ALBERT

Description: Southern Africa and its southern continental margin offer an unrivalled region, where continental accretion, continental breakup and magmatic processes over a period of more than 3.5 billion years can be studied. The Agulhas-Karoo Geoscience Transect is part of the South African - German cooperative research project "Inkaba ye Africa", which aims to investigate this part of the continent and ocean in a cone-shaped sector from core to space. Geophysical and geological data and samples were collected along this transect which spans from the Agulhas Plateau across the Agulhas-Falkland fracture zone, the Outeniqua Basin, the Cape Fold Belt, the Namaqua-Natal Belt onto the Karoo Craton. A combined onshore-offshore deep crustal seismic reflection and refraction survey as well as several magnetotelluric surveys provide information about tectonic and magmatic structures and constraints for physical parameters from sedimentary sequences to the upper mantle. The main objectives include an understanding of the crustal nature of the Agulhas Plateau, the processes accompanying and succeeding the crustal shearing process along the Agulhas-Falkland Transform/Fracture Zone, the offshore basin formation in relation to breakup, the deep-seated tectonics of the Cape Fold Belt, and the geometries and sources of the Beattie Magnetic Anomaly and the Southern Cape Conductivity Belt. The geophysical data are integrated with geological, petrological and geochemical analysis on rock composition, age and alteration history to form an overarching geodynamic model of the evolution of this region and its sedimentary, tectonic and magmatic units.

Description: 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 numer 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 basin, the lower Paleozoic Cape Fold Belt, and the Eocanbrian Kango and Kaaimans inliers. They also cross the Beattie Magnetic Anomaly, a large east-west orientated crustal feature within the upper crust, and 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 uncontormity between the Cape Supergroup and the Namaqua-Natal Metamorphic Complex. The normal listric geometry of the Kango and Gramtoos Faults is clearly seen to a minimum depth of 15 km. We also observe a high velocity anomaly within the NNMC at ~ 10 km depth that we relate to the source of the source of the Beattie Magnetic Anomaly.

Description: A Neqar 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 ~500 m spacing and receiver points at 100 m nominal spacing in an 18 km spread. Trael 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 ~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, vielding 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. Althouth 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.

Description: A controlled source Near Vertical Reflection (NVR) Seismic experiment along a ~ 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. Décollement 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 ~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 ~10 km wide tectonically complex zone confined to the upper mid-crust. The underlying lower crustal layer is wedge-shaped: ~24 km thick in the north and decreasing 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 upper 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 ~43 km in the north, with a nick point at ~42 km depth, ~35 km along the profile, and then deepens to ~45 km in the south beneath the tectonic front of the Cape Fold Belt. A possible ~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.