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Southern African Topography and Erosion History: Plumes Or Plate Tectonics?

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Southern African Topography and Erosion History: Plumes Or Plate Tectonics? doi: 10.1111/j.1365-3121.2009.00887.x Southern African topography and erosion history: plumes or plate tectonics? Andy Moore,1,2 Tom Blenkinsop3 and Fenton (Woody) Cotterill4 1African Queen Mines Ltd., Box 66, Maun Botswana; 2Department of Geology, Rhodes University, Grahamstown, South Africa; 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 ABSTRACT The physiography of southern Africa comprises a narrow young from the margin to the interior, correlate closely with coastal plain, separated from an inland plateau by a horse- major reorganizations of spreading regimes in the oceanic shoe-shaped escarpment. The interior of the inland plateau is ridges surrounding southern Africa, suggesting an origin from a sedimentary basin. The drainage network of southern Africa stresses related to plate motion. Successive epeirogenic uplifts is characterized by three river divides, broadly parallel to the of southern Africa on the axes, forming the major river coastline. These features contrast strongly with the broad divides, initiated cyclic episodes of denudation, which are dome and radial drainage patterns predicted by models which coeval with erosion surfaces recognized elsewhere across ascribe the physiography of southern Africa to uplift over a Africa. deep mantle plume. The drainage divides are interpreted as axes of epeirogenic uplift. The ages of these axes, which Terra Nova, 21, 310–315, 2009 elevation, is interpreted as a poly- middle divide separates the Orange Introduction genetic landform (Burke and Gunnell, River basin from the Limpopo, and In broad outline, the physiography of 2008); this topographic anomaly is cuts across an abandoned former link southern Africa is characterized by a attributed to the persistence of a between the Molopo-Nossib and the narrow coastal plain of low relief, Large Low Shear Velocity Province Orange Rivers (Moore, 1999). The separated by a horseshoe-shaped (LLSVP) at the Core-Mantle bound- innermost divide separates the major escarpment from an inland plateau ary which is postulated to have Limpopo and Zambezi drainage standing at more than 1000 m above generated plumes under the African basins in Zimbabwe, and the Limpopo sea level (Bond, 1979; Fig. 1). The plate (Torsvik et al., 2006; Burke from fossil endoreic river lines in elevation of this plateau is atypically et al., 2008). In concert with a grow- Botswana. high compared with the average of ing chorus of general objections to the In contrast to the observed topo- 400–500 m for cratons on other con- plume hypothesis (e.g. Bailey, 1993; graphy, geophysical models attempt- tinents (Lithgow-Bertelloni and Silver, Westaway et al., 2003; Anderson and ing to explain the unusually elevated 1998; Gurnis et al., 2000), and forms Natland, 2005, 2006; Foulger, 2007), topography of southern Africa in part of a belt of high ground that we draw attention to major inconsis- terms of a plume-sustained model, extends to east Africa, constituting the tencies between the actual topography predict that this region will be char- ÔAfrican SuperswellÕ (Nyblade and of southern Africa and the dynamic acterized by a broad domal upwarp, Robinson, 1994). Recent attempts to topography predicted by plume-based and thus fail to explain the interior account for this elevated topography models. Cenozoic Kalahari sedimentary basin have generally invoked variations of (Fig. 1). Predicted low ground at the the mantle plume concept (Nyblade southern margin of the plume-mod- Observed vs. modelled topography and Robinson, 1994; Burke, 1996; elled dome (Lithgow-Bertelloni and of southern Africa Ebinger and Sleep, 1998; Lithgow- Silver, 1998) is in fact the location of Bertelloni and Silver, 1998; Gurnis As a first order generalization, areas the highest ground on the sub-conti- et al., 2000). Indeed, modelled mantle of elevated topography in southern nent, in the Drakensberg-Maluti density and viscosities (Gurnis et al., Africa are associated with the mar- mountain-land. The model predicts 2000) assume uplift in southern Africa ginal escarpment, while the interior is relatively high ground over the bulge to have been plume-related. The the site of the Cenozoic Kalahari of the low-lying (<200 m) Mocam- African erosion surface, associated sedimentary basin (Fig. 1). The drain- bique coastal plain, while the mark- so closely with AfricaÕs anomalous age network is characterized by a edly elevated ground of the Khomas remarkable pattern of river divides Highlands in Namibia and Bie´ Correspondence: Dr Tom G Blenkinsop, (Fig. 2), which define three horseshoe- Plateau