Thermo-Compositional Structure of the South American Platform Lithosphere

It remains unanswered how cratons - the ancient cores of the continents - formed, remained stable, and occasionally lose their deep roots. Constraints on the structure of cratonic lithosphere are essential to improve our understanding of cratons. Here, we performed 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. Our results indicate thick lithospheric roots below the Amazonian, São Francisco, and Rio de la Plata cratons (220-295 km). These cratonic regions require the presence of shallow lithospheric metasomatism and, in some places, a layer of eclogite, which is probably a signature of collision during their assembly in the Archean to Neoproterozoic. Other regions underlain by thick lithosphere are the Paleozoic Parecis and eastern Paraná intracratonic basins (300-315 km). These regions require more pervasive metasomatism, probably related to plume activity leading to infiltration of magmatic fluids into the cratonic keel. In contrast, the western and southern part of the Paraná Basin, which follow a Neoproterozoic suture zone, are characterised by a thinner lithosphere (98-146 km) and a shallow layer of eclogite. This layer of high density composition is probably stabilised by the remaining high-viscosity lithosphere and it is most likely a remnant of the Neoproterozoic subduction. The regions along the foreland basins and the coastal edge of the platform (92-154 km) cannot be matched with an isostatic model, indicating that they are being affected by dynamic topography, probably due to the Andean subduction in the west and edge-driven convection along the passive margin in the east. Thus, the variable geophysical structure of the platform lithosphere reflects a history that involves besides some stable keels, significant modification and thinning.