southern african topography and erosion history: plumes or ... blenkinsop, cotterill - 2009... ·...

Southern African topography and erosion history: plumes or platetectonics?

Andy Moore,1,2 Tom Blenkinsop3 and Fenton (Woody) Cotterill41AfricanQueenMines Ltd., Box 66,MaunBotswana; 2Department ofGeology, RhodesUniversity, Grahamstown, SouthAfrica; 3School of Earth

and Environmental Sciences, James Cook University, Townsville 4811, Qld, Australia; 4AEON – Africa Earth Observatory Network, and

Department of Geological Sciences, and Department of Molecular and Cell Biology, University of Cape Town, Rondebosch 7701, South Africa

Introduction

In broad outline, the physiography ofsouthern Africa is characterized by anarrow coastal plain of low relief,separated by a horseshoe-shapedescarpment from an inland plateaustanding at more than 1000 m abovesea level (Bond, 1979; Fig. 1). Theelevation of this plateau is atypicallyhigh compared with the average of400–500 m for cratons on other con-tinents (Lithgow-Bertelloni and Silver,1998; Gurnis et al., 2000), and formspart of a belt of high ground thatextends to east Africa, constituting the�African Superswell� (Nyblade andRobinson, 1994). Recent attempts toaccount for this elevated topographyhave generally invoked variations ofthe mantle plume concept (Nybladeand Robinson, 1994; Burke, 1996;Ebinger and Sleep, 1998; Lithgow-Bertelloni and Silver, 1998; Gurniset al., 2000). Indeed, modelled mantledensity and viscosities (Gurnis et al.,2000) assume uplift in southern Africato have been plume-related. TheAfrican erosion surface, associatedso closely with Africa�s anomalous

elevation, is interpreted as a poly-genetic landform (Burke and Gunnell,2008); this topographic anomaly isattributed to the persistence of aLarge Low Shear Velocity Province(LLSVP) at the Core-Mantle bound-ary which is postulated to havegenerated plumes under the Africanplate (Torsvik et al., 2006; Burkeet al., 2008). In concert with a grow-ing chorus of general objections to theplume hypothesis (e.g. Bailey, 1993;Westaway et al., 2003; Anderson andNatland, 2005, 2006; Foulger, 2007),we draw attention to major inconsis-tencies between the actual topographyof southern Africa and the dynamictopography predicted by plume-basedmodels.

Observed vs. modelled topographyof southern Africa

As a first order generalization, areasof elevated topography in southernAfrica are associated with the mar-ginal escarpment, while the interior isthe site of the Cenozoic Kalaharisedimentary basin (Fig. 1). The drain-age network is characterized by aremarkable pattern of river divides(Fig. 2), which define three horseshoe-shaped arcs, broadly concentric withthe coastline (Moore, 1999). The outerarc follows the crest of the GreatEscarpment, and separates relativelyshort coastal rivers from the maindrainages of the inland plateau. The

middle divide separates the OrangeRiver basin from the Limpopo, andcuts across an abandoned former linkbetween the Molopo-Nossib and theOrange Rivers (Moore, 1999). Theinnermost divide separates the majorLimpopo and Zambezi drainagebasins in Zimbabwe, and the Limpopofrom fossil endoreic river lines inBotswana.In contrast to the observed topo-

graphy, geophysical models attempt-ing to explain the unusually elevatedtopography of southern Africa interms of a plume-sustained model,predict that this region will be char-acterized by a broad domal upwarp,and thus fail to explain the interiorCenozoic Kalahari sedimentary basin(Fig. 1). Predicted low ground at thesouthern margin of the plume-mod-elled dome (Lithgow-Bertelloni andSilver, 1998) is in fact the location ofthe highest ground on the sub-conti-nent, in the Drakensberg-Malutimountain-land. The model predictsrelatively high ground over the bulgeof the low-lying (<200 m) Mocam-bique coastal plain, while the mark-edly elevated ground of the KhomasHighlands in Namibia and BiePlateau in Angola (Fig. 1) both occuron the margins of the plume-mod-elled dome. The plume modelalso implicitly requires that southernAfrica should be characterized by aradial drainage pattern, and fails topredict or explain the remarkable

