Southern African topography and erosion history: plumes or plate tectonics?

Andy Moore^1,2, Tom Blenkinsop³ & Fenton (Woody) Cotterill⁴

¹African Queen Mines Ltd., Box 66, Maun Botswana.

²Dept of Geology, Rhodes University, Grahamstown, South Africa, andy.moore@info.bw

³School of Earth and Environmental Sciences, James Cook University, Townsville, QLD4811, Australia, thomas.blenkinsop@jcu.edu.au

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, fcotterill@gmail.com

This webpage is a summary of: Moore, A., Blenkinsop, T. & Cotterill, F. Southern African topography and erosion history: plumes or plate tectonics?, Terra Nova, 21, 310-315, 2009.

ABSTRACT

The physiography of southern Africa comprises a narrow coastal plain, separated from an inland plateau by a horseshoe-shaped escarpment. The interior of the inland plateau is a sedimentary basin. The drainage network of southern Africa is characterized by three river divides, broadly parallel to the coastline. These features contrast strongly with the broad dome and radial drainage patterns predicted by models which ascribe the physiography of southern Africa to uplift over a deep mantle plume. The drainage divides are interpreted as axes of epeirogenic uplift. The ages of these axes, which young from the margin to the interior, correlate closely with major reorganizations of spreading regimes in the oceanic ridges surrounding southern Africa, suggesting an origin from stresses related to plate motion. Successive uplifts of southern Africa were focused along respective epeirogenic axes, forming the major river divides. These events initiated cyclic episodes of denudation, which are coeval with erosion surfaces recognized elsewhere across Africa.

The interior of southern Africa forms part of a belt of elevated ground, extending to East Africa, termed the “African Superswell” (Nyblade & Robinson, 1994), which is anomalously high (>1000 m) relative to average elevations of 400-500m for cratonic areas on other continents (Lithgow-Bertelloni & Silver, 1998; Gurnis et al., 2000). The latter two studies conclude, on the basis of theoretical geophysical modelling, that the anomalous elevation of southern Africa is related to the dynamic effects of the extant “African Superplume”. Their models predict that the plume-sustained topography of southern Africa will approximate to a broad dome, with an implied radial drainage pattern. However, this is completely at odds with the observed first-order topography, with the interior of the country being the site of the relatively low-lying Kalahari sedimentary basin, surrounded by a horseshoe arc of high ground that is closely associated with the marginal escarpment (Figure 1). Moreover, the drainage system of southern Africa defines a remarkable pattern of three concentric river divides, broadly parallel to the continental margin (Figure 2), and completely at odds with the radial drainage pattern implied by the plume model.

Figure 1: SRTM digital elevation image for southern Africa. The highest elevations (purple-grey tones) are associated with the marginal escarpment and the central Zimbabwe watershed. This high ground surrounds the Cenozoic Kalahari sediments, whose extent is depicted by dotted line. Elevations in meters.

Figure 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 major river divides are interpreted to reflect epeirogenic uplift Axes. EGT Axis = Etosha-Griqualand-Transvaal Axis; OKZ Axis = Ovambo-Kalahari-Zimbabwe Axes. Data from USGS EROS.

The three major river divides in southern Africa cut across boundaries between Archaean cratons and surrounding Proterozoic mobile belts, as well as other major structural features such as the Great Dyke (Zimbabwe), Okavango Dyke Swarm (Botswana) and late-Proterozoic Damara belt (Namibia) (Figure 3). This argues against a primary lithological control, and they have been interpreted rather as axes of epeirogenic flexure (du Toit, 1933; King, 1963; Moore, 1999). They are designated, from the coast inlands, the Escarpment Axis, Etosha-Griqualand-Transvaal (EGT) Axis and Ovambo-Kalahari-Zimbabwe (OKZ) Axis respectively (Figure 2). The EGT and OKZ axes are closely associated with the margins of the Kalahari basin (Figure 4), indicating that the latter was controlled by uplift along these two flexures (du Toit, 1933).

Figure 3. Loci of major river divides, inferred to reflect axes of epeirogenic flexure, in relation to the geology of southern Africa. These divides all cross boundaries separating Archaean cratons from the surrounding Proterozoic terrains. In Namibia, they traverse the northeast-trending late Proterozoic Damara belt at a high angle. In Zimbabwe, the O-K-Z axis transects the granite-greenstone terrain of the Zimbabwe craton, cuts across the NNE-trending Great Dyke, and continues across the Okavango Dyke swarm in Botswana. The central E-G-T axis crosses the north-trending Kheiss belt at right angles, while much of the eastern section of the Escarpment axis traverses readily eroded horizontal Karoo sediments. Geology of southern Africa after De Wit et al. (2004).

Figure 4. Distribution of the Kalahari Formation (Kalahari Sands) in relation to the epeirogenic axes defined by the major river divides. Note how the EGT Axis encircles the southern margin of the Kalahari basin. Further to the north, the eastern and western margins of the basin are bounded by the OKZ Axis.

