An Integrated Geophysical and Geological Interpretation of the Southern African Lithosphere

Corner, Branko; Durrheim, Raymond J.

doi:10.1007/978-3-319-68920-3_2

Branko Corner⁸ &
Raymond J. Durrheim⁹

Part of the book series: Regional Geology Reviews ((RGR))

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Abstract

Southern Africa, here taken as the region that comprises the Kalahari and southern Congo Cratons, and the important orogenic belts that surrounded or separated them during the assembly of Gondwana, was situated in the heart of the supercontinent. As such, the region is an ideal site to study the lithospheric structure, composition and evolution of the supercontinent. A plethora of data sets, both geological and geophysical, are available in the public domain, including outcrop mapping, drilling results, aeromagnetic, gravity and magnetotelluric surveys, allowing mapping of extensive regions under cover. Deeper penetrating seismic reflection, refraction and teleseismic data, and also magnetotelluric data, have allowed the lithospheric interpretation to be extended to the middle and lower crust, and to the upper mantle. Interpretation has included, inter alia, mapping, or refinement of existing mapping, of the craton boundaries and associated terranes; major faults, structural lineaments and ring structures; specific features which have a geophysical expression, such as the Witwatersrand Basin, the Xade Complex and the tectonostratigraphic zones of the Damara-Ghanzi-Chobe Orogenic Belt; the Namaqua-Natal Belt and extensions thereof as the Maud Belt in Antarctica, as well as associated features such as the Beattie Magnetic Anomaly and Southern Cape Conductivity Belt; and the Namibian volcanic passive margin. Interpretations of the large-scale seismic, electrical resistivity, geomagnetic induction and magnetotelluric data by many workers have revealed conductive zones, terrane boundaries and continental-scale shear zones concealed by younger strata, and yielded important insights into the deep structure and evolution of the subcontinent. The topography of the Moho and the Lithosphere–Asthenosphere Boundary has also been mapped, showing that the Archaean Kaapvaal, Zimbabwe and Congo Cratons have deep roots that are relatively cold. As far as possible, the interpreted features honour the geological and geophysical data sets within the resolution of the data. Integration of these results in the unified interpretation map presented here brings new insights into both the disposition of selected geological features under cover, and the evolution of the Precambrian geology of southern Africa, extending into Antarctica within a Gondwana framework.

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1 Introduction and Chapter Layout

Southern Africa is an ideal site to study the structure, composition and evolution of the Gondwana supercontinent. Geophysical methods, integrated with the results of geological field mapping and exploratory drilling, play an increasingly important role in mapping rocks and structures, concealed by younger cover, from the near surface to the upper mantle. The Archaean-to-Palaeoproterozoic Kaapvaal, Zimbabwe and Congo Cratons are surrounded by Proterozoic metamorphic belts, platforms and basins, preserving more than 3600 million years of the Earth’s history (Hunter et al. 2006). These terranes are penetrated by kimberlite intrusions, which provide samples of the lower crust and upper mantle (Skinner and Truswell 2006). Hart et al. (1981, 1990a, b) propose that the 2023 Ma Vredefort meteorite impact near the centre of the Kaapvaal Craton turned the crust ‘on edge’, providing a window into the deeper crystalline basement. Geophysical studies of the structure of the continental crust and upper mantle have also been driven by the search for hydrocarbons, diamonds, base metals, and precious metals. While the resolution and accuracy of geophysical images and models has improved as technology and knowledge have advanced, early studies still remain relevant because they provide important constraints on lithospheric models, especially in areas where surveys have not been repeated. De Beer (2015a, b, c) provides a comprehensive review of the history of geophysics in South Africa. We briefly also review the historic investigations that were conducted 20 or more years ago. More recent work is discussed in greater detail, as well as new aspects of interpretation of magnetic and gravity data sets. The scope of coverage is enormous, thus the mapping is selective, focusing on geophysically evident features and selected geological units that may not have a geophysical expression but which are relevant to the interpretation.

The early regional aeromagnetic and gravity data sets covering southern Africa have proved invaluable in mapping regional structure. More recently, higher-resolution data sets have become available in Namibia and Botswana, as well as magnetotelluric (MT), seismic reflection and teleseismic array studies. The interpretation presented here follows the norm of working from known, mapped geology, as published by the various geological surveys and geoscience councils, to mapping extensions thereof under cover using the geophysical data sets presented here, either by us or as referenced. Emphasis is placed on the mapping of large-scale structures, including major faults, lineaments and ring structures, and extensions of specific lithologies and tectonostratigraphic zones, many of which were not recognized in past studies of these geoscience data sets. Much of what is presented here is, in the first instance, observational, identifying and mapping many new features and thus creating a basis for further research on their nature, genesis and geological evolution. Although potential field data has formed the basis of the presented interpretation, and includes consideration of the MT and seismic results, we recognize that complete integration of the data sets has not been fully achieved. The prime reason for this is that the various geophysical studies had different key questions, and depths of investigation that did not always overlap, ranging from the near surface to the mantle. In addition, aspects of the chapter address potentially different readership interests, particularly in Sects. 2.2 and 2.3. Figure 2.1 shows the extent of the interpretation area within southern Africa.

The coverage of the chapter, bearing the above comments in mind, is based on the type of geophysical data used—that is, commencing with potential field data integrated with known geology, progressing to published magnetotelluric and seismic data and interpretations, as summarized briefly below:

Interpretative mapping of specific Archaean and Proterozoic geological features and their boundaries, in areas covered by Phanerozoic or younger sediments, largely using magnetic and gravity data sets integrated with published outcrop geology—that is:
- Kaapvaal, Zimbabwe, Kalahari and southern Congo Cratons, their boundaries and intracratonic belts or zones (e.g., Limpopo Terrane (LP) and Magondi Belt);
- aspects of crustal magnetization;
- the Witwatersrand Basin and its extensions;
- the Xade Complex in Botswana;
- the Meso-Proterozoic Sinclair-Rehoboth Groups, and Grootfontein Metamorphic Complex;
- tectonostratigraphic zones of the Damara-Ghanzi-Chobe Belt, and the Gariep Belt;
- the Rehoboth Terrane, and deep Neo- and Late Meso-Proterozoic basins in Namibia, Botswana and Angola;
- the Karas Impact Structure;
- the offshore Namibian passive volcanic margin;
- the Namaqua-Natal Belt, the Khoisan Province, the Beattie Magnetic Anomaly and the Southern Cape Conductivity Belt;
- extensions of the Namaqua-Natal Belt within Gondwana—the Maud Belt, Antarctica;
- major, geophysically evident faults and structural lineaments.
Electrical resistivity, magnetotelluric, and regional seismic investigations which complement the above interpretation and extend it to the deeper crust and upper mantle:
- reflection seismic data sets which have been used to search for reservoirs containing oil and gas on land and at sea, and also to map and explore for extensions of the Witwatersrand Basin and Bushveld Complex;
- electrical and seismic characteristics which have been used to determine the thickness of the crust and lithosphere and to map the boundaries between cratons and mobile belts;
- several important regional zones of anomalous conductivity and magnetization have been discovered, as noted also above—that is, the Southern Cape Conductivity Belt, Damara-Chobe Conductivity Belt and the Beattie Magnetic Anomaly. Considerable research has been conducted to map, and determine the cause of, these anomalous zones.

2 Potential Field Data Sets: An Integrated Interpretation

2.1 Magnetic and Gravity Data

The regional aeromagnetic data sets covering southern Africa, mostly acquired pre-1990, have proved invaluable in mapping stratigraphic units, specific lithologies and regional structure under the cover sequences. These data, shown as a reduced-to-the-pole (RTP, of the total magnetic intensity) image in Fig. 2.2, were acquired at flight-line spacings varying from mostly 1–4 km, under contract to the various geological surveys and geoscience councils of the countries covered by the map. The more recent medium-resolution Botswana aeromagnetic data (200–250 m line spacing, degraded to a 500 m grid interval) was merged into this grid. The image also includes an offshore aeromagnetic survey, flown under contract to the National Petroleum Corporation of Namibia (Pty) Ltd (NAMCOR) over the continental shelf area at a line spacing of 25 km. Some of the interpreted geological features are derived from the higher-resolution data sets in Botswana and Namibia. Also used in the interpretation was the Worldwide Earth Magnetic Anomaly data set, compiled from merged satellite, airborne and marine magnetic data (EMAG2; Maus et al. 2009). Given the relatively low 2 arc-min grid spacing, the higher-frequency (shallower) anomalies are degraded but the grid preferentially enhances some of the regional structures (Fig. 2.3). The offshore areas of this data set, compiled from satellite and marine magnetic data sets, provide a wealth of structural and seafloor spreading information.

The Bouguer Anomaly image shown in Fig. 2.4, to which a regional-residual separation filter has been applied, was derived from mostly ground-based national gravity data sets of Botswana, Namibia, Zambia, Mozambique, Swaziland and Lesotho, as well as from an older-generation South African data set of the Geological Survey (now the Council for Geoscience). Also shown in Fig. 2.4 is the satellite-derived NAMCOR offshore Free-Air anomaly data set covering the continental shelf of Namibia. Further gravity data sets used for interpretation were extracted from the World Gravity Map and related products, compiled by the Bureau Gravimetrique International (Balmino et al. 2012; Bonvalot et al. 2012) from satellite, airborne and ground data. Four data products were available in XYZ format at a 2 arc-min grid spacing—that is, the Bouguer Anomaly data shown in Fig. 2.5, and the Isostatic Anomaly, surface Free-Air Anomaly data and the ETOP-1 Topographic data, which are not shown here owing to space constraints. These data sets were gridded at a 3500 m interval. Although the resolution is reduced with the Bureau Gravimetrique International gravity data sets, the regional structure is relatively clear, as with the aeromagnetic data. Of particular value is the gravity data over Angola, where more detailed private sector ground and airborne data sets are not available.

2.2 Interpretation Methodology

The interpretation maps in Figs. 2.6 and 2.7 show geological units, stratigraphy and tectonostratigraphic zones that have been derived from either (1) published outcrop mapping, as indicated in the caption to the interpretation map (Fig. 2.6) or (2) from geophysical interpretation by the authors and their co-authors, or other authors as referenced in each case. Interpretation was aided in part by numerous magnetotelluric (MT) surveys discussed in Sect. 2.3. A number of filters were applied to the above potential field data sets prior to interpretation so as to enhance structure, lithological fabric and contact locations. In the case of the magnetic data, Reduction-to-the-Pole (RTP) of the Total Magnetic Intensity (TMI) was applied for specific local-scale interpretations. Filters applied to the RTP data set included the First Vertical Derivative, Gaussian residual filters, Analytical Signal, and Total Horizontal Derivative. In the case of the gravity data sets, a Gaussian regional-residual separation filter was applied to each of the three Bureau Gravimetrique International gravity grids in order to reduce high-frequency noise, evident in the data, before further filtering to enhance lithological anomalies. The interpretation methodology followed the norm of working from the known, mapped geology and structure to a projection thereof under areas of cover using the above geophysical data sets. The derivative and residual filters provided the resolution required to map both local- and regional-scale structures. However, owing to the scale of presentation, many of the smaller, local structures have been excluded from the interpretation maps presented here. All features shown in the maps honour both the mapped geology and the geophysical data sets as best possible within the resolution of the data sets.

The interpretation maps (Figs. 2.6 and 2.7) show numerous geophysically evident faults and lineaments, many of which have been previously identified and discussed (e.g., Corner 2008). The term ‘lineament’ is used here, in the definition of Richards (2000), to denote a large-scale fault zone, or structural corridor, having a much broader swathe of manifestation up to 50 km in width, either continuous or disrupted. Geophysically, lineaments reflect approximately linear structures or zones with anomalous physical properties that range in depth from surface to 5 km or more, depending on the size of the source bodies and their physical property contrast. The lineaments, shown as lines on the interpretation maps, thus reflect the locus of a much broader structural corridor.

A number of ring features have also been identified (Corner 2000, 2008), which are considered to be the manifestation of ring fractures or faults, or alteration aureoles. Their origins may be varied and possibly include:

ring fractures or faults associated with magmatism and associated intrusions;
alteration aureoles associated with intrusions;
alteration aureoles associated with exhalative vents;
meteorite impact;
craton-scale ring structures resulting from plate rotation or changes in lithospheric thickness.

Examples of ring features are provided and discussed by Corner (2008) and Corner et al. (1997). For their arcuate geometry to be preserved, which may cut across older structural fabric, the ring features would have to be post- or late-tectonic in the first instance. It is also considered possible, as with many major faults, that the above foci or causative sources may have been reactivated through geological time owing to crustal weakness. In this sense, a ring feature may be evidence of an older reactivated focal source. A craton-scale source (last item in list above) is best exemplified by the Kaapvaal Ring Structure (KVRS; Fig. 2.7), which may have resulted from rheological or structural variations in the deep crust or upper mantle. It is evident in the south as the arcuate Namaqua-Natal Front, separating the Kaapvaal Craton and the Namaqua-Natal Belt, whereas its northern arcuate sector is evidenced through fault-trace analysis of the aeromagnetic data. Figure 2.16 shows an overlay of the KVRS on the 200 km P-wave velocity model depth slice. It clearly encompasses the high-velocity root of the Kaapvaal Craton and is interpreted here to arise from relative movement between this root and the surrounding lower velocity zones, perhaps initiated during plate movement along the zones of competency contrast.

Examples of ring structures that are interpreted to arise from meteorite impact events are, firstly, due to the Morokweng Impact event (MIS; Fig. 2.6; MRS; Fig. 2.7; e.g. Corner 1994a; Andreoli et al. 1995; Corner et al. 1997) and, secondly, due to the interpreted Karas Impact event (KIS; Fig. 2.6; KRS; Fig. 2.7; Corner 2008; Section 2.2.7.2)

2.3 Archaean and Palaeoproterozoic Cratons

2.3.1 Introduction to the Interpretation of the Archaean and Proterozoic Geology

The various Archaean and Proterozoic stratigraphic units shown in the interpretation map (Fig. 2.6a, Legend 2.6b) are derived, first, from mapped geology as published by the relevant geological surveys, geoscience councils and institutes of the countries covered, and, second, from the interpretation of magnetic and gravity data sets, constrained by the outcrop data and limited published borehole data in the areas of cover. Cover sequences that have not been shown in this interpretation are defined here as being Phanerozoic in age, ranging approximately from the Cambrian to recent, with one exception—that is, the magnetic basalt flows and dolerite sills of Karoo age, including the Etendeka basalts. The distribution of these rocks is shown so as to indicate those areas where interpretation of the underlying suboutcrop geology is compromised as a result of the presence of these highly magnetized strata in the cover sequence.

Much geophysical research has been conducted in the southern African region. Aspects that are highlighted and referenced in this review include (1) interpretation of gravity and magnetic data, particularly of Precambrian features which have a clear expression in this data; (2) geomagnetic induction, Magnetotelluric (MT) and deep electrical resistivity studies; (3) deep seismic reflection surveys, conducted both by industry in its quest to locate extensions to the Witwatersrand Basin and by the South African National Geophysics Programme; (4) deep seismic refraction surveys; and (5) teleseismic studies of the cratons, underlying mantle, and adjacent polymetamorphic terranes. Many geological publications on the Precambrian geology of southern Africa show variable boundaries or delineations of the cratons and surrounding terranes in the areas of cover, as well as of key regional structures. Often, the detail which geophysical data can and does give has been ignored or at best loosely interpreted. We have sought to be rigorous in the mapping of these features, carefully honouring the detail of geological, magnetic and gravity data sets on both regional and local scales. The latter interpretations are not shown here owing to the scale of this presentation, but have been used to constrain the regional mapping shown here, as referenced where appropriate. The lines indicating craton and terrane boundaries should be considered to represent broader complex swathes of varying structural style and dip, drawn at their shallowest manifestation.

2.3.2 Kaapvaal Craton

The Kaapvaal and Zimbabwe cratons were formed and have grown through accretion, thus comprising crustal blocks of different ages with different structural styles, from the Archaean to the Proterozoic. The boundaries thereof are mostly clearly revealed by the gravity and magnetic data. The assumption is made here that major structures or structural zones, as evidenced in the geophysical data, with geological control where available, constitute the craton boundaries. This applies similarly to the surrounding polymetamorphic terranes. De Beer and Meyer (1984) were the first to geophysically map a portion of the Kaapvaal Craton boundary covered by Phanerozoic rocks, delineating the arcuate southern craton margin by modelling a number of gravity profiles across it, where it is juxtaposed against the Namaqua-Natal Belt.

