Effects of the subducting Nazca Plate on South American upper-mantle dynamics, inferred from adjoint waveform tomography

We use 3D spectral-element seismic wave simulations and data from 112 earthquakes recorded by 1311 seismic stations to construct an adjoint waveform tomography model of South America. We performed 23 iterations using exponentiated instantaneous phase measurements, which optimize the cycle skip problem and the information extracted from each time series. Our final model (SAAM23, South American Adjoint Model - iteration 23) shows a 50% decrease in the misfit relative to its 3D starting model. We assessed SAAM23 by cross-correlation traveltime measurements using 53 earthquakes not included in the adjoint inversion and point-spread function tests. The Nazca Slab is well imaged and is shown to be continuous in the 300-500 km depth range following the Peruvian flat-slab segment. Beneath northern South America, the slab crosses the mantle transition zone and continues into the lower mantle. In the central and southern part of South America, the slab appears to flatten near the 650 km discontinuity before continuing into the lower mantle. In the stable Precambrian platform, both exposed cratons (Amazonian and São Francisco), as well as covered cratonic blocks beneath the intracratonic Paraná and Parnaíba basins (Paranapanema and Parnaíba, respectively), show high velocities at lithospheric depths. The seismic Lithosphere/Asthenosphere boundary (LAB) agrees well with that obtained by S-wave receiver functions. In the Amazonian Craton, the positive lithospheric S-wave velocity anomalies and LAB depth increase with the average age of the geochronological provinces. No high-velocity anomalies were found beneath the Río de la Plata Craton.