ABSTRACT

The physiography of southern Africa comprises a narrowcoastal plain, separated from an inland plateau by a horse-shoe-shaped escarpment. The interior of the inland plateau isa sedimentary basin. The drainage network of southern Africais characterized by three river divides, broadly parallel to thecoastline. These features contrast strongly with the broaddome and radial drainage patterns predicted by models whichascribe the physiography of southern Africa to uplift over adeep mantle plume. The drainage divides are interpreted asaxes of epeirogenic uplift. The ages of these axes, which

young from the margin to the interior, correlate closely withmajor reorganizations of spreading regimes in the oceanicridges surrounding southern Africa, suggesting an origin fromstresses related to plate motion. Successive epeirogenic upliftsof southern Africa on the axes, forming the major riverdivides, initiated cyclic episodes of denudation, which arecoeval with erosion surfaces recognized elsewhere acrossAfrica.

Terra Nova, 21, 310–315, 2009

Correspondence: Dr Tom G Blenkinsop,

School of Earth and Environmental

Sciences, James Cook University, Towns-

ville Campus, Townsville 4811, Qld,

Australia. Tel.: 00 61 7 4781 4318; fax: 00

617 4725 1501; e-mail: thomas.blenkinsop@

jcu.edu.au

310 � 2009 Blackwell Publishing Ltd

doi: 10.1111/j.1365-3121.2009.00887.x

observed concentric pattern of riverdivides in southern Africa. In short,the first-order topography of south-ern Africa does not correspond withthe dynamic topography predicted bythe plume model. Therefore theobserved topography demands alter-native explanations.

Evolution of the topography ofsouthern Africa

The three major river divides allcut across geological boundaries andstructural lineaments (Fig. 3), andtherefore do not reflect simple litho-logical controls. Rather, the divides

have been interpreted to reflect axes ofepeirogenic flexure (Du Toit, 1933;King, 1963; Moore, 1999), designated,from the coast inland, the EscarpmentAxis, the Etosha–Griqualand–Trans-vaal (EGT) Axis and the Ovambo–Kalahari–Zimbabwe (OKZ) Axis. Theevidence that the three major riverdivides represent lines of epeirogenicflexures is very compelling. TheEscarp-ment Axis is well modelled as a line ofisostatic flexure, related to the erosionof the coastal plain following breakupof Gondwana (Gilchrist and Summer-field, 1991; Moore and Blenkinsop,2006). Relative uplift along the EGTAxis is reflected by the preservation of adismembered relic of a fossil drainageline straddling the flexure at MahuraMuthla (Fig. 2). The abandoned linkbetween the Molopo and OrangeRivers has a convex-up profile whereit crosses this axis (Moore, 1999). Thiscontrasts with the concave-up profileexpected fromheadward erosion, but isconsistent with uplift across the rivercourse (Du Toit, 1933; Partridge,1998). In the case of the OKZ Axis,there is clear evidence for reversal ofdrainage flow directions across this lineof flexure in Botswana (Moore, 1999).In Zimbabwe, this axis was responsiblefor beheading an earlier drainagenetwork established in the Permian(Moore and Moore, 2006; Mooreet al., 2009). The EGT Axis forms thesouthern boundary of the KalahariFormation, which crosses a variety ofbedrock formations (Haddon, 1999,2001), demonstrating that the axis isnot lithologically controlled (Fig. 4).Further north, the locus of the OKZAxis corresponds closely with the east-ern and western margins of this sedi-mentary unit. This underlines theobservation made by Du Toit (1933)that subsidence of the Kalahari Basinwas closely associated with uplift alongthe river divides that define these twoaxes.Several lines of evidence show that

the flexures forming the three majorriver divides in southern Africa wereinitiated in discrete episodes (Moore,1999). Uplift along the EscarpmentAxis was initiated by the opening ofthe Indian and Atlantic Oceans atc. 126 Ma (King, 1963; Moore andBlenkinsop, 2006). This is reflected bya major Early Cretaceous erosionalevent, particularly marked along themargins of southern Africa (Brown

Fig. 1 SRTM digital elevation image for southern Africa. Note that the highestelevations (purple-grey tones) are associated with the marginal escarpment and alsothe central Zimbabwe watershed. This high ground surrounds the Cenozoic KalahariBasin (KB). Elevations in metres.