The Kalahari Lineament (Kal-L; Fig. 2.6; Kalahari Line, Reeves 1978), interpreted as the western Meso-, Palaeo-Proterozoic boundary of the Kaapvaal craton, is one of the most dramatic features in the aeromagnetic image of southern Africa (Figs. 2.2 and 2.6), separating relatively shallow basement to the east (with mostly less than 1 km of cover) from extremely deep magnetic basement beneath the Rehoboth Terrane (RT) to the west, where cover thicknesses have been determined to vary from 6 to 10 km (Reeves 1978; Corner 2008). The magnetic signature of the Kal-L changes towards the south, where it bounds the Kheis Province (comprising the Olifantshoek Supergroup rocks) in the west, but the craton boundary is nevertheless clear as mapped in Fig. 2.6. Moen (1999) places the western limit of the Olifantshoek Supergroup at the Dabeep fault, which lies roughly centrally between the craton boundary and the Kheis Lineament (Kh-L) in Fig. 2.6. No justification for a major structural boundary is seen in the geophysical data in this central area. Recent gravity data (Botswana Geoscience Institute) indicates that a gravity high is associated with the Kal-L, although not ubiquitously. This suggests that the Kal-L magnetic signature results in part from mafic intrusions, probably mostly of late Mesoproterozoic Umkondo age (c. 1.1 Ga; Meixner and Peart 1984; Hanson et al. 2006; Cornell et al. 2011), and in part from magnetization interpreted to arise from hydrothermally altered granite-gneiss, associated with this major suture, where no clear gravity high is evident (Corner 2008). Localized high magnetic anomalies evident within the anomalous gravity zone are most likely associated with smaller-scale mafic intrusions, such as the Tshane Complex.

A number of features hallmark the Kal-L as a major crustal structure, including the enormous change in depths to basement, as described above, and apparent separation of Precambrian stratigraphy and terranes of differing structural styles, from its east to its west. It has been inferred to be a zone of major collision (Meixner and Peart 1984) or a zone of major transpression (Cornell et al. 2011). Its age has been inferred to be post-Waterberg (Olifantshoek Supergroup equivalent)—that is, post-Eburnean (Reeves 1978). Early interpretations of the Kal-L (e.g., Reeves 1978) identify a separate crustal terrane, the Okwa basement, north of the Makgadikgadi Fault, where it cuts the Kal-L. Many authors interpret the Zoetfontein fault to be the northern boundary of the Kaapvaal craton, constituting the southern boundary of the larger Okwa terrane north thereof (e.g., De Wit and Tinker 2004), a view most likely based on the original interpretation by Reeves (1978), who separated the basement north of the Zoetfontein fault from cratonic basement to the south. Corner (1998) does not support the interpretation of the larger Okwa terrane (i.e., north of the Zoetfontein fault, Kaapvaal Craton Okwa Block; Fig. 2.6) as a crustal entity separate from the Kaapvaal Craton, since it is bounded in the west by the uninterrupted Kal-L. That the Zoetfontein fault is a major early fault, with post-Karoo faulting as its youngest manifestation, is beyond question. However, as with the Makgadikgadi fault, there is no disruption of the Kal-L by the Zoetfontein fault. The continuity of the Kal-L thus suggests that the crustal blocks accreted north and south of the Zoetfontein fault are at least of Meso-, Palaeo-Proterozoic age, if not earlier. Nevertheless, interpretation of the aeromagnetic data confirms a clear change in basement fabric north and south of the Zoetfontein fault, the two most dramatic examples being the truncation of the north-northwest-trending Pilanesberg dykes against it, with minor continuation north thereof beneath Karoo basalt, and the east-west disposition of the interpreted feeder dykes to the Xade Complex to its north (Figs. 2.2 and 2.6; Corner et al. 2012). A change in crustal level within the Kaapvaal Craton is thus inferred across the Zoetfontein fault zone. The Okwa terrane, as described above, is referred to as the Okwa Block here, and interpreted to be part of the Kaapvaal Craton. Other authors also recognize the possibility that the Okwa Block is one of a mosaic of blocks making up the Kaapvaal Craton (e.g., Eglington and Armstrong 2004). Further subdivisions internal to the Kaapvaal Craton have been published. For example, Schmitz et al. (2004) recognize separate entities west (Witwatersrand Block) and east (Kimberley Block) of the Colesberg Lineament.

Reeves (1978) identified two sub-basins west of the Kal-L, overlying the Rehoboth Terrane—that is, the Ncojane Basin north of the Makgadikgadi fault, and the Nosob Basin to the south. The distinction between these two sub-basins is not recognized here, other than a probable change in deformation northwards as the Tsumis-Ghanzi-Chobe Belt is approached (T-G-C-B; Fig. 2.6). The northern margin of the Kaapvaal Craton, adjoining the Kal-L, is clearly evidenced by a similar dramatic change in depth to basement beneath the Passarge Basin (PB; Fig. 2.6), which is underlain by the T-G-C-B. Further east, the Palala shear zone (PaSZ; Fig. 2.6) is seen to trend north-westwards towards the PB, in both the gravity and magnetic data sets, constituting the boundary between the Kaapvaal Craton and the Limpopo Terrane (LT; Fig. 2.6). The eastern craton margin is well delineated by the Lebombo Belt (LB; Fig. 2.6), both geophysically and geologically.

The southeastern boundary of the Kaapvaal Craton, extending through Lesotho to the Tugela Thrust Front (TTF; Fig. 2.6), is a broad complex tectonic zone of inherited Archaean and Mesoproterozoic crust separating the craton from the Natal Province. This zone has been termed the Tugela Allochthon (TA; Fig. 2.6), based on geophysical and isotopic studies by Barkhuizen and Matthews (1990), De Wit and Tinker (2004), Eglington and Armstrong (2004); Schmitz and Bowring (2004). Although no magnetic data is available for Lesotho, the full gravity coverage has been merged into the residual Bouguer Gravity image of Fig. 2.4. Two gravity highs are evident in Lesotho, roughly parallel to the craton margin. At first sight these might be taken to be part of the Kaapvaal Craton, roughly on-strike with, although detached from, the gravity high (and coincident magnetic high) flanking the southern Witwatersrand Basin in the south. Alternatively, these highs might be interpreted to be relicts of the (unknown) source of the Bethlehem Gravity High (BGH; Fig. 2.6). However, the latter is of much higher amplitude and lacks any magnetic expression. Schmitz and Bowring (2004) have shown, from geochronological and isotopic data on lower crustal xenoliths from the Lesotho kimberlites located just northeast of the two Lesotho gravity highs, that granulitization of the lower crust was a relatively young phenomenon, c. 1.0–1.1 Ga, which affected pre-existing Archaean to Mesoproterozoic crust. The gravity highs are thus assumed to be within the allochthonous zone (e.g., of De Wit and Tinker 2004), as are the smaller-scale gravity highs further east (excluding the known mafic intrusive sources). The Tugela Thrust Front clearly demarcates the northern boundary of the TA in the northern sector of the Natal Province. A possible continuation of the Kaapvaal Craton into Dronning Maud Land, Antarctica, where the Grunehogna Craton has been identified, is discussed in Sect. 2.2.10.3.

2.3.3 Zimbabwe Craton, Limpopo Terrane, Magondi Belt and the Kalahari Craton

The disposition of the Zimbabwe Craton (ZC; Fig. 2.6), and associated Magondi-Gweta Belt (MGB; Fig. 2.6) and Limpopo Terrane (LT; Fig. 2.6), has been revisited here in as much detail as the mapped geology and geophysical data sets allow. The boundary between the ZC and LT is relatively well defined from geological mapping in southern Zimbabwe, northern South Africa and eastern Botswana, and from a clear change in magnetic fabric across the boundary. Working westwards, this magnetic fabric loses clarity under the cover of Karoo basalts, although it is still evident in places. The MGB has a semilinear magnetic fabric that is not dissimilar to that of the LT. A faulted contact is suggested between the two belts in Fig. 2.6, but this may be a local feature, and the possibility thus exists that the LT and MGB constitute a continuous deformation zone encompassing the ZC in the south and west. Mapping of the eastern boundary of the ZC is limited by the both the paucity and the coarseness of the geophysical data.

The Mwembeshi Shear Zone in southern Zambia (MSZ; Fig. 2.6), a major roughly east–west dislocation zone, has been extended using the aeromagnetic data, as shown in Fig. 2.6 (see also Fig. 2.2)—that is, in the east it is seen to curve southeastwards toward the ZC boundary. Two other regional structures, with a similar roughly east–west trend, are mapped to its south. Fault mapping, derived from both the geological mapping and the geophysical interpretation, shows a continuation of these regional structures eastward into Zimbabwe. One of these (in northern Zimbabwe) is a major post-Karoo fault, at its youngest manifestation, preserving a deep Karoo basin to the north—that is, the Lower Zambezi Zone of the Cabora Bassa Basin. These regional fault zones appear to curve into the ZC boundary, suggesting possible later transpressional movement along the boundary. Of interest is that the northeast-trending magnetic fabric of the Magondi-Gweta Belt in Zimbabwe is seen to change direction dramatically, truncating against the above east–west-striking fault zone, and trending westwards into Zambia south of the Mwembeshi Shear Zone, initially following the east–west trend of this zone. Deep-seated magnetic sources are also shown within the basement in Zambia (Fig. 2.6), which mimic the interpreted westward trend of the Magondi-Gweta Belt. We find no evidence in the geophysical data for any continuity between the LT and the so-called Okwa Block, as inferred by a number of workers.

Jacobs et al. (2008) discuss the evolution of the Kalahari Craton, describing it as ‘having been spawned from a small Archaean core which grew by prolonged crustal accretion in the Palaeoproterozoic … to form the Proto-Kalahari Craton by 1750 Ma’. They include the Grunehogna Craton in Dronning Maud Land, Antarctica, in their definition of this Archaean core (as discussed in Sect. 2.2.10.3). From c. 1400 Ma, all margins of the Proto-Kalahari Craton recorded intense tectonic activity, and by c. 1050 Ma the Proto-Kalahari nucleus was almost completely rimmed by voluminous Mesoproterozoic crust, becoming a larger entity, the Kalahari Craton (Jacobs et al. 2008). The outline of the Proto-Kalahari Craton, as defined by Jacobs et al. (2008) above, is shown in Fig. 2.1.

2.3.4 Congo Craton in Namibia and Botswana

McCourt et al. (2013) describe the Angolan Shield, c. 2.0 Ga, as being a Palaeoproterozoic basement terrane dominated by granitoids, together with a limited amount of Neo-Archaean crust, which extends from south of Lubango in Angola into Namibia, and eastward under cover into Zambia. This defines the southwest section of the Congo Craton. The craton was intruded by the c. 1385 Ma Kunene Complex, the areal extent of which indicates an extensive period of Mesoproterozoic crustal extension (McCourt et al. 2013). Delineation of the southern boundary of the Congo Craton in northern Namibia, and its continuation into Botswana and Zambia, has been the subject of much speculation in view of, firstly, the extensive Karoo and Kalahari cover in the central and eastern areas of northern Namibia and, secondly, the complex structural evolution that hallmarks the Kaoko Zone in the northwest (see Sect. 2.2.6.3 ). One is left with geophysical signatures (magnetic and magnetotelluric), and in places a lack thereof, which have often been variably interpreted, to map the craton boundary.

A significant crustal-scale magnetic anomaly, characterized by deep-seated high-amplitude anomalies arising from the crust beneath the Namibian Northern Platform and a southern dominant magnetic low, strikes northeastwards across northern Namibia, as is readily evident in Fig. 2.2. Eberle et al. (1995) modelled a number of magnetic profiles across key structural features in Namibia, of which three traversed this regional magnetic anomaly. Their modelling, based on dipping prism-shaped bodies with induced (normal) magnetization, consistently suggested the presence of a regional-scale antiformal structure situated beneath the southern portion of the carbonate platform. This may indicate the southern abutment of the Congo Craton (CC). We thus interpret this regional magnetic anomaly and associated structure to delineate the boundary of the Congo Craton, as shown in Fig. 2.6, which is a refinement of an earlier interpretation which placed the boundary further north (Corner 2008). The magnetotelluric work of Khoza et al. (2013a, b), and seismic tomography studies of Raveloson et al. (2015), provide support for the boundary mapped here. The Khorixas-Gaseneirob Lineament (KG-L; Miller 2008a; Fig. 2.6) is thus considered to constitute the near-surface manifestation of the southern boundary of the Congo Craton. The continuity of the southern east–west-trending portion of the craton boundary has been disrupted by a number of major structural lineaments, as mapped in Fig. 2.6, including the Welwitschia (We-L) and Kudu (Ku-L) Lineaments in particular. This relatively well-defined regional magnetic signature of the southern Congo Craton boundary disappears northwestwards, where the craton boundary is inferred to continue along the north-northwest-trending Purros Shear Zone (PuSZ; Fig. 2.6). Similarly, in the east, the Congo Craton boundary, as evidenced in the magnetic data, appears to terminate abruptly in northwestern Botswana against what is newly interpreted here as the north-trending Tsodilo Lineament (Ts-L; see also Sect. 2.2.6.3 ).

The Mesoproterozoic Grootfontein Inlier (GI; also known as the Grootfontein Metamorphic Complex; Miller 2008b) is located in north-northeastern Namibia and extends eastward into Botswana, where it is inferred to continue as the Quangwadum Inlier (QI; Fig. 2.6) and to include the Chihabudum Complex. A singular combination of geophysically evident regional-scale features occurs within the Inlier—that is, an annular zone of high magnetization encompasses a low-magnetic central zone, referred to as the Omatako remanent anomalies (OR; Fig. 2.6; Corner 2000), which suggest deep, probably remanent, sources. These, and the encompassing annular high-magnetic zone, are collectively referred to as the Omatako Ring Structure (ORS; Fig. 2.7 ). The ORS occurs at the intersection of a number of major lineaments, including the Omaruru and Kudu Lineaments, and lies within the swathe of the west-northwest-trending Botswana dyke swarm that crosses the subcontinent (BDSL; Fig. 2.6). A number of deep, roughly linear magnetic anomalies radiate to the east, north and west of the OR/ORS, forming part of a much larger regional structural feature. The Grootfontein Inlier is mostly characterized by strongly magnetic gneisses. The radial anomalies are thus interpreted to arise from radial faults within the Complex, with a focus on the ORS. The area is covered by Kalahari and Karoo sediments, as well as by Karoo basalts, thus the origin of the ORS and associated features is unknown. Possible causes include a major volcanic eruptive centre, or even a meteorite impact site.

McCourt and Jelsma (this volume) discuss the Angolan Craton and, of relevance here, age aspects of the Congo Craton in Namibia. They describe the continental crust forming the Grootfontein Inlier as comprising plutonic rocks of dominantly alkaline/calc-alkaline composition. The protolith of granitic gneisses from the related Tsumkwe and Quangwadum Inliers (TI, QI; Fig. 2.6) have been dated at 2022 ± 15 Ma (Hoal et al. 2000) and at 2051 ± 1 Ma (Singletary et al. 2003), which are compatible with inherited zircon grains at 2052 ± 44 Ma and 1987 ± 4 Ma in granite and felsic lava in the Kamanjab Inlier (KI; Fig. 2.6). They take this as evidence of correlation between the older crust, interpreted to be present at a depth below the KI, and the granite gneisses exposed in the Grootfontein, and related Quangwadum and Tsumkwe, Inliers. Archaean ages within the Palaeoproterozoic basement inliers of northern Namibia and Botswana have been reported in the Epupa Inlier (e.g., 2585.4 ± 1.2 Ma and 2645 ± 6 Ma; Seth et al. 1998; EI; Fig. 2.6) and in the Tsodilo Hills area associated with the QI (2548 ± 65 Ma, Gaisford 2010). The Archaean localities are shown as red stars in Fig. 2.6.

2.3.5 Crustal Magnetization

Interpretation of the aeromagnetic and gravity data covering the Witwatersrand Basin (Figs. 2.6), and its potential extensions to the south and west of the main basin, led to some fundamental new insights into the underlying craton (Corner et al. 1986a, b; Corner et al. 1990). As part of these studies, long-wavelength magnetic anomalies within the craton, relating to deep sources, were interpreted through forward modelling. The Vredefort Impact Structure (VIS; Fig. 2.6) has an annular highly magnetic zone in its gneissic basement core which, accepting the crust-on-edge model for the structure, would thus lie some 8 km beneath the West Rand Group of the Witwatersrand Basin. This zone was interpreted, using a typical P-wave velocity for the basement, to correspond with seismic reflectors in the granite-gneissic basement evident in a reflection seismic line traversing the basin west of VIS (Durrheim et al. 1991). These reflectors are parallel, or subparallel to the base of the basin. Projection of this intermediate-crustal magnetic, reflective zone, through forward modelling of the magnetic data, to its suboutcrop beneath the Karoo Sequence to the southwest of the Witwatersrand Basin, suggested that the deep, high-amplitude semilinear magnetic anomalies trending northward from Colesberg (Co-L; Fig. 2.6) were due to this same level of magnetized basement. Drilling by a mining company (Goldfields SA, pers. comm.) confirmed this interpretation. Corner et al. (1986c) named this anomalous belt the Colesberg Trend, here termed Lineament (Co-L; Fig. 2.6). This is inferred to be the locus of an early orogenic belt west of the Witwatersrand Basin, along which a major section of the upper crust has been eroded, possibly being the main source of the sediments that filled the western portion of the basin.

