Study of the lithospheric structure beneath Northeast Brazil with P-wave multiple frequency seismic tomography

Summary We used the multiple-frequency seismic tomography method to image the upper mantle beneath Northeast Brazil by processing P-wave broadband seismograms in six frequency passbands. The data comprised 87 896 relative traveltime residuals for P and PKIKP phases, simultaneously inverted to obtain 3D models of P-velocity anomalies. We conducted resolution tests using checkerboard patterns with horizontal dimensions of 312×312 km and 390×390 km. For the 312×312 km structures, we observed good horizontal recovery beneath the areas of the Borborema province and northern São Francisco craton at the depths of 136 and 226 km, with the areas of the São Luís craton and Parnaíba basin also presenting recovery, albeit with reduced amplitudes. For the 390×390 km structures we observed good horizontal and amplitude recovery throughout the entire study area. Our model was unable to recover the sharp vertical transitions between the anomalies. We present the preferred model for Parnaíba basin and Borborema province. The model shows a fragmented basement for the Parnaíba basin, with two strong high-velocity anomalies consistent with the Parnaíba and Granja blocks, and another slight high-velocity anomaly north of the basin, consistent with the São Luís craton. To the west of the basin, the Parnaíba block appears separate from another high-velocity anomaly associated with the Amazonian craton. A strong high-velocity anomaly south of the Borborema province is interpreted as the northern portion of the São Francisco craton. The São Francisco craton anomaly presents strong high-velocity anomalies, interpreted as a thickening of certain portions of the craton, separated by a weaker positive anomaly, interpreted as the Paramirim Aulacogen. The Borborema province is characterized by a low-velocity anomaly. The central and northeastern portions of this anomaly presented even lower velocities, which was interpreted as lateral flow in the asthenosphere, originating from the passage of a plume to the north of the province. A low-velocity anomaly located west of the Borborema province strikes roughly NE-SW and separates the São Francisco craton from the Parnaíba block and the Amazonian craton. It is interpreted as the Transbrasiliano Lineament. To test the capability of our data to resolve the limits of large-scale structures, we created four synthetic models simulating the presence of different cratonic nuclei. The models show good horizontal recovery, with the fourth model, based on our findings, presenting the best correlation between the real and recovered models. Seismicity in the study region is mainly correlated to low-velocity anomalies.