Two geophysical onshore-offshore lines on the southern margin of Africa form the Agulhas-Karoo Geophysical Transect (AKGT) and cross prominent geological features such as the Karoo Basin, Cape Fold Belt (CFB) and the Beattie Magnetic Anomaly (BMA). Geophysical data acquired along this AKGTransect between 2004 and 2007 within the Inkaba yeAfrica (IyA) framework, provide the platform for constructing a deep crustal section (IyA-200501) for the centre 100 km of the western AKG'r-transect in order to resolve these features at depth. We present a detailed deep crustal model constructed from the joint interpretation of:i. archive data comprising surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions and regional seismic reflection profiles, andii. line coincident newly acquired high-resolution geophysical data consisting of near vertical seismic reflection data, shallow P- and S-wave velocity data, wide-angle refraction data, high resolution magnetotelluric data and impedance spectroscopy measurements on borehole samples.Our model differentiates four components in the up to 45 km thick crust:1. a similar to 2 to 5 km thick folded Karoo Supergroup, disrupted by low-angle thrust faults rooted in a zone of local decollements in the lower Ecca Group and resting paraconformably on2. a continuous undeformed sub-horizontal similar to 1.5 to 10 km thick wedge of the Cape Supergroup (CSG). This CSG wedge stretches from the Escarpment in the north to the tectonic front of the CFB in the south, and rests on an unconformity that dips about three degrees to the south. The angular unconformity is interpreted as an erosional peneplain that separates the CSG wedge from component3. the similar to 13 to 21 km mid-crust basement below. The mid-crust contains a distinct north-clipping seismic fabric, here interpreted as similar to 1.4 to 1.0 Ga Mesoproterozoic Namaqua-Natal Metamorphic Belt (NNMB) crust. A south-dipping mid-crustal detachment, interpreted as a ductile thrust zone, separates the mid-crust from component4. a highly reflective similar to 10 to 24 km thick lower crust. The latter is interpreted as an older Palaeoproterozoic section of the NNMB (or even Archean cratonic basement), and bounded by a similar to 2 to 5 km thick, highly reflective bottom layer below that lies sub-parallel to a clear Moho. This bottom layer is interpreted as a mafic underplate, metasomatic reaction zone, or lower-crust to mantle transition zone.Collectively the seismic reflection and wide-angle refraction data support an interpretation that the NNIMB mid-crustal layer contains the BMA source, possibly connected to two zones of strong reflectivity: a similar to 10 to 12 km wide northern zone and a similar to 5 to 7 km wide southern zone, both about 5 km thick and 7 to 8 km below surface. We interpret the BMA source to be at least in part, a Namaqua-like massive to disseminated, deformed/metamorphosed stratiform sulphide-magnetite ore body with metasomatic overprint.The seismic reflection and -refraction data support an interpretation that a Pan-African suture zone at the BMA is absent and that instead, the NNMB continues below the CFB tectonic front, probably up to the continental margin and the Agulhas Fracture Zone. The seismic reflection data also supports a thin-skinned tectonic thrust model for the evolution of the CFB without significant fore-deep stratigraphic thickening of the Karoo Basin strata. A compatible tectonic model implies a Palaeozoic collision orogen setting, coupled to a south verging subduction zone much farther south of the CFB. Similarly, the geophysical data support a south clipping subduction zone during the amalgamation of the NNMB in the Mesoproterozoic.Current reconstructions of the Rodinia supercontinent link the NNMB and the Grenville Province of North America across the Grenville-Kibaran orogen. Our seismic section tests this reconstruction through a direct comparison with seismic profiles on the opposite flank of the orogen. Although the once adjacent continental blocks are now 1000s of kilometres apart, the seismic images show a good correlation and support the reconstruction.

The deep crust of the southernmost margin of Africa contains unresolved tectonic features such as the Paleozoic Cape Fold Belt (CFB), the Paleozoic-Mesozoic Karroo Basin and the largest terrestrial magnetic anomaly, the Beattie Magnetic Anomaly (BMA). Without resolving these structures, our understanding of the evolution of the southern margin will be incomplete and limited. Under the auspices of the Inkaba yeAfrica framework, several geophysical datasets were acquired from 2004 to 2007, along two transects across the margin and its unique tectonic features. This research presents a tectonic model and crustal geometry, at the centre 100 km of the western transect. The model is derived from the joint interpretation of: surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions, impedance spectroscopy measurements on borehole samples, near vertical reflection seismic data (NVR), shallow P- and S-wave velocity data, wide angle refraction data and magnetotelluric data. The model differentiates a four component ~ 42 to 45 km thick crust and constrains the two part BMA to a ~10 to 12 km wide northern zone, and a ~5 to 7 km wide southern zone, both at ~7 to 8 km below surface, continuing for a depth of ~5 km, and, contained in the Mesoproterozoic Namaqua-Natal mid-crust. The BMA source is interpreted to be a Namaqua-like massive to disseminated, deformed/metamorphosed strataform sulphide ore body. The model presents evidence in support of a thin-skinned tectonic thrust model for the evolution of the flat-based CFB and shows no significant fore-deep stratigraphic thickening in the Karroo Basin towards the CFB front. The tectonic model suggests a Meso-Proterozoic collision orogen setting, overprinted by a mid-Phanerozoic thin-skinned fold and thrust belt coupled to the far-field accretion/subduction margin to the south.

Our understanding of the tectonic evolution of the Cape Fold Belt, southern Karoo Basin and associated geophysical Beattie Magnetic Anomaly (BMA), may be challenged by the new ~ 100 km Near Vertical Reflection (NVR) seismic image which reveals ~42 to 45 km of complex crust and a sharp crust-mantle transition. The 5 to 10 km upper crust consists of dipping reflectors interpreted as the folded sequences of the Karoo Basin, underlain by continuous flat-lying reflectors that represent the Cape Supergroup (CSG) which may contain thrusts.

