Seismic velocity anomalies beneath SE Brazil from P and S wave travel time inversions

Abstract

[1] We present the result from teleseismic travel time inversions for P and S wave data mainly recorded at portable broadband stations in SE Brazil. The stations were deployed at 45 sites within an area 1000 Â 1700 km during the years 1992-1999. More than 10,000 relative P and S wave arrival times, including core phases, were obtained from the waveforms using a new coherence functional. These P and S relative phase times are independently inverted for slowness perturbations, earthquake relocations, and station corrections. The final models represent the least amount of structure required to explain the residuals within a defined standard error. The robust and consistent features in the velocity anomaly models are interpreted and their resolution is tested with synthetic case inversions. We confirm the existence of a cylindrical low-velocity anomaly beneath the Paraná basin, which has been interpreted by VanDecar et al. [1995] as the fossil conduit through which the initial Tristan da Cunha plume head traveled to generate the Paraná -Etendeka flood basalts about 130 Ma. We now show that this low-velocity cylindrical structure seems to be confined to the upper mantle. Beneath the upper mantle, the velocity anomalies show a N-S oriented pattern, which we interpret as due to the Nazca plate subducted slab. At lithospheric depths, the Archean, southern part of the São Francisco craton, shows high velocities down to 200-300 km. All areas with Late Cretaceous postrift alkaline intrusions are characterized by low velocities at lithospheric depths.INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 8180 Tectonophysics: Tomography; 9360 Information Related to Geographic Region: South America; KEYWORDS: seismic regional tomography, SE Brazil, Paranã-Etendeka flood basalts, Tristan da Cunha plume, subducted Nazca plate, alkaline intrusions Citation: Schimmel, M., M. Assumpção, and J. C. VanDecar, Seismic velocity anomalies beneath SE Brazil from P and S wave travel time inversions,

An overview of the Mesozoic-Cenozoic magmatism and tectonics in Eastern Paraguay and central Andes (Western Gondwana): Implications for the composition of mantle sources

“…Moreover, neither a geoid anomaly nor a surface expression of the Tristan da Cunha thermal anomaly has been recognized in this region (Molina and Ussami, 1999;Ernesto et al, 2002). Schimmel et al (2003) argued that low velocities of seismic waves at lithospheric depths in South America characterize all the areas with late Cretaceous post-rift alkaline intrusions. If this is correct, the late Cretaceous alkaline intrusions may extend to the Apoyaya complex (NW Bolivia; Schultz et al, 2004) through the regions of Goiás and Mato Grosso in Brazil (Sousa et al, 2005) and southeastern Bolivia (Comin-Chiaramonti et al, 2005b).…”

“…Van Decar et al (1995) and Schimmel et al (2003) identified a "cylindric" low-velocity volume in the upper mantle and mantle transition zone beneath the northeastern Paraná basin (Iporá late Cretaceous magmatic province -San Francisco craton), and this was interpreted as a thermal anomaly corresponding to the "fossil" Tristan da Cunha plume that had moved with the lithospheric plate. In contrast, Liu et al (2003) suggested that the thermal anomaly does not extend into the mantle transition zone or, alternatively, that the observed anomaly is not primarily thermal, but dominantly compositional in origin (e.g., "veined mantle").…”

Tomografia sísmica com ondas P e S para o estudo do manto superior no Brasil

“…Nos trabalhos anteriores de tomografia sísmica regional (VanDecar et al, 1995;Schimmel et al, 2003) A tl a n ti c O c e a n Amazon ian Craton…”

“…Este movimento está relacionado ao fluxo astenosférico, o qual, estaria, segundo Assumpção et al (2006), desviado localmente pela topografia da interface litosfera/astenosfera (quilha) sob o Cráton do São Francisco. Schimmel et al (2003) observaram anomalias de alta velocidade para ondas P e S no manto superior abaixo da porção oeste da Bacia do Paraná. Esta anomalia poderia indicar a presença de um núcleo cratônico (Bloco Paraná - Cordani et al, 1984).…”

“…No Brasil, este método vem sendo aplicado desde 1995 (VanDecar et al, 1995;Schimmel et al, 2003;Escalante, 2002;Rocha, 2003). Vários domínios tectônicos foram revelados nos trabalhos anteriores, no entanto, devidoà distribuição esparsa de estações, não se alcançou uma resolução adequada em algumas regiões.…”