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The amalgamation and breakup of the West Gondwana shaped the South American platform. The dynamics during the processes can be reflected by crust anisotropy of the platform, but there are no specialized crustal anisotropic measurements yet. Splitting analysis of Moho-converted shear waves in P -wave receiver functions (Pms) can reveal crustal-scale anisotropy, which is important for understanding the dynamic evolution of the crust and for the interpretation of mantle anisotropy from splitting analysis of core–mantle refracted shear waves (XKS phases). This study measured crustal anisotropy for the old and stable South American platform by Pms splitting analysis. The splitting times vary mainly in the range of 0–0.5 s, with a regional mean of 0.2 s, slightly lower than that observed in tectonically active regions. The detected crustal anisotropy shows distinct characteristics and spatial zoning, providing insights into tectonic processes. (1) Fast polarization directions at stations close to the Transbrasiliano Lineament (TBL) are oriented NNE–SSW, generally consistent with the strike of the TBL but inconsistent with the maximum horizontal compressive stress, implying that they might be formed by dynamic metamorphism during the formation of the TBL. (2) Crustal anisotropy along the passive continental margin in the east and northeast is weak. Still, the fast polarization directions tend to be oriented along the margin, implying the existence of fossil extensional crustal fabrics formed during the continental rifting of West Gondwana. (3) The Paraná Basin, one of the world's largest Large Igneous Provinces (LIP) covered by continental flood basalts, shows distinctively strong anisotropy, with fast polarization directions highly aligned with mantle anisotropy, implying that synchronous crust–mantle deformation occurred in these regions as a result of magmatism during the breakup of West Gondwana. [ABSTRACT FROM AUTHOR]

The crust of the South American platform recorded imprints of dynamic processes related with the opening of the Central and South Atlantic but has not been well measured. Crustal structure can be retrieved from teleseismic receiver functions using H – κ stacking, but nearly parallel stripes of high stacking values existing in stacking images for seismic stations in sedimentary area cause difficulties in identifying solutions. We show that some seemingly spurious stripes that do not point to any layer solution are helpful in the identification of the solution position. With the aid of the auxiliary stripes, we retrieved thicknesses and Vp / Vs of sedimentary and crystalline crust for 65 permanent stations of the Brazilian Seismographic Network and six new portable seismic stations in Brazil and Uruguay. The resulted sedimentary thickness and Vp / Vs exhibit a good correlation with the Phanerozoic sediments in the South American basins. The crust of Paraná–Etendeka Large Igneous Province (LIP) had been expected to be more mafic since it had ever been penetrated by mantle magma in the Cretaceous related to the south Atlantic opening. However, we found very low Vp / Vs (1.67) in the crystalline crust beneath the LIP, implying a more felsic crust and that no significant mafic intruding/underplating has occurred in the region. The more felsic crust may be formed in a special evolution early than the magmatic event, or during the magmatic event by releasing crustal volatiles. The resulted sedimentary thickness and Vp / Vs ratios exhibit a good correlation with the Phanerozoic sediments in the South American basins, which implies that Triassic–Jurassic and Cretaceous magmatism did not cause significant metamorphism in sediments formed before the magmatic events. [ABSTRACT FROM AUTHOR]

Ambient seismic noise is now routinely used to study the Earth's interior. For an isotropic homogeneous medium, the basic assumption to extract seismic phases from a station pair is that the sources of seismic noise are distributed in such a way that there is a uniform energy flux around the station pair. In general, however, some particular azimuthal directions may dominate the energy flux, which directly affects the extracted interstation empirical Green's function (EGF). To solve this problem, we analysed synthetic cross-correlation functions (CCFs) from seismic pulses propagated in isotropic and anisotropic heterogeneous half-space media towards a station pair under the assumptions of uniformly and non-uniformly distributed sources of noise. A reliable EGF signal can be extracted by applying three processing steps: (1) normalizing the number of repeated stationary sources, (2) normalizing the energy of each excited source and (3) selecting coherent CCFs in the final stacking. In this way, three different classes of station pairs were identified based on the number of CCFs used in the stacking procedure. We introduced and applied a new method based on weighted root-mean-square stacking (WRMS) to the CCFs of more than 33 months of ambient noise recorded from January 2016 to September 2018 at 75 broad-band stations in West-Central Brazil. In the case of non-uniform distribution of source of noise, simple classical linear stacking of CCFs produces distorted EGFs. However, the waveform extracted by the WRMS method is very similar to the Rayleigh waves excited by an earthquake (on 2017 January 3) near one receiver observed at the other receiver. Moreover, synthetic tests and a comparison between extracted and earthquake signals show that although the WRMS method can extract the main part of the signal that is propagated on the shortest path, it cannot recover the energy parts propagated on multipath. Despite the N–W directionality in the geometry of the array, the rose-diagram results indicate no significant spatial variations in the energy level of EGFs extracted by the WRMS stacking, whereas the EGFs extracted by the classical linear stacking indicate the extreme directionality of energy flow in different period ranges. Rayleigh wave group and phase velocity tomographic maps resolved by the EGFs derived from the WRMS method indicate a clear boundary along the Asuncion and Rio Grande Arches between the Chaco-Paraná and the Paraná basins at the shorter period, while the tomographic maps in the same periods which were calculated by other stacking methods cannot clearly separate basins and arcs. Our tomographic maps at longer periods indicate variations of Moho depth and lithospheric velocities. [ABSTRACT FROM AUTHOR]