Integrated geophysical study of the conjugate passive margins of South West Africa and South America

Passive margins are classified as volcanic or non-volcanic based on the amount of volcanism occurring during rifting. Volcanic margins are characterized by seaward dipping reflectors on seismic reflection data and underplated material with anomalously high seismic velocities (>7.2 km/s) and densities (3100 kg/m 3). This study examines changes in the nature of a volcanic margin with along strike distance from a hotspot source.; Gravity, magnetic and seismic data have been used to examine the crustal structure of the conjugate South Atlantic margins south of the Walvis Ridge-Rio Grande Rise hotspot tracks. Crustal-scale gravity models constrained by seismic data were constructed over the ocean-continent transition across both margins. Regional gravity was used to extend these models along-strike with distance from the hotspot. Changes in character of gravity anomalies have been attributed to variations in crustal thinning, sedimentation and magmatic underplating. To investigate variations in the lower crust and upper mantle, contributions to the gravity anomalies from shallow sources were removed. The resulting residual anomalies delineate distinct crustal domains associated with crustal thinning and regions of underplating corresponding well with those independently identified from limited refraction data.; The analysis of residual gravity anomalies shows a southward decrease in the extent of underplating with distance from the hotspot, from ∼240 km near the hotspot on both sides to ∼150 km at 33°S on the SW African margin, and a somewhat greater decrease to ∼100 km at 38°S on the South American side. There is a marked conjugate margins asymmetry in the character of the gravity anomalies and the residual gravity gradients over the transition. The change from a gradual transition on the SW African margin to a steeper transition on the South American side indicates differences in the amount of underplating on conjugate margins.; Simple shear extension models predict considerably less melts beneath a lower plate margin than the underplating of the upper plate margin. The decrease along-strike in the width of underplating on the conjugate margins is therefore consistent with an asymmetric simple shear model, with the upper plate on the SW African margin and the lower plate on the South American conjugate.