Waveform tomography of the South Atlantic Ocean and the African and South American Continents

The South Atlantic Ocean and adjacent African and South American continents share a fascinating geological history, but also a relatively poor sampling with respect to seismic data. Recent growth in broadband seismic station coverage has significantly improved data sampling across the region, enabling us to resolve its lithospheric and upper mantle structure at a new level of detail. We assembled a very large regional dataset of vertical-component seismograms, complemented with global data, and extracted structural information from over 1.2 million seismogram fits, using the Automated Multimode Inversion of surface, S- and multiple S-waves. Each waveform fit provides a set of linear equations describing 1D average wavespeed perturbations within approximate sensitivity volumes, with respect to a 3D reference model. We then combine all equations into a linear system and invert for a model of S- and P-wave speeds and azimuthal anisotropy within the crust, lithosphere and underlying mantle. The model reveals the detailed structure of the African and South American cratonic lithosphere. In west Africa, two separate high-velocity units underlie the Reguibat and Man-Lèo Shields. The mantle root of the Congo Craton comprises three main units beneath Gabon-Cameroon, Bomu-Kibali and Kasai Shields. In South America, we observe the roots of the Amazon and São Francisco Cratons. High-velocity roots are also present underneath the Paranà and Parnaìba basins. In northern Africa, low-velocity anomalies beneath the Afar Hotspot and the East African Rift Zone extend down to the bottom of the upper mantle. Strong low velocities are also seen beneath the Ahaggar, Tibesti and Darfur Hotspots at 56-260 km depth. In the South Atlantic, the model reveals the patterns of interaction between the Mid-Atlantic Ridge (MAR) and nearby hotspots, i.e. Ascension, Tristan da Cunha, Shona and Bouvet. Low-velocity anomalies beneath major hotspots extend substantially deeper than those beneath the MAR. The Vema Hotspot, in particular, displays a pronounced low-velocity anomaly under the thick, cold lithosphere of the Cape Basin. A strong low velocity anomaly also underlies the Cameroon Volcanic Line and its offshore extension, from Africa towards the MAR.