Crustal and Uppermost Mantle Azimuthal Anisotropy beneath West and SE Brazil using Ambient Seismic Noise

Abstract

Seismic azimuthal anisotropy within the crust and upper mantle offers important information of past and present tectonic deformation. We used ambient seismic noise to map azimuthal anisotropy in the lithosphere beneath W and SE Brazil, providing new insights into the amalgamation history of the various cratonic blocks in SW Gondwana, which are now partly buried by Phanerozoic basins. We used 72 stations from January 2016 to September 2018. To correct the non-uniform distribution of the energy flow around each inter-station path, the weighted rms (WRMS) stacking method was applied. The inter-station empirical Green's functions provided Rayleigh-wave group and phase velocity dispersion curves, which were used in a tomographic inversion to obtain the fast anisotropy directions, and the isotropic (mean) group and phase velocities in the period range of 4–70 s. At the shortest period, both group and phase low-velocity anomalies are observed in the sedimentary basins, while the fast direction is parallel to the deformation in the surrounding fold belts (e.g., beneath the shallow Pantanal basin). At 40 s period, group and phase velocities are affected by crustal thickness variations. During the longest period of the 70s, the fast anisotropy directions are mostly N-S, in general agreement with the azimuthal anisotropy of the global model of Debayle et al.(2016, updated to 2022), which is interpreted as due to compressional deformation in the lithospheric lid. This deformation-induced anisotropy suggests that the final Neoproterozoic collision occurred between the two groups of cratonic blocks: (I) the Amazon craton, the Rio Apa, and the Rio Tebicuary cratonic blocks in the Amazon domain, and (II) the Paranapanema block on the Atlantic domain. The isotropic V_S model generally agrees with the proposed West Paraná Suture zone (inferred from gravity and magnetotelluric data). In the lower crust (20 to 35 km), predominantly low velocities are seen in the central and southern part of the Paraná basin, and higher velocities are observed around the Pantanal basin, in general agreement with Cedraz et al. (2020) proposal of underplating in that region.