Independent geological evidence and apatite fission track (AFT) dating, summarized in the original paper by Moore et al. (2009), shows that the three axes represented by the major river divides are of different ages (Escarpment: early Cretaceous; EGT: Upper-Cretaceous to early Palaeogene; OKZ: late Palaeogene respectively), and thus young from the coast towards the interior. Their ages in turn correspond closely with major reorganizations of the spreading ridges surrounding Africa (Figure 5). Thus, initiation of the Early Cretaceous Escarpment Axis matches the opening of the Atlantic and Indian at ~126 Ma (McMillan, 2003). The age of the EGT Axis corresponds closely with major changes of the Atlantic and Indian ridge spreading poles at ~84 and ~90 Ma respectively (Nürnberg & Müller, 1991; Reeves & de Wit, 2000). The late Palaeogene OKZ axis is broadly coeval with a major spreading reorganization in the Indian Ocean (Reeves & de Wit, 2000) as well as a marked increase in spreading rate at the Mid-Atlantic Ridge (Nürnberg & Müller, 1991; Figure 5). These temporal correlations, coupled with the broad parallelism of the concentric river divides and the oceanic spreading ridges surrounding southern Africa, suggests that uplift along the continental flexures was linked to deformation events associated with plate reorganization. This implies long-range transmission of stresses, through the lithosphere, from the ridges into the continental interiors.

Figure 5. Comparison of the geological events that constrain the ages of uplift axes, Indian and Atlantic Ocean opening histories, offshore basin erosion histories and ages of alkaline volcanic rocks (based on Moore et al., 2008). Geologic events are: 1 – Start of Atlantic opening (McMillan, 2003); 2 – Maximum/minimum age bracket for disruption of Mahura Muthla paleo-drainage (Partridge, 1998); 3 – Increased sedimentation in the major Zambezi and Limpopo River deltas (Walford et al., 2005; Burke & Gunnell, 2008). Offshore unconformities data are from McMillan, (2003) within the Kwa Zulu, Algoa, Gamtoos, Pietmos, Bredasdorp, and Orange basins respectively (from top to bottom). Indian Spreading History from McMillan (2003) and Reeves & de Wit (2000): 1 – Initial rifting between Africa and Antarctica; 2 – Commencement of spreading; 3 & 4: Changes in Indian Spreading regime recognized by Reeves & de Wit (2000). Atlantic Spreading History (from Nürenberg & Muller, 1991; Dingle & Scrutton, 1974): 1- Rifting extends into southern Atlantic Ocean; 2 – Commencement of opening of Atlantic (drift sequence); 3: Estimated time of separation of Falkland Plateau and Agulhas bank, based on assumed spreading rates; 4 – Major shift in pole of rotation of African/South American plates; 5 – Beginning of progressive shift in pole of rotation of African/South American plates. Sources of volcanic ages are quoted in Table 1 of Moore et al. (2008). Dashed lines and question marks are for the Chameis Bay pipes, denoting the two different ages indicated by field relationships and very limited radiometric dating. Click here or on image for enlargement.

The three ages of epeirogenic flexure that initiated the major river divides are all broadly contemporaneous with episodes of alkaline volcanism in southern Africa. However, while the axes young from the coast towards the interior, volcanic activity migrated in the reverse sense, from the interior towards the coastal margins. This inverse relationship is not readily explained by the plume hypothesis, and we conclude that volcanic activity was triggered by lithospheric stresses. In concordance with the mechanisms proposed by Oxburgh & Turcotte (1974), the broad upwarps represented by the flexure axes would be associated with relative tensional stresses in the upper surface of the plate. In contrast, the lower plate surface would experience relative tension beneath the basins surrounding the axes.

Our observations have an important bearing on one of the most celebrated debates in geomorphology. This is the concept of erosion cycles, championed by Lester King in papers published in 1949, 1955 and 1963, who recognized relics of successive erosion surfaces of different ages in southern Africa, including the African Surface of continent-wide distribution. Many of the criticisms of this model focussed on the underlying model to account for these surfaces, rather than the evidence for their existence. Successive uplifts along the axes represented by the major river divides would each result in episodes of drainage rejuvenation, thus initiating a new cycle of erosion. This provides a series of triggers that could account for the development of erosion surfaces of different ages. The ages of the axes correspond also closely in age to major unconformities recognized in the Congo basin (Cahen & Lepersonne, 1952; Girisse, 2005; Stankiewicz & de Wit, 2006), pointing to continent-wide episodes of erosion, as postulated by King (1963).

Acknowledgments

We thank Tyrel Flugel for producing the Digital Elevation image of southern Africa, and Dr. Marty McFarlane, Paul Green and two anonymous reviewers for their constructive comments on the manuscript that formed the basis for this web page.

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