No abstract

We present velocity anomalies for the upper mantle beneath the Pantanal, Paraná, and Chaco-Paraná basins, using teleseismic P-wave tomography. Three hundred thirty-nine stations were used to record 4,989 events for P and PKIKP phases, during the years 1992-2017. A new temporary deployment with 34 stations improved the coverage in that region. A high-velocity anomaly beneath the Paraná Basin was interpreted as a cratonic basement. Its northern portion is consistent with the cratonic block presented by Cordani et al. (1984), and the southern portion is consistent with that presented by Mantovani et al. (2005, https://doi.org/10.1016/s1342-937x(05)71137-0). Low velocities are consistent with the limits of the Rio de la Plata craton, proposed by Rapela et al. (2011, https://doi.org/10.1016/j.gr.2011.05.001). A low-velocity anomaly under the Pantanal Basin correlates with the seismicity, suggesting lithospheric thinning. This result is not consistent with an extension of the Rio Apa Block beneath the Pantanal Basin. We observed high velocities separating the Pantanal and South-Paraguay seismic zones. The Western Paraná Suture (as proposed by Dragone et al., 2017, https://doi.org/10.1016/j.precamres.2017.01.029) shows no correlation with the seismic tomography anomalies. The thick lithosphere in the central part of the Paraná Basin is consistent with deviation of mantle flow, as suggested by SKS fast polarization (Melo & Assumpção, 2018, https://doi.org/10.1093/gji/ggy288). Synthetic tests show low resolution of the model for structures smaller than 200 x 200 km in the southwest portion of the study area but good resolution for large structures. the Transbrasiliano Lineament (TBL; Figure 1): The Amazonian Domain, including mainly the Amazonian Craton and related to the supercontinent of Laurentia, and the Extra-Amazonian (or Brasiliano) Domain, related to the supercontinent of West Gondwana. The Brasiliano Domain was formed by several paleocontinental fragments, where the largest are the São Francisco and Rio de La Plata (RDLP) cratons and the Paranapanema Block.The understanding of the tectonic evolution of the SAP requires a study of how these various paleocontinental fragments are related and how this mosaic of fragments reached its present state. This is more important for the Extra-Amazonian Domain, where these fragments are smaller than for the main representative of the Amazonian Domain, the Amazonian Craton. Defining the limits and geometry of these fragments at depth using seismic tomography is one way to understand the processes that allowed the formation of the SAP, especially when those limits are covered by sedimentary basins, hindering direct geological observations. Several geological and geophysical studies have been conducted to better understand the layout and geometry of the various cratonic nuclei of the SAP. Cordani et al. (1984), based on regional geology and radiometric dating from deep boreholes, suggested the first limits of the cratonic block related to the Paraná Basin basement. Mantovani et ...

No abstract

SummaryThe São Francisco Paleocontinental Block (SFPB) represents part of the Congo-São Francisco Paleocontinent (CSFP), amalgamated around 2 Ga. In the Neoproterozoic, a branched continental rift system evolved to ocean basins around most edges of the SFPB that remained only partially linked to the Congo Paleocontinent by means of the Bahia-Gabon Continental Bridge. After the Brasiliano—Pan-African orogeny, two relatively preserved CSFP sectors formed the São Francisco and Congo cratons, surrounded by Neoproterozoic orogenic belts. Recent results of upper mantle P-wave seismic tomography allowed us to suggest a delimitation in lithospheric depths of the Neoproterozoic SFPB, which comprise the São Francisco Craton, and that this would have been connected with the Congo Paleocontinent along the Araçuaí Belt. It is characterized by high-velocity anomalies and its boundaries with other blocks are marked by low-velocity anomalies at lithospheric depths. We tested the resolution of the tomographic results through synthetic models obtained by a ray tracing scheme using the observed ray configuration. We observe that the lateral resolution is adequate, but the method used was not able to set the depth reached by the SFPB. Our results indicate that the SFPB area in lithospheric depths is larger than the surface area ascribed to the São Francisco craton, and thus, the SFPB basement deeply extends beneath neighboring orogenic regions, suggesting that these Neoproterozoic mobile belts, such as Araçuaí Orogen and the Brasilia Fold Belt, reworked the continental crust. We observe a low-velocity anomaly in the SFPB central region, corresponding to the Pirapora aulacogen. Our results have a good spatial correspondence with the low Bouguer anomalies used to define the SFPB in previous studies. The limits of the SFPB are consistent with deviation of the mantle flow, as suggested by SKS fast polarization.