Crosby Lectures in Geology - Africa's Past 30 Million Years II

Small volumes of volcanic rocks and evidence of basement uplift indicate that the source of the African plate < 30 Ma igneous rocks is not a very hot region (as the Ethiopian trap source must have been to make such a huge volume). A shallow source is compatible with this consideration The Dakar swell at the edge of the continent raises basement ca.2 km. Volcanism has been episodic on the swell over the past 24 My. Erosion has kept the surface close to sea-level. Sediment derived from that erosion has been deposited in deep water. Elevation of swells relates to erosion (fn rainfall over 30 Ma). Highest swells in (S & E Africa) are in driest regions Locations of Dakar, Ahaggar And Jos Swells. Chad basin is surrounded by volcano crested swells. The volcanoes describe an ellipse presumably indicating something about underlying convection The Ahaggar (Hoggar) swell rises to almost 3 km asl but the volcanic rocks on the swell, (mainly basalts and trachytes), are thin. The plume sources of the shallow convection are too cool to generate much magma. Sub-swells trend NE as part of the circum-Chad ellipse The small volcanoes of Jos lie on the crest of the topographic swell NW volcano trend as but the part of circum-Chad Bouguer ellipse gravity Anomaly and the relief are modified by low *Abuja density (and hard to erode) Jurassic granites On Maio In the Cape Verde Islands Jurassic Ocean floor outcrops In the center of a Miocene volcano The ocean floor basement is elevated by > 4km. Only a small volume of basalt has been erupted by the volcano. Jurassic ocean Floor also outcrops in Lanzarote in the in shore Canary islands The top of the Ocean floor and the seismic MOHO rise toward the island of Principe in the Cameroons. The island is on a swell. Doubt exists in the minds of some as to whether the volcanoes of the Oceanic part of the Plate are loads or basement is elevated. A compromise is shown here! What a contrast to Hawaii which depresses the lithosphere because It is a huge load In summary: Igneous rocks of < 30 Ma on the African Plate are of small volume and concentrated on swells. They may reflect structure in a shallow underlying convection system. Distribution shows plate is at rest. What can be learned about the Basin& Swell structure from the elevated areas themselves ? As always in geomorphology the Time over which processes have Operated is very important. The great changes of the past 30 My Have sometimes been underemphasized African and S. American hypsometry Other continents have small areas of very high ground, as in the Andes and large areas close to sea level as in the Amazon basin. Africa has a large area at intermediate elevation (ca 1.5km), no very high ground (> 6km) and only a Elevation of a low-lying Africa in very small area near past 30 My can account for sea level. Africa’s present unusual pattern of relief. Southern Africa is a good place to start: much high ground in a few great swells, a peripheral “Great Escarpment” and a lot of information of diverse kinds. Weaknesses include: No young igneous rocks (because cratonal) not much deep-water offshore seismic data. One strength is the recent recognition of a shallow LVZ which is compatible with the idea of shallow mantle convection and , if it has developed in the past 30 My, accounts for Southern Africa’s widespread elevation of ca.1km AGE OF SOUTHERN AFRICA’S RELIEF In Southern Africa there are two schools of thought: One that stresses that the relief has developed since 30 Ma ( e.g.Burke 1996, Burke et al 2003) and another that treats the relief, and particularly the Great Escarpment, as having endured since the Cretaceous and Perhaps since the continental margins formed at ca.125 Ma. That “old relief” model is used for two main reasons: (1) It is traditional (as for example in all but the last writings of the late L.C.King). (2) It fits in with the popular idea of “Old escarpments at continental Margins”(e.g. Beaumont, also Matmon in Geology ). The main evidence cited by supporters of the “old relief” school is from the interpretation of apatite fission tracks (AFT) of the partially annealed zone (PAZ). See papers by Gleadow or R.Brown •The Great Escarpment (GE) runs parallel to the coast 100 to 200 km inland. •Red and yellow AFT < ca.125 lie below the GE. •Older AFT lie above the GE. This can be interpreted as indicating no erosion of high ground which has been high since the margin rifted( 125 Ma) . Only areas below GE have been eroded and retreat has been slow. It can also be interpreted on a “Two Event” model Gallagher’s figures show how: From Brown et al. YOUNG AFT AGES FORM A BELT CLOSE TO THE COAST Great Escarpment on west coast of S. Africa (and a small length in SE of image). Orange River traverses much of image. Orange History of continental margin escarpments (Modified from Gallagher et al., 1998, AREPS) • • • AFT reset near coast when rift shoulders eroded (maybe 5 km) in tens of My. Too little erosion (<2km) in interior to reset AFT because AFT PAZ insensitive below 60o.(It is not clear that Gleadow accepts that) Africa low by 70 Ma is consistent with old AFT ages above scarp. • A second event: Present Gt. Escarpment is new:Swells initiated at 30 Ma. Erosion is nearly all outboard of newly formed Great Escarpment.Too little erosion <2km to reset AFT since 30 Ma • Up to 5 km were eroded from the rift shoulders when the ocean began to form because it was hot below. AFT were reset. Inboard erosion took < 2 km by 70 Ma and AFT were not reset. The flat continent was domed up in a swell to make the Great Anticline of South Africa beginning at 30 Ma. The Great Escarpment that we see today was formed and began to Retreat after 30 Ma. Because margin was cold no AFT resetting. < 2km have been eroded. From Brown et al. HOW THE TWO STAGE MODEL ACCOMMODATES YOUNG AFT AGES IN A BELT CLOSE TO THE COAST How long do escarpments at the continental margin last? Coward et al. (1998) showed: South Atlantic rifts formed 145 ± 5 Ma. Rift deposition (pink) persisted in rifts and on ocean floor under air till sea water spilled in to form evaporites. After evaporites were deposited, carbonate deposition (blue) followed ending at 2035 M.y. after ocean floor began to form. “Rift shoulder time”. Rift shoulders had been eroded by the time silici-clastic deposition (green) began at 105-90 Ma. Long-lived escarpments did not persist On the South Atlantic margins Conclusion: Escarpments last a few tens of millions of years.