extension.
Open fractures are widespread and are a result of tension gashes. These fractures are mainly syn-depositional. Generally the horizontal transversal throw component dominates. The tension fractures can be filled by breccias or magma immediately upon formation. Other open fractures occurring in the EARS include tail-crack and horse-tail structures, which are found at the ends of oblique-slip fault, and result in the formation of volcanoes and calderas.
Volcanoes are rooted on open fractures. Their magmas are alkaline to hyperalkaline, and the magamatism is linked with the asthenospheric ascent. An asthenospheric wedge ascends diapirically during lithospheric extension (related to dyking), leading to the formation of basaltic melts. [1]
Below are a geologic maps of the Afar Triple Junction, and the Eastern and Western branches, in which some of the structures discussed above can be seen in greater detail than in the first geologic map.
Refer to the geologic map at the top of the page.
The EARS is made up of several types of basins. Most of these were caused by rift grabens, and are asymmetric, having a roll-over structure. They have one thermally-uplifted shoulder, a result of asthenospheric intrusion. A second kind of basin correspond with the transform zones linking different parts of the rift system, and are made up of strike-slip faults. These basins do not show shoulder uplift. A third kind of basin are the half-graben basins which cam be found in the Omo-Turkana relay zone. These basins are not a result of asthenospheric ascension, since no uplifted shoulders can be observed. Finally, there exists appended basins, which are not found along the main rift line. These are just crustal structures and have no uplifted shoulders [1].
The main structures are the normal faults, though the presence of strike-slip, oblique-slip, and reverse faults can also be noted. Most of the rift faults have three throw components: vertical, horizontal strike-slip and horizontal transverse. In general, the normal faults are interpreted as listric, and the largest faults are found on one side of the graben basins. They penetrate the middle-lower crust and connect with the low-angle ductile-fragile transition zone at around 20-30 km in depth. There is usually only one major normal and detachment fault per graben segment, resulting in an asymmetric roll-over structure. Since the normal faults have a notable oblique-slip component, rhomb box faulting and strike-ramps can be observed. The listric faults with an open gap near the surface are filled with sedimentary breccias. Transcurrent faults (which are dominantly strike-slip) run over 500 km distances, and folds can occur related to these faults. [1]
The rift basins are connected through large NW-striking transform fault zones. These zones have a major strike-slip component. The Tanganyika-Rukwa-Malawi fault zone is a right-lateral transform fault zone that connects two sections of the Western branch (see figure in section 4). Along the zone, both low plains (synclines), and high relief parts (anticlines) can be observed (instead of the usual high shoulders), since folds can develop along the narrow sections between the faults. The Aswa transform zone connects the Kenyan Rift Valley and the Eastern branch, and contains left-lateral strike-slip faults (resulting in strong earthquakes). The transform zone is roughly 200 km long, and has en echelon faulting and fissuring to make up for differences in crustal extension from one end to the other. This system of rifts and transform zones resembles oceanic ridges and ocean transform faults (respectively). In fact the EARS is expected to develop into an ocean. [1]
Within a rift basin, transfer faults and accommodation zones can be found linking faults of changing geometries. The transfer faults apply to changes in fault geometries of the same age, therefore the fault is in the direction of the extension in the basin, while the accommodation zones apply to changes in fault geometries of different ages, and its faults segment are not parallel to extension.
Open fractures are widespread and are a result of tension gashes. These fractures are mainly syn-depositional. Generally the horizontal transversal throw component dominates. The tension fractures can be filled by breccias or magma immediately upon formation. Other open fractures occurring in the EARS include tail-crack and horse-tail structures, which are found at the ends of oblique-slip fault, and result in the formation of volcanoes and calderas.
Volcanoes are rooted on open fractures. Their magmas are alkaline to hyperalkaline, and the magamatism is linked with the asthenospheric ascent. An asthenospheric wedge ascends diapirically during lithospheric extension (related to dyking), leading to the formation of basaltic melts. [1]
Below are a geologic maps of the Afar Triple Junction, and the Eastern and Western branches, in which some of the structures discussed above can be seen in greater detail than in the first geologic map.
