A terrane boundary close to the Nile in the Sudan, detected by radar from the Space Shuttle: the Keraf Suture. From NASA
Undoubtedly the best exposed and one of the biggest examples, the accretionary orogen of the Arabian-Nubian Shield (ANS) is a witness to the creation of a supercontinent from the remnants of an earlier one. At about 1 Ga, most of the Earth’s continental material was clumped together in the Rodinia supercontinent that existed for a quarter of a billion years. At a time of massive mantle upheaval that left most crust of that age affected by basaltic magmatism, in the form of lava flows and dyke swarms, Rodinia began to break up at 800 Ma to scatter continental fragments. Subduction zone accommodated this continental drift to form many ocean and continental-margin volcanic arcs. The ANS is a repository for many of these arcs which episodically accreted between earlier cratons to the west in Africa and those comprising Somalia and the present Indian subcontinent. Primarily the terranes are oceanic in origin and formed in the aftermath of the dismemberment of Rodinia, although a few slivers of older, reworked crust occur in Saudi Arabia and Yemen. Among the various components are ophiolites marking sutures and other major tectonic features of the orogen. The shape of the Shield is unlike that of any other major orogen of later times, for it shrinks from a width estimated at ~2000 km in Arabia to the north to vanish just south of the Equator in southern Kenya. This ‘pinched’ structure has suggested to some that the bulk of the new crust was forced laterally northwards when the African and Indian cratons collided, in the manner of toothpaste from a trodden-on tube.
Today the ANS is a harsh place, some off-limits to geologists either for political reasons or the sheer hostility and remoteness of the environment. Yet a picture has emerged, bit by bit, over the last 30 years. So a detailed review of the most extensive and varied part from 7° to 32°N and 26° to 50°E – in Egypt, Saudia Arabia, eastern Sudan, Eritrea, Yemen and northern Ethiopia is especially welcome (Johnson, P.R. et al. 2011. Late Cryogenian–Ediacaran history of the Arabian–Nubian Shield: A review of depositional, plutonic, structural, and tectonic events in the closing stages of the northern East African Orogen. Journal of African Earth Sciences, v. 61, p. 167-232). Peter Johnson himself compiled a vast amount of information during his career with the US Geological Survey Mission in Saudi Arabia and has blended the inevitably diverse ideas of his 7 co-workers – but by no means all the ideas that are in the literature. The result is a readable and well illustrated account of how the ANS assembled tectonically during times when a near-global glaciation took place, and the first macroscopic animals appear in the fossil record. Tillites and other glaciogenic rocks from the Marinoan ‘SnowBall’ occur from place to place in the ANS, as do banded iron formations that made a surprise return after a billion-year or longer absence in the Cryogenian Period . Coincidentally, glacial conditions returned to the region twice in Ordovician and Carboniferous to Permian times, forming distinctive, tectonically undisturbed sediments in the Phanerozoic cover that unconformably overlies the Neoproterozoic orogen.
Except in a few areas only recently explored, geologists have assiduously dated events in the ANS, showing nicely that all the basement rock formed after 800 Ma, and that orogenic events culminated before the start of the Cambrian period, although one or two unusual granites intruded as late as the Ordovician. The deformation is immense in places, with huge nappes, often strike-slip shear zones and exposure ranging from the lowest metamorphic grade to that in which water and granitic magma was driven from the lower Pan African crust. The range of exposed crustal levels stems partly from the tectonics, but owes a lot to the 2-3 km of modern topographic relief, unique to NE Africa and Arabia. Yet it is not uncommon to come upon delicate features such as pillowed lavas, conglomerates and finely laminated volcanoclastic tuffs. Following tectonic welding, more brittle deformation opened subsiding basins that contain exclusively sedimentary rocks derived from the newly uplifted crust, both marine and terrestrial in formation (basins of this type, in Eritrea and Ethiopia, unfortunately do not figure in the regional maps). Much of the ANS is currently the object of a gold rush, encouraged by a rising world price for the ‘inflation-proof’ comfort blanket provided by the yellow metal. Consequently, newcomers to the stampede will be well advised to mug-up on the regional picture of occurrences and gold-favourable geology provided in the review, and may be interested by other exploration possibilities for rare-earth metals and other rising stars on the London Metal Exchange, such as tin, which are often hosted in evolved granites, that stud the whole region.