Fig. 2 Drainage system of southern Africa. Colours denote stream rank from red (1)to purple (5). M, Molopo River; N, Nossob River; MM, Mahura Muhtla. The majorriver divides are interpreted to reflect epeirogenic uplift Axes. EGT Axis, Etosha–Griqualand–Transvaal Axis; OKZ Axis, Ovambo–Kalahari–Zimbabwe Axes. Datafrom USGS EROS, http://eros.usgs.gov/products/elevation/gtopo30/hydro/af_streams.html

Terra Nova, Vol 21, No. 4, 310–315 A. Moore et al. • Topography and erosion history: plumes or plate tectonics?

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� 2009 Blackwell Publishing Ltd 311

et al., 1990; McMillan, 2003) (Table 1and Fig. 5). Erosion of the coastalplain resulted in the progressive inlandmigration of the axis as an isostaticflexure (Gilchrist and Summerfield,1991) which continues to the present,albeit at very slow rates (Fleminget al., 1999; Brown et al., 2002).An upper Cretaceous (85–65 Ma)

age for the EGT Axis is inferred onthe basis of fossil wood preserved inthe Mahura Muthla palaeo-channelastride the axis (Partridge and Maud,

1987; Partridge, 1998) (Fig. 5). Recentsupporting evidence for this timingcomes from an Apatite Fission Track(AFT) study in Zimbabwe (Belton,2006). Three episodes of acceleratederosion were identified in the LimpopoValley in the south of the country(c. 125 Ma, c. 83 Ma and 44–33 Ma)(Table 1 and Fig. 5). The first wasascribed to erosion triggered bydisruption of Gondwana. Uplift alongthe EGT Axis, which forms the south-ern watershed of the Limpopo Basin,

would have rejuvenated drainages withheadwaters on this divide, thus initiat-ing the mid-Cretaceous erosion event.In agreement with this interpretation,the Zambezi basin does not recordaccelerated denudation at this time.We propose that the youngest (latePalaeogene) erosion event was trig-gered by uplift along the OKZ Axis,that forms the central Zimbabwewatershed, separating the Limpopoand Zambezi Basins (Moore et al.,2009). Coeval erosion in the Zambezibasin (Belton, 2006) (Table 1) is con-sistent with this interpretation, whichfurther explains the marked Oligoceneincrease in sediment flux identified inthe deltas of the Zambezi (Walfordet al., 2005) and Limpopo (Burke andGunnell, 2008), ascribed to epeirogeniccontinental uplift by both studies.Thus, the ages of all three axes areconstrained by independent geologicaland AFT evidence. In the Congosedimentary basin, major unconformi-ties have been recognized in the LowerCretaceous, Late Cretaceous andmid-Tertiary (Cahen and Lepersonne,1952; Giresse, 2005; Stankiewicz anddeWit, 2006).These correspond closelyin timing with the initiation of the threeflexure axes in southern Africa. Thesetemporal correlations recall Goldfin-ger�s dry observation: �Once is happen-stanceMr. Bond. Twice is coincidence.But the third time, it is enemy action.�(Fleming, 1959). Collectively, theevidence argues for coeval continent-wide uplifts.The timing of uplift along the

three flexures (Early Cretaceous, mid-Cretaceous andLate Palaeogene) over-lap episodes of alkaline volcanism insouthern Africa (Moore et al., 2008)(Fig. 5). Nevertheless, while the ages ofthe Axes young inland from the coast,alkaline volcanism in southern Africashows a pattern of younging in theopposite direction, towards the coast(Moore et al., 2008).In summary, southern Africa is

characterized by a remarkable patternof roughly concentric river divides,broadly paralleling the coastline andof different ages, younging inlandfrom the coast. Each flexure wasassociated with an episode of alkalinevolcanism in southern Africa. Anymodel which invokes mantle plumesto account for the topography ofsouthern Africa must address boththe remarkable pattern of the river