The confirmation of this interpretation led Corner (1998) to propose a model that attributed many of the high-amplitude, regional long-wavelength magnetic anomalies to arise from intermediate-crustal magnetization, particularly to the west of the Witwatersrand Basin, including the Colesberg Lineament (Figs. 2.2 and 2.6). Comparisons were made with other cratonic areas, particularly with studies of the structure of Russian cratons based on numerous deep crustal seismic refraction profiles (Pavlenkova 1987). These studies led Pavlenkova to propose a generalized three-layer craton model, distinguished on the basis of geological structure, seismic boundaries and P-wave velocities. Corner (1998) compared the above magnetization model with both the Pavlenkova three-layer craton model and the deep electrical resistivity results for southern Africa (Van Zijl 1978; De Beer and Meyer 1984; De Beer and Stettler 1988). Corner (1998) noted that the three-layer craton model could fully explain the extremely low resistivities observed in the deeper crust, which would thus occur below the upper, brittle, resistive crust, in an underlying hydrofractured, aseismic, probably fluid-filled middle crust (in the three seismic-layer model) where horizontal displacement stresses predominate. It is in this electrically conductive intermediate-crustal zone that Corner proposed the development of magnetization, through the growth of magnetite from iron in the protolith at elevated temperatures (below the Curie temperature for magnetite), in the presence of fluid.

Invoking the presence of an intermediate-crustal zone of magnetization, not necessarily ubiquitous throughout the craton but certainly prevalent in many areas particularly along major structures that may have allowed fluid movement, readily allows the interpretation, through forward modelling of both magnetic and gravity data sets, to explain many of the deeper features, such as the long-wavelength anomalies west of the Witwatersrand Basin near Wolmaransstad and Vryburg, the Colesberg Lineament (Co-L; Fig. 2.6), the Strydenburg Lineament to the west of the Colesberg Lineament (St-L; Fig. 2.6) and possibly portions of the Kal-L in Botswana (Corner 1998).

2.4 The Witwatersrand Basin

The main Witwatersrand (Wits) Basin is situated roughly centrally on the Kaapvaal Craton (Fig. 2.6). Economic exploitation since the discovery of gold in 1886 has seen the development of more than 150 mines, some of which have yielded uranium as a by-product. In terms of value of metal recovered, the basin’s mineralization must rank as one of the most valuable mineral deposits ever found. However, more than 90% of the basin is covered by younger sequences, ranging from Neo-Archaean, through the Palaeoproterozoic, to Phanerozoic. Geophysics played a critical role, at an early stage, in the discovery of new mines, in particular through the application of magnetic and gravity techniques. More recently, reflection seismic techniques have also been very successfully applied, both to locate new extensions and to map, in 3D, structures on a mine scale (e.g., Pretorius et al. 1989). This technology was further applied to mapping and evaluation of the Bushveld Igneous Complex in the search for platinum (e.g., Pretorius et al. 2010).

In terms of mapping the basin under cover, Borchers (1964) produced the first geological map based on outcrop, mining and drilling data. A more recent map, based mostly on additional geological data acquired since that time, partially constrained by geophysical data, was published by Pretorius (1986). Using this map and associated data as a base to work from, Corner and Wilsher (1989) conducted a rigorous reinterpretation of the basin, using both the aeromagnetic and ground gravity data to further upgrade the mapping. Corner et al. (1986a, b) also published the first ever digitally printed colour images, in the public sector, of the aeromagnetic and gravity data covering the basin. The mapping of Corner and Wilsher (1989) is replicated in Fig. 2.6. Although older than the overlying Ventersdorp and Transvaal Supergroups, the main Wits Basin and outliers are placed on top of these in Fig. 2.6 so as to provide the reader with a view of their actual extent and disposition. Corner et al. (1986c) extended their interpretation southwards, covering the southern portion of the Kaapvaal Craton. They identified the possible presence of outliers of Witwatersrand rocks southeast of the main basin in the Bethlehem area, and south of Bloemfontein, confirming what many geologists had proposed in the past. High gold prices at the time resulted in extensive exploration activities commencing in these areas.

The ensuing quest, by major and junior mining companies, for extensions of the Wits Basin in the Bethlehem area and south of Bloemfontein, using aeromagnetic, gravity and extensive seismic reflection surveys, resulted in many successful boreholes being drilled which intersected Witwatersrand Supergroup rocks, and Waterberg-equivalent rocks, in previously untested areas. Unfortunately, most of the intersections were in the barren West Rand Group and, in time, these activities were terminated. One such exploration programme was conducted by AfriOre (Pty) Ltd, which built on the data available from other companies that had withdrawn from the area. The first author (Corner) was a member of a team that interpreted these data as well as a more recent aeromagnetic survey flown by AfriOre (McCarthy et al. in preparation). Drilling intersected both Witwatersrand and Transvaal Supergroup rocks east and southeast of the Trompsburg Complex. Integrated interpretation, in a team approach, of all data resulted in the delineation of major Wits and Transvaal sub-basins underlying the Trompsburg Complex south of Bloemfontein, extending eastwards to Lesotho and to the Bethlehem area north of Lesotho (Fig. 2.6; McCarthy et al. in preparation). Both the Wits and the Transvaal sub-basins are separated from the main Wits Basin by the Bloemfontein Arch (BA; Fig. 2.6). This is a major contribution, built on all past exploration programmes, to the mapping of Archaean and Palaeoproterozoic rocks beneath an extensive cover of Karoo Supergroup rocks, in excess of 1000 m in thickness, in the southern portion of the Kaapvaal Craton.

2.5 Xade Complex

The Xade Complex (XC), situated in central Botswana, is a large singularly anomalous feature in the aeromagnetic and gravity images of southern Africa (Figs. 2.2, 2.4 and 2.6), occurring under a complete cover of sediments of the Kalahari Group and Karoo Supergroup, including Karoo volcanics in places, with a combined thickness that varies from 220 to 1000 m, probably extending to greater depths in the north beneath the Passarge Basin (PB). As such, it has drawn much attention both academically and from a minerals exploration point of view. It was first identified during the regional aeromagnetic survey of the country in 1975–1977 (Reeves 1978; Meixner et al. 1984; Figs. 2.2 and 2.4). The Xade Complex was originally interpreted to comprise a high-amplitude kidney-shaped zoned magnetic anomaly with two semilinear anomalies extending to the northwest and northeast in a Y-shaped form (e.g., Meixner et al. 1984). It is also evidenced by a coincident Bouguer gravity anomaly. Historical work was limited, with only three cored-boreholes having been drilled. Two of these were drilled as part of the Kalahari Drilling Project in the early 1980s, following interpretation of the aeromagnetic data (Meixner et al. 1984). One borehole intersected gabbroic rocks at 815 m, and the other a weathered basalt at 419 m, passing into dolerite. A third borehole was drilled by the Anglo American Corporation (Ambot 1998), which held exploration licences over the complex in the late 1990s. Amygdaloidal lava was intersected at 621 m, passing into dolerite, and shales assigned to the Waterberg Group. An U-Pb zircon age of 1109.0 ± 1.3 Ma, which is coeval with the Umkondo Igneous episode, has been published for the gabbroic unit intersected in the first borehole (Hanson et al. 2004).

A junior exploration company, Manica Minerals Ltd, held prospecting licences over the Xade Complex from 2005. Its exploration activities, conducted in joint venture partnerships with two other companies, included use of both the medium-resolution aeromagnetic data, acquired under contract to the Botswana Geoscience Institute at a 250 m line spacing, and the Institute’s ground Bouguer gravity data. An additional higher-resolution aeromagnetic survey was conducted over a portion of the complex. Detailed ground gravity surveys and time domain electromagnetic soundings were conducted on selected profiles traversing the complex. Forward modelling of the magnetic and gravity profiles helped constrain the zoning and structure of the complex. Three boreholes were subsequently drilled (Corner et al. 2012). A parallel interpretation was conducted using the Institute’s data, but without the benefit of the further exploration data, by Pouliquen and Key (2007).

The interpretation of the work of Corner et al. (2012) showed for the first time that the Xade Complex comprises two lobes: a Southern Lobe (XC-SL; Fig. 2.6), which is the historically identified kidney-shaped zoned magnetic anomaly, as well as a hitherto unrecognized large Northern Lobe (XC-NL; Figs. 2.2 and 2.6). The NL is mostly deeply buried in the north and northwest beneath the Neo-Proterozoic PB, as evidenced by deep magnetic and gravity anomalous sources, but its southern and eastern margins partially suboutcrop beneath Karoo sediments, forming the Y-shaped anomalies north of the SL. An apparent transgressive contact between the two lobes is indicated by the aeromagnetic data, suggesting that the NL may be slightly younger. Inversion depths to the complex range from 220 to 1000 m beneath the Kalahari and Karoo sediments, and greatly in excess of this beneath the PB. Forward modelling, of both magnetic and gravity data along a number of sections traversing the both lobes, indicates that they are lopolithic features with a depth extent of approximately 4 km. This is supported by the interpretation of Pouliquen and Key (2007) for the SL, as well as from dips derived from the drill cores. The total of four boreholes drilled into the SL shows that it comprises a volcanic sequence with subordinate gabbro (Corner et al. 2012). The basalts are partly highly magnetic, giving rise to the zoned high-amplitude anomalies of the larger kidney-shaped anomaly, and partly magnetically subdued owing to less magnetic basalts that appear to underlie the rocks of the main SL anomaly. The three boreholes drilled into the NL margins, as published to date, indicate that it comprises a texturally heterogeneous and magmatically differentiated sequence of gabbroic rocks, with minor dioritic and monzonitic rocks, as well as basalt (Corner et al. 2012).

The interpretation of Corner et al. (2012) has also identified a dyke system associated with the NL, which may represent either feeder or exit magmatic conduits. The interpretation further shows that the Xade Complex is located in a craton margin setting—that is, the SL lies on the northern margin of the Kaapvaal Craton, whereas the eastern suboutcrop of the NL extends along the margins of the Kaapvaal and Zimbabwe cratons (Fig. 2.6). The combined extent of both lobes of the Xade Complex is approximately a third the size of the Bushveld Complex, making it the largest Late-Mesoproterozoic magmatic complex in southern Africa, with a potential for nickel-copper mineralization.

2.6 Tectonostratigraphic Zones of the Damara-Chobe Orogenic Belt

2.6.1 Regional Aspects of the Interpretation

The tectonostratigraphic zones of the Namibian Damara Belt and its Mesoproterozoic basement, extending eastward into Botswana as the Damara-Chobe Orogenic Belt, were mapped under Kalahari cover to the eastern border with Botswana by Corner (2000, 2008), based on published mapping and stratigraphy, known bounding regional structures, and the overall internal magnetic signature of the zones. The aeromagnetic data used at that time was the national regional data set compiled from surveys flown at line spacings varying between 1 and 4 km. The interpretation has since been refined and updated, as shown in Fig. 2.6, being more accurate in local detail, based on the higher-resolution aeromagnetic data (200 m flight line spacing) and mapped geology at a 1:250 000 scale (all data from the Geological Survey of Namibia). Ongoing extension of this work into Botswana was facilitated by the publication of the 1998 edition of the National geological map of Botswana, in both digital and hard copy form; the work of Key and Ayers (2000), Singletary et al. (2003) and Rankin (2015); and the availability of the medium-resolution National aeromagnetic data of Botswana. These products have been reviewed, revised in places, and integrated with the Namibian interpretation shown in Fig. 2.6. Brief lithological, stratigraphic and structural summary descriptions of each tectonostratigraphic zone shown in the interpretation map of Fig. 2.6 follow below. For the Damara Belt in Namibia, these are based on detailed descriptions by Corner (2008) and Miller (2008a). Furthermore, an aeromagnetic survey covering the continental shelf of Namibia, flown under contract to NAMCOR (Fig. 2.2), although relatively coarse, has facilitated mapping of the extension of some of the tectonostratigraphic zones offshore, up to the seismically interpreted hinge zone (Sect. 2.2.9). It should be noted that the systematics of stratigraphic classification generally do not take cognizance of geophysical responses. Stratigraphic boundaries may thus differ from geophysically apparent boundaries. The Damara Sequence does, however, show a strong correlation in that the lower units, which include diamictites, psammitic rocks and volcanics of the Nosib and lower Swakop Groups, are often strongly magnetic, while the overlying pelitic and carbonate sequences of the Swakop Group tend to be relatively subdued magnetically.

2.6.2 Mesoproterozoic Sinclair-Rehoboth Zone

The Mesoproterozoic Sinclair-Rehoboth Zone (SRZ; Fig. 2.6) comprises the southern Damara Basement and is dominated by volcano-sedimentary cycles of the Rehoboth Group and the Sinclair Supergroup, as well as by granitic and mafic rock suites. The overall magnetic signature of this terrane is visibly different from that of the Gordonia Subprovince to the south, being dominated by relatively high-amplitude, curvilinear magnetic anomalies arising from the volcanic sequences. Some of the granites, granodiorites and orthogneisses are also magnetic. In contrast, the Gordonia Subprovince displays highly variable magnetic responses owing to a range of rock types, which include ortho- and para-gneisses, granites, granodiorites, ultramafics, charnockites, metasediments and metavolcanics of the Namaqua Metamorphic Complex, the Orange River Group and the Vioolsdrif Intrusive Suite. Both the Gordonia Subprovince (the northern extension of the Namaqua Province; GSP; Fig. 2.6) and Sinclair Supergroup are characterized by numerous remanent magnetic anomalies, which are distinctly different from the induced anomalies arising from the Sinclair basalts. These result from gabbroic, ultramafic and charnockite bodies, although their individual signatures are indistinguishable. Their magnetization is of uncertain age but is expected to be post-Sinclair, possibly being set at the time of emplacement during the c. 1000 Ma Namaqua metamorphic event.

2.6.3 Pan-African Tectonostratigraphic Zones

2.6.3.1 Gariep Group

The current geophysical interpretation has not as yet been extended to include the Gariep Belt, thus what is shown in Fig. 2.6 is based entirely on published mapping (e.g., Frimmel 2008). However, the offshore geophysical data sets, although of low resolution, have facilitated mapping of the offshore extensions of the Port Nolloth Zone, and the Marmora Terrane with its associated Schakalsberge volcanics (Fig. 2.6).

2.6.3.2 Tsumis Group of Namibia and the Ghanzi Group of Botswana

The Tsumis Group comprises sediments of the Doornpoort, Eskadron and Klein Aub Formations, which post-date the period of large-scale Sinclair-Rehoboth Mesoproterozoic igneous activity. The group has been considered as either encompassing both the lower Nosib Group and the Klein Aub Formation (e.g., Schalk 1988), or as occurring within the Sinclair Supergroup (e.g., Miller 2008c). However, Hoffman (1989a) and Becker et al. (2005), based on field observations and new geochronological evidence, place the Tsumis Group in unconformable contact with, and thus younger than, the Sinclair Supergroup. Although overlaid para- to dis-conformably by the Damaran Nosib Group, the Tsumis Group is considered to constitute the lowermost part of the Damara Sequence (Hoffmann 1989a). The latter interpretation, although equivocal, is favoured in this review and is shown as such in Fig. 2.6.

Intra-Tsumis stratigraphic units display clear magnetic signatures, which allow mapping of these units under cover, eastward in Namibia and into Botswana. The main stratigraphic units, with the Namibia nomenclature given first, followed by the Botswana nomenclature and general magnetic signature, are: Doornpoort Formation = Ngwako Pan Formation (low, quiet magnetic response); Klein Aub Formation = D’Kar Formation (strongly magnetic fabric); Nosib Group = Mamuno Formation (intermediate to low magnetic fabric). These units, mapped with outcrop control where available, are grouped together in Fig. 2.6 as the T-G-C-B (Fig. 2.6). This belt is also commonly referred to as the Kalahari Copperbelt in view of its numerous copper-silver occurrences (e.g., Maiden and Borg 2011). The T-G-C-B thus constitutes the southern flank of the Damara-Chobe Orogenic Belt.

2.6.3.3 Southern Margin Zone of Namibia and Its Extension into Botswana

Miller (1983) describes the Southern Margin Zone (SMZ) as comprising ‘two subzones, a southern, less intensely deformed subzone containing mainly thrust slices of pre-Damaran rocks, and a northern subzone consisting of complex thrust sheets containing both Damaran and pre-Damaran rocks’. Stratigraphic units present in the SMZ thus include pre-Damara basement, gneissic basement, the Nosib Group and the lithologically variable passive margin succession of the Hakos Group (Miller 2008a). The northern subzone of the SMZ contains relatively little-deformed cover sequences of the early Damaran Nosib Group along the northeastern margin (Hoffman 1983). The SMZ is bounded in the south by the Frontal Thrust and in the north by the Gomab River Line (Corner 2008; Miller 2008a). High-amplitude magnetic fabric is associated with the Chuos diamictites within the SMZ. Magnetic fabric, characterized by lower levels of magnetization, is associated with the Nosib Group. This is particularly evident in the higher-resolution data.

The extension of the SMZ under cover eastwards to the Botswana border, as interpreted by Corner (2008) working from the western and central and delineation thereof by Miller (1983, 2008a), shows that it continues into Botswana as what has been mapped as the Roibok Formation by Key and Ayres (2000). The interpretation of Rankin (2015) does not appear to support continuity of the SMZ throughout the southern Damaran boundary, but does support the continuation of a portion thereof in eastern Namibia, associated with high-amplitude magnetic anomalies. His interpretation favours mapping the extension of the Botswana Roibok Formation into Namibia, where it also inter-fingers with these high amplitude magnetic anomalies. This package is considered here to be an integral part of the SMZ, extending into Botswana as mapped in Fig. 2.6, recognizing that the Roibok Formation may only be part of a much more complex SMZ extension in Botswana.

