The East African Orogen: Neoproterozoic tectonics on display

Over a period of about 300 Ma the fragmentation of a supercontinent, Rodinia, drove a round of sea-floor spreading and continental drift that culminated in reassembly of the older continental pieces and entirely new crust in a new supercontinent, Gondwana. The largest source of evidence for this remarkable tectonic turnaround is a belt stretching N-S for over 3000 km from southern Israel through East Africa to Mozambique. At its widest the belt exposes Neoproterozoic  rocks and structures for some 1700 km E-W from west of the Nile in northern Sudan almost to Riyadh in Saudi Arabia.  This Arabian-Nubian Shield tapers southwards to thin out completely in northern Tanzania between far older cratons and in a state of high-grade metamorphism.

This East African Orogen has long been considered the best exposed bowels of former mountain building that there are: results of continent-continent collision and the bulldozing together of many oceanic arcs and remnants of oceanic lithosphere that once separated the cratons. This was much more complex than a case of head-on tectonics, the northward-swelling Arabian-Nubian Shield showing all the signs of being like a gigantic ‘pip’ squeezed out northwards from two cratonic jaws during the last stages of what is often called the Pan African Orogeny. Interestingly, the line of the orogen is roughly followed by East Africa’s other giant feature, the Rift Valley; actually two of them following Pan African terranes. A continental scale anisotropy has been reactivated and subject to extensional tectonics, and maybe in future a new round of sea-floor spreading as has begun in the Red Sea, some half a billion years after it formed.

Simplified geological map of the East African Orogen courtesy of the authors of Fritz et al 2013
Simplified geological map of the East African Orogen courtesy of the authors of Fritz et al 2013

Now there is an opportunity for anyone to download and read a digest of East African orogenic processes compiled by researchers from several countries along the belt and their colleagues from North America, Europe and Australia who have been privileged to work in this vast area (Fritz, H and 13 others 2013. Orogen styles in the East African orogen: A review of the Neoproterozoic to Cambrian tectonic evolution. Journal of African Earth Sciences, v. 86, p. 65-106 Click on the link, scroll to the Open Access article to download the 12 Mb PDF version). The authors present superb simplified geological maps of each major part of the orogen, a vast array of references and well-written accounts of its sector-by-sector tectonic and metamorphic evolution, variations in style and broad tectonic setting.

Homes for hominin evolution

African savannah exhibit at the National Zoolo...
Typical African savannah. Image via Wikipedia

Friedrich Engels’s notion in The Part Played by Labour in the Transition from Ape to Man (1876), encouraged by Darwin’s The Descent of Man (1871), that the road to modern humans began with walking on two legs, thereby freeing the hands for work and tool making has been central to discussion of human evolution for more than a century. The ‘descent from the trees’ that bipedalism signifies has long been supposed to stem from the replacement of tropical forests in East Africa by open woodland or savannah, but evidence to support that environmental change has been difficult to glean from the fossil record  since the Late Miocene. Even in terrestrial sediments plant remains are rare, so that much has rested on animal fossils in relation to the habitats of their living descendants: opinion is divided.

There is a round-about means of resolving this central issue: using the carbon-isotope record in fossil soils that depends on the fractionation effects of broadly different kinds of plants that once grew in the soils and the signature of that fractionation in carbonate nodules that formed in the soils. The d13C value (crudely the difference between the 13C/12C ratio of a sample and that of a carbon-rich standard) found in C4 plants (many grasses) is -16 to -10 ‰ whereas that in C3 plants (including almost all trees) it is much more depleted in the heavier 13C isotope (-33 to -24‰). Exchange of carbon between living and dead organic matter, and carbonates that are precipitated from soil waters through the intermediary of gases in the soil should leave a d13C signature in the carbonates that reflects the overall proportions of different photosynthetic plant groups living at the time the soil formed. The approach was developed in the early 1990s by Thure Cerling and Jay Quade of the US universities of Utah and Arizona respectively.

After a long gestation period, involving calibration using soils from different modern ecosystems, the soil C-isotope method has been applied painstakingly to palaeosols in which African hominin remains have turned-up (Cerling, T.E. and 9 others 2011. Woody cover and hominin environments in the past 6 million years. Nature, v. 476, p. 51-56). All the famous hominin sites from the Awash and Omo Valleys of Ethiopia and around Lake Turkana in Kenya, figure in this important study, in which the authors devise a proxy for ‘palaeo-shade’ based on their carbonate d13C data from 76 modern tropical soils: a good ‘straight-line’ plot of d13C against the fraction of woody cover at the different calibration sites. Applying the proxy to their 1300 samples of palaeosols they show convincingly that since about 6 Ma tree cover rarely rose above 40% in the homelands of all the East African hominins. From the times of Ardepithecus ramidus (~4 Ma) at Aramis in Ethiopia, through those of ‘Selam’ and ‘Lucy’, the 2.5 Ma first stone tools at Gona, the times when Africa was dominated by Homo erectus(1.8 to 1 Ma) to perhaps the first signs of modern human cranial remains (those with chins!) around 1 Ma, all hominins strode through open, grassy environments. One can imagine pleasured nods from the shades of Darwin and Engels now their prescience has finally been confirmed.