Arabian-Nubian Shield: Wikis

  

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The Arabian-Nubian Shield (ANS) 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, Ethiopia, Yemen, and Somalia. The ANS in the north is exposed as part of the Sahara Desert and Arabian Desert, and in the south in the Ethiopian Highlands, Asir province of Arabia and Yemen Highlands.

True-color Aqua MODIS image from September 29, 2004, showing the Red Sea and surrounding deserts of NE Africa and Arabia. Clockwise from upper left, the nations are Egypt, Israel, Jordan, Saudi Arabia, Yemen, Eritrea (on the coast of the Red Sea), Ethiopia (bottom image edge), and Sudan. The Nile River flows northwards through Egypt and Sudan in the upper left.

The ANS was the site of some of man's earliest geologic efforts, principally by the Egyptians to extract gold from the rocks of Egypt and NE Sudan. This was the most easily worked of all metals and does not tarnish. All of the gold deposits in Egypt and northern Sudan were found and exploited by Egyptians, but new gold discoveries have been found in Sudan, Eritrea, and Saudi Arabia. Pharonic Egyptians also quarried granite near Aswan and floated this down the Nile to be used as facing for the pyramids. The earliest preserved geologic map was made in 1150 BCE to show the location of gold deposits in Eastern Egypt; it is known as the Turin papyrus. The Greek name for Aswan, Syene; is the type locality for the igneous rock syenite. The Romans followed this tradition and had many quarries especially in the northern part of the Eastern Desert of Egypt where porphyry and granite were mined and shaped for shipment. Precious and industrial metals, including gold, silver, copper, zinc, tin, and lead, have been mined in Saudi Arabia for at least 5,000 years. The most productive mine in Saudi Arabia, Mahd adh Dhahab ("Cradle of Gold"), has been periodically exploited for its mineral wealth for hundreds or even thousands of years and is reputed to be the original source of King Solomon's gold. Today, mining at Mahd adh Dhahab is conducted by the Saudi Arabian Mining Company, Ma'aden.

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Red Sea rifting and exposure of the Arabian-Nubian Shield

The Arabian-Nubian Shield is representative of the continental crust that underlies NE Africa, SW Asia, and Arabia. Geophysical studies inform us that ANS crust is about 40–45 km thick, typical for continental crust elsewhere. Geologists are interested to study this crust because it provides an outstanding opportunity to learn how continental crust forms. Elsewhere in Arabia and NE Africa the crust is buried beneath kilometers of younger, Phanerozoic sediments and can only be reached by scientific drilling. Glaciers scoured this land during one of the Neoproterozoic Snowball earth episodes (~630 Ma) and the remarkable Arabian-Nubian peneplain is overlain by the greatest sand sheet on Earth, the Cambro-Ordovician Siq/Saq sands. This is exposed in spectacular cliffs like the site of Petra in Jordan. The Arabian sandstones are overlain by thick deposits of younger sediments that dip and thicken eastwards into the Persian Gulf. These sands are missing in most of Egypt and Sudan, instead the ANS is overlain by the late Mesozoic Nubian sandstones. All of these sediments were peeled off by erosion to expose the ANS, and much of this sand was reworked by desert winds to make great sand dunes in western Egypt and the Ar Rub al Khali (Empty Quarter) of Arabia. The embryonic ocean called the Red Sea has slowly widened by rifting over the past 25 million years. Rifting typically shoulders up its margins, and the covering sediments were removed by erosion as the Red Sea widened and its margins lifted up, allowing us to examine the crust of this region. This examination is favored also by the desert climate of the region - there is no soil, little vegetation, and few cities or roads to obscure the geology.

Basic dyke cutting granite, Arabian-Nubian Shield

Formation of Arabian-Nubian Shield crust

The Arabian-Nubian Shield is the northern half of a great collision zone called the East African Orogen. This collision zone formed near the end of Neoproterozoic time when East and West Gondwana collided to form the supercontinent Gondwana. The most intense part of the collision occurred in what is today southern Africa, where older crust in Tanzania. Mozambique, and Madagascar was remobilized to form the Mozambique Belt. This great collision was responsible for the Pan-African orogeny near the end of Neoproterozoic time. The crust of the Mozambique Belt is quite different from that of the Arabian-Nubian Shield, which is predominantly 'juvenile' crust, that is, crust that formed from partial melting of Earth's mantle, although much older Archean and Paleoproterozoic crustal materials is exposed west of the Nile in Egypt, in the SE part of the shield in Arabia, in eastern Ethiopia, and in Yemen.

The ANS took about 300 million years to form. The oldest rocks associated with the formative cycle of the ANS crust formed by coalescence of Island arcs and back-arc basins and perhaps oceanic plateaus. The oldest rocks associated with this cycle are about 870 million years old and are found in eastern Sudan and SE Arabia. Some of the oldest rocks are ophiolites, which testify that formation of ANS continental crust began with formation of oceanic crust by seafloor spreading, followed by the development of subduction zones and Island arcs. The various island arcs collided and these tectonic terranes sutured together during the time period 780 to 620 Ma to form an increasingly broad and thick nucleus of juvenile continental crust. This thickening resulted in the formation of several suture zones, marked by obduction of ophiolites and intense deformation. Crustal thickening was also accompanied by melting and magmatic fractionation of mafic magmas that ponded deep in the crust. These melts rose upwards to be emplaced as granitic plutons. Magmatism during this episode is characterized by tholeiites and calc-alkaline suites.

The welt of juvenile ANS crust was trapped between great tracts of converging continental crust. A protracted episode of continental collision started at about 610 Ma ago and continued for about 50 million years. Collision was more intense in the south, in the Mozambique Belt, but it also strongly affected the ANS. N-S oriented upright folds and shear zones deformed the arc terranes and sutures in the southern ANS, forming elongate structures such as the Hamisana Shear Zone in NE Sudan. Farther north and east, the ANS was affected by the formation of the great NW-SE trending Najd system of strike-slip faults. The composition of igneous rocks became distinctively more evolved as the collision continued and the crust continued to thicken. Deep erosion, possibly by a continental ice sheet, happened during this time. All tectonic and magmatic activity ended by the time the Cambrian sandstones were deposited, about 530 million years ago.

A number of features have been ascribed to late stage extensional tectonics including a widespread NE-SW trending dyke swarm, NE-SW trending normal faults and NW-SE trending sedimentary basins filled with post-orogenic molasse deposits [1]

Astronaut photograph (ISS006-E-43186) of the Arabian-Nubian Shield in eastern Sudan, looking NE, with the Red Sea in the background. The N-S structure in the center of the image is the Hamisana Shear Zone. The ANS exposures in eastern Egypt to the north can also be seen, as well as part of the Nile (left) and Arabia (right).
Astronaut photograph (STS088-719-3) of N-S trending Hamisana Shear Zone, NE Sudan, illustrating typical patterns of folding and faulting in the ANS. South is towards the upper left corner of this image. Semicircular features are granitic plutons.

References

  1. ^ Blasband,B., White,S., Brooijmans,P., De Boorder,H. & Visser,W. 2000. Late Proterozoic extensional collapse in the Arabian-Nubian Shield. Journal of the Geological Society, 157, 615-628. [1]
  • P. R. Johnson and B. Woldehaimanot, 2003. 'Development of the Arabian-Nubian Shield: perspectives on accretion and deformation in the northern East African Orogen and the assembly of Gondwana.' In M. Yoshida, S. Dasgupta, and B. Windley (eds) Proterozoic East Gondwana: Supercontinent Assembly and Breakup. Geological Society of London Special Publication 206, pp. 289-325.

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