Analysis of MAGSAT magnetic contrasts across Africa and South America

Comparisons of MAGSAT magnetic contrasts and geology across the Mesozoic assembly of Africa and South America provide new insight into the interpretation of the long-wavelength magnetic anomalies near the present continental margins. Across continental Africa and South America, the MAGSAT magnetic contrasts can be correlated with geologic provinces formed before the Mesozoic separation of the continents. On the continents, areas affected by significant Mesozoic hotspot tectonism display negative magnetic contrasts suggesting a causative relationship between hotspot tectonism and the origin of the observed magnetic contrasts. The magnetic characteristics of a portion of the lower crust in these areas appear to have been significantly altered during the Mesozoic hotspot epeirogeny. By analogy with the processes of continental rifting, it is suggested that the magnetic mineralogy of the intruded lower crust may be dominated by weakly- to non-magnetic titanomagnetites. Oceanic magnetic contrast comparisons show that the positions of magnetic anomaly highs over the Rio Grande-Walvis Ridge System of the South Atlantic are consistent with the interpretation of their evolution over the Walvis Hotspot. However, only the parts of these ridges that were formed during the Cretaceous normal polarity geomagnetic epoch bear strong magnetic contrasts. Remanent magnetization thus appears to be an important contributor toward the MAGSAT magnetic anomalies of these features.