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Aeromagnetic profiles show two types of smooth magnetic zone in the South Atlantic symmetrically positioned about the Mid-Atlantic Ridge. The first borders the continental margins off Argentina and the Union of South Africa and the second extends over part of the Argentine-Brazil and Angola-Cape basins. The origin of the first zone may be related to the tectonic and/or diagenetic processes associated with the initial rifting of South America from Africa and the second can be simply explained by seafloor spreading during Middle-Late Cretaceous time when the polarity of the Earth's field was relatively constant.