- Arabian-Nubian Shield
The Arabian-Nubian Shield (ANS) is an exposure of
Precambriancrystalline rocks on the flanks of the Red Sea. The crystalline rocks are mostly Neoproterozoicin 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 Desertand Arabian Desert, and in the south in the Ethiopian Highlands, Asirprovince of Arabia and Yemen Highlands.
The ANS was the site of some of man's earliest geologic efforts, principally by the
Egyptiansto extract goldfrom 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 granitenear Aswanand floated this down the Nile to be used as facing for the pyramids. The earliest preserved geologic mapwas 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 granitewere 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.
Red Sea rifting and exposure of the Arabian-Nubian Shield
The Arabian-Nubian Shield is representative of the
continental crustthat 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. Geologistsare 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, Phanerozoicsediments and can only be reached by scientific drilling. Glaciers scoured this land during one of the Neoproterozoic Snowball earthepisodes (~630 Ma) and the remarkable Arabian-Nubian peneplainis overlain by the greatest sand sheet on Earth, the Cambro-Ordovician Siq/Saq sands. This is exposed in spectacular cliffs like the site of Petrain 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 dunesin western Egypt and the Ar Rub al Khali ( Empty Quarter) of Arabia.The embryonic ocean called the Red Seahas slowly widened by riftingover 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.
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 Madagascarwas remobilized to form the Mozambique Belt. This great collision was responsible for the Pan-African orogenynear 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 Archeanand Paleoproterozoiccrustal 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 crustbegan with formation of oceanic crustby seafloor spreading, followed by the development of subduction zonesand 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 obductionof 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 graniticplutons. Magmatism during this episode is characterized by tholeiites and calc-alkalinesuites.
The welt of juvenile ANS crust was trapped between great tracts of converging continental crust. A protracted episode of
continental collisionstarted 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 tectonicsincluding a widespread NE-SW trending dyke swarm, NE-SW trending normal faults and NW-SE trending sedimentary basinsfilled with post-orogenic molassedeposits [ 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. [http://www.utdallas.edu/~dxt038000/Egypt%20Website/PDFs/General/BlasbandJGSL00.pdf] ]
*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.
* [http://weekly.ahram.org.eg/1999/417/special.htm The Eastern Desert of Egypt in Ancient Times]
* [http://nabataea.net/ppark.html The Petra Archeological Park; Petra was carved into lower Paleozoic sandstone.]
* [http://www.gondarlink.org.uk/guide/travelguide/tigray.shtml The Rock-Hewn Churches of Tigre (N. Ethiopia); these are in lower Paleozoic sandstone]
* [http://www.dmmr.gov.sa/geology-ksa.php Geology of Saudi Arabia]
* [http://www.pubs.usgs.gov/gip/gold/gold.pdf USGS pamphlet on gold, including early mining in Egypt and Arabia]
* [http://www.sgs.org.sa/ Website for the Saudi Geological Survey, which does a lot of work on the Arabian-Nubian Shield.]
* [http://whc.unesco.org/en/list/954 UNESCO World Heritage site for St. Catherine monastery area. This is built upon ANS exposures in Sinai.]
* [http://www.metmuseum.org/toah/hd/aksu/hd_aksu_1.htm Axum, Ethiopia. This ancient capitol was built on ANS exposures.]
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