Teleseismic P Wave Tomography Beneath the Pantanal, Paraná, and Chaco‐Paraná Basins, SE South America: Delimiting Lithospheric Blocks of the SW Gondwana Assemblage

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 ...

Lithospheric Architecture of the Paranapanema Block and Adjacent Nuclei Using Multiple‐Frequency P‐Wave Seismic Tomography

“…Our proposed southern limit (Figure 11) diverges from Rocha, Assumpção, et al. (2019) for the southeastern portion, being placed north of the RGA. This is the result of the improved resolution of the tomographic method used here when compared to the ray‐theory methods.…”

“…In this model, the São Francisco Paleocontinental Block (SFPB—that incorporates the SFC) was modeled based on the results of Rocha, Azevedo, et al. (2019), with a maximum depth of 226 km. The Paranapanema Block and the Luiz Alves and Rio Apa cratons were simulated based on the final tomographic model (Figure 7).…”

New measurements of crustal and lithospheric thickness for the South American platform using the receiver function method, toward a 3D velocity model

“…The eastern edge of Pantanal Basin presents a thinner LAB of ∼80 km; however, the BDQN station, located on the Rio Apa block, has a deeper LAB of ∼110 km. Rocha et al (2019) also founded a positive anomaly of 1% of P-wave velocity at 100 km depth in the Pantanal Basin, which reinforces our results. Toward Chaco Basin, the lithosphere is slightly thinned to ∼100 km (Figure 3.13c).…”

“…Some authors interpreted the first arrival close to ∼10 seconds as the LAB in younger areas, and as the MLD in older and more stable regions Rychert and Shearer (2009); Fischer et al ( 2010); Kind and Yuan (2018) which leads us to believe that our interpretation of the second arrival as the LAB is correct. Besides, tomographies of S-wave and P-wave (Rocha et al, 2019) do not show negative velocity anomalies at 100 km depth in the Paraná Basin. Only with SRF, it is harder to establish a clear distinction of the LAB and MLD conversions, we believe that it is necessary to do more geophysical studies to reach a conclusion.…”

“…With the growth of the Brazilian Permanent Network (RSBR) and the temporary deployment of the XC subnet, many studies have been carried out by the Seismology group of the USP and UnB obtaining local and regional P-and S-wave velocity models for the crust and upper mantle. Unfortunately, the local P-wave velocity models derived from tomographies cover mainly only the area of the Pantanal, Paraná, and Chaco Basins, and do not have enough information of sediments and crustal velocities (Dragone and Bianchi, 2019;Rocha et al, 2019). A prominent regional P-and S-wave velocity model for South America is under development by adjoint tomography (Ciardelli et al, 2019), the preliminary version of this model shows better resolution at crustal depths, but it still has some inconsistencies as velocity inversion with the depth; however, it shows a good fit with mantle discontinuities.…”