Adjoint Waveform Tomography of South America

We used 3D spectral-element seismic wave simulations and data from 112 earthquakes and 1,311 seismic stations, totalizing 20,884 unique ray paths, to construct an adjoint waveform tomographic model of South America. We performed 23 conjugate-gradient iterations using exponentiated phase (EP) measurements. Our final model (SAAM23, South American Adjoint Model—iteration 23) shows a 50% decrease in the EP misfit relative to its 3D starting model. We further assessed the phase misfit reduction by using cross-correlation travel-time measurements of 53 earthquakes not included in the inversion. We estimated SAAM23 resolution using point-spread function tests and density coverage analysis. The Nazca Slab is well imaged and is shown to be continuous in the 300–500 km depth range. Beneath northern South America, the slab traverses 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 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 published values 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 lithospheric high-velocity anomalies were found beneath the Río de la Plata Craton.