Saharan Metacraton
Encyclopedia
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 event
, but where the characteristics of the original craton are still identifiable. The geology of the continent has only been partially explored, and other names have been used to describe the general area that reflect different views of its nature and extent. These include "Nile Craton", "Sahara Congo Craton", "Eastern Saharan Craton" and "Central Saharan Ghost Craton".
This last term is because the older rocks are almost completely covered by recent sediments and desert sands, making geological analysis difficult.
to the west, the Congo craton
to the south, the Arabian-Nubian Shield
to the east, and the northern African continental margin.
The southern boundary is not well-defined, but may be considered to run along the northern edge of the Oubangides orogenic belt in the southwest and the Aswa Shear Zone further east.
It lies under southern Egypt
and Libya
and western Sudan
, northern Kenya
, Uganda
and Congo
, the Central African Republic
, Cameroon
and eastern Nigeria
and Chad
.
The oldest rocks in Egypt date to about 2700 Ma.
Rocks in southern Libya and eastern Chad have been dated between 2900 Ma and 2600 Ma.
Some geologists date a rifting episode to between 1200 Ma and 950 Ma.
The Pan-African orogeny
saw the cratons that had formed the Rodinia
super-continent come together in a completely new arrangement into the Gondwana
super-continent around 500 Ma.
The "Pharusian Ocean
", which separated the West African craton from the Saharan craton around 800 Ma, began to close about 730 Ma, and about 635 Ma these two cratons collided, with the basement of the Saharan craton undergoing extensive reworking in the process.
The Saharan craton may have been decratonized, perhaps oceanized, during the Pan-African event.
At the end of this period the craton was extended and rifts appeared.
By about 500 Ha the Saharan craton was sutured with the Congo craton during the final assembly of Gondwana
. Although Gondwana later split up, the core African Plate
remained intact.
A different theory of the area's evolution has the Nile craton in southern Libya and eastern Chad as an extension of the Congo craton, remaining as an intact unit during the break-up of Rodinia between 1000 Ma and 700 Ma, and during the subsequent Pan-African event.
In this scenario, a fragment of craton found in the Hoggar Mountains, dating from the late Proterozoic
, is called the East Saharan craton, originally far from the Nile craton with which it was joined only during the Pan-African event.
Perhaps the simplest view is that the "craton" is an assembly of microcontinental blocks with different origins that were swept together during the Pan African orogeny.
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
in the north-central part of Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
. Where a craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
is an old and stable part of the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
, the term "metacraton" is used to describe a craton that has been remobilized during an orogenic event
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
, but where the characteristics of the original craton are still identifiable. The geology of the continent has only been partially explored, and other names have been used to describe the general area that reflect different views of its nature and extent. These include "Nile Craton", "Sahara Congo Craton", "Eastern Saharan Craton" and "Central Saharan Ghost Craton".
This last term is because the older rocks are almost completely covered by recent sediments and desert sands, making geological analysis difficult.
Extent
The "metacraton" covers an area of about 5,000,000 km2. It lies between the Tuareg shieldTuareg Shield
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...
to the west, the Congo craton
Congo craton
The Congo craton, covered by the Palaeozoic-to-recent Congo basin, is an ancient Precambrian craton that with four others makes up the modern continent of Africa. These cratons were formed between about 3.6 and 2.0 billion years ago and have been tectonically stable since that time...
to the south, the Arabian-Nubian Shield
Arabian-Nubian Shield
The Arabian-Nubian Shield is an exposure of Precambrian crystalline rocks on the flanks of the Red Sea. The crystalline rocks are mostly Neoproterozoic in age. Geographically - and from north to south - the ANS includes the nations of Israel, Jordan. Egypt, Saudi Arabia, Sudan, Eritrea,...
to the east, and the northern African continental margin.
The southern boundary is not well-defined, but may be considered to run along the northern edge of the Oubangides orogenic belt in the southwest and the Aswa Shear Zone further east.
It lies under southern Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
and Libya
Libya
Libya is an African country in the Maghreb region of North Africa bordered by the Mediterranean Sea to the north, Egypt to the east, Sudan to the southeast, Chad and Niger to the south, and Algeria and Tunisia to the west....
and western 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...
, northern Kenya
Kenya
Kenya , officially known as the Republic of Kenya, is a country in East Africa that lies on the equator, with the Indian Ocean to its south-east...
, Uganda
Uganda
Uganda , officially the Republic of Uganda, is a landlocked country in East Africa. Uganda is also known as the "Pearl of Africa". It is bordered on the east by Kenya, on the north by South Sudan, on the west by the Democratic Republic of the Congo, on the southwest by Rwanda, and on the south by...
and Congo
Democratic Republic of the Congo
The Democratic Republic of the Congo is a state located in Central Africa. It is the second largest country in Africa by area and the eleventh largest in the world...
, the 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 ,...
, 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...
and eastern Nigeria
Nigeria
Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...
and 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...
.
Chronology
The craton may have formed during a period of accelerated crustal growth between 3000 Ma and 2000 Ma (Ma: million years ago).The oldest rocks in Egypt date to about 2700 Ma.
Rocks in southern Libya and eastern Chad have been dated between 2900 Ma and 2600 Ma.
Some geologists date a rifting episode to between 1200 Ma and 950 Ma.
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....
saw the cratons that had formed the Rodinia
Rodinia
In geology, Rodinia is the name of a supercontinent, a continent which contained most or all of Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago, in the Neoproterozoic era...
super-continent come together in a completely new arrangement into the Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
super-continent around 500 Ma.
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:...
", which separated the West African craton from the Saharan craton around 800 Ma, began to close about 730 Ma, and about 635 Ma these two cratons collided, with the basement of the Saharan craton undergoing extensive reworking in the process.
The Saharan craton may have been decratonized, perhaps oceanized, during the Pan-African event.
At the end of this period the craton was extended and rifts appeared.
By about 500 Ha the Saharan craton was sutured with the Congo craton during the final assembly of Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
. Although Gondwana later split up, the core African Plate
African Plate
The African Plate is a tectonic plate which includes the continent of Africa, as well as oceanic crust which lies between the continent and various surrounding ocean ridges.-Boundaries:...
remained intact.
A different theory of the area's evolution has the Nile craton in southern Libya and eastern Chad as an extension of the Congo craton, remaining as an intact unit during the break-up of Rodinia between 1000 Ma and 700 Ma, and during the subsequent Pan-African event.
In this scenario, a fragment of craton found in the Hoggar Mountains, dating from the late Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
, is called the East Saharan craton, originally far from the Nile craton with which it was joined only during the Pan-African event.
Perhaps the simplest view is that the "craton" is an assembly of microcontinental blocks with different origins that were swept together during the Pan African orogeny.