2.6.3.4 Southern Zone or Khomas Zone

The Southern Zone (SZ), bounded by the SMZ in the south and the Okahandja Lineament in the north, comprises a thick, deformed succession of Kuiseb schists arising from both active and passive margins of the Khomas Sea, which are placed into separate formations—that is, the Khomas and Hureb Formations (Miller 2008a). The SZ is also often referred to as the Khomas Zone or Khomas trough. This succession mostly shows a low-order relatively quiet magnetic fabric in the higher-resolution data, but it is significantly more magnetic in the lower stratigraphic sequence, particularly north and northeast of Windhoek, and also due to the magnetic quartzites associated with the Matchless Amphibolite Belt.

The eastward extension of the SZ into Botswana, based on the magnetically quiet signature, follows on north of the SMZ (Roibok Formation) in Botswana, where it is seen to pinch out (Fig. 2.6). The offshore extension of the SMZ and SZ, southwards toward the Gariep Belt, has been interpreted as a package, from both the offshore aeromagnetic and Free Air gravity data sets. The location of the boundary between these two zones may not be exact as a result of the poor data quality, but the trend of the package is clear, also abutting against the hinge zone (Fig. 2.6).

2.6.3.5 Deep-Level Southern Zone

The magnetic signature of the Southern Zone (SZ) and southern Central Zone (SCZ, described below) changes dramatically in the Steinhausen area and northeast thereof (note the Steinhausen anomaly, SMA, for location in Fig. 2.6). Here, the Kuiseb Formation schists display high-amplitude magnetic anomalies within the SZ. Magnetic rock types in the area also include epidosite and gabbro, uncommon for the Kuiseb schist sequence elsewhere. To the north, a belt of very high-amplitude magnetic anomalies striking roughly east–west and cutting across the SZ and SCZ fabric correlate with Fe–Mn reefs, and both magnetic and glassy quartzites. Hoffman (1989b) and K Kasch (pers. comm. 2006) have questioned the inclusion of these strata in the Damara Supergroup and have suggested that they form part of the pre-Damara basement. They are thus tentatively mapped in Fig. 2.6 as the Deep-level Southern Zone (Corner 2008), characterized by intense thrusting and deep stratigraphic levels (Kasch 1986). The Deep-level Southern Zone is largely centred between the Okahandja and Kudu Lineaments (Ok-L and Ku-L; Fig. 2.6), continuing east of the latter with a significant change in strike, suggesting a possible fault throw controlled by the latter lineament.

2.6.3.6 Southern and Northern Central Zones

The Central Zone is subdivided into northern and southern parts—that is, the Northern Central Zone (NCZ) and the Southern Central Zone (SCZ), as shown in Fig. 2.6—both of which are characterized by dome structures with an overall northeast elongation (Miller 2008a). Voluminous syn- to post-tectonic granite plutons occur in both. The SCZ is a regional horst, bounded by the Okahandja and Omaruru Lineaments (Ok-L and Om-L; Fig. 2.6), and characterized by high-amplitude magnetic anomalies arising from the exposure of deeper-level sequences, primarily of the Nosib Group, lower Swakop Group (Chuos Formation diamictites) and the Meso- to Palaeo-Proterozoic basement. These units are exposed in dome and anticlinal structures with a pronounced northeasterly trend. The relatively magnetically quiet lower Swakop Group carbonates and schists are preserved in the intervening synclines. Some of the granite phases are magnetic, particularly if derived in part from the basement and Nosib Group. In the western portion of the SCZ, where metamorphic grades are higher, the Etusis and Khan Formations of the Nosib Group, as well as the granitic derivatives therefrom (and also from basement), are strongly magnetic with high-amplitude anomalies that have retained the Damaran remanence (Corner 1983). This magnetic signature of the Khan Formation constitutes an important geophysical marker horizon for the uraniferous granites (e.g., at the Rössing and Husab mines), which mostly occur immediately above it in the Rössing Formation.

Much higher stratigraphic levels are exposed in the NCZ which largely comprises rocks of the Swakop Group. It is bounded by the Omaruru and Autseib Lineaments (Om-L and Au-L; Fig. 2.6). Regional-scale downthrow, or rapid deepening, to the north of the Omaruru Lineament, as also modelled magnetically by Corner (1983), has preserved this thick succession of higher stratigraphic-level Karibib Formation carbonates and Kuiseb Formation schists, deformed in numerous basin and dome structures. The dome structures are generally not cored by granitic basement but rather by Karibib Formation marbles and Usakos Subgroup schists, calcsilicates and marbles. The Swakop Group rocks are relatively magnetically inert, giving an overall quiet magnetic signature to the NCZ. Some low-order fabric is nevertheless seen in the higher-resolution data. Extensive syn- and post-tectonic granite emplacement has taken place, although less so in the areas where the highest levels of the Kuiseb Formation are exposed (Miller 2008a).

The offshore continuation of the SCZ and NCZ, westward up to the hinge zone where they are dramatically truncated, is clearly visible in the aeromagnetic image of Fig. 2.2. Eastwards, the northeast trend of the Damaran Belt is of particular interest as it changes dramatically east of the Kudu Lineament (Ku-L; Fig. 2.6) in the following respects:

The SCZ terminates against the Ku-L, whereas the NCZ changes strike east of the Om-L trending east-southeastwards up to the Ku-L, thereafter regaining a northeasterly trend, pinching out in Botswana (as does the SZ). An alternative interpretation could be considered, given the relatively quiet, thus ambiguous, magnetic signature of both the SZ and the NCZ—that is, that the NCZ also terminates against the Ku-L. Thus what is shown in Fig. 2.6 as NCZ (blue) east of the Ku-L may in fact constitute part of the SZ.
Recent interpretations of an area, focused on mineral exploration, which straddles the Ku-L in the vicinity of the Steinhausen Magnetic Anomaly (SMA; Fig. 2.6) reveal a significant change in the Damaran stratigraphy eastwards as the Ku-L is crossed—that is, with a much deeper level of erosion to the west thereof, exposing basement domes such as the Ekuja dome and deeper-level southern zone stratigraphy (e.g., Corner 2008; Naudé 2012; K. Hartmann, pers. comm.). Thus, not surprisingly, the Matchless Amphibolite Belt (MAB; Fig. 2.6) terminates against the Ku-L, and is not expected to continue on-strike further to the east.
The width of the Damara-Chobe Mobile Belt taken from the SMZ to NMZ is approximately 350 km, west of the Ku-L. The NMZ, NZ, NCZ and SCZ appear to truncate against, or are in disconformable contact with, the Grootfontein Inlier (GI; Fig. 2.6). East of the Ku-L, this width decreases dramatically to less than 150 km, striking roughly parallel to the contact with the GI, pinching out altogether as it progresses further northeastwards into Botswana towards the Tsodilo Lineament. Similarly, the Okahandja Lineament (Ok-L) appears to terminate against the Ku-L and cannot be definitively followed east thereof in the aeromagnetic data.

2.6.3.7 Northern Zone

The Northern Zone (NZ; Fig. 2.6), comprising rocks of both the Nosib and Swakop Groups, has been thrust northwards onto Otavi, Mulden and pre-Damara rocks along the Khorixas-Gaseneirob Thrust (KG-L; Fig. 2.6; Miller 2008a). The Autseib Lineament (Au-L; Fig. 2.6), in part including the Autseib Thrust, forms the southern boundary of the highly and complexly deformed NZ, which shows an overall strongly magnetic signature owing largely to the highly magnetic diamictites of the Chuos and Ghaub Formations and the mafic volcanics of the Askevold Formation. The NZ, together with the northern part of the NCZ, formed the floor of the Outjo Sea during spreading (Miller 2008a).

2.6.3.8 Northern Margin Zone

The NMZ, constituting in-part the Khorixas-Gaseneirob Lineament (the inferred Congo Craton boundary), is a narrow transition zone which Miller (2008a) describes as the northern platform foreslope region where the rather uniform facies of the Otavi Group to the north become more variable and include deep-water-facies carbonates of the Otavi Group. Mulden Group rocks are preserved in tight synclines within this zone, which, although generally magnetically quiet, display high-amplitude remanent magnetic anomalies. Corner (2008) ascribes these to secondary magnetization arising from the development of pyrrhotite in the phyllites through the passage of fluids along the lineament.

2.6.3.9 Northern Otavi Platform, and the Tsodilo Lineament

Damaran rocks are only exposed along the southern and western edges of the Northern Platform (NP) but continue northwards and eastwards below Karoo and Kalahari cover. The limits of the NP in Namibia, as shown in Fig. 2.6, are determined by the distribution of the shallow-water facies of the Otavi Group, extending westwards to the Sesfontein thrust (ST; Fig. 2.6; Miller 2008a). Predominant east–west-trending anticlinal structures are seen in the magnetic data in the southern sector of the NP. These separate expansive magnetically quiet areas that comprise higher stratigraphic levels, including Mulden Formation strata, in the synclines. The antiforms have an anomalous magnetic signature largely due to the exposure, or shallow suboutcrop, of the diamictites and possibly volcanics associated with the lower Damara strata. The tight folding of the antiforms close to the NMZ becomes progressively more open to the north.

Geophysical mapping of the eastward extension of the NP under cover in Namibia, and into Botswana, is complicated by the largely highly magnetic basement rocks of the Grootfontein Inlier. Outcrops of the NP do nevertheless occur at the Aha Hills close to the Botswana border, which allows continuity to be inferred. The intervening area is hatched in Fig. 2.6 to indicate the ambiguity of continuity owing to the low-magnetic signature of the overlying NP carbonates. On strike to the northeast of the Aha hills, in Botswana, is the Xaudum Group, which correlates with the Aha Hills Formation (e.g., Rankin 2015), and hence with the NP in Namibia. Extension of the NP into Botswana is indicated as such in Fig. 2.6. The Tsodilo Hills Group (Figs. 2.6a, b), however, is of uncertain correlation. Miller (1983), Breitkopf (1988) and Bűhn et al. (1992) correlate it with the Chuos Formation in Namibia in view of the abundant iron formations and ferruginous quartzites. Rankin (2015), based on further interpretation using potential field data, concluded that the Tsodilo Hills Group cannot be confidently correlated with a single tectonostratigraphic zone of the Damaran Belt, and that it may even be a correlative of units in Angola or Zambia. What appears to be singularly unique in the area is the dramatic change in structural style displayed by the Tsodilo Hills Group, from the expected northeast trend of the Damara-Ghanzi-Chobe Mobile Belt to tight folding and faulting striking north-northwest (e.g., Rankin 2015). The Tsodilo Hills Group is thus specifically separately colour coded in Fig. 2.6.

The question may thus be asked as to the reason for this dramatic change in structural style. In the current interpretation, we single out the regional Bouguer Gravity low in the Botswana Geosciences Institute data, with which the Tsodilo Hills Group is associated (Fig. 2.4; see also Rankin 2015), as providing the clue. We have the further benefit of the WGM Bouguer Gravity data (Fig. 2.5), which shows that the gravity low trends further north-northwestwards and northwards into Angola. The gravity low thus appears to hallmark a major, roughly north–south lineament which we have termed the Tsodilo Lineament (Ts-L; Fig. 2.6). That it constitutes a major regional structural zone is not only evidenced by the structural style displayed by the Tsodilo Hills Group and environs, but also provides an explanation as to why the regional east-northeast-trending magnetic low in northern Namibia, correlated with the southern Congo Craton margin, abruptly terminates in northwestern Botswana at the Tsodilo Hills gravity low. Viewed in a regional context, it is possible that the Ts-L was the northward continuation of the Kalahari Lineament (Kal-L) in pre-Pan-African times, as they are virtually on strike with each other, given some local north-northwest deformation in the Tsodilo Hills area. The continuity between the two may thus have been interrupted by the Damaran Orogeny (see also Sect. 2.2.8).

2.6.3.10 Kaoko Belt

The Kaoko Belt is subdivided into four subzones (Goscombe et al. 2003, 2005; Miller 2008a): the Southern, Eastern, Central and Western Kaoko Zones:

The pelitic sequences of the Southern Kaoko Zone (SKZ), which are tightly folded in north–south-striking chevron folds, display a low-amplitude magnetic fabric, giving an overall quiet appearance to this zone which nevertheless has enabled better definition of this subzone’s boundaries. In contrast, the strong, roughly north-northwest magnetic fabric of the Western, Central and Eastern Kaoko Zones results from complex deformation and variable grades of metamorphism (Goscombe et al. 2003).
The Eastern Kaoko Zone (EKZ) comprises upright folds of subgreenschist-facies shelf carbonates (Goscombe et al. 2003). These rocks are relatively non-magnetic, but where lower stratigraphic levels are exposed or are in shallow suboutcrop, a strong magnetic fabric is seen. Miller (2008a) points out that the stratigraphy of the EKZ is almost identical to that of the Northern Platform (NP), but that the two regions are structurally distinct. This is also observed in the magnetic data, which shows a distinct magnetic, and hence tectonic, eastern boundary between the EKZ and the NP. The Sesfontein Thrust (ST) marks the western boundary of the EKZ.
The Central Kaoko Zone (CKZ) comprises east-vergent nappes of Swakop Group passive margin rocks (Goscombe et al. 2003; Miller 2008a). Within the CKZ, the Damara stratigraphic units, the Okapuka Formation and the Palaeoproterozoic crystalline basement of the Epupa Inlier show variable magnetic responses, in some places subdued and in other places with a slight magnetic fabric. Higher-amplitude anomalies arise from the lower Swakop Group diamictites and amphibolites, and from amphibolitic and gabbroic units within the pre-Damara basement. The CKZ has no clear eastern boundary with the EKZ as evidenced in the magnetic data. Based on geological premises, the boundary is thus set, as above, at the Sesfontein Thrust (ST; Fig. 2.6; Miller 2008a). The western boundary of the CKZ is the Purros Shear Zone (PuSZ; Fig. 2.6), which comprises a series of closely spaced, steeply westward-dipping ultramylonites (Miller 2008a).
The Western Kaoko Zone (WKZ) is predominantly a deep basin facies sequence of high metamorphic grade, intruded by numerous granites, which has experienced intense wrench-style deformation along steep, crustal-scale shear zones (Goscombe et al. 2003). Much of the magnetic fabric results from the mylonitic shear zones. The Purros Shear Zone constitutes the eastern boundary of the WKZ, which is further subdivided into a Coastal Terrane and an Orogen Core. The latter comprises a number of internal subdomains, which are separated by the Three Palms Mylonite Zone (Goscombe et al. 2005; Miller 2008a).

2.7 Rehoboth Terrane

2.7.1 Sedimentary Sequences

The Rehoboth Terrane (RT) has a unique expression in the aeromagnetic image of southern Africa (Figs. 2.2 and 2.6). It is hallmarked by long-wavelength anomalies that reflect deep magnetic basement sources, an overlying thick package of magnetically-inert sedimentary rocks including pre-Nama, Nama, Karoo and Kalahari sequences, and some shallow, high-frequency magnetic anomalies arising from Karoo basalts and sills. The interpreted deep-source magnetic and gravity anomalies which underlie the above sedimentary package (Corner 2008), are shown in Fig. 2.6.

The RT, which forms the western Palaeoproterozoic core of the Kalahari Craton (Jacobs et al. 2008; Fig. 2.1), is bounded in the east by the Kal-L. The change in depth to magnetic basement on either side of the Kal-L is dramatic, from ~500 m east thereof to 8–10 km west thereof (Corner 2008). Isopach contour levels, derived by Corner (2008) from a depth-to-basement analysis, of >4000 m (i.e., pre-Karoo sediments) and >6000 m, are shown in Fig. 2.8. This analysis is in agreement with the earlier, more regional, or localized, interpretation of Reeves (1978) and of an interpretation by Petro-Canada (a hydrocarbon exploration group which conducted seismic surveys in western Botswana and an aeromagnetic interpretation; Botswana Geoscience Institute archives, Wright and Hall 1990). In the southwest, the RT is bounded by the northwestward extension of the Namaqua Lineament (Naq-L; Fig. 2.6; an extension of the Namaqua-Natal Front), which also shows a dramatic increase in depth to basement across it into the RT, in excess of 4000 m to the northeast (Fig. 2.8). A considerable package of sediments, up to 10 km in total thickness, is thus preserved within the RT. Karoo and Nama strata, or Kalahari cover, are evident at surface. However, the deepest section of Nama rocks yet drilled was in the Masethleng Pan borehole in western Botswana, passing through the base of the Nama at 3844 m. Underlying red beds were intersected in both this and the Tses borehole in Namibia. Known red beds, outcropping to the northwest and west of the Nama Group, belong to the Doornpoort and Aubures Formations, respectively, which have been variably correlated with either the Tsumis or Sinclair Groups (Becker et al. 2005; Miller 2008a). The implication is thus that a red bed sequence of Mesoproterozoic age, and possibly older strata, may form a remarkably thick succession of up to approximately 6 km beneath the Nama Group.

