- © 2007 Geological Society of South Africa
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.