Tuareg Shield
Encyclopedia
The Tuareg Shield is an area lying between the West African craton
and the Saharan Metacraton
in West Africa
. It has complex geology, reflecting the collision between these cratons and later events. It covers parts of Algeria
, Niger
and Mali
.
or Paleoproterozoic
terrane
s and Neoproterozoic
terranes that amalgamated during the Pan African orogeny when the West African craton and the Saharan metacraton converged.
The shield originated in oceanic island arc
s that formed on the cratons during 900 Ma to 680 Ma, of which relics remain as thrust sheets on top of more rigid bodies.
In Mali, the Tilemsi belt is a complex of intraoceanic arcs that formed above a subduction zone sloping down to the east and were accreted to the shield during the Pan-African orogeny
.
The western part of the Hoggar massif is made of material from the Pharusian Ocean
including oceanic basalts, arc volcanic and sedimentary rocks and sediments that were shed into the Pharusian Ocean
by the West African craton and the eastern Hoggar.
These blocks were separated by subvertical shear zones.
The fractured shield was penetrated by volcanic magmas that formed the Hoggar in Algeria
, Adrar des Iforas in Mali
and Aïr Mountains
in Niger
.
The Trans-Saharan orogenic belt which lies along the eastern margin of the West African craton is generally thought to have been caused by the collision between that craton and the Tuareg shield.
the shield was eroded and covered by Ordovician
sandstones. More recently, around 95 Ma - 90 Ma, the area was partly submerged by a seaway linking the Mediterranean and the proto-Atlantic. Since then, starting around 30 Ma the area rose by as much as 3000 meters.
Volcanic activity started around 35 Ma - 30 Ma, and continues sporadically to this day.
The higher areas consist of the uplifted Precambrian
basement overlaid with volcanic rocks.
West African craton
The West African craton is one of the five large masses, or cratons, of the Precambrian basement rock of Africa that make up the African Plate, the others being the Kalahari craton, Congo craton, Saharan Metacraton and Tanzania craton. These land masses came together in the late Precambrian and...
and the Saharan Metacraton
Saharan Metacraton
The Saharan Metacraton is a term used by some geologists to describe a large area of continental crust in the north-central part of Africa. Where a craton is an old and stable part of the lithosphere, the term "metacraton" is used to describe a craton that has been remobilized during an orogenic...
in West Africa
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...
. It has complex geology, reflecting the collision between these cratons and later events. It covers parts of Algeria
Algeria
Algeria , officially the People's Democratic Republic of Algeria , also formally referred to as the Democratic and Popular Republic of Algeria, is a country in the Maghreb region of Northwest Africa with Algiers as its capital.In terms of land area, it is the largest country in Africa and the Arab...
, Niger
Niger
Niger , officially named the Republic of Niger, is a landlocked country in Western Africa, named after the Niger River. It borders Nigeria and Benin to the south, Burkina Faso and Mali to the west, Algeria and Libya to the north and Chad to the east...
and Mali
Mali
Mali , officially the Republic of Mali , is a landlocked country in Western Africa. Mali borders Algeria on the north, Niger on the east, Burkina Faso and the Côte d'Ivoire on the south, Guinea on the south-west, and Senegal and Mauritania on the west. Its size is just over 1,240,000 km² with...
.
Origins
The shield is mainly composed of ArcheanArchean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
or Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
s and Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
terranes that amalgamated during the Pan African orogeny when the West African craton and the Saharan metacraton converged.
The shield originated in oceanic island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
s that formed on the cratons during 900 Ma to 680 Ma, of which relics remain as thrust sheets on top of more rigid bodies.
In Mali, the Tilemsi belt is a complex of intraoceanic arcs that formed above a subduction zone sloping down to the east and were accreted to the shield during the Pan-African orogeny
Pan-African orogeny
The Pan-African orogeny was a series of major Neoproterozoic orogenic events which related to the formation of the supercontinents Gondwana and Pannotia about 600 million years ago....
.
The western part of the Hoggar massif is made of material from the Pharusian Ocean
Pharusian Ocean
The Pharusian Ocean is an ancient ocean that existed from 800 to 635 million years ago, between the break-up of the Rodinia supercontinent and the start of formation of the Gondwana supercontinent.-Opening and closure:...
including oceanic basalts, arc volcanic and sedimentary rocks and sediments that were shed into the Pharusian Ocean
Pharusian Ocean
The Pharusian Ocean is an ancient ocean that existed from 800 to 635 million years ago, between the break-up of the Rodinia supercontinent and the start of formation of the Gondwana supercontinent.-Opening and closure:...
by the West African craton and the eastern Hoggar.
Fracturing
During the oblique collision of the West African craton with the Saharan metacraton, the shield was torn into separate but rigid moving blocks.These blocks were separated by subvertical shear zones.
The fractured shield was penetrated by volcanic magmas that formed the Hoggar in Algeria
Algeria
Algeria , officially the People's Democratic Republic of Algeria , also formally referred to as the Democratic and Popular Republic of Algeria, is a country in the Maghreb region of Northwest Africa with Algiers as its capital.In terms of land area, it is the largest country in Africa and the Arab...
, Adrar des Iforas in Mali
Mali
Mali , officially the Republic of Mali , is a landlocked country in Western Africa. Mali borders Algeria on the north, Niger on the east, Burkina Faso and the Côte d'Ivoire on the south, Guinea on the south-west, and Senegal and Mauritania on the west. Its size is just over 1,240,000 km² with...
and Aïr Mountains
Aïr Mountains
The Aïr Mountains is a triangular massif, located in northern Niger, within the Sahara desert...
in Niger
Niger
Niger , officially named the Republic of Niger, is a landlocked country in Western Africa, named after the Niger River. It borders Nigeria and Benin to the south, Burkina Faso and Mali to the west, Algeria and Libya to the north and Chad to the east...
.
The Trans-Saharan orogenic belt which lies along the eastern margin of the West African craton is generally thought to have been caused by the collision between that craton and the Tuareg shield.
Later events
At the start of the PhanerozoicPhanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
the shield was eroded and covered by Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
sandstones. More recently, around 95 Ma - 90 Ma, the area was partly submerged by a seaway linking the Mediterranean and the proto-Atlantic. Since then, starting around 30 Ma the area rose by as much as 3000 meters.
Volcanic activity started around 35 Ma - 30 Ma, and continues sporadically to this day.
The higher areas consist of the uplifted Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
basement overlaid with volcanic rocks.