GESGB London Evening Lecture – July 2024 (Hybrid)

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Speaker: Duncan Macgregor – MacGeology

Topic: The History of the African Plate since the Permian: a Consolidation of PESGB Africa Conferences

Date: Tuesday 30 July 2024

Timings: 17:30 – Doors open / 18:00-19:00 – Lecture / 19:00-20:00 – Networking drinks

Meeting Venue: The Bedford Hotel – 83-95 Southampton Row, London WC1B 4HD
(Ground Floor Meeting Room – next to the garden)

Catering: Tea/coffee/biscuits to be provided.

Networking Drinks: The Swan – 7 Cosmo Place, London, WC1N 3AP
(Self-funded bar)

The presentation will be available to view via live stream on Zoom.
Login details will be sent to registered attendees one day prior to the event.

The History of the African Plate since the Permian: a consolidation of PESGB Africa Conferences

Duncan Macgregor, MacGeology & Colin Reeves, Earthworks

We are compiling an online atlas illustrating the tectonic, climatic, topographic, erosional and depositional histories of the African Plate since the Permian. The objective of this work is to provide a free product to all workers on African geology to illustrate the large scale controls on the projects they are working on. Multiple themed maps are being prepared on 19 geological levels ranging from the Early Permian to Present Day. The latest versions are available on www.africageologicalatlas.com, while the Sub-Saharan plate model on which these are based can be accessed on www.reeves.nl.  As much of African subsurface geology, particularly on the deep offshore margins, is poorly documented in peer reviewed publications, we aim specifically to document interpretations derived from the contributions provided by the 22 years of PESGB/HGS Africa conferences and similar Geological Society events. Talks there have included hundreds of interpretations that have never been published. Examples include unpublished seismic illustrating the deep structure of African continental margins and the dating, through drilling and seismic, of the hundreds of rifts within the continent.

In this talk, we shall concentrate on the tectonic history (Figure 1). The oldest margin is the northern (Tethyan) margin. Neotethys propagates into the northern part of the eastern Mediterranean in the Late Triassic, reaching offshore Egypt in the late Early to early Middle Jurassic. Horsts and grabens in the hyperextended Levantine Basin trend NNE-SSW, implying transform movement along the sharp Egyptian margin. The predominantly transform margin thus created later accommodated the Azores-Gibraltar transform, which bounds additional slightly younger Tethyan oceans and the incipient Central Atlantic. Associated transtensional rifting occurs in the Western Desert and Cyrenaica. The Gulf of Sirt margin is a dip-slip salient between two large transforms.

Following a major magmatic event at the Triassic-Jurassic boundary, the first Central Atlantic oceanic crust is thought to have been emplaced around 190Ma, though north of the Blake Spur only. This ocean then leaps south to Guyana in the Oxfordian, forming a combination of hyperextended and volcanic margins.

Following Toarcian rifting, the northern part of the Indian ocean opens in the Middle Jurassic, commencing a ‘smile’ shaped unzipping process around the remainder of the African plate, that reaches the southernmost tip of the South Atlantic in the Valanginian, Gabon/Angola in the Aptian, and leading to a final connection with the Central Atlantic in the early Cenomanian. The nature of the margins created varies considerably. The Somali Basin originally opens as an oblique margin, then switches sharply to a transform margin with the creation of the Davie transform, which in its northern part lies within oceanic crust. The Mozambique Basin is underlain by a wide zone of seaward dipping reflectors, i.e. is largely a volcanic margin, though this zone is bounded to the north and south by large transforms. The Agulhas margin opens as a large transform, with some activity preceding the first opening of the South Atlantic in the Early Cretaceous, as evidenced by the westwards directed first marine transgressions in the Agulhas Basins. The Namibian margin is again a volcanic margin, part of a pattern of frequent and repeated volcanism over the southern part of the African Plate, covering margins, intra-plate and intra-ocean settings. Propagation northwards of the South Atlantic continues through the Aptian and Albian. A particular source of debate in Africa conferences has been the age of first true oceanic crust emplacement off Angola and south Gabon . Here it is interpreted that the first oceanic crust emplacement in the southern Angola salient precedes Late Aptian salt deposition, while that in north Gabon probably postdates the salt. This interpretation is controlled by evidence for a connected hypersaline lake covering Brazil and West Africa in the early to mid Aptian and for separate salt basins bounded by oceanic ridges in the Late Aptian. The break-up ‘process’, commencing with the flow of the lower crust, does however commence much earlier, reflected in Barremian-early Aptian sag tectonics. The development of isolated E-W transforms between the Marajo Basin of Brazil through to Sudan commences in the Valanginian, progressively combining to form a near-continuous transform in the Aptian. This process continued to give oceanic crust off the equatorial margin in the Late Albian and the final separation of Africa from South America in the Early Cenomanian.

