The lithospheric architecture of passive margins is crucial for understanding the tectonic processes that caused the break-up of Gondwana. We highlight the evolution of the South Atlantic passive margins by a simple thermal lithosphere-asthenosphere-boundary (LAB) model based on rifting time, crustal thickness, and stretching factors. We simulate the different rifting stages that caused the opening of the South Atlantic Ocean and pick the LAB as the T=1330 °C isotherm, which is calculated by 1D advection and diffusion. In a synthetic example, we demonstrate that the initial crustal thickness has the largest effect on the thermal LAB. For the South American passive margin, our modeled LAB shows a deep and smooth structure between 110-150 km depth at equatorial latitudes and a more variable LAB between 50-200 km along the southern part. This division reflects different stages of the South Atlantic opening: initial opening of the southern South Atlantic causing substantial lithospheric thinning, followed by rather oblique opening of the equatorial South Atlantic accompanied by severe thinning. The modeled LAB reflects a high variability associated with tectonic features on a small scale. Comparing the LAB of the conjugate South American and African passive margins in a Gondwana framework reveals a variable lithospheric architecture for the southern conjugate margins. Along selected conjugate margin segments stark differences up to 80 km of the LAB depths correlate with strong gradients in margin width. This mutual asymmetry suggests highly asymmetric melting and lithospheric thinning prior to rifting.