The Tuareg Shield is a geological formation lying between the West African craton and the Saharan Metacraton in

West Africa West Africa, also known as Western Africa, is the westernmost region of Africa. The United Nations geoscheme for Africa#Western Africa, United Nations defines Western Africa as the 16 countries of Benin, Burkina Faso, Cape Verde, The Gambia, Gha ...

. Named after the

Tuareg people The Tuareg people (; also spelled Twareg or Touareg; Endonym and exonym, endonym, depending on Tuareg languages#Subclassification, variety: ''Imuhaɣ'', ''Imušaɣ'', ''Imašeɣăn'' or ''Imajeɣăn'') are a large Berbers, Berber ethnic group, ...

, it has complex a geology, reflecting the collision between these

craton A craton ( , , or ; from "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of contine ...

s and later events. The landmass covers parts of

Algeria Algeria, officially the People's Democratic Republic of Algeria, is a country in the Maghreb region of North Africa. It is bordered to Algeria–Tunisia border, the northeast by Tunisia; to Algeria–Libya border, the east by Libya; to Alger ...

Niger Niger, officially the Republic of the Niger, is a landlocked country in West Africa. It is a unitary state Geography of Niger#Political geography, bordered by Libya to the Libya–Niger border, north-east, Chad to the Chad–Niger border, east ...

and

Mali Mali, officially the Republic of Mali, is a landlocked country in West Africa. It is the List of African countries by area, eighth-largest country in Africa, with an area of over . The country is bordered to the north by Algeria, to the east b ...

Origins

The Tuareg Shield is mainly composed of

Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

Paleoproterozoic The Paleoproterozoic Era (also spelled Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the Proterozoic eon, and also the longest era of the Earth's geological history, spanning from (2.5–1.6 Ga). It is further sub ...

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its d ...

s and

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

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 arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...

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 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. This orogeny is also known as the Pan-Gondwanan or Saldanian Oro ...

. The western part of the

Hoggar Mountains The Hoggar Mountains (; Berber: ''idurar n Ahaggar'') are a highland region in the central Sahara in southern Algeria, along the Tropic of Cancer. The mountains cover an area of approximately 550,000 km2. The Hoggar Mountains are home to ...

(Ahaggar Mountains) is made of material from the

Pharusian Ocean The Pharusian Ocean (; ) is an ancient ocean that existed on Earth from , between the break-up of the Rodinia supercontinent and the start of formation of the Pannotia supercontinent. Opening and closure The Pharusian Ocean opened around 800 milli ...

. This comprises oceanic basalts, arc volcanic and sedimentary rocks and sediments, which were shed into the Pharusian Ocean 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 Mountains in

Adrar des Ifoghas The Adrar des Ifoghas (also Adrar des Iforas; Tamasheq: ⴰⴷⵔⴰⵔ ⵏ ⵉⴼⵓⵖⴰⵙ in Tifinagh; Adrar n Ifoghas; Ifoghas' Mountains) is a massif located in the Kidal Region of Mali, reaching into Algeria. It has an area of aroun ...

and

Aïr Mountains The Aïr Mountains or Aïr Massif (Air Tamajeq language, Tamajăq: ''Ayǝr''; Hausa language, Hausa: Eastern ''Azbin'', Western ''Abzin'') is a triangular massif, located in northern Niger, within the Sahara. Part of the West Sa ...

. The Trans-Saharan

orogenic Orogeny () is a mountain-building process that takes place at a convergent plate margin when plate motion compresses the margin. An or develops as the compressed plate crumples and is uplifted to form one or more mountain ranges. This involv ...

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

Phanerozoic The Phanerozoic is the current and the latest of the four eon (geology), geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and ...

the shield was eroded and covered by

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

sandstones. More recently, around 95 Ma - 90 Ma, the area was partly submerged by a seaway linking the

Mediterranean The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern ...

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 The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...

basement overlaid with volcanic rocks.

References

{{Major African geological formations Geology of Africa Historical geology Sahara Shields (geology)