Constraints on the structure of cratonic lithosphere are essential to improve our understanding of craton formation, evolution and long-term stability. Here, we perform a joint inversion for the thermal and compositional structure of the mantle lithosphere below the South America Platform, using Rayleigh wave group velocities, elevation, and geoid height. Thick thermal lithosphere (200-300 km) is found below the southern Amazonian and São Francisco Cratons and adjoining Parecis Basin and northern Paraná Basin. The southern Rio de la Plata Craton also retains a 200-250 km thick keel. Compositionally, Amazonian, São Francisco and Rio de la Plata lithosphere has a metasomatic and possibly eclogite signature similar to that of North American Proterozoic collision belts. Parecis and northern Paraná lithosphere has likely been altered by Mesozoic plume activity throughout most of its depth, while the rest of the Paraná Basin and the Chaco and Patanal basins appear to have lost the lithospheric root below ~100 km depth that was there during intracratonic basin formation. The low elevation and high geoid of the western Paraná Basin requires a dense (eclogite) layer within the crust/shallow lithosphere, possibly associated with the NeoProterozoic western Paraná Suture Zone and/or Mesozoic plume activity, while topography and geoid of the basins further west and of the western Rio de la Plata craton seem affected by dynamic (subduction-related) topography. Thus, the variable geophysical structure of the platform lithosphere reflects a history that involves besides some stable keels, significant modification and thinning.