Defense
Student: Thereza Mayra de Souza Fialho
Program: Geophysics
Title: "Upper Mantle Anisotropic Structure Beneath South America from XKS Shear-Wave Splitting"
Advisor: Prof. Dr. Carlos Alberto Moreno Chaves
Judging Comitee:
- Prof. Dr. Carlos Alberto Moreno Chaves IAG/USP - (Presidente e Orientador)
- Prof. Dr. Marcelo Peres Rocha - UnB
- Prof. Dr. Gilberto Leite Neto ON
Other Members:
- Prof. Dr. George Sand Leão Araújo de França - IAG/USP
- Dr. André Vinícius de Sousa Nascimento ON
- Prof. Dr. Aderson Farias do Nascimento - UFRN - Co-advisor
- Profa. Dra. Cintia Rocha da Trindade UFOPA
Abstract:
Seismic anisotropy constitutes a tool for evaluating the elastic anisotropy of upper mantle rocks, which manifests as a directional variation in the propagation velocity and polarization of seismic waves in response to the lattice preferred orientation (LPO) of olivine, developed by viscous deformation, and to fossil lithospheric structures preserved since past tectonic events. This dissertation presents a systematic analysis of seismic anisotropy in the South American upper mantle through XKS shear-wave splitting, based on records of 555 teleseismic events processed at stations of the BL, BR, XC, XV, and Y4 networks. The splitting parameters, fast polarization direction (ϕ) and delay time (δt), were estimated by the transverse component energy minimization method and evaluated by a multivariate quality scoring system (Quality Score), which classified 77.5% of the stations in the highest reliability categories. The results reveal a dominant pattern of ENE–WSW to E–W orientations, with δt varying between 0.6 and 1.3 s depending on the geological domain. Comparison with the S-wave tomography model of Nascimento et al. (2024) indicates a negative correlation between δt and seismic velocity, with a correlation coefficient of −0.624 at 80 km for the highest-quality stations. Comparison with the lithosphere–asthenosphere boundary (LAB) depth model SAAM23 (Ciardelli et al., 2022) revealed a moderate negative correlation (r = −0.51, p < 0.001) for the highest-quality stations, indicating that thick lithospheric domains restrict the asthenospheric channel and reduce the integrated δt, while thin lithospheric domains favor a more expressive asthenospheric contribution. Comparison with the lattice preferred orientation (LPO) field modeled by Hu et al. (2017) demonstrates increasing agreement with the XKS vectors as depth increases from 100 to 200 km, consistent with the dominance of asthenospheric flow induced by Nazca Plate subduction over the integrated anisotropic signal. The central result of this dissertation consists of the identification and interpretation of an anomalous anisotropic pattern in the Pantanal Basin, the western portion of the Tocantins Province, and the region west of the Western Paraná Suture (WPS). In this region, the XKS vectors show angular divergence of ∼ 30 – 45 ° relative to the E – W to ESE – WNW directions predicted by the model of Hu et al. (2017), with ENE – WSW orientations that geometrically coincide with the absolute plate motion (APM) of the South American plate, but whose coincidence is here interpreted as non-causal. The proposed hypothesis is that these orientations reflect asthenospheric flow channeled through the thinner lithosphere zone associated with the Brasília Belt, interposed between the lithospheric keels of the Paranapanema block and the São Francisco Craton. This mechanism has direct physical support in the principle that viscous asthenospheric flow follows the path of least resistance: beneath thick cratonic keels, effective viscosity is higher and the asthenospheric channel is reduced, whereas beneath thin Neoproterozoic mobile belt lithosphere the channel is thicker and flow resistance is lower. Upon reaching the Amazon Craton to the northwest, the flow would encounter an additional lithospheric barrier, being then redirected toward the NE–SW direction in the thinned lithospheric region west of the WPS. This mechanism is analogous to that numerically documented for the Caribbean–South American margin, where the South American cratonic keel deflects and concentrates asthenospheric flow into a narrow channel, and for East Asia, where flow from Tibet is blocked by the Ordos and Sichuan cratons and redirected along the thinner lithosphere zone between them. The pattern is also consistent with the limitation acknowledged by Hu et al. (2017): the absence of adequate deep keels for the São Francisco Craton and the Paranapanema block in the geodynamic model degrades the fit in this region, and verification of the proposed hypothesis would require geodynamic simulations with updated lithospheric geometry, incorporating the cratonic roots imaged by the models of Nascimento et al. (2024) and Ciardelli et al. (2022).
Keywords: Seismic anisotropy, XKS shear-wave splitting, South America, asthenospheric flow, LPO, lithosphere, Paranapanema, São Francisco Craton.