Continental rifts of the world

Continental rifts are regions of extensional deformation where the entire thickness of the lithosphere has deformed under the influence of deviatoric tension.
Rifts represent the initial stage of continental break-up where extension may lead to lithospheric rupture and the formation of a new ocean basin.

1. Continental Rifts of the World By: FARHAN HAMEED

2. Continental rifts Continental rifts are regions of extensional deformation where the entire thickness of the lithosphere has deformed under the influence of deviatoric tension. Rifts represent the initial stage of continental break-up where extension may lead to lithospheric rupture and the formation of a new ocean basin.

3. How Do Continental Rifts Form? Continental rifts initiate with doming.

4. The doming induces tensional stress in the upper crust which results in normal faulting.

5. There is increased heat flow. The heat flow increases as the crust thins by ductile shear of the lower crust and lithosphere and normal faulting in the brittle upper crust. Bimodal (basaltic and rhyolitic ) volcanism are results from the increased heat flow.

6. If rifting continues the crust/lithosphere will thin to zero and seafloor spreading is initiated forming a new ocean basin between two drifting continents.

7. Case Studies

8. The East African Rift system The East African Rift (EAR) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22-25 million years ago. In the past, it was considered to be part of a larger Great Rift Valley that extended north to Asia Minor.

9. A map of East African Rift Zone.

10. Competing theories on geologic evolution of EAR Over time, many theories have tried to clarify the evolution of the East African Rift. In 1972 it was proposed that the EAR was not caused by tectonic activity, but rather by differences in crustal density. Others proposed an African superplume causing mantle deformation. The most recent and accepted view is the theory put forth in 2009, that magmatism and plate tectonics have a feedback with one another, controlled by oblique rifting conditions.

11. At that time it was suggested that lithospheric thinning generated volcanic activity, further increasing the magmatic processes at play such as intrusions and numerous small plumes. These processes further thin the lithosphere in concentrated areas, forcing the thinning lithosphere to behave like a mid-ocean ridge.

12. Geologic Evolution of EAR Prior to rifting, enormous continental flood basalts erupted on the surface and uplift of the Ethiopian, Somalian , and East African plateaus occurred. The first stage of rifting of the EAR is characterized by rift localization and magmatism along the entire rift zone. Periods of extension alternated with times of relative inactivity.

13. There was also the reactivation of a pre-Cambrian weakness in the crust, a suture zone of multiple cratons , displacement along large boundary faults, and the development of deep asymmetric basins. The second stage of rifting is characterized by the deactivation of large boundary faults, the development of internal fault segments, and the concentration of magmatic activity towards the rifts.

14. Today, the narrow rift segments of the East African Rift system form zones of localized strain. These rifts are the result of the actions of numerous normal faults which are typical of all tectonic rift zones. As voluminous magmatism and continental flood basalts characterize some of the rift segments, while other segments, such as the Western branch, have only very small volumes of volcanic rock.

15. Volcanism The East African Rift Zone includes a number of active as well as dormant volcanoes, among them. Active volcanos include Erta Ale, DallaFilla , and Ol Doinyo Lengai , the former of which is a continuously active basaltic shield volcano in the Afar Region of northeastern Ethiopia. When DallaFilla erupted in 2008 it was the largest volcanic eruption in Ethiopia in recorded history. The Ol Doinyo Lengai volcano is currently the only active natrocarbonatite volcano in the world.

16. The magma contains almost no silica, making the flow viscosity extremely low. “Its lava fountains crystallize in midair then shatter like glass.” (National Geographic) Approximately 50 volcanic structures in Ethiopia alone have documented activity since the onset of the Holocene.

17. Seismicity The EAR is the largest seismically active rift system on Earth today. The majority of earthquakes occur near the Afar Depression, with the largest earthquakes typically occurring along or near major border faults. Seismic events in the past century are estimated to have reached a maximum moment magnitude of 7.0

18. The seismicity trends parallel to the rift system, with a shallow focal depth of 12–15 km beneath the rift axis. Further away from the rift axis, focal depths can reach depths of over 30 km.

19. Rio Grande Rift The Rio Grande Rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuahua, Mexico in the south.

20. Locality map showing the Rio Grande Rift

21. Geology The Rio Grande Rift represents at the eastern most manifestation of widespread extension in the western U.S. during the past 35 million years. The rift consists of three major basins and many smaller basins, less than 100 km. The three major basins (from northernmost to southernmost) are the San Luis, Espanola, and Albuquerque basins. Further south, the rift is defined by a network of smaller, less topographically distinct alternating basins and ranges. The distinction between these smaller basins and those of the Basin and Range Province becomes blurred in northern Mexico.

22. Basin size generally decreases to the north in the rift, where the Espanola covers approximately 120 km north-south and 40 km east-west, and the San Luis is roughly 120 km by 80 km. The Albuquerque basin is the largest of the three basins, spanning 160 km north-south and 86 km east-west at its widest points. It is the oldest of the three major basins, and contains 7,350 m of Paleocene clastic sediments deposited on Precambrian basement.

23. Baikal Rift Zone The Baikal Rift Zone is a divergent plate boundary centered beneath Lake Baikal in southeastern Russia. Along it form a series of basins more than 2,000 kilometers long. To its west are the Eurasian Plate and the Siberian platform and to its east is the Amur Plate which is moving away from the rift toward Japan at about 4 mm per year.

24. Gulf of Suez Rift The Gulf of Suez Rift is a continental rift zone that was active between the Late Oligocene (28 Ma) and the end of the Miocene (5 Ma). It represented a continuation of the Red Sea Rift until break-up occurred in the middle Miocene, with most of the displacement on the newly developed Red Sea spreading centre being accommodated by the Dead Sea Transform.

25. Satellite image of the Gulf of Suez

26. Blue Nile rift The Blue Nile rift is a major geological formation in the Sudan, a rift with a NW trend that terminates on the Central African Shear Zone. It was formed through crustal extension during the break-up of Gondwana .

27. White Nile rift The White Nile rift is one of several rifts in central Sudan running in a NW direction and terminating in the Central African Shear Zone. The rift is a Cretaceous/Tertiary structure that has similar tectonic characteristics to the Southern Sudan Rift.

28. Rifts in Sudan and Kenya

29. West Antarctic Rift The West Antarctic Rift is a major, active rift valley lying between East and West Antarctica. It encompasses the Ross Sea, the area under the Ross Ice Shelf and a part of West Antarctica. Its evolution is due to lithospheric thinning of the non- cratonic area of West Antarctica.

30. Some Other Rifts Bahr el Arab rift Red Sea Rift Anza trough Melut Basin Muglad Basin

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