The Central African Shear Zone (CASZ) (or Shear System) is a wrench fault system extending in an ENE direction from the Gulf of Guinea through Cameroon into Sudan. [1] The structure is not well understood. As of 2008 [update] , there was still no general agreement about how the individual shears along the lineament link up. [2]
The shear zone dates to at least 640 Ma (million years ago). [3] Motion occurred along the zone during the break-up of Gondwanaland in the Jurassic and Cretaceous periods. [1] Some of the faults in the zone were rejuvenated more than once before and during the opening of the South Atlantic in the Cretaceous period. [3]
It has been proposed that the Pernambuco fault in Brazil is a continuation of the shear zone to the west. [3] In Cameroon, the CASZ cuts across the Adamawa uplift, a post-Cretaeous formation. The Benue Trough lies to the north, and the Foumban Shear Zone to the south. [4] Volcanic activity has occurred along most of the length of the Cameroon line from 130 Ma to the present, and may be related to re-activation of the CASZ. [5] The lithosphere beneath the CASZ in this area is thinned in a relatively narrow belt, with the asthenosphere upwelling from a depth of about 190 km to about 120 km. [6] The Mesozoic and Tertiary movements have produced elongated rift basins in central Cameroon, northern Central African Republic and southern Chad. [3]
The CASZ was formerly thought to extend eastward only to the Darfur region of western Sudan. [3] It is now interpreted to extend into central and eastern Sudan, with a total length of 4,000 km. [1] In the Sudan, the shear zone may have acted as a structural barrier to development of deep Cretaceous-Tertiary sedimentary basins in the north of the area. Objections to this theory are that the Bahr el Arab and Blue Nile rifts extend northwest beyond one proposed line for the shear zone. [7] However, the alignment of the northwestern ends of the rifts in this areas supports the theory. [8]
Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence creates a regional depression that provides accommodation space for accumulation of sediments. Over millions or tens or hundreds of millions of years the deposition of sediment, primarily gravity-driven transportation of water-borne eroded material, acts to fill the depression. As the sediments are buried, they are subject to increasing pressure and begin the processes of compaction and lithification that transform them into sedimentary rock.
The African Plate, also known as the Nubian Plate, is a major tectonic plate that includes much of the continent of Africa and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American Plate to the west ; the Arabian Plate and Somali Plate to the east; the Eurasian Plate, Aegean Sea Plate and Anatolian Plate to the north; and the Antarctic Plate to the south.
In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley, which may be filled by water forming a rift lake. The axis of the rift area may contain volcanic rocks, and active volcanism is a part of many, but not all, active rift systems.
The Cameroon line is a 1,600 km (1,000 mi) long chain of volcanoes that includes islands in the Gulf of Guinea and mountains on the African mainland, from Mount Cameroon on the coast towards Lake Chad on the northeast. They form a natural border between eastern Nigeria and the West Region of Cameroon. The islands, which span the equator, have tropical climates and are home to many unique plant and bird species. The mainland mountain regions are much cooler than the surrounding lowlands, and also contain unique and ecologically important environments.
Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.
Darfur Dome or Darfur Volcanic Province is an area of about 100x400 km in Western Sudan. As well as its best-known and central feature, Deriba Crater, it encompasses the surrounding Marra Mountains and Tagabo Hills, both formed around 16–10 Ma, and the Meidob Hills, formed around 6.8 Ma.
The Andean orogeny is an ongoing process of orogeny that began in the Early Jurassic and is responsible for the rise of the Andes mountains. The orogeny is driven by a reactivation of a long-lived subduction system along the western margin of South America. On a continental scale the Cretaceous and Oligocene were periods of re-arrangements in the orogeny. The details of the orogeny vary depending on the segment and the geological period considered.
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, Blue Nile rift and Atbara rift. These rifts follow similar trends, and terminate in a line at their northwestern ends. Probably this line is an extension of the Central African Shear Zone through the Sudan.
The Trans Brazilian Lineament (TBL), or Transbrasiliano Lineament, is a major shear zone that developed in the Precambrian period, and that has been reactivated several times since then, mostly recently during the Mesozoic. Movement along the shear zone helps explain how the South American continent could have fitted tightly to the African continent before the breakup of Gondwana.
The Kandi fault zone is a southern extension of the Hoggar fault zone in West Africa, with splays in Benin, Togo and southeastern Ghana. It lies at the southern end of the Trans Saharan belt, a lineament that extends in a southwest direction from Algeria to Benin. The Kandi fault zone is identified with the Sobral fault in northeastern Brazil, considered to be the northern section of the Trans Brazilian Lineament.
The Foumban Shear Zone, or Central Cameroon Shear Zone (CCSZ), is a fault zone in Cameroon that has been correlated with the Pernambuco fault in northeastern Brazil, which splays from the Trans-Brazilian Lineament. It is part of the Central African Shear Zone (CASZ) and dates to at least 640 million years ago. The zone was rejuvenated several times, usually with a dextral movement, before and during the opening of the South Atlantic in the Cretaceous period.
The Ngaoundéré Plateau is a highland area in the Adamawa Region of Cameroon. It contains Tchabal Nganha, a large volcano, and many other recent volcanic structures.
The Pernambuco Fault or Pernambuco Shear Zone in northeastern Brazil is a fault radiating from the Trans Brazilian Lineament eastwards to the coast, a major geological feature.
The North Sea is part of the Atlantic Ocean in northern Europe. It is located between Norway and Denmark in the east, Scotland and England in the west, Germany, the Netherlands, Belgium and France in the south.
The West and Central African Rift System (WCARS) is a rift system composed of two coeval Cretaceous rift sub-systems, the West African Rift sub-system (WAS) and the Central African Rift sub-system (CAS). These are genetically related, but are physically separated and show structural differences. The Logone Birni Basin constitutes a transitional area between the two sub-systems.
The geology of Cameroon is almost universally Precambrian metamorphic and igneous basement rock, formed in the Archean as part of the Congo Craton and the Central African Mobile Zone and covered in laterite, recent sediments and soils. Some parts of the country have sequences of sedimentary rocks from the Paleozoic, Mesozoic and Cenozoic as well as volcanic rock produced by the 1600 kilometer Cameroon Volcanic Line, which includes the still-active Mount Cameroon. The country is notable for gold, diamonds and some onshore and offshore oil and gas.
The geology of Sudan formed primarily in the Precambrian, as igneous and metamorphic crystalline basement rock. Ancient terranes and inliers were intruded with granites, granitoids as well as volcanic rocks. Units of all types were deformed, reactivated, intruded and metamorphosed during the Proterozoic Pan-African orogeny. Dramatic sheet flow erosion prevented almost any sedimentary rocks from forming during the Paleozoic and Mesozoic. From the Mesozoic into the Cenozoic the formation of the Red Sea depression and complex faulting led to massive sediment deposition in some locations and regional volcanism. Sudan has petroleum, chromite, salt, gold, limestone and other natural resources.
The Canadian Arctic Rift System is a major North American geological structure extending from the Labrador Sea in the southeast through Davis Strait, Baffin Bay and the Arctic Archipelago in the northwest. It consists of a series of interconnected rifts that formed during the Paleozoic, Mesozoic and Cenozoic eras. Extensional stresses along the entire length of the rift system have resulted in a variety of tectonic features, including grabens, half-grabens, basins and faults.
The plate theory is a model of volcanism that attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field. The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of the stress field are:
Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.