Fig. 3 Loci of major river divides, inferred to reflect axes of epeirogenic flexure, inrelation to the geology of southern Africa. These divides all cross boundariesseparating Archaean cratons from the surrounding Proterozoic terranes. In Namibia,they traverse the northeast-trending late Proterozoic Damara belt at a high angle. InZimbabwe, the OKZ Axis transects the granite–greenstone terrain of the Zimbabwecraton, cuts across the NNE-trending Great Dyke, and continues across theOkavango Dyke swarm in Botswana. The central EGT Axis crosses the north-trending Kheiss belt at right angles, while much of the eastern section of theEscarpment Axis traverses readily eroded horizontal Karoo sediments. Geology ofsouthern Africa after De Wit et al. (2004).

Topography and erosion history: plumes or plate tectonics? • A. Moore et al. Terra Nova, Vol 21, No. 4, 310–315

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divides, and the timing of their initi-ation. It must also account for theinland younging of the flexures andcoastward migration of alkalinevolcanism in southern Africa, sincethe disruption of Gondwana. As thedomal uplifts predicted by the plumemodel fail to account for theseobservations, it is necessary to seekalternative explanations.The three southern African flexure

axes are not only parallel to thecoastline, but also to the surroundingocean-spreading ridges. There areclose correlations between the ages ofinitiation of the three flexures andmajor episodes of plate reorganizationin the Atlantic and Indian Ocean(Moore et al., 2008) (Fig. 5). Thus,initiation of the coastal EscarpmentAxis in the Early Cretaceous corre-sponds to the inception of the driftsequence in sedimentary basins sur-rounding southern Africa (McMillan,2003). Continental instability follow-ing separation of the Agulhas Bankand Falkland plateau at c. 100 Mamay have rejuvenated erosion of thecoastal plain and thus uplift along the

Escarpment Axis (Moore et al., 2008).The upper age limit (c. 85 Ma) for theinitiation of the mid ⁄ late-CretaceousEGT Axis closely matches the timingof a major shift in the pole of rotationof the Atlantic at Chron 34 (c. 84 Ma)(Nurnberg and Muller, 1991), andfollows shortly after a reorganizationof spreading in the Indian Oceandated at c. 90 Ma (Reeves and deWit, 2000). The late Palaeogene OKZAxis correlates closely in age with areorganization of the Indian Oceanspreading regime (Reeves and deWit, 2000), as well as a markedincrease in spreading rate in the mid-Atlantic (Nurnberg and Muller, 1991)(Fig. 5).These correlations suggest that the

flexure axes, and thus topography insouthern Africa, could be primarilylinked to deformation events associ-ated with these plate reorganizations.While the exact mechanisms are sub-ject matters involving speculation andaccordingly require further investiga-tion, major episodes of erosion in theBritish Isles have been correlated withplate boundary deformation (Hillis

et al., 2008; Holford et al., 2009).The projection of compressional stres-ses over distances >1000 km fromoceanic ridges into the interiors ofcontinental plates �can account for abroad spectrum of shortening-relatedintraplate deformation styles whichvary in scale from upper crustal fold-ing … to whole lithosphere buckling�(Holford et al., 2009).The disparity between the actual

topography of southern Africa andplume-based numeric models does notnecessarily rule out the existence of theputative African Superplume. Never-theless, the evidence presented in thisarticle demonstrates that the moderntopography of southern Africa reflectsa complex interplay of processes,linked to plate motions, that haveoperated over a long period of time ascompared with the short time-scalessuggested for plume-sustained topog-raphy. Numerical models, whichattempt to account fully for Africa�sanomalously elevated topography,should not consider the possible influ-ence of a single extant deep-mantleplume in isolation from these long-termprocesses.

