Imaging the South American Continental Interior with Waveform Tomography

The South American continent consists of an active mountain range on the west, formed by the subduction of the oceanic Nazca slab, and a large stable platform region, mainly composed of the Precambrian basement. Within South America, we find the cratons, blocks of differentiated continental lithosphere, characterized by their cold and buoyant behavior, and surrounding the cratons, mobile belts mostly from the Neoproterozoic form a complex collage network. The lithosphere and asthenosphere underlying a continent record most past tectonic events as much as control the different dynamic episodes of current deformation, magmatism, assembly, and large-scale rifting leading to break-up. However, our understanding of South America and how it has been affected by the underlying mantle processes is limited by the availability of both geophysical and geological data, hindered by the presence of thick sedimentary covers, dense forests, and large water masses.

Seismic tomography can resolve the 3D distribution of seismic-wave velocity, sensitive to temperature and composition in the crust and upper mantle. Until recently, seismic data sampling in South America was highly uneven, and high-resolution models were obtained mainly regionally. Here, we assembled all available seismic data including the data from the FAPESP “3-Basins Thematic Project.” The massive dataset includes data from the temporary deployments in South America that became available recently and is complemented by data from all over the globe.

We compute a new S-velocity tomographic model of the upper mantle of South America and surrounding oceans using the Automated Multimode Inversion of surface, S- and multiple S-waves. The increase in the data coverage of the model combined with the optimized tuning of the inversion parameters on the continent allows us to identify for the first time the fine details present in the lithospheric structure. We observe that regions of thinner lithosphere inside cratons correspond to areas where rifting has been proposed in previous tectonic cycles. Inside the boundaries of the Amazon craton, we image two cratonic blocks, separated by the Amazon basin. In this area, an aborted rift system preceded the formation of the Amazon basin in the Neoproterozoic, and rift reactivation occurred with the break-up of Pangea in the Mesozoic. Similarly, in the São Francisco Craton, we image a significantly thinner lithosphere in the Paramirim Aulacogen area, a Paleoproterozoic intracontinental rift system. We also image high-velocity lithospheric blocks under sedimentary basins. East of the Amazon craton, we image a high-velocity anomaly known as the Parnaíba block, and under the Paraná basin, a fragmented Paranapanema block. Finally, by imaging an accurate boundary of the cratonic units, we can analyze the distribution of magmatic events and large igneous provinces and how they correlate with our model’s seismic velocities at lithospheric and asthenospheric depths.