The lithosphere of South America from seismic tomography: Structure, evolution, and control on tectonics and magmatism

Abstract

The thickness, temperature and mechanical strength of the lithosphere vary greatly across South America and have controlled tectonic and magmatic processes during its evolution. Here, we introduce a new tomographic model of South America’s lithosphere and underlying mantle, SACI-24, and analyse this and other state-of-the-art models together with other geological and geophysical data. The new model is obtained by waveform inversion of surface, S and multiple S waveforms globally, but is optimised for South America and the surrounding oceans. SACI-24 is constrained by ∼ 970,000 seismograms from 9259 stations and maps detailed structure of cratonic lithosphere across the continent, highlighting previously unknown complexities and fragmentation. Within the Amazon Craton, the Guiana and Central Brazil blocks show high Vs exceeding 4.8 km/s in the 80–150 km depth range but are separated by lower velocities below the Amazon Basin, matching the locations of ancient rifting and the flood basalts of the Central Atlantic Magmatic Province. In the São Francisco Craton, high velocities extend north and southwest beyond the previously proposed boundaries. Warmer, thinner lithosphere underlies the Paramirim and Pirapora aulacogens, locations of magmatic and rifting cycles. A fragmented cratonic root underlies the Paraná Basin, with thinner lithosphere along its central rift. High velocities south of the Paraná Block indicate the northern Rio de la Plata Craton’s thick root. The Cenozoic intraplate basalts in the Borborema province, along the southern Atlantic coast and in Paraguay occurred in areas of thin lithosphere and near thick-thin lithospheric boundaries. Most flood basalts of the Phanerozoic large igneous provinces also map on low velocity areas. Smaller flood-basalt portions sit atop cratonic lithosphere and offer new evidence on lateral flow of flood-basalt lava and magma within the crust. About 80 % of known diamondiferous kimberlites are on thick lithosphere, with the exceptions indicating cratonic erosion and thinning since their emplacement.