Implications for continent-wideerosion cycles and the origin ofuplifts

Our observations have an importantbearing on one of the most celebrateddebates in geomorphology – the con-cept of cyclic erosion episodes. Thiswas championed by Lester King(1949, 1955, 1963), who recognizedrelics of successive erosion surfaces ofdifferent ages in southern Africa,including the senile African Surface,with a continent-wide distribution.A major criticism of the concept of

cyclic erosion surfaces has alwaysbeen the question of the mechanismsresponsible for their initiation. Thisproblem is resolved by successiveuplift along axes located progressivelyinland from the coast. Each episode ofepeirogenic tectonism would haverejuvenated the drainage network ofsouthern Africa, thus providing aseries of triggers to initiate new cyclesof erosion in the continental interior.This in turn offers a theoretical frame-work to account for the succession oferosion cycles previously recognized insouthern Africa (King, 1963; Lister,1987; Partridge and Maud, 1987).

Fig. 4 Distribution of the Kalahari Formation (Kalahari Sands) in relation to theepeirogenic axes defined by the major river divides. Note how the EGT Axis encirclesthe southern margin of the Kalahari basin. Further to the north, the eastern andwestern margins of the basin are bounded by the OKZ Axis.

Table 1 Major post-Gondwana erosion episodes in southern Africa indicated by

apatite fission track evidence.

Locality Studied

(from north to south) Palaeogene Mid-Cretaceous Early Cretaceous Reference

Zambezi Valley, Zimbabwe 47–37 Ma 133–124 Ma Belton (2006)

Limpopo Valley, Zimbabwe 48–38 Ma 93–73 Ma 128–118 Ma Belton (2006)

Northwest coastal plain 130–100 Ma Brown et al., 1990

Northeast coastal plain Early Cretaceous Brown et al., 2002

Cape Fold Belt, South coast 100–83 Ma 140–120 Ma Tinker et al. (2008)


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Close correlation between the ages ofthese flexure axes and the major un-conformities in the Congo Basin lendssupport for the development of con-temporaneous erosion surfaces overwide areas of Africa, as postulated byKing (1963). A re-evaluation of ero-sion surfaces in southern African inrelation to the three flexure axes offersa potential framework for refiningunderstanding of their chronologyand interrelationships.It has been noted that the ages of

the three flexure axes also correspondto continent-wide episodes of alkalinevolcanism recognized by Bailey(1993), underlining the link betweenepeirogenesis and associated litho-spheric stresses, and alkaline volca-nism (Bailey, 1993; Moore et al.,2008). The broad upwarps representedby the flexure axes would be associ-ated with relative tensional stresses inthe upper surface of the plate. Incontrast, the lower plate surface wouldexperience relative tension beneath the

basins surrounding the axes. This linkbetween the uplift axes and the distri-bution of sub-lithospheric stressescould explain the coastward youngingof alkaline volcanism in southernAfrica, which contrasts with theinland age progression of the threeaxes.In addition to the flexures repre-

sented by the major drainage divides insouthern Africa, a number of lines ofPlio-Pleistocene uplift have affectedtopography to a lesser extent (Du Toit,1933; Partridge, 1998; Moore, 1999),but a discussion of their origin isbeyond the scope of this paper.In summary, the observed topo-

graphy of Southern Africa does notcorrelatewellwithdynamictopographypredicted by plume models. Rather,topography is largely determined byconcentric flexural uplift axes, eachcoeval with an episode of plate-boundaryreorganization.Thissuggeststhat the dominant influence on themodern topography in southern Africa

reflects stresses associated with platekinetics, rather thanmantle plumes.

Acknowledgements

Tyrel Flugel is thanked for producing theDigital Elevation image of southern Africa,and Dr Marty McFarlane, Paul Green andtwo anonymous reviewers for their con-structive comments on the manuscript.

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Received 5 January 2009; revised versionaccepted 18 May 2009


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