Intra-plate tectonics are dominated by extension, with fold belts confined to  the southern and northern limits of the plate (Cape Fold Belt : Permo-Trias collision of Patagonia) and Atlas Fold Belt (Neogene collision of Kabylies/Alboran Plates). In addition, there are a set of ‘shock’ events creating inversions in the north of the plate in the Aptian, Maastrichtian, early Oligocene and early-mid Miocene, plus a more regional one in the Santonian.  Most of these ‘shocks’ can be tied to Tethyan/Alpine Belt events . The Santonian event is tied to the commencement of an anticlockwise rotation of Africa relative to Europe,and to consequential Anatolian collisions.

Active intra-plate rifting is interpreted on all 19 geological levels mapped, though with peaks in the Mid Permian to Early Triassic, Late Triassic, the Early Cretaceous and Neogene. Many rifts are composite in age. These commence with the Permian ‘Karoo’ transtensional rifts. These propagate northwards into predominantly Early Triassic rifts in northeast Africa, whereas the Late Trias represents the peak of rift activity in north Africa. The whole of the African plate would appear to have been experienced NE-SW stretching in the Early Cretaceous, creating a series of NW-SE dip slip rifts, bounded by E-W trending transforms. Such rifts in Sudan and Niger are active through to the Oligocene, at which time fault orientations migrate to a N-S trend. Consistent with this stress direction change, we now see the formation of the first rifts of the East African Rift System in Kenya. The Afar Plume Oligocene volcanics precede the opening of the Red Sea, while mid Miocene Kenyan volcanics precede the opening of the greater part of the EARS. An unzipping trend of rifting in the Red Sea advanced northwards with time, with a peak of rift activity around 20Ma. At 14Ma, the Dead Sea Transform was created, ending syn-rift conditions in the Gulf of Suez. EARS rifts propagate north and south from Kenya through the Mid and Late Miocene, all varying in age and likely fill, with the peak extent of rifting and greatest vertical movements in the last 5 My. New branches have recently been created offshore east Africa, and along a  trend exploiting Permian rifts in Zambia, Zimbabwe and Namibia. The African plate, which starts to fragment with the opening of the Red Sea in the Early Pliocene, may now be starting to split further with the distinction of the Somali, Victoria and possibly the San plate (south of the Zambian rift trend). On the northern margin, incipient collision is occurring with Anatolia, while the Western Mediterranean may be starting to subduct below Africa.

In addition to the crustal tectonics described above, Africa has since at least the Late Cretaceous lain over a dynamic mantle, which has controlled the development of long wavelength ‘basin and swell’ tectonics. In the absence of compressional tectonics, such processes have been the primary influence on the paleotopography and hence the paleodrainage, of the continent. The South African plateau, which here is proposed to have a composite Late Cretaceous and Neogene origin, is the largest such ‘swell’ in the world. Additional uplifts within and on the margins of the continent have developed mainly in Oligocene to Recent times, though some enigmatic elongate uplifts along continental margins, developed at least in part by isostatic compensation (i.e. shifts in crustal weight due to erosion and deposition) seem to be older.

Duncan Macgregor – MacGeology

Duncan Macgregor is a geologist specialising in the regional geology of the African plate. After 22 years at BP, he worked as a consultant for a series of companies, including PGS, Sasol, Newmed and Neftex, specialising in regional African exploration. He has edited three publications on African geology, written around 25 papers and was the Technical Chair of the PESGB Africa Conference for 18 years. He is now retired from consultancy work but still spends time working on the africageologicalatlas website and giving talks and training courses.