Restoring pan-African-Brasiliano connections: More Gondwana control, less Trans-Atlantic corruption

in Geological Society Special Publication (2008), 294

The concept of South America and Africa as rigid continents during the formation, growth and motion of their respective plates has frustrated reconstruction of a tight, geologically economic fit between ... [more ▼]

The concept of South America and Africa as rigid continents during the formation, growth and motion of their respective plates has frustrated reconstruction of a tight, geologically economic fit between these two fragments in their Gondwana framework. We recognize that (1) internal strains released during and following Gondwana break-up have distorted their actual shapes within Gondwana and (2) these two continents comprise mosaics of smaller microblocks, or platelets, of relatively undistorted Precambrian terrains that experienced modest, episodic relative motions along rift zones that separate them. This permits a fresh approach to quantitative reconstructions of palaeo-continents. Former geological ties forged at the time of Gondwana amalgamation, now exposed at the continental margins of the South Atlantic as piercing points, provide robust anchors for new paleo-cartographic experiments. We present two new tectonic maps of the Brasiliano and Pan-African structures of West Gondwana on which we identify ten piercing points that, if re-joined simultaneously, could facilitate quantification of a well-substantiated Gondwana fit and help retrace the evolution of its continental margins with greater accuracy than has been achieved until now. This has significant bearing on understanding the origin and evolution of passive continental margins, and the geodynamics of Gondwana break-up. [less ▲]

Deep crustal seismic reflection experiment across the southern Karoo Basin, South Africa

in South African Journal of Geology (2007), 110(2-3), 419-438

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 ... [more ▼]

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 midcrust. 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 [less ▲]

A proposed drainage evolution model for Central Africa - Did the Congo flow east?

in Journal of African Earth Sciences (2006), 44(1), 75-84

River networks of southern Africa: Scaling laws governing their geometry and deviations from scaling

in Geochemistry, Geophysics, Geosystems (2005), 6(9),

Africa is tectonically and hypsometrically different to all other continents and therefore provides a unique natural laboratory for the study of river network geometry. Southern Africa is one of the more ... [more ▼]

Africa is tectonically and hypsometrically different to all other continents and therefore provides a unique natural laboratory for the study of river network geometry. Southern Africa is one of the more unusual geomorphological regions of this continent, hence our interest in the geometry of the rivers draining this region. The 14 river basins analyzed here come from the larger networks of the Orange and Limpopo rivers, as well as from the southernmost section of the continent where rivers meander through an exhumed belt of folded Paleozoic mountains. Basic scaling laws, such as Horton’s laws and Hack’s law, hold for the rivers studied here, but the parameters governing these relationships vary between networks. A strong inverse correlation between Horton’s ratio of stream numbers and mean source stream slope is observed. We believe this is the first time such a relation has been reported. Horton’s ratio of basin areas is proportional to that of stream numbers but always greater than it; this contradicts existing models that equate the two parameters. An expression for the ratio of stream lengths in terms of the ratio of stream numbers is derived theoretically and verified for the available data. The parameter governing Hack’s law, h, varies not only between networks but also with scale inside each network. Existing models provide up to four scaling regimes. Given the spatial resolution of this study, the two regimes at the middle scales are visible here. These are termed the short-range and randomness regimes, with a crossover occurring at a drainage area of approximately 800 km2 . The values for h in the smaller short-range regime have an inverse correlation with the roughness of the topography. Observed values for h imply basins appear more elongated with increasing area; this would be more pronounced in rivers draining a smooth topography. In the randomness regime the values of h are lower, mostly between 0.5 and 0.6, with a weak positive correlation to topography roughness. This range of h corresponds to self-similar basins, but a more detailed analysis of individual stream fractality is necessary before such a conclusion can be made. [less ▲]