in Angola (Fig. 1) both occur School of Earth and Environmental shaped arcs, broadly concentric with on the margins of the plume-mod- Sciences, James Cook University, Towns- the coastline (Moore, 1999). The outer elled dome. The plume model ville Campus, Townsville 4811, Qld, arc follows the crest of the Great also implicitly requires that southern Australia. Tel.: 00 61 7 4781 4318; fax: 00 Escarpment, and separates relatively Africa should be characterized by a 617 4725 1501; e-mail: thomas.blenkinsop@ short coastal rivers from the main radial drainage pattern, and fails to jcu.edu.au drainages of the inland plateau. The predict or explain the remarkable 310 Ó 2009 Blackwell Publishing Ltd Terra Nova, Vol 21, No. 4, 310–315 A. Moore et al. • Topography and erosion history: plumes or plate tectonics? ............................................................................................................................................................. have been interpreted to reflect axes of epeirogenic flexure (Du Toit, 1933; King, 1963; Moore, 1999), designated, from the coast inland, the Escarpment Axis, the Etosha–Griqualand–Trans- vaal (EGT) Axis and the Ovambo– Kalahari–Zimbabwe (OKZ) Axis. The evidence that the three major river divides represent lines of epeirogenic flexures is very compelling. The Escarp- ment Axis is well modelled as a line of isostatic flexure, related to the erosion of the coastal plain following breakup of Gondwana (Gilchrist and Summer- field, 1991; Moore and Blenkinsop, 2006). Relative uplift along the EGT Axis is reflected by the preservation of a dismembered relic of a fossil drainage line straddling the flexure at Mahura Muthla (Fig. 2). The abandoned link between the Molopo and Orange Fig. 1 SRTM digital elevation image for southern Africa. Note that the highest Rivers has a convex-up profile where elevations (purple-grey tones) are associated with the marginal escarpment and also it crosses this axis (Moore, 1999). This the central Zimbabwe watershed. This high ground surrounds the Cenozoic Kalahari contrasts with the concave-up profile Basin (KB). Elevations in metres. expected from headward erosion, but is consistent with uplift across the river course (Du Toit, 1933; Partridge, 1998). In the case of the OKZ Axis, there is clear evidence for reversal of drainage flow directions across this line of flexure in Botswana (Moore, 1999). In Zimbabwe, this axis was responsible for beheading an earlier drainage network established in the Permian (Moore and Moore, 2006; Moore et al., 2009). The EGT Axis forms the southern boundary of the Kalahari Formation, which crosses a variety of bedrock formations (Haddon, 1999, 2001), demonstrating that the axis is not lithologically controlled (Fig. 4). Further north, the locus of the OKZ Axis corresponds closely with the east- ern and western margins of this sedi- mentary unit. This underlines the observation made by Du Toit (1933) Fig. 2 Drainage system of southern Africa. Colours denote stream rank from red (1) that subsidence of the Kalahari Basin to purple (5). M, Molopo River; N, Nossob River; MM, Mahura Muhtla. The major was closely associated with uplift along river divides are interpreted to reflect epeirogenic uplift Axes. EGT Axis, Etosha– the river divides that define these two Griqualand–Transvaal Axis; OKZ Axis, Ovambo–Kalahari–Zimbabwe Axes. Data axes. from USGS EROS, http://eros.usgs.gov/products/elevation/gtopo30/hydro/af_ Several lines of evidence show that streams.html the flexures forming the three major river divides in southern Africa were initiated in discrete episodes (Moore, observed concentric pattern of river 1999). Uplift along the Escarpment Evolution of the topography of divides in southern Africa. In short, Axis was initiated by the opening of southern Africa the first-order topography of south- the Indian and Atlantic Oceans at ern Africa does not correspond with The three major river divides all c. 126 Ma (King, 1963; Moore and the dynamic topography predicted by cut across geological boundaries and Blenkinsop, 2006). This is reflected by the plume model. Therefore the structural lineaments (Fig. 3), and a major Early Cretaceous erosional observed topography demands alter- therefore do not reflect simple litho- event, particularly marked along the native explanations. logical controls. Rather, the divides margins of southern Africa (Brown Ó 2009 Blackwell Publishing Ltd 311 Topography and erosion history: plumes or plate tectonics? • A. Moore et al. Terra Nova, Vol 21, No. 4, 310–315 ............................................................................................................................................................. would have rejuvenated drainages with headwaters on this
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