We have interpreted 27,550 km of deep-penetrating, 2D-seismic reflection profiles across the South Atlantic conjugate margins of Uruguay/Southern Brazil and Namibia. These reflection profiles reveal in unprecedented detail the lateral and cross-sectional, asymmetrical distribution of voluminous, postrift volcanic material erupted during the Barremian-Aptian (129–125 Ma) period of early seafloor spreading in the southernmost South Atlantic. Using this seismic grid, we mapped the 10–200 km wide, continental margin-parallel limits of seaward-dipping reflector (SDR) complexes — that are coincident with interpretations from previous workers using seismic refraction data from the South American and West African conjugate margins. Subaerially emplaced and tabular SDRs have rotated downward 20° in the direction of the mid-Atlantic spreading ridge and are up to 22 km thick near the limit of continental crust. The SDR package is wedge shaped and thins abruptly basinward toward the limit of oceanic crust where it transitions to normal, 6–8 km thick oceanic crust. We have developed a model for the conjugate rifted margins that combine diverging tectonic plates and northwesterly plate motion relative to a fixed mantle position of the mantle plume. Our model explains an approximately 30% higher volume of SDRs/igneous crust on the trailing Namibian margin than on the leading Brazilian margin during the syn- and postrift phases. Our model for volcanic margin asymmetry in the South Atlantic does not require a simple shear mechanism to produce the asymmetrical volcanic material distribution observed from our data and from previously published seismic refraction studies. Determining the basinward extent of the extended continental basement is crucial for understanding basin evolution and for hydrocarbon exploration. Although these conjugate margins have evolved asymmetrically, their proximity during the early postrift stage suggests a near-equivalent, early basin evolution and similar hydrocarbon potential. Understanding the tectonic and magnetic processes that produce these observed asymmetries is critical for understanding volcanic passive margin evolution.