2.7.2 Interpreted Basement Geology, the Karas Impact Structure

The only southern area proximal to the RT where basement is exposed is in the 130 km-wide north-northeast-trending Karas Horst (KH; Fig. 2.6), with which a dyke swarm is also associated. The Karas Horst forms a beautiful mountain land in southern Namibia, indicating relatively recent reactivation along the north-northeast bounding faults. A focus of attention arising from the deep magnetic and gravity sources mentioned in Sect. 2.2.7.1 is a roughly circular set of magnetic anomalies in the southern portion of the RT proximal to the Namaqua Lineament, approximately on strike with the Karas Horst (Fig. 2.6; Corner 2008). Concentric gravity highs collar the magnetic anomalies in the south and east but centrally the circular magnetic anomalies and their core correlate with a strong gravity low. This group of magnetic and gravity anomalies is also the centre of focus of a number of possible far-field ring features which were mapped by Corner (2008) from a number of the filtered magnetic data sets (Karas Ring Structure (KRS); Fig. 2.7). Despite the coarseness of gravity data, this combination of features, together with numerous apparently associated radial faults, has been interpreted to arise from a major meteorite impact named the Karas Impact Structure (KIS; Fig. 2.6; Corner 2008). The depth solutions, discussed above, of ~6 km in this area suggest a pre-Aubures (Sinclair Group) age. Furthermore, the deep magnetic anomalies appear to be largely terminated against the Namaqua Lineament, possibly also suggesting a Mesoproterozoic or earlier age.

2.8 Deep Neo- and Meso-Proterozoic Basins in Namibia and Angola

The extent, and interpreted thickness of up to 10 km, of the magnetically inert sedimentary succession which overlies the basement of the Rehoboth Terrane is described in Sect. 2.2.7.1 (Fig. 2.8). Note that the depth-to-basement contours, which commence at 4000 m and extend to greater than 6000 m, largely reflect the thickness of a pre-Nama Mesoproterozoic succession. Similar sedimentary successions have been interpreted in northern Namibia and southern Angola from, firstly, a preliminary interpretation of Angolan airborne gravity and magnetic data (Sonangol & NAMCOR AAPG presentation 2009) and, secondly, a hydrocarbon exploration interpretation of the Owambo Basin in northern Namibia by Hoak et al. (2014). The 4000 m and 6000 m isopach contours are approximately merged in Fig. 2.8 to show the full extent of a combined northern sedimentary succession. The former (Angolan) interpretation stopped at the eastern border with Zambia. An eastward thinning of the succession was nevertheless evident, thus the extent of the basin in Zambia, as shown in Fig. 2.8, is schematic. Structural controls of the basin are clearly evidenced from structures mapped in Namibia and extended into Angola using the World Bouguer Gravity data shown in Fig. 2.5. The basin is largely constrained by the Opuwo and Kudu Lineaments (Op-L and Ku-L; Fig. 2.6) in the northwest and southeast, respectively. The latter lineament has a more complex, broader swathe as indicated in Figs. 2.6 and 2.7. Whether these lineaments were active during sedimentation, or subsequently controlled the interpreted distribution of the sedimentary succession through faulting, is unknown.

Viewed in a regional context, there is a remarkable similarity in the extent, thickness of sedimentary pile, and location of the two basins with respect to the Kalahari (Ka-L) and Tsodilo (Ts-L) Lineaments which bound them in the east (Fig. 2.8). As noted in Sect. 2.2.6.3, it is possible that the Ts-L was the northward continuation of the Kal-L in pre-Pan-African times, and that the continuity between the two was interrupted by the Pan-African Damaran Orogeny.

2.9 West Coast Offshore Domain and the Namibian Passive Volcanic Margin

Corner and Swart (1997) and Corner et al. (2002) interpreted the Namibian offshore airborne and ship-track magnetic data, satellite Free Air gravity data, as well as numerous seismic lines that traverse the continental shelf. The offshore magnetic and gravity data sets are merged with the onshore data in Figs. 2.2 and 2.4. Significant contributions to the interpretation of the offshore domain have been made by Clemson et al. (1997) and Bauer et al. (2000). Corner and Swart (1997) used more recent reflection seismic surveys covering the Hinge Zone (HZ; Fig. 2.6) to update the mapping by Clemson et al. (1997) of the onset of the basalt-related seaward-dipping reflectors (SDR). Two boundaries were mapped—that is, the HZ itself, and the feather edge of the SDRs to the east of the Hinge Zone (Fig. 2.6). The continental crust occurring offshore east of the Hinge Zone was found not to have undergone any observable extension during the break-up of Gondwana. The extent of this block to the south of the Khorixas-Gaseneirob Lineament is clearly evident in the offshore aeromagnetic data. East of the HZ, the offshore extension of the Damara Orogen was clearly evidenced by the magnetic data for the first time, allowing the offshore extensions of the NCZ, SCZ, SZ and SMZ to be mapped (Fig. 2.6). A further significant result is that the mapped Mesozoic gravity and magnetic anomalies have been clearly offset along the offshore continuations of a number of Pan-African lineaments—for example, the Autseib (Au-L), Omaruru (Om-L) and Welwitschia (We-L) Lineaments (Corner et al. 2002, Fig. 2.6). The abrupt termination of Damaran rocks at the Au-L offshore is particularly dramatic. This observation suggests that these Late Proterozoic to Early Palaeozoic structures not only determined the architecture of the extended crust but were likely reactivated during the Late Mesozoic, having directions favourable for the initiation of transform faults. An important implication arising from this interpretation is that these structures may have provided potential pathways for the larger drainage systems and hence the focus of major offshore sedimentation, controlling the evolution of the offshore basins.

The M-type apparent spreading-related magnetic anomalies of previous workers (e.g., Rabinowitz and LeBrecque 1979; Gladczenko 1994) are readily evident in the recent offshore data—for example, M2 and M4 in Fig. 2.6. These are now more definitively delineated, and are seen to converge towards the northern Namibian and Angolan coastlines, in contrast to their previous placement much further out to sea. This has major implications for relative rates of extension from the southern to northern offshore areas, and for the relative ages of the offshore sedimentary sequences. The interpretation of Corner et al. (2002) fundamentally clarifies, through forward modelling of the magnetic and gravity data, the origin of the classical M-type anomalies. These high-low pairs are shown to arise from shallow westerly dipping wedges of lava (SDRs) at their suboutcrop feather edge, as opposed to the classical steep-dyke seafloor-spreading model. Such dykes are not expected to occur senso stricto on the continental shelf or within the extended continental crust, thus negating the ‘M’ nomenclature.

At least four coast-parallel gravity highs are seen in the offshore data, the highest amplitude being closest to shore (shallowest). These are also structurally disrupted along offshore extensions of the major onshore Pan-African lineaments, similarly signifying either later reactivation of these lineaments or the control by these early lineaments of the architecture of the extended crust. Corner et al. (2002) showed, through modelling, that the eastern, and most prominent, gravity high is caused primarily by the onset of a major package of SDRs that define the HZ. The amplitude of this gravity high increases as the lava sequence starts to thicken, but disappears completely further seaward as the overlying low-density sedimentary pile progressively neutralizes the effect of the deeper lava-related gravity high. The gravity response to the resultant centres of mass may thus be offset from the associated magnetic anomalies, which delineate the shallower feather edge of the basalts. The interpretation does not take cognizance of the long-wavelength component, which is likely to be associated with deep magmatic underplating, but does explain the anomalies very well in the first order. This interpretation is in contrast with that of Watts and Fairhead (1999), who applied a process-orientated approach to modelling ‘edge effect’ anomalies such as the HZ gravity high. The processes which they considered included rifting, sedimentation and magmatic underplating. By quantifying these, they were able to model the ‘edge effect’ gravity anomalies. However, they did not consider the effect of the basaltic seaward-dipping reflectors and associated magnetic anomalies.

A number of probable offshore Cretaceous igneous complexes were interpreted, the largest being the Walvis Bay Complex (Wb; Fig. 2.6), situated close to the Om-L, and hallmarked by significant gravity and magnetic anomalies (Corner and Swart 1997; Corner et al. 2002). The inland Cretaceous Erongo Complex (Er) also occurs along the Om-L. The offshore Cape Seal Complex (Cs) and the offshore extension of the Cape Cross Complex (Cc) were also newly identified. The offshore magnetic and gravity data adjacent to South Africa have not been rigorously interpreted, but the significant anomalies and possible faults have been mapped from the EMAG2 and World Gravity Map data, as shown in Figs. 2.6 and 2.7. Noteworthy is the presence of the only Cretaceous igneous complex associated with the South African west coast region—that is, the Koegelfontein Complex (Kf; Fig. 2.6; Whitehead et al. 2016).

2.10 Namaqua-Natal Belt and Its Extension as the Maud Belt in Antarctica

2.10.1 Namaqua-Natal Belt

The Namaqua-Natal Belt (Namaqua-Natal Metamorphic Belt (NNB)) is an arcuate orogenic belt roughly 1400 km in length which bounds the Kalahari Craton to the south (Figs. 2.2, 2.6 and 2.7). It comprises igneous and metamorphic rocks formed or metamorphosed during the Proterozoic, with four periods of activity being evident at ~1.8–2.00 Ga, ~1.2 Ga, ~1.15 Ga and ~1.06 Ga, (Cornell et al. 2006; Eglington 2006)—that is, essentially two phases of activity of Palaeo- and Meso-Proterozoic age. The extensive outcrops in the west and east constitute the Namaqua and Natal Provinces, respectively (NamqP and NatP; Fig. 2.6), the former extending into Namibia as the Gordonia Subprovince (GSP; Fig. 2.6). The region between the outcrop areas is covered by Phanerozoic Karoo sediments, which hide the boundary between the NNB and the younger Palaeozoic rocks of the Cape Supergroup. The boundary is expected to be complex in view of the late Permian to early Triassic Cape Orogeny. It is thus only possible to map the southern boundary of the NNB using geophysical techniques.

The NNB comprises a number of tectonostratigraphic terranes (i.e., areas of common lithostratigraphy and structural fabric bounded by shear zones), which were assembled during the Namaqua Orogeny (Cornell et al. 2006). However, the structural fabric of the Namaqua and Natal Province differs considerably (e.g., Cornell et al. 2006), as is also evidenced in the magnetic images of Figs. 2.2 and 2.3. The Natal Province may be subdivided into three distinct, structurally complex, discontinuity-bound terranes: the northern Tugela, central Mzumbe and southern Margate Terranes. These are bounded by major shears and thrusts: the Tugela (thrust) Front, Lilani-Matigulu Shear Zone, the Lovat Shear Zone and the Melville Thrust (Thomas 1989; Jacobs et al. 1993; Jacobs and Thomas 2004; Cornell et al. 2006). Thrust tectonics are extensively preserved within the Tugela Terrane, where rocks were obducted northwards onto the Kaapvaal Craton during the Namaqua Orogeny, whereas the Mzumbe and Margate Terranes are dominated by major wrench faults (Matthews 1972; Jacobs and Thomas 2004). The thrust and shear zones are clearly evidenced by high-amplitude, linear magnetic anomalies, as seen in Figs. 2.2, 2.6 and 2.7 (Corner 1989; Corner et al. 1991; Hunter et al. 1991; Thomas et al. 1992). This structural fabric, as seen in the aeromagnetic data, is in contrast to that of the approximately coeval Namaqua Province, which has an irregular, unstructured or curvilinear fabric. The magnetically anomalous zone correlating with the mapped portion of the Lovat Shear Zone (LSZ; Fig. 2.6) in the Natal Province can be followed west-southwestwards in the aeromagnetic data, and has been mapped as such in Figs. 2.6 and 2.7.

The continuity of the NNB from the Namaqua to the Natal Provinces has commonly been accepted since the early recognition thereof by Nicolaysen and Burger (1965). More recently, Eglington and Armstrong (2003) conducted geochronological and isotope studies on cores from four deep boreholes drilled by SOEKOR (Ka, Kc, Qu and We; Fig. 2.6), comparing these with known signatures from the Namaqua and Natal Provinces. In particular, this study showed that the four boreholes reflect a Palaeoproterozoic crustal history similar to that of the Namaqua Province, and that reworked crust of this age is not evident in the Natal Province nor in the Cape Meredith Complex of the Falkland Islands. They thus concluded that there must be a significant terrane boundary between the exposures of the Natal Province and the easternmost borehole studied (We; Fig. 2.6; Eglington and Armstrong 2003). They further concluded that this clear evidence for isotopically distinct terranes during the Mesoproterozoic needs to be taken into account in reconstructions of the NNB, and its easterly extensions within Gondwana.

In the present study, a variety of regional-residual separation and upward continuation filters were used to enhance the sub-Karoo magnetic anomalies, including the Beattie Magnetic Anomaly (Sect. 2.2.10.2), to assess the relationship between the Namaqua and Natal Provinces (Figs. 2.2 and 2.6). It is clear that the central area within the NNB has a distinctly different structural fabric to either the Namaqua or Natal Provinces. This central area, here named the Khoisan Province (KP; Fig. 2.6), is hallmarked by a disrupted but nevertheless roughly continuous belt of magnetic anomalies, of longer wavelength in the south as a result of the thicker Karoo cover. The KP shows apparent closure, through possible folding or thrusting in the west, where it adjoins the Namaqua Province; whereas in the southeast it appears to be discordantly truncated against the westward extension of the Lovat Shear Zone. The distribution of residual gravity anomalies in Figs. 2.4 and 2.6 also suggests apparent closure in the west, despite the coarseness of the data. In the east, the KP appears to terminate against a north-northeast-striking fault just west of the SOEKOR borehole We (Fig. 2.6). A further major north-northeast-striking fault occurs east of borehole We, which appears to bound the Natal Province in the west. This fault is here named the Elliot fault (EF; Fig. 2.6; after the nearby town of Elliot). In part, the northern sector of the Elliot fault coincides with the escarpment, and it could be argued that it is simply the manifestation of a dramatic increase in terrain clearance during the airborne survey east of the fault. However, the fault is also associated with a long-wavelength anomaly at its northern end, and is interpreted to continue southwards away from the escarpment. The intermediate area between these two faults is hallmarked by a zone of high-frequency anomalies arising from Karoo-age sills. The isotopic results of borehole We discussed above (Eglington and Armstrong 2003) suggests a correlation of this area with the Namaqua Province. No clear deeper sources, from either the KP or the Natal Province, were evident in this intermediate area in any of the filter products applied to the magnetic data. None of the four SOEKOR boreholes was sited within the KP, and its origin thus remains unknown. Given the location of the KP, and its offset with respect to the Kaapvaal Craton, it could be speculated that it may contain relicts of the southern portion of a proto-Kaapvaal Craton, and associated Archaean basins, assimilated during the Namaqua Orogeny. In the first instance, it is thus proposed here that, firstly, the Khoisan Province constitutes a subterrane of the Namaqua Province and, secondly, the Elliot fault is the western boundary of the Natal Province.

2.10.2 Beattie Magnetic Anomaly and the Southern Cape Conductivity Belt

Two remarkable, broadly spatially coincident, continental-scale geophysical anomalies are evident within the NNB: the Beattie Magnetic Anomaly (BMA; Beattie 1909) and the Southern Cape Conductivity Belt (SCCB; Gough et al. 1973; De Beer and Gough 1980; De Beer et al. 1982a; De Beer and Meyer 1984). The BMA extends for more than 950 km, has an apparent suboutcrop width in the order of 50 km, and correlates spatially with the northern margin of the broader SCCB, which extends from the western to the eastern coastal margins for more than 1100 km (Figs. 2.6 and 2.12). Their origin is enigmatic, with some earlier authors suggesting a common source owing to their apparent correlation in approximate location and scale, while more recent studies indicate separate sources, but with a common evolutionary origin. The SCCB is discussed in more detail in Sect. 2.3.2, but reference is made to it here where relevant. Many geophysical studies have been directed at mapping and understanding the SCCB and the BMA, including interpretations of aeromagnetic data, gravity data, magnetotelluric data, reflection seismic data and wide-angle seismic refraction studies (e.g., Gough et al. 1973; De Beer and Gough 1980; De Beer et al. 1982a; De Beer and Meyer 1984; De Beer and Stettler 1988; Corner 1989; Pitts et al. 1992; Thomas et al. 1992; Lindeque et al. 2007, 2011; Weckmann et al. 2007a, 2007b; Stankiewicz et al. 2008; Quesnel et al. 2009; Scheiber-Enslin et al. 2014).

The BMA is not a single magnetic anomaly but rather comprises a series of subparallel anomalies that may be separated with spectral filtering into residual anomalies reflecting the shallower (in a relative sense, but nevertheless deep) suboutcrop of magnetic sources beneath Phanerozoic Karoo strata, and deeper sources within the crystalline crust, which together give the appearance of a broad magnetically anomalous belt. There is no gravity anomaly associated with the BMA. Scheiber-Enslin et al. (2014) have mapped the extent of the residual magnetic anomalies, extending into the Natal Province. In parallel studies, Corner (e.g., 2015) conducted more rigorous mapping of the residual anomalous sources, an interpretation which is further refined and presented in Figs. 2.6 and 2.10. Forward models of magnetic profiles over the BMA, conducted by a number of researchers (e.g., Maher and Pitts 1989; Du Plessis and Thomas 1991; De Beer and Stettler, unpublished) show similar results—that is, the BMA is readily modelled with prism-shaped sources dipping at a shallow angle to the south. These models are indicative of a possible northward-directed thrust zone, as discussed further below. An early model for the origin of the BMA was that of a dipping slab of oceanic crust, representing a Pan-African suture zone, with serpentinization being invoked to explain the absence of an associated gravity high (De Beer et al. 1982a; De Beer and Meyer 1984).

