Central African Shear Zone
Encyclopedia
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
.
The structure is not well understood.
As of 2008 there was still no general agreement about how the individual shears along the lineament
link up.
dates to at least 640 MA (million years ago).
Motion occurred along the zone during the break-up of Gondwanaland in the Jurassic
and Cretaceous
ages.
Some of the faults in the zone were rejuvenated more than once before and during the opening of the South Atlantic in the Cretaceous
era.
The Pernambuco fault
in Brazil
is a continuation of the shear zone to the west.
In Cameroon, the CASZ cuts across the Adamawa uplist, a post-Cretaeous formation. The Benue Trough
lies to the north, and the Foumban Shear Zone
to the south.
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.
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.
The Mesozoic
and Tertiary
movements have produced elongated rift basins in central Cameroon, northern Central African Republic
and southern Chad
.
region of western Sudan.
It is now known to extend into central and eastern Sudan, with a total length of 4,000 km.
In the Sudan, the shear zone may have acted as a structural barrier to development of deep Cretaceous-Terriary sedimentary basins in the north of the area.
Objections to this theory are that the Bahr el Arab
and Blue Nile rift
s extend northwest beyond one proposed line for the shear zone.
However, the alignment of the northwestern ends of the rifts in this areas supports the theory.
Gulf of Guinea
The Gulf of Guinea is the northeasternmost part of the tropical Atlantic Ocean between Cape Lopez in Gabon, north and west to Cape Palmas in Liberia. The intersection of the Equator and Prime Meridian is in the gulf....
through Cameroon
Cameroon
Cameroon, officially the Republic of Cameroon , is a country in west Central Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the...
into Sudan
Sudan
Sudan , officially the Republic of the Sudan , is a country in North Africa, sometimes considered part of the Middle East politically. It is bordered by Egypt to the north, the Red Sea to the northeast, Eritrea and Ethiopia to the east, South Sudan to the south, the Central African Republic to the...
.
The structure is not well understood.
As of 2008 there was still no general agreement about how the individual shears along the lineament
Lineament
See also Line A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will comprise a fault-aligned valley, a series of fault or fold-aligned hills, a straight coastline or indeed a combination of these...
link up.
Description
The shear zoneShear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
dates to at least 640 MA (million years ago).
Motion occurred along the zone during the break-up of Gondwanaland in the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
and Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
ages.
Some of the faults in the zone were rejuvenated more than once before and during the opening of the South Atlantic in the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
era.
The Pernambuco fault
Pernambuco Fault
The Pernambuco fault or Pernambuco Shear Zone in northeastern Brazil is a splay fault radiating from the Transbrasiliano lineament eastwards to the coast, a major geological feature....
in Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
is a continuation of the shear zone to the west.
In Cameroon, the CASZ cuts across the Adamawa uplist, a post-Cretaeous formation. The Benue Trough
Benue Trough
The Benue Trough is a major geological formation underlying a large part of Nigeria and extending about 1,000 km northeast from the Bight of Benin to Lake Chad.It is part of the broader Central African Rift System.-Location:...
lies to the north, and the Foumban Shear Zone
Foumban Shear Zone
The Foumban Shear Zone, or Central Cameroon Shear Zone , is a fault zone in Cameroon that has been identified with the Pernambuco fault in northeastern Brazil, which splays from the Trans-Brazilian Lineament....
to the south.
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.
The lithosphere beneath the CASZ in this area is thinned in a relatively narrow belt, with the asthenosphere
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
upwelling from a depth of about 190 km to about 120 km.
The Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
and Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
movements have produced elongated rift basins in central Cameroon, northern Central African Republic
Central African Republic
The Central African Republic , is a landlocked country in Central Africa. It borders Chad in the north, Sudan in the north east, South Sudan in the east, the Democratic Republic of the Congo and the Republic of the Congo in the south, and Cameroon in the west. The CAR covers a land area of about ,...
and southern Chad
Chad
Chad , officially known as the Republic of Chad, is a landlocked country in Central Africa. It is bordered by Libya to the north, Sudan to the east, the Central African Republic to the south, Cameroon and Nigeria to the southwest, and Niger to the west...
.
Sudan
The CASZ was formerly thought to extend eastward only to the DarfurDarfur
Darfur is a region in western Sudan. An independent sultanate for several hundred years, it was incorporated into Sudan by Anglo-Egyptian forces in 1916. The region is divided into three federal states: West Darfur, South Darfur, and North Darfur...
region of western Sudan.
It is now known to extend into central and eastern Sudan, with a total length of 4,000 km.
In the Sudan, the shear zone may have acted as a structural barrier to development of deep Cretaceous-Terriary sedimentary basins in the north of the area.
Objections to this theory are that the Bahr el Arab
Bahr el Arab rift
The Bahr el Arab rift is a major geological feature in the southwest Sudan.The Bahr el Arab rift is made up of the Baggara graben, between the Central African Republic and the Nuba Mountains to the east, and the Sudd graben further south...
and Blue Nile rift
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....
s extend northwest beyond one proposed line for the shear zone.
However, the alignment of the northwestern ends of the rifts in this areas supports the theory.