Upper-mantle seismic structure beneath SE and Central Brazil from P- and S-wave regional traveltime tomography

Abstract

S U M M A R YWe present models for the upper-mantle velocity structure beneath SE and Central Brazil using independent tomographic inversions of P-and S-wave relative arrival-time residuals (including core phases) from teleseismic earthquakes. The events were recorded by a total of 92 stations deployed through different projects, institutions and time periods during the years 1992-2004. Our results show correlations with the main tectonic structures and reveal new anomalies not yet observed in previous works. All interpretations are based on robust anomalies, which appear in the different inversions for P-and S-waves. The resolution is variable through our study volume and has been analyzed through different theoretical test inversions. High-velocity anomalies are observed in the western portion of the São Francisco Craton, supporting the hypothesis that this Craton was part of a major Neoproterozoic plate (San Franciscan Plate). Low-velocity anomalies beneath the Tocantins Province (mainly fold belts between the Amazon and São Francisco Cratons) are interpreted as due to lithospheric thinning, which is consistent with the good correlation between intraplate seismicity and lowvelocity anomalies in this region. Our results show that the basement of the Paraná Basin is formed by several blocks, separated by suture zones, according to model of Milani & Ramos. The slab of the Nazca Plate can be observed as a high-velocity anomaly beneath the Paraná Basin, between the depths of 700 and 1200 km. Further, we confirm the low-velocity anomaly in the NE area of the Paraná Basin which has been interpreted by VanDecar et al. as a fossil conduct of the Tristan da Cunha Plume related to the Paraná flood basalt eruptions during the opening of the South Atlantic.

Lithospheric Features of the São Francisco Craton

“…2.4. Previous P-wave tomography in SE Brazil (Schimmel et al 2