Other forward models, conducted more recently, do not necessarily agree with southward dips of the BMA but are based on more simplistic source geometry models such as horizontal prisms and spheres (Quesnel et al. 2009) or a very coarse polygonal body interpreted to mimic a deeper-level resistive crust (Weckmann et al. 2008). On the strength of the model of Maher and Pitts (1989), Corner et al. (1991) proposed that the linear magnetic anomalies in the Natal Province associated with the known major craton-directed thrust zones may share a common origin with the BMA. Palaeomagnetic studies in northern Natal (Corner and Maccelari unpublished) confirmed the presence of highly magnetic mylonites associated with the Lilani-Matigulu Thrust Zone (LMSZ; Fig. 2.6). Thomas et al. (1992) further confirmed this, also indicating that the thrusts and shears of the Natal Province are associated with linear magnetic anomalies, giving a magnetically ‘striped appearance’ to the province, and named the anomaly near Amanzimtoti the Williston anomaly (ASZ/WA; Figs. 2.6 and 2.10). They also supported the possibility that these may be associated with the BMA, which is present south of the Williston anomaly. Based on this model, Corner and Groenewald (1991), Corner et al. (1991) and Hunter et al. (1991) proposed that the regional-scale, high-amplitude magnetic anomalies in Dronning Maud Land (DML), Antarctica, were the extension of the BMA within a Gondwana framework. Fieldwork in DML, including palaeomagnetic sampling (Corner, unpublished), showed strong similarities with northern Natal: highly magnetic mylonites associated with craton-directed thrusts were observed in the Kirwanveggen, where the DML anomaly was observed in outcrop (see also Sect. 2.2.10.3). Large euhedral magnetite grains were also observed to be pervasive in the gneisses adjacent to the thrusts. The similarity of signature between the magnetized thrust zones of the Natal Province and the BMA was confirmed in a comprehensive study by Scheiber-Enslin et al. (2014). The finding that high-amplitude magnetization is pervasively associated with thrust zones and associated gneisses in both the Natal Province and DML, as well as the observation of the similar strike and linear magnetic signature of units within the BMA and in DML, is the best evidence we have to date on the origin of the BMA. A reflection seismic section crossing the BMA (Figs. 2.11 and 2.13, discussed in Sect. 2.3.3 ) shows a bean-shaped zone of strong reflections at the sub-Karoo location of the BMA. No diagnostic dips or depth extent could, however, be determined.

2.10.3 Extensions of the Namaqua-Natal Belt Within Gondwana: The Maud Belt, Antarctica

Early regional-scale aeromagnetic surveys were carried out in Antarctica over Dronning Maud Land (DML) and the Weddell Sea by the Polar Marine Geological Research Expedition (PMGRE, VNIIOkeangeologia, USSR) during the 1970s and 1980s, at a flight-line spacing of 20 km, with a higher-resolution 5 km flight-line spacing survey being flown over the Kirwanveggen region (Golynsky et al. 2000a). Regional aeromagnetic data sets had also been acquired by the German Federal Institute for Geosciences and Natural Resources (BGR) and Alfred Wegener Institute (AWI). A collaborative interpretation of these combined data sets showed that the major magnetic anomalies in DML were likely to be the continuation of counterparts in southern Africa and the Falkland Plateau (Corner 1989; Fűtterer 1989). In addition, the geophysical components of the South African National Antarctic Research Programme (SANARP) included limited, but nevertheless diagnostic, regional surface gravity profiles traversing the Grunehogna and Maud Provinces, limited helicopter-borne magnetic surveys, and palaeomagnetic studies (Hodgkinson 1989; Hunter et al. 1991; Corner 1994b; Jones et al. 2003). More recently, higher-resolution surveys, with a mostly 10 km flight-line spacing, were undertaken in specific areas by the British Antarctic Survey (BAS) in the Jutulstraumen region of DML, by the BGR and AWI (Johnson et al. 1992; Riedel et al. 2013). The area of early aeromagnetic coverage in DML was partly reflown, and extended, as part of the AWI VISA project during the period 2001–2005, at flight-line spacings of 10 km (mostly) and 20 km (Riedel et al. 2013).

A zone of regional-scale linear or semi-linear en-echelon magnetic lineaments (c. 40 km wide and extending over 700 km), revealed by the early and subsequent aeromagnetic surveys, was termed the H.U. Sverdrupfjella-Kirwanveggen Anomaly (SKA; Golynsky and Aleshkova 2000; Golynsky et al. 2000a). Golynsky et al. (2000a, 2000b) did not conduct a detailed analysis of the anomalies owing to the regional scale of their interpretation, but they attributed the anomalies to high-grade meta-igneous gneisses of 1000–1200 Ma (Grenvillian/Kibaran age), thrust or shear zones, and other larger crustal structures and zones of weakness. The associated magnetic anomalies were seen to be interrupted by changes in strike and inferred faulting. Later deformation and magmatism associated with the Ross orogeny (Pan-African) is described as being considerably less intense, with contrastingly weaker manifestation in the magnetic data (Moyes et al. 1993; Golynsky et al. 2000b). Importantly, Golynsky et al. (2000b) also recognized the additional complexity of interpretation, in that many magnetic anomalies could arise from Mesozoic magmatism associated with the break-up of Gondwana. The recent interpretation of Riedel et al. (2013) is more rigourous, given the larger area of coverage, the acquisition of data at a closer flight-line spacing (other than over the Kirwanveggen), the development and use of new data-filtering techniques, and the benefit of more fieldwork having been conducted in the interim.

The digital PMGRE aeromagnetic data set covering a portion of DML was passed on to SANARP by PMGRE in 1989 as part of a collaborative research programme, and interpreted by Corner (1989), Corner and Groenewald (1991), Corner et al. (1991), Hunter et al. (1991) and Corner (1994b). The aeromagnetic data is shown in Fig. 2.9 and the interpretation is presented in Fig. 2.10, juxtaposed with southern Africa within a Gondwana framework. That early interpretation does not differ substantively from the more recent interpretation of Riedel et al. (2013), but it benefits in detail locally from the higher-resolution 5 km PMGRE flight-line coverage over the Kirwanveggen, where outcrop occurs and ice cover is relatively thin in places. By comparison, the interpretation of Riedel et al. (2013) benefits from the much larger area of coverage of the VISA Project in areas outside the higher-resolution PMGRE survey.

The continuation of the Namaqua-Natal Belt of southern Africa into DML, as the Maud Belt (MB; Fig. 2.10) within Gondwana, thus comprising the larger Namaqua-Natal-Maud Belt (NNMB), has been established geologically for some time (e.g., Groenewald et al. 1991; Hunter et al. 1991; Jacobs et al. 1993; Riedel et al. 2013). The interpretation of Corner (1989) above was based, firstly, on the similarity of the Beattie and SKA anomalies, in terms of scale and signature; and secondly, on the fact that the former is clearly truncated by the Agulhas Fracture Zone, implying possible eastward extension in Gondwana. The Gondwana reconstruction (Martin and Hartnady 1986), initially used by Corner (1989), aligned the SKA with the extension of the Beattie into DML. The intervening gap, known as the Natal Embayment (Jacobs and Thomas 2004), was occupied by the Maurice Ewing Bank (MEB) microplate of the Falkland Plateau, over which marine magnetic data revealed a linear magnetic anomaly which, if the MEB microplate is rotated slightly as suggested in Figs. 2.9 and 2.10, would also constitute a possible continuation of the Beattie anomaly, and a link between the BMA and SKA (Figs. 2.9 and 2.10). In the interpretation of Corner (in Hunter et al. 1991), refined here, magnetic units and faulting within the larger SKA zone were mapped using higher-resolution residual-filtered aeromagnetic data. One aspect that complicated this interpretation was the presence of numerous magnetic anomalies owing to the Mesozoic basalts associated with the break-up of Gondwana, progressively dominating westwards from the SKA towards the Explora Escarpment (EE; Fig. 2.10). However, these appeared to have a strike direction different from that of the SKA anomalous units, and were thus subjectively separated, as shown in Fig. 2.10. This interpretation may be equivocal in the zone where both of these anomalous sources are present. The closer fit of DML to southern Africa of Grantham et al. (1988) has been relaxed in Fig. 2.10 so as to accommodate the interpreted Mesozoic basalt anomalies. It is thus an indicative, rather than rigorous, fit for the purposes of this discussion. The continuation of the Namaqua-Natal Belt and BMA into DML in Antarctica has been further supported by the more recent work of Golynsky and Jacobs (2001); Jacobs and Thomas (2004) and Riedel et al. (2013).

The postulated preservation of an Archaean cratonic microplate in DML (the Grunehogna Craton), Antarctica, is supported by Archaean ages of c. 3000 Ma (Halpern 1970), and 3067 Ma (Marschall et al. 2010), determined on isolated exposures of basement at the Juletoppane and Annandagstoppane nunataks, respectively (red star; Fig. 2.10; see also Barton et al. 1987; Hunter et al. 1991). Corner (1994b), in interpreting the DML aeromagnetic data and ground gravity profiles, outlined the extent in DML of this microplate (Figs. 2.1 and 2.10) and juxtaposed it with the eastern margin of the Kaapvaal craton within a Gondwana reconstruction, suggesting the possibility that the cratonic microplate constituted the eastward extension of an early Kaapvaal craton. However, the location of the Kaapvaal Craton within Rodinia, and subsequently Gondwana, has been the topic of much controversy. An early view was that the DML Archaean microplate (Grunehogna Craton) is not related to the Kaapvaal-Zimbabwe Province (Barton et al. 1987; Barton and Moyes 1990). Dalziel (1991, 2000) and Moores (1991) hypothesized that Laurentia and East Antarctica were juxtaposed in an early Neoproterozoic supercontinent. This was named the SWEAT hypothesis (Southwest United States—East Antarctica). Moyes et al. (1993) examined the hypothesis by comparing coeval magmatism, regional isotopic resetting and structural deformation styles, and they concluded that this data neither supports nor contradicts the SWEAT hypothesis. Storey et al. (1994) compared geochronological, isotopic and aeromagnetic data between Coats Land and DML, confirming rocks of Grenvillian age in both, and concluded that this data supports the SWEAT hypothesis. On the other hand, Golynsky et al. (2000b) investigated the tectonic development of Coats Land and western DML using aeromagnetic data and correlative outcrop data, and concluded that Coats Land was never part of the Kaapvaal-Zimbabwe craton. Overall support for or against the SWEAT hypothesis is thus equivocal. It has also been suggested that Western Australia was a collision partner for the Kalahari Craton. This hypothesis was investigated by Ksienzyk and Jacobs (2015), who could not find support from a geochronological point of view. In summary, much recent work supports the extension of the Kaapvaal-Zimbabwe Cratons into DML, and the existence of a continuous Grenvillian/Kibaran belt bounding this proto-Archaean craton to the south and east.

3 Electrical Resistivity, Magnetotelluric and Regional Seismic Investigations

3.1 Introduction

Over the past half-century, a series of passive and active seismic experiments, and resistivity, geomagnetic induction and magnetotelluric (MT) surveys, have been conducted in southern Africa. Most of the early work was stimulated by curiosity, with the objective of learning more about the structure and evolution of the African continent. Much of the later work was done with commercial intent, with the aim of discovering new gold, platinum, diamond and hydrocarbon resources. In particular, reflection seismology surveys, while much more expensive to carry out than gravity and magnetic surveys, are able to produce far better images of the subsurface. The locations of MT and broadband seismic stations, Schlumberger resistivity soundings, seismic refraction, seismic reflection and MT profiles are superimposed on the geological interpretation map in Fig. 2.11.

In Sect. 2.2, magnetic and gravity data was used to map features of the southern African crust (e.g., Fig. 2.6). These regional surveys were mostly conducted by geological surveys, geoscience councils and other government agencies with the aim of stimulating exploration for metals and minerals. Exploration and mining companies use these surveys to identify target areas, secure prospecting licences, conduct higher-resolution geophysical and geochemical surveys, and, depending on the outcome, drill boreholes. In this section we use the electrical resistivity, MT and seismic data and their interpretations to validate and extend the interpretation of the potential field data. We recognize that the data sets have not been fully integrated. The prime reason for this is that the various geophysical studies addressed different key questions. Consequently, the survey footprints and depths of investigation (ranging from the near surface to the mantle) do not always overlap. Nevertheless, we believe that there is merit in providing the reader with a brief but comprehensive review of all the major geophysical surveys that have been conducted in southern Africa.

3.2 Electrical Resistivity and Magnetotelluric Studies

3.2.1 Deep Electrical Resistivity and Geomagnetic Induction Soundings

Between 1967 and 1986 the Council for Scientific and Industrial Research in South Africa (CSIR) conducted 11 ultradeep electrical resistivity soundings using the Schlumberger array with electrode spacings of up to 1200 km, probing the crust and upper mantle in the Kaapvaal, Zimbabwe and Congo cratons, the Bushveld Complex, and the Limpopo, Namaqua-Natal, Gariep and Damara Belts (De Beer 2015b; Fig. 2.11). In general, ignoring the surficial weathered layer, a five-layer electrical structure was found in both cratons and mobile belts:

A high-resistivity layer (ρ > 30,000 Ωm) extending to about 10 km was found in ‘massive terrains’—that is, in the granitic, Archaean cratonic nuclei.
A moderate-resistivity zone (2000 Ωm < ρ < 20,000 Ωm), indicating the presence of water-filled fractures associated with deformed metamorphic rocks in mobile belts, was observed at a depth range of 0–30 km and in the middle crust in Archaean cratons.
A highly conductive zone (ρ < 100 Ωm), observed in the depth range 25–30 km, was speculated to consist of serpentinized ultramafic mantle rock.
The uppermost mantle was found to be highly resistive (ρ > 20,000 Ωm).
Thereafter the resistivity was found to decrease gradually as mantle temperatures increase with depth.

The CSIR supplemented the resistivity surveys with a series of geomagnetic induction campaigns (e.g., De Beer 2015c). Some 26 three-component magnetometers were deployed in 1971 in a triangular array over central South Africa, which straddled the boundary between the Kaapvaal Craton and the Namaqua-Natal Belt (Gough et al. 1973; Fig. 2.11). No significant resistivity difference was found between these domains, but a significant induction anomaly was evident under the southern edge of the array. In 1977 an array of 52 magnetometers was used to map this electrical conductivity anomaly, named the Southern Cape Conductive Belt (SCCB; Fig. 2.12). This coincides in part with the Beattie Magnetic Anomaly (De Beer and Gough 1980; see also Sect. 2.2.10.2 for further discussion).

A magnetometer array was deployed in northeastern Namibia, northern Botswana and northeastern Zimbabwe in 1971/1972 (De Beer et al. 1976; Fig. 2.11). A zone of low resistivity was discovered that runs from the Zambezi Valley to south of the Okavango Delta, and into the Damara Orogenic Belt (Fig. 2.12). Further surveys were conducted in the Damara Belt in 1977 (De Beer et al. 1982b), which tracked the Damara conductor to the Atlantic coast. The geomagnetic surveys were complemented by more than 40 Schlumberger soundings with maximum electrode spacings of 40 km, which showed that the northeast-striking conductor has steep sides, is 3–10 km deep and at least 20 km thick, and has a resistivity of less than 20 Ωm (Van Zijl and De Beer 1983; Fig. 2.12).

Magnetotelluric (MT) surveys conducted in northern Zimbabwe (Losecke et al. 1988, cited by Weckman 2012) detected highly conductive layers in the Lower Zambezi Valley situated in the Zambezi Mobile Belt (lying between the Congo and Kalahari cratons), which were tentatively linked with the conductive structures of the Damara Belt. In the late 1990s a series of MT surveys were carried out in shallow Karoo-age basins in the Zambezi Valley (Whaler and Zengeni 1993; Bailey et al. 2000a, b). The underlying cratonic rocks had quite high resistivities, which Bailey et al. (2000a, b) interpreted to indicate that cratons were not affected by the basin-forming processes in the adjacent mobile belts.

3.2.2 SAMTEX and Inkaba Ye Africa Magnetotelluric Experiments

Between 2003 and 2008 the Southern African Magnetotelluric Experiment (SAMTEX) deployed more than 740 stations at a nominal spacing of 20 km on lines, with a total length of some 15,000 km, straddling the major tectonic provinces (Evans et al. 2011; Fig. 2.11). SAMTEX was led by Dr A Jones of the Dublin Institute for Advanced Studies and involved many African and international scientists and institutions. MT arrays were also deployed under the auspices of the Inkaba ye Africa programme (Weckmann 2012). The principal findings of these investigations are reviewed below.

Kaapvaal Craton and Rehoboth Terrane (see also Sect. 2.2.7 ): The MT observations were integrated with various geophysical and petrological observables (viz. elevation, surface heat flow, xenoliths) to derive an electrical conductivity model (Fullea et al. 2011). The depth of the present-day thermal lithosphere–asthenosphere boundary (LAB) was estimated to be at depths of 230–260 km and 150 km for the western block of the Kaapvaal Craton and Rehoboth Terrane, respectively. (It is important to note that the thermal LAB may differ from the chemical and mechanical LABs.)