3. Deformation contributing to the formation of the EARS (also refer to relevant information in sections 1 and 2)
It is widely accepted that the evolution of the EARS is due to extension, however the initial trigger is more debated. As in the West European rift system, the rifting could be due to collision. However, in the south of Africa, the Karroo compression ended in the Jurassic, before the formation of the EARS, and compression events in the Mediterranean were too old and localized. Compression from oceanic ridges is also unlikely, since both the Central Indian oceanic ridge and the Atlantic ridge formed too early. [1]
The rupture of the lithosphere occurs either by active rupture (due to forced intrusion of the asthenosphere, a result of its plume movements) or passive rupture (in which the asthenosphere rises to fill a gap in the lithosphere caused by the extension caused by plate boundaries). The tectonic rupture could be due to a combination of simple shear (caused by planar faults) in the upper brittle layer, delamination in the lower crust, and pure shear (and plastic deformation) in the lithospheric mantle. [1]
The lithosphere first failed in the north, which corresponds to the onset of the plume, under Lake Tana. The plume was 1000 km wide around 30 Ma, weakening a large area by heating and thinning it. The Pan-African suture zone was also present and also contributed to the lithospheric failure. These weakening effects and the tension caused by the tectonic plates led to the first tension fractures and faults in the Afar (after the formation of the Red Sea-Gulf of Aden opening), resulting in the Afar triple junction. Later, the plume moved to the south, arriving at the Kenyan dome before 20 Ma, where crustal up-warping and flood volcanism followed at 15 Ma. The failure that started in the Afar (as a result of the weakening discussed above) spread to the south causing the MER (main Ethiopian rift) as it followed the suture zone. Another suture zone located more to the south allowed the failure to spread to the Kenyan dome. Since the MER and the Kenyan rifts followed different suture zone, they could not link and formed splayed graben basins between them. As it spread to the south, the failure was stopped by the thicker lithosphere, where it formed the north Tanzanian divergence. After the plume had traveled down the western branch, the failure spread to the Kivu-northern Tanganyika area. The rifts eventually linked with the eastern branch through the Aswa transform zone. In general, important parts of the rift system were formed far from the plume, since the lithospheric thinning, which helped initiate failures and asthenospheric intrusions was initiated by the plume but continued along ancient weak lines. [1]
4. Stress contributing to the formation of the EARS
The deformation contributing to the formation of the rift system (a lithospheric opening in the African continent) is the extension caused by the divergence of large blocks. The direction of movement of the extension is debated, no consensus has been reached through analysis of local paleostress fields. There are two types of movements affecting the faults. First, there is the NW-SE movement of the large continental blocks. Second, smaller local movements are triggered along the major border faults, where the high relief creates gravity gliding effects. Local extension occurs in the E-W direction, especially in the eastern branch. The combination of these movements explains the changing pattern of stress and deformation, even though the kinematic schemes do not vary. [1]
It is widely accepted that the evolution of the EARS is due to extension, however the initial trigger is more debated. As in the West European rift system, the rifting could be due to collision. However, in the south of Africa, the Karroo compression ended in the Jurassic, before the formation of the EARS, and compression events in the Mediterranean were too old and localized. Compression from oceanic ridges is also unlikely, since both the Central Indian oceanic ridge and the Atlantic ridge formed too early. [1]
The rupture of the lithosphere occurs either by active rupture (due to forced intrusion of the asthenosphere, a result of its plume movements) or passive rupture (in which the asthenosphere rises to fill a gap in the lithosphere caused by the extension caused by plate boundaries). The tectonic rupture could be due to a combination of simple shear (caused by planar faults) in the upper brittle layer, delamination in the lower crust, and pure shear (and plastic deformation) in the lithospheric mantle. [1]
The lithosphere first failed in the north, which corresponds to the onset of the plume, under Lake Tana. The plume was 1000 km wide around 30 Ma, weakening a large area by heating and thinning it. The Pan-African suture zone was also present and also contributed to the lithospheric failure. These weakening effects and the tension caused by the tectonic plates led to the first tension fractures and faults in the Afar (after the formation of the Red Sea-Gulf of Aden opening), resulting in the Afar triple junction. Later, the plume moved to the south, arriving at the Kenyan dome before 20 Ma, where crustal up-warping and flood volcanism followed at 15 Ma. The failure that started in the Afar (as a result of the weakening discussed above) spread to the south causing the MER (main Ethiopian rift) as it followed the suture zone. Another suture zone located more to the south allowed the failure to spread to the Kenyan dome. Since the MER and the Kenyan rifts followed different suture zone, they could not link and formed splayed graben basins between them. As it spread to the south, the failure was stopped by the thicker lithosphere, where it formed the north Tanzanian divergence. After the plume had traveled down the western branch, the failure spread to the Kivu-northern Tanganyika area. The rifts eventually linked with the eastern branch through the Aswa transform zone. In general, important parts of the rift system were formed far from the plume, since the lithospheric thinning, which helped initiate failures and asthenospheric intrusions was initiated by the plume but continued along ancient weak lines. [1]
4. Stress contributing to the formation of the EARS
The deformation contributing to the formation of the rift system (a lithospheric opening in the African continent) is the extension caused by the divergence of large blocks. The direction of movement of the extension is debated, no consensus has been reached through analysis of local paleostress fields. There are two types of movements affecting the faults. First, there is the NW-SE movement of the large continental blocks. Second, smaller local movements are triggered along the major border faults, where the high relief creates gravity gliding effects. Local extension occurs in the E-W direction, especially in the eastern branch. The combination of these movements explains the changing pattern of stress and deformation, even though the kinematic schemes do not vary. [1]
References:
[1] Chorowitz, J., 2005. The East African rift system. J. Afr. Earth Sci. 43, 379–441.
[2] Corti, G., 2009. Continental rift evolution: From rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa. Earth-Science Reviews. 96, 1-53.
[3] Isola et al., 2014. Spatial variability of volcanic features in early-stage rift settings: the case of the Tanzania divergence, East African rift system. Terra Nova. 26. 461-468.
[4] MacGregor, D., 2015. History of the development of the East African Rift System: A series of interpreted maps through time. J. Afr. Earth Sci. 101, 232-252.
[1] Chorowitz, J., 2005. The East African rift system. J. Afr. Earth Sci. 43, 379–441.
[2] Corti, G., 2009. Continental rift evolution: From rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa. Earth-Science Reviews. 96, 1-53.
[3] Isola et al., 2014. Spatial variability of volcanic features in early-stage rift settings: the case of the Tanzania divergence, East African rift system. Terra Nova. 26. 461-468.
[4] MacGregor, D., 2015. History of the development of the East African Rift System: A series of interpreted maps through time. J. Afr. Earth Sci. 101, 232-252.