One of the projects within the framework of Inkaba yeAfrica, an earth system science initiative between German and South African research communities, is the Agulhas-Karoo transect. This 800 km north-south off-onshore transect runs from the offshore Agulhas Plateau onto the South African coast, across the Cape Fold Belt, Beattie Magnetic Anomaly, the Karoo Basin, the Great Escarpment and into the Kaapvaal Craton. Among the number of geophysical measurements taken along the transect are two wide-angle on-shore seismic lines collected in April and May 2005. The lines run roughly parallel to each other approximately 200 km apart, starting at Mossel Bay and St. Francis, and running about 200 km north to Fraserburg and Graaf Reinet, respectively. At each line 48 seismic receivers were used to record data from 13 shots. The profiles cross a wide variety of geological terrains, such as the siliciclastic sequences of the Paleozoic - Mesozoic Karoo and Oudtshoorn basins, the lower Paleozoic Cape Fold Belt, and the Eocambrian Kango and Kaaimans inliers. They also cross the Beattie Magnetic Anomaly, a large east-west orientated crustal feature within the upper crust, anti more than 1000 km long, first reported almost a century ago, but still not fully understood. The overall quality of seismic data is very good. First (P-wave) arrivals were manually picked on the available traces, and tomographic inversion was done using these travel times. The ray coverage made it possible to create the P-wave velocity model to depths of up to 25 km. We find excellent correlation of the shallow features with surface rock type. Deeper down we can identify both stratigraphic and tectonic contacts between geological groups. These include an inferred possible blind Paleozoic thrust fault, and the unconformity between the Cape Supergroup and the Namaqua-Natal Metamorphic Complex. The normal listric geometry of the Kango and Gamtoos Faults is clearly seen to a minimum depth of 15 km. We also observe a high velocity anomaly within the NNMC at similar to 10 km depth that we relate to the source of the Beattie Magnetic Anomaly.

A controlled source Near Vertical Reflection (NVR) Seismic experiment along a similar to 100 km profile yields the first high quality seismic image of the crust and Moho across the southern Karoo Basin in South Africa. The highly reflective crust comprises upper, middle and lower layers. In the upper crust, folded and gently south-dipping continuous reflectors up to the Escarpment, represent the bedding of the Karoo and Cape Supergroups respectively. Decollement structures occur locally along carbonaceous shales of the Whitehill Formation. A well-defined mid-crustal layer that hosts the Beattie Magnetic Anomaly (BMA), occurs below a seismically imaged unconformity. The mid-crustal layer is similar to 20 km thick in the vicinity of the BMA and is likely to be a subsurface continuation of the 1.0 to 2.0 Ga granitoid gneisses of the Bushmanland sub-province in the 1.2 to 1.0 Ga Namaqua-Natal Orogenic Belt. The internal seismic fabric of this layer is interpreted as a tectonic fabric dipping to the north. The probable source of the BMA appears at 7 to 15 km depth, as a narrow feature in a similar to 10 km wide tectonically complex zone confined to the upper mid-crust. The underlying lower crustal layer is wedge-shaped: similar to 24 km thick in the north and decreasing to similar to 12 km thick beneath the Cape Fold Belt. This lower crustal layer may represent granulite-gneisses of the Namaqua sub-province. The internal seismic fabric in the tipper part of this layer dips both to the north and south, but a north-dipping fabric dominates the lower part. A clearly imaged undulating Moho occurs at a depth of similar to 43 km in the north, with a nick point at similar to 42 km depth, similar to 35 km along the profile, and then deepens to similar to 45 km in the south beneath the tectonic front of the Cape Fold Belt. A possible similar to 1 to 2 km thick lowermost crustal layer of high seismic reflectivity, overlies the Moho and may represent underplated mafic material. The reflectivity seen in this NVR seismic image bears similarities to seismic transects across the coeval Mesoproterozoic Grenville orogen in Canada.

A Near Vertical Reflection (NVR) Seismic profile from Prince Albert to Slingersfontein across the Karoo Basin of South Africa was generated using 182 controlled source shots at similar to 500 m spacing and receiver points at 100 m nominal spacing in an 18 km spread. Travel time data of seismic phases are used as input data to derive shallow velocity models of compressional waves (P) and shear waves (S). Using First Arrival Seismic Tomography (FAST) software, we derive shallow tomographic P- and S-velocity models for the upper 1 to 1.5 km. Checkerboard tests indicate good resolution down to 1 km depth. The models of P- and S-velocity and Vp/Vs ratio show an abrupt change at similar to 50 km, dividing the model into two regions. A southernmost region consists mainly of steeply dipping in-situ sedimentary bedrock with an east-west strike, approximately perpendicular to the profile. The northern 50 km region comprises predominantly unconsolidated sediment and/or highly weathered rock, yielding a high Vp/Vs ratio. The tomographic models compare well with corresponding lateral variation in the surface geology of the Permian Karoo Supergroup sedimentary rocks (Dwyka, Ecca and Beaufort Groups) deformed at the Cape Fold Belt front. The correlation between the velocity models and outcrops is stronger for the southernmost 25 km of the NVR seismic profile. Although the surface geology is more uniform from kilometre 25 to 50, the velocity models and Vp/Vs ratio suggest continued systematic lateral variation. Based on this, we infer a sub-surface continuation of tight folding not seen in outcrops. The S-velocity model supports this theory, as velocity variations correlate well with the location of major fold axes in the regional scale gentle tight folding of the Ecca Group (Abrahamskraal Formation). In the northern part of the model, from 50 to 100 km, minimal change in the velocity models indicate more uniform and undisturbed lithologies.