Kaapvaal Craton — Limpopo Terrane — Zimbabwe Craton (see also Sect. 2.2.3.3 ): Three profiles were used to investigate the electrical structure of this region (Khoza et al. 2013b; Fig. 2.11). The 30 km-wide Sunnyside-Palala-Tshipise shear zone (PaSZ; Fig. 2.6) was found to be a subvertical conductive feature that was interpreted to be the collisional suture between the Kaapvaal and Zimbabwe cratons.

Congo Craton — Damara Belt (see also Sects. 2.2.3.4 and 2.2.6.3 ): The boundary between the Congo Craton and the Damara Belt is largely concealed by younger sediments. Four semiparallel MT profiles were used to investigate the electrical structure (Khoza et al. 2013a; Fig. 2.11). The Damara Belt lithosphere was found to be considerably thinner and more conductive than the Congo Craton lithosphere. Resistive features in the upper crust are interpreted as igneous intrusions emplaced during the Pan-African orogenic event, while highly conductive zones within the Central Zone of the Damara Belt are believed to be related to graphite- and possible sulphide-bearing stratigraphic units. The boundary between the Congo Craton and Damara Belt was shifted southwards, compared to prior models. A local study across the Waterberg Thrust/Omaruru Lineament (WT/Om-L; Fig. 2.6) found a 10 km-wide and at least 14 km-deep zone of anisotropic conductivity in the shallow crust parallel to the WT/Om-L (Ritter et al. 2003; Weckmann et al. 2003) and was interpreted to be the exhumed deep roots of ancient active shear zones.

Zimbabwe Craton — Magondi Belt — Ghanzi - Chobe Belt (see also Sect. 2.2.3.3 ): Kalahari sands cover most of northeastern Botswana and little was known about lithospheric structure and thickness prior to the study by Miensopust et al. (2011). A 600 km-long profile (Fig. 2.11) was interpreted. The Zimbabwe Craton is characterized by thick (~220 km) resistive lithosphere; the Tsumis-Ghanzi-Chobe Belt (TGCB; Fig. 2.6) by a somewhat thinner (~180 km) resistive lithosphere; while two lower- to mid-crustal conductors were found in the intervening Magondi Belt (MB; Fig. 2.6). The terrane boundary between the Magondi and Ghanzi-Chobe Belts was interpreted to be further north than previously inferred from regional potential field data.

Kaapvaal Craton — Namaqua Natal Belt (see also Sect. 2.2.10.1 ): Karoo Supergroup strata conceal the contact between the Kaapvaal Craton and the Namaqua Natal Belt, but a large contrast in resistivity (>5000 Ωm and ~30 Ωm, respectively) makes it possible to map the contact (Weckmann 2012).

Southern Cape Conductivity Belt (see also Sect. 2.2.10.2 ): The source of the SCCB, discovered by De Beer et al. (1982a; Fig. 2.12), was studied in detail at specific locations by Weckmann et al. (2007a, b; Fig. 2.10). The Namaqua-Natal and Cape Fold Belts were found to have a generally high ‘background’ conductivity (~30 Ωm). Several distinct zones of high conductivity were identified, one 5–10 km below the surface trace of the Beattie Magnetic Anomaly, and the other linked to an extensive 50–70 m-thick pyritic-carbonaceous shale formation in the Karoo Basin (the White Hill Formation). Weckmann (2012) suggests that the SCCB, largely mapped using a sparse magnetometer array, may be the integrated result of the generally conductive NNB and the conductivity anomalies within it, or overlying it.

Cape Fold Belt: MT imaging of the structure of the CFB (Fig. 2.6) showed deep and resistive roots that are incompatible with a major crustal detachment zone and thick-skinned tectonics (Weckmann et al. 2012).

Lithospheric Mantle: Large variations in maximum resistivity at depths of 200–250 kmarine seismic refraction surveysm were found to relate directly to the age and provenance of upper crustal structure (Evans et al. 2011)—for example, beneath the central parts of the Kaapvaal craton the resistive mantle extends to depths as great as 230 km, while the mantle beneath the Bushveld Complex was found to be highly conductive at 60 km depth.

One of the main objectives of the extensive MT work was to investigate the use thereof to identify regions that are prospective for diamondiferous kimberlites. Images of electrical resistivity and electrical resistivity anisotropy at depths of 100 km and 200 km show that resistive regions correlated with the Kaapvaal, Zimbabwe and Congo (Angola) cratons, and more conductive regions correlated with the neighbouring mobile belts and the Rehoboth Terrane (Jones et al. 2009; Muller et al. 2009). Known diamondiferous kimberlites are inferred to lie mainly within the resistive or isotropic regions, and on their boundaries with conductive or anisotropic regions. Comparison with seismic body and surface wave tomographic images (see also Sect. 2.3.3.5 and Fig. 2.16) show that high-velocity regions are resistive and low-velocity regions are conductive.

3.3 Seismic Surveys

3.3.1 Refraction Seismic Surveys Using Mining-Induced Tremors

Seismic investigations of the Kaapvaal Craton, using recordings of earth tremors induced by deep gold mining, began in the 1940s (Willmore et al. 1952; Gane et al. 1956; Hales and Sacks 1959; Green 1980). Seismometers were deployed on profiles radiating from the Witwatersrand gold fields. The longest profile extended as far as Lusaka (Green 1980). The crust–mantle boundary (known as the Mohorovičić discontinuity or simply the Moho) was found to be at a depth of 33–36 km beneath the Kaapvaal Craton, with unusually high subcrustal P- and S-wave velocities of about 8.2 and 4.8 km/s, respectively. Long-period surface wave studies were conducted as part of the international Upper Mantle Project, in which a five-station long-period seismograph array was deployed on the Kaapvaal Craton (Bloch et al. 1969). Analysis of the dispersion of the surface waves produced by earthquakes in Malawi and Zambia confirmed that the shear wave velocities in the upper mantle beneath the Kaapvaal Craton were unusually high. The approach was used again in the 1980s with more modern technology (Durrheim and Green 1992). Seismographs were deployed at c. 10 km intervals on profiles linking the Klerksdorp, East Rand and Free State gold fields (Fig. 2.11); seismograms were recorded on magnetic tape and digitized; and ray-tracing and the reflectivity method were used to interpret the travel times and amplitudes. A P- and S-wave velocity model of the crust, upper mantle and crust–mantle transition zone was derived and integrated with xenolith and geochemical data to produce a model of evolution of the central Kaapvaal Craton (Durrheim and Mooney 1994). They postulated that there was a fundamental change in lithosphere-forming processes at the end of the Archaean. Hotter Archaean mantle temperatures led to the eruption of komatiitic lavas and the formation of a lithosphere that was ultradepleted in FeO, intrinsically buoyant and sufficiently cool for diamonds to form. As the Earth cooled, the mantle temperature passed through a critical point and komatiitic volcanism ceased. The fertile Proterozoic mantle lithosphere had an FeO content similar to the asthenosphere and was prone to partial melting during heating events. Thickening of the Proterozoic crust occurred by the extrusion of basalt and underplating, the latter forming a high-velocity layer at the base of the crust.

3.3.2 Marine and Onshore Refraction Seismic Surveys Using Artificial Sources

Early marine seismic refraction surveys were conducted over the Agulhas-Bank, -Basin and -Plateau, and the Transkei Basin (Green and Hales 1966; Spence 1970; Chetty and Green 1977; Hales and Nation 1972; Barrett 1977; Tucholke et al. 1981; Fig. 2.11). The Agulhas Bank crust was found to be 33 km thick, with a high-velocity lower-crustal layer (7.2 km/s). The 2.5 km-deep Agulhas Plateau, to the south, was interpreted to be continental in origin, which was confirmed by dredging, while the crust beneath the 5 km-deep Agulhas Basin was interpreted to be oceanic in origin.

Three refraction profiles parallel to the axis of the Damara Orogen in Namibia were surveyed in 1975 (Baier et al. 1983; Green 1983; Fig. 2.11). The Moho was found to be at a depth of 36 km beneath the Pan-African Damara Orogen, deepening to 50 km along the southern margin of the orogeny, and to 60 km beneath the Proterozoic Kalahari Craton. A 290 km-long refraction seismic profile over the Proterozoic Namaqua Province (see also Sect. 2.2.10.1) was shot in 1983 (Green and Durrheim 1990; Fig. 2.11). The Namaqualand crust was found to be 42 km thick, with a lower crust P-wave velocity of 6.6–6.9 km/s, indicating the presence of rocks of intermediate composition.

3.3.3 Reflection Seismic Surveys for Hydrocarbons

From 1965 to 1979, about 11,000 line kilometres of seismic reflection data were acquired on land by the Southern Oil Exploration Corporation Ltd (SOEKOR) over the Karoo, Algoa, Gamtoos and Zululand basins in South Africa, and 138 boreholes were drilled (Wood 2015). The seismic surveys were complemented by aeromagnetic, gravity and deep resistivity investigations. It was concluded that the quality of sandstone reservoirs was poor, potential structural traps were small and high-quality source rocks were absent. Surveys of the continental shelf also commenced in the 1960s. The first discoveries of gas in southern South African waters were made in 1970 about 60 km offshore Plettenberg Bay and 90 km offshore Mossel Bay. Black oil was discovered in the Bredasdorp basin in 1986. Surveys for oil were also conducted in the Nosob Basin in Botswana (Wright and Hall 1990; NoB; Fig. 2.6), the Owambo Basin in northern Namibia (Hoak et al. 2014) and the coastal plain and continental shelf of Mozambique (Brownfield 2016).

3.3.4 Deep Continental Reflection Seismic Surveys

In the 1980s it was realized that the reflection seismic method, routinely used to search for hydrocarbons, could effectively probe the entire continental crust. Many countries established deep reflection seismic programmes, such as COCORP (US), Lithoprobe (Canada), DEKORP (Germany), ECORS (France) and BIRPS (UK). The South African National Geophysical Programme (NGP) was launched in 1985 and six deep reflection seismic profiles were surveyed, targeting some of the region’s most interesting and important geological features. Other deep seismic reflection profiles were surveyed during mineral and hydrocarbon exploration programmes and the Inkaba ye Africa programme.

Agulhas Bank: The first deep reflection profile in southern Africa, shot in 1985 on the Agulhas Bank by SOEKOR, straddled the Cape Seal Arch (Durrheim 1987). The folded Cape and Kaaimans sediments gave rise to occasional strong reflections, and strong reflections at 9–10 s two-way time (TWT) were interpreted to arise from the Moho.

Central Kaapvaal Craton (see also Sects. 2.2.3.2 and 2.2.4 ): Beginning in 1982, reflection seismics was used to explore for extensions of the gold-bearing Archaean Witwatersrand Basin and the platinum-bearing Bushveld Complex. Tens of thousands of line kilometres were surveyed (Pretorius et al. 2003; Campbell 2011). In 1988 the NGP surveyed a 112 km-long 16 s TWT profile from the Ventersdorp dome, across the Potchefstroom syncline, to the Vredefort Dome (Fig. 2.11). Using this data, Durrheim et al. (1991) mapped the present-day structure of the Witwatersrand Basin and the crystalline basement between the Ventersdorp dome and Potchefstroom, and found that it is characterized by several zones of strong, subhorizontal reflections (see also Sect. 2.2.3.5). Other geophysical data sets used to constrain interpretations of the reflection seismic profile included gravity and magnetics, and refraction seismics (Green and Chetty 1990; Durrheim and Green 1992). Between 1983 and 1994 the Gold Division of Anglo American Corporation of South Africa (now Anglogold Ashanti Ltd) acquired more than 16,000 kilometres of reflection data across the Kaapvaal Craton. As the main objective of these surveys was to explore for deeply buried gold-bearing Witwatersrand sediments, most of the sections were 5 or 6 s TWT. However, several 16 s TWT profiles were acquired, of which eight were made available for research and publication (Tinker 2001; Tinker et al. 2002; de Wit and Tinker 2004; Fig. 2.11). Seven of these profiles were projected onto a common section that strikes northeast–southwest across the central part of the Kaapvaal Craton, stretching from the Vredefort Dome to Vryburg and straddling the Colesberg Lineament (see also Sect. 2.2.3.5). The seismic data suggests a tectonically stacked series of crustal slivers that were thrust over the Kaapvaal Craton during the Neoarchaean. The eighth profile crosses the boundary between the Kaapvaal Craton and the Namaqua-Natal Belt (NNB) in the Eastern Cape, and indicates that the NNB is thrust over the Kaapvaal Craton (location not shown in Fig. 2.11, see also Sect. 2.2.3.2).

Bushveld Complex: The NGP acquired two profiles over the Bushveld Complex (Fig. 2.11). In 1986 a 50 km-long profile was surveyed along a line starting west of Pretoria and running northwards across the Bushveld Complex (Odgers et al. 1993). In 1989 a 117 km-long profile was surveyed across the eastern lobe of the Bushveld Complex. Odgers and du Plessis (1993) interpreted the seismic profiles and concluded that the data supported a model of a sheet-like structure for the Rustenburg Layered Suite. They interpreted the Malope Dome to be a diapir of Nebo Granite Suite rocks that rose through the denser Rustenburg Layered Suite.

Limpopo Terrane (see also Sect. 2.2.3.3 ): In 1987 the NGP acquired a 200 km-long profile from Pont Drift (on the Limpopo River), traversing the Central and Southern Marginal Zones of the Limpopo Terrane (LT; Fig 2.6), and ending on the Kaapvaal Craton near Polokwane (Fig. 2.11). De Beer and Stettler (1992) found that the reflection seismic data provided excellent information about shallow structure, but lacked detail on the lower crustal and the crust–mantle boundary structures. Combining the seismic reflection data and results from geoelectrical studies, De Beer and Stettler (1992) concluded that the high-grade metamorphic rocks of the Southern Marginal Zone were thrust southward over the low-grade cratonic rocks. The reflection data were reprocessed and reinterpreted by Barker (1992), and subsequently integrated with gravity observations and refraction profiles surveyed from Johannesburg to Musina (Gane et al. 1956), Polokwane to Lusaka (Green 1980) and across the Limpopo Belt (Stuart and Zengeni 1987). It was found that the crust ranges in thickness from about 30 km in the centre of the LT to 40 km in the Zimbabwe Craton (Durrheim et al. 1992).

Kaapvaal Craton — Namaqua Natal Belt (see also Sects. 2.2.3.2 and 2.2.10.1 ): In 1991 a 226 km-long profile was surveyed under the auspices of the NGP between Sishen and Keimoes in the Northern Cape Province, straddling the western Kaapvaal Craton boundary (Fig. 2.11). Following the seismic survey, eleven magnetotelluric sounding stations were located at 20 km intervals along the same profile. Stettler et al. (1998, 1999) integrated the seismic, magnetotelluric, magnetic and gravity data along this traverse. In their interpretation, the reflective package west of Sishen is located on the western margin of the Kaapvaal Craton, coinciding with conductive, dense and partially magnetized material, which may be indicative of an ophiolitic sequence that was thrust onto the Craton margin to form an accretionary wedge between the Kheis Terrane and the Kaapvaal Craton.

Southern Karoo — Cape Fold Belt (see also Sect. 2.2.10 ): Loots (2013) reprocessed and interpreted the 105 km-long Karoo seismic reflection profile that was acquired under the auspices of the NGP in 1992 (Figs. 2.11 and 2.13). The upper crust consists of Karoo and Cape Supergroup rocks that dip slightly to the south. The middle crust is interpreted to consist of granitic gneisses belonging to the Bushmanland Terrane, part of the Namaqua-Natal Belt (NNB). The middle crust also hosts the source of the Beattie Magnetic Anomaly, which is characterized by a bean-shaped cluster of strong reflections, ~10 km wide, with a thickness of ~8 km and an apparent greatest depth at ~8 km. The Moho is encountered at ~37 km at one section of the profile, but no clear Moho reflections are seen elsewhere.

3.3.5 Southern African Seismic Experiment

The Kaapvaal Project, initiated in 1996, consisted of a range of geological, geophysical, geochemical and petrological investigations (Carlson et al. 1996). A key component was the Southern African Seismic Experiment (SASE), which comprised two broadband seismograph arrays: the regional Kaapvaal Seismic Array (KSA) of 82 stations along an 1800 km × 600 km transect that stretched from the Cape to Zimbabwe (James and Fouch 2002, Fig. 2.11); and the dense Kimberley Telemetered Array (KTA; Fig. 2.11), a six-month deployment of 32 stations in a 65 × 50 km area covering a region with several kimberlite intrusions. We review the main findings of a range of seismological investigations that were conducted by members of the Kaapvaal Seismic Group, and, following the release of the data into the public domain, by the international seismological community (Fig. 2.14).

African Superswell and Superplume Anomaly: Southern Africa is characterized by a high (~1.5 km) inland plateau, the ‘African Superswell’ (Nyblade and Robinson 1994). Geomorphological and geochronological studies indicate that the rise occurred between 5 and 30 Ma (McCarthy and Rubidge 2005). One of the largest shear wave anomalies in the lower mantle, the ‘African Superplume’ or simply the ‘African Anomaly’, is found beneath southern Africa. It is a 7000 km-long, 1000 km-wide and 1500 km-high structure, striking roughly northwest and characterized by a 3% drop in the shear-wave velocity Vs (Ni and Helmberger 2003; Wang and Wen 2007). An investigation of the phenomena that caused the Superswell (e.g., buoyancy caused by heating and/or convection forces) and the Superplume (e.g., thermal and/or chemical variations in the lower mantle), and the connection (if any) between the two features, were important drivers of SASE.

