Lithospheric structure velocity in the Southern Brazilian margin from surface wave analysis

Sedimentary basins in the southern Brazilian continental margin have gained special attention in recent years due to hydrocarbon research. Detailed geodynamic models help to understand the basins' evolution, and the choice of the right constraints helps to improve them. However, the oceanic lithosphere structure in this area is poorly investigated by seismological methods, because of the lack of stations. We study the variations in the lithospheric velocity structure along the southern Brazilian passive margin using two types of databases elaborated from the records of 23 stations of the Brazilian Seismographic Network, between 2011 and 2023. The first database is composed of 4781 group velocity dispersion curves generated by 547 teleseismic events of M>5.0 that occurred in the mid-Atlantic ridge and Sandwich Islands, these curves show periods between 15 and 400s, however for higher periods than 100s there are greater uncertainties which makes the dispersion velocities too unstable to be interpreted. The second database contains 226 dispersion curves derived from 30 local events, with magnitudes between 3.0 and 4.0M, that occurred in the offshore region of the Brazilian margin, these curves show periods between 4 and 12 seconds, which sample shallow depths providing important detailed information of the stretched platform region, that can not be sampled by teleseismic data because the absence of short periods.

The regionalization was performed in both databases to identify regional patterns and obtain velocities at different points of interest, between 20 and 40 km are observed velocities with more continental signatures to the south of 20°S, which may be associated with the extension of the continental shelf, while to the north of 20°S are observed higher velocities, indicating a more oceanic lithosphere. Deeper than 40 km, the areas closer to the margin present higher velocities that decrease up to ~100km depth, as far as we have an acceptable resolution. We also observe less strong velocity anomalies with depth, reflecting a more homogeneous lithosphere. Close to 20ºS latitude is observed a negative anomaly for depths greater than 40 km, suggesting a correlation with Trindade's plume. The regionalized curves extracted in points closest to the platform are consistent with PREM continental velocities up to 30 km, while depths between 30 and 70 km present higher velocities than PREM reference model. Previous studies also found similar velocity patterns in the continental margin; however, these do not present results at shallower depths, such as close to the bottom of the crust and the top of the mantle lithosphere.

The regionalization of the local database gives us essential information up to 10km depth, where important basins and the pre-salt region are located. Despite the low quantity of curves, it is possible to observe a clear positive velocity anomaly at 5km depth, that matches with the pre-salt limits as well as with the Brazilian offshore seismicity. The final 1D inverted velocity models will be interpreted and linked with existing geological and geophysical information to improve the knowledge of the basins in the Brazilian margin.