Crust: Receiver functions, which rely on the conversion of P-wave to S-wave energy at the Moho, have been used by several workers to map the structure and thickness of the crust (Harvey et al. 2001; Midzi and Ottemőller 2001; Nguuri et al. 2001; Stankiewicz et al. 2002; Kgaswane et al. 2009, 2012). It was found that the crust beneath undisturbed parts of the Archaean Kaapvaal and Zimbabwe Cratons is relatively thin (35–40 km), unlayered and characterized by a strong velocity contrast across a relatively sharp Moho. This contrasts with the crust beneath post-Archaean terranes or Archaean regions affected by large-scale Proterozoic events (e.g., Bushveld Complex), where the crust is relatively thick and the Moho is complex. The KTA enabled more detailed investigations to be performed in the Kimberley area. It was found (Fig. 2.15) that the crust is 35 km thick with an average Poisson’s ratio of 0.25; the lower crust has a density of 2.86 g/cc, Vp of 6.75 km/s and Vs of 3.9 km/s, indicating a felsic to intermediate composition; the Moho transition is less than 0.5 km, with a 15% density contrast across it; and the uppermost mantle has Vp of 8.2 km/s and Vs of 4.79 km/s (Niu and James 2002; Stankiewicz et al. 2002; James et al. 2003).

Upper Mantle: Tomographic inversion of 3D body wave delay times (P, PKP, S, SKS) and surface wave phase and group velocities showed that high-velocity mantle roots extend to depths of at least 250 km and locally to 300 km beneath the Kalahari craton (comprising the Kaapvaal and Zimbabwe Cratons, and the Limpopo and Rehoboth Terranes, as shown in Fig. 2.1; Ritsema and Van Heijst 2000; James et al. 2001; James and Fouch 2002; Fouch et al. 2004). An example of a seismic-tomography P-wave-anomaly depth slice at 150 km is shown in Fig. 2.16 (after James and Fouch 2002). The mantle beneath the Bushveld Complex has relatively low velocities. Evidence for a seismic low-velocity zone (LVZ) in the upper mantle was eagerly sought in order to test the hypotheses regarding the cause of the Superswell. A LVZ is regarded as evidence of the presence of warm and/or low-density material. However, the body wave tomography did not provide evidence for a LVZ. Several surface wave studies were carried out using a variety of methods. Chevrot and Zhao (2007) confirmed that high-velocity cratonic roots are confined to the Archaean cratons, and that the roots extend to at least 250 km. Larson et al. (2006) found that Vs decreases slightly with depth from 4.7 km/s in the uppermost mantle to 4.60 km/s at 200 km. No evidence was found of Vs < 4.55 km/s shallower than 250 km. Larson et al. (2006) concluded that the present-day mantle velocity structure is similar to that at the time of kimberlite eruption at 70–90 Ma. Li and Burke (2006) imaged a fast, lithospheric lid beneath most parts of southern Africa, ranging in thickness from c. 80 km beneath the Namaqua-Natal Belt to 180 ± 20 km beneath the Kaapvaal Craton. Relatively low velocities were observed under the Cape Fold Belt and in the shallow upper mantle beneath the tectonic border regions. The basement to the Bushveld Complex was found to be relatively slower than its surroundings above 100 km. A LVZ was observed at depths of 160–260 km across southern Africa with an average velocity of 4.5 km/s. This layer, although not absolutely slow, is 4% slower than the fast lithosphere above it. Li and Burke (2006) suggested that the LVZ is largely caused by high temperature associated with sublithospheric mantle convection, which could contribute to the high elevation of southern Africa, rather than compositional and petrologic effects.

3.4 Agulhas Bank—Cape Fold Belt—Namaqualand Geotransects

The Cape Fold Belt-Agulhas Bank Geotransect (Central Geotransect, Fig. 2.11), compiled by Hälbich (1993), integrated much of the prior work in the region. The 600 km north-northwest-south-southeast-trending transect stretches from the central Karoo, across the Cape Fold Belt to the edge of the continental shelf, traversing several unusual features, such as the Southern Cape Conductivity Belt, the Beattie Magnetic Anomaly, a belt of Jurassic Karoo dolerites and the Agulhas Fracture Zone. Geophysical data sets used to compile the transect included seismic reflection profiles acquired by Soekor to image the offshore Cretaceous basins, including a deep (12 s two-way time) profile (Durrheim 1987); a combined refraction seismic and gravity profile along 21°50′E (Hales and Nation 1972); a refraction seismic profile in Namaqualand (Green and Durrheim 1990); and Bouguer gravity, magnetic and magnetotelluric data. The main conclusion drawn from the study was that old sutures, manifested by linear magnetic and conductive belts, probably played an important role in the subsequent geotectonic evolution of the continental crust.

A similar transect, east of the above, was compiled more than 20 years later under the auspices of the German-South African multidisciplinary earth science project Inkaba ye Africa. The 800 km Agulhas-Karoo Geoscience Transect runs north-northwestwards from the Agulhas Plateau across the Agulhas Passage, Agulhas-Falkland Transform Fault and Agulhas Bank to the coastline, then across the Cape Fold Belt, Beattie Magnetic Anomaly, the Karoo Basin, the Great Escarpment, and onto the Namaqua Natal Belt. The 457 km offshore profile, from the Agulhas Passage to the Agulhas Bank, was shot in 2005 (Parsiegla et al. 2007). It was extended landward using 48 seismic stations spaced over 240 km (Stankiewicz et al. 2007). The offshore profile shows a 52 km-wide transition from oceanic to continental crust, with crustal thickness increasing from 7 to 30 km. The upper and lower crust has P-wave velocities of 5.6–6.6 km/s and 6.4–7.1 km/s, respectively. The onshore profile enabled a P-wave velocity model to be derived to a depth of 25 km. A 100 km-long reflection seismic line was shot along the same line, beginning at Prince Albert and terminating near Fraserburg (Lindeque 2008; Lindeque et al. 2007), clearly imaging the Karoo and Cape Supergroup supracrustal strata. The unconformity at the base of these strata deepens from ~5 km in the north to ~10 km in the south. The mid-crustal layer is ~20 km thick, and the source of the Beattie Magnetic Anomaly is associated with reflectors 7–15 km deep. Reflections, that were interpreted to represent the Moho, range in depth from ~43 km in the north to ~35 km in the centre, thickening to ~45 km beneath the tectonic front of the Cape Fold Belt.

4 Summary and Conclusions

The scope of this chapter is vast, presenting an integrated geophysical-geological interpretation of the major Archaean and Proterozoic domains concealed beneath Phanerozoic and younger cover in southern Africa. The interpretation is focused on specific regional-scale features that have been the subject of numerous investigations over the last five decades and current work by the authors. Where no geophysical interpretation has been conducted or sourced, the published geological mapping is shown for the sake of completeness. As far as possible, the interpreted features honour all geological and geophysical data sets within the resolution of the data. Integration of these results in the unified interpretation map presented here brings new insights into the disposition and evolution of the Precambrian geology of southern Africa, extending into Antarctica within a Gondwana framework. Improvement of this work will, no doubt, be further facilitated by in-house mining and exploration data when this becomes available. Interpretation highlights are summarized below:

The craton boundaries have been mapped, or existing interpretations refined. Although indicated as ‘line’ boundaries, they reflect the locus of what is likely to be a much more complex structural swathe. What has often been referred to as the Okwa basement, excluding correlations with the Magondi-Gweta Belt by some workers, is considered to be an integral part of the Kaapvaal Craton, here termed the Okwa Block. The Magondi-Gweta Belt is suggested by the potential field data to be a separate entity, and is mapped extending westwards into Zambia. A possible link between this and the Limpopo Belt is suggested but is difficult to define owing to the cover of Karoo basalts. The high P-wave velocity anomaly associated with the Magondi-Gweta Belt, at a depth of 150 km, appears to continue southwards into the mapped western portion of the Limpopo Belt. The Congo Craton is interpreted to have a definable but complex southern boundary. Mapping the extent thereof into Angola is, however, hampered by extensive cover and lack of published detailed aeromagnetic and gravity data.
Major structural lineaments have been mapped. As with the craton boundaries, the traces should be considered to be the locus of broader structural swathes or corridors. Many of the lineaments have been mapped previously and documented. However, the present study identifies a previously unrecognized lineament, here termed the Tsodilo Lineament, which extends from northwest Botswana into Angola, trending north-northwest to north. It is suggested that it may have been an early, northern extension of the Kalahari Lineament prior to the development of the Damaran Orogenic Belt. Many faults have been mapped from the potential field data, but only a select few are shown owing to the scale of presentation. It should be borne in mind that the data sets reflect sources at a range of depths. The fault traces should thus be considered as structural trends that include, but are not exclusive to, faults mapped at surface.
A number of ring features have been mapped which may be associated with a variety of sources, such as intrusions, basalt vents and meteorites. Examples of the latter include the Morokweng and proposed Karas meteorite impact sites. The preservation of the arcuate geometry of ring structures, cutting across older structural fabric, suggests that they would have to be post- or late-tectonic features, or foci of older causative sources which have been reactivated through geological time as a result of associated crustal weakness. In this sense, a ring feature may be evidence of an older focal source. Further possible sources may result from rheological or structural variations in the deep crust or upper mantle. In particular, the Kaapvaal Ring Structure (KVRS) is thought to be such a feature, encompassing the high-velocity root of the Kaapvaal Craton. It is suggested that movement along a probable competency contrast between low and high mantle velocity zones may have resulted in the near-surface manifestation of the KVRS. A number of significant kimberlites, or clusters thereof, are associated with the KVRS.
Mapping of the extent and structure of the Witwatersrand Basin, in previous studies by a number of workers, has been manifestly improved and documented here, not only with regard to the main basin but also through the identification and mapping of a southern extension of the main basin close to the southern boundary of the Kaapvaal Craton. Southern extensions of the Transvaal Supergoup have also been mapped.
The Xade Complex in Botswana, a layered sequence of mafic intrusive and extrusive rocks forming part of the more extensive Umkondo magmatic province, has been found to comprise Northern and Southern Lobes. The Northern Lobe was not previously recognized as a separate entity, since it extends under a thick succession of sediments in the Passarge Basin, where it continues with diminished gravity and magnetic signatures. The Xade-related magmatic event is interpreted to have extended locally westwards to include the Tsetseng and Tshane intrusions, as well as along the northern portions of the Kalahari Lineament.
On the Zimbabwe Craton, the residual gravity highs, despite the coarse data, show strong correlation with the greenstone belts as a result of their volcanic and banded iron sequences.
The tectonostratigraphic zones of the Damara-Chobe Belt have been refined, and built on, from previous interpretations by a number of workers, both westwards to offshore Namibia and northeastwards into Botswana. Major features of the belt are, firstly, the apparent disconformable truncation of these zones against the interpreted Congo Craton boundary and the disruption of continuity across this boundary into the Northern Platform; secondly, the dramatic change in the apparent width of the belt, as well as stratigraphic level, as the Kudu Lineament is crossed eastwards; and, thirdly, the rapid pinching out of the belt, as bounded by the Southern Margin Zone in the south, eastwards into Botswana. The northeastward extension of the belt in northern Botswana, and into Zambia, where it is almost entirely covered, has been the subject of much research and debate. The identification in northwestern Botswana of the Tsodilo Lineament, a structural corridor transverse to the strike of the belt, serves to explain the north-northwest structural disposition of the Tsodilo Hills Group seen in the aeromagnetic data, as well as the apparent abrupt termination of the eastern Congo Craton boundary. This structural corridor is thought to have played a significant role in controlling the extent and continuity of the Damaran-Katangan strata into southern Zambia south of the Mwembeshi Shear Zone.
An aspect of interest within the Rehoboth Terrane is the identification of a potential meteorite impact site termed the Karas Impact Structure. The structure is possibly of Mesoproterozoic age, but the interpretation suggests that it was the focus of continued crustal weakening and reactivation until recent times.
Published interpretations of depth to magnetic basement west of the Kalahari and Tsodilo Lineaments suggest the preservation of two extensive, thick sedimentary successions in excess of 6000 m, probably of Neoproterozoic age or older. Of interest is, firstly, their common occurrence west of these respective Lineaments; secondly, the possible control of a number of regional structures, which appear to constrain the basins, on the development of the basins or of their subsequent structural disruption; and, thirdly, that they straddle the Damara-Ghanzi-Chobe Belt to the north and south, suggesting that they may have formed a continuous basin in pre-Pan-African times.
Improved definition of the Beattie Magnetic Anomaly (BMA), separating it into individual magnetic zones, suggests that the significant strike change from roughly east–west in the west to east–northeast in the east may have resulted from a change in transpressive direction affecting the eastern sector, possibly in Pan-African times. In addition, the parallelism of strike in the east with known highly magnetic thrust zones that crop out in Natal suggests that the origin of the BMA is thrust related. Furthermore, continuation of the belt of BMA-type anomalies into Dronning Maud Land (DML) in Antarctica, within the more extensive Namaqua-Natal-Maud Belt (NNMB) in a Gondwana framework, has provided additional field evidence for similar thrust-related magnetization seen to be associated with these anomalies in DML. The sense of thrusting at all observed field sites is craton directed, which is supported by the magnetic modelling of dips of the anomalous units in areas of cover.
The interpretation identifies an area, under Phanerozoic cover, between the Namaqua and Natal Provinces of the Namaqua-Natal Belt, which has magnetic and gravity signatures distinctly different from either of these provinces. This is here termed the Khoisan Province which, based on a single deep borehole east thereof (SOEKOR borehole WE), is interpreted to be affiliated to the Namaqua Province, rather than to the Natal Province.
The Southern Cape Conductivity Belt (SCCB), one of the largest deep crustal conductivity anomalies on Earth, remains enigmatic in its origin. Although the BMA lies within its swathe, paralleling its northern boundary, the SCCB is interpreted to lie deeper than the BMA. The majority of workers believe that the two anomalous belts do not have a common source but may be the result of a common evolutionary process. It has also been proposed that the SCCB, mapped largely from sparse magnetotelluric stations, results from an integrated effect of the elevated conductivity of the Namaqua-Natal basement, of higher conductivity zones therin, and of the high conductivities observed in overlying Phanerozoic strata.
Interpretation of the aeromagnetic and satellite gravity data offshore Namibia has provided significant insights into the evolution of this passive volcanic margin. Firstly, the tectonostratigraphic zones of the Damara Orogen are clearly evidenced and mapped up to the offshore hinge zone where seaward-dipping reflectors (SDR), associated with basalt flows, first appear. Secondly, the mapped offshore Mesozoic gravity and magnetic anomalies are seen to be clearly offset along the offshore continuations of a number of Pan-African or older lineaments, suggesting that these structures not only determined the architecture of the extended crust but also were reactivated during the late Mesozoic, having directions that favoured the initiation of transform faults offshore. Thirdly, the M-type apparent spreading-related magnetic anomalies have been better delineated, and are seen to converge towards the northern Namibian and Angolan coastlines, in contrast to their previous placement much further out to sea. This has significant implications for the relative rates of extension from the southern to northern offshore areas, and hence for the relative ages of the offshore sedimentary sequences. Forward modelling of the magnetic data has clarified the origin of the apparent M-type anomalies. These are shown to arise from the feather-edge suboutcrop of lava flows with a very shallow westerly dip, rather than to the classical vertical dyke-like seafloor spreading model.
Several large-scale seismic, electrical resistivity, geomagnetic induction and magnetotelluric investigations have been conducted over the past five decades. They have yielded important insights into the deep structure and evolution of the subcontinent, and provided constraints on magnetic and gravity interpretations. The resultant published data sets have made it possible to extend our view of shallow crustal features to greater depths, and into the upper mantle. Conductive zones were recognized, and sedimentary basins, igneous intrusions, terrane boundaries and continental-scale shear zones concealed by younger strata were mapped. The topography of the Moho and the Lithosphere-Asthenosphere Boundary was mapped, showing that the Archaean Kaapvaal, Zimbabwe and Congo Cratons have deep roots that are relatively cold.

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Manica Minerals Ltd., Swakopmund, Namibia
Branko Corner
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
Raymond J. Durrheim

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GZG—Structural Geology and Geodynamics University of Göttingen, Göttingen, Germany
Siegfried Siegesmund
Geosciences Institute, University of São Paulo, São Paulo, Brazil
Miguel A. S. Basei
Departamento de Geodinámica Interna, Universidad de la República, Montevideo, Uruguay
Pedro Oyhantçabal
GZG—Structural Geology and Geodynamics University of Göttingen, Göttingen, Germany
Sebastian Oriolo

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Corner, B., Durrheim, R.J. (2018). An Integrated Geophysical and Geological Interpretation of the Southern African Lithosphere. In: Siegesmund, S., Basei, M., Oyhantçabal, P., Oriolo, S. (eds) Geology of Southwest Gondwana. Regional Geology Reviews. Springer, Cham. https://doi.org/10.1007/978-3-319-68920-3_2

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DOI: https://doi.org/10.1007/978-3-319-68920-3_2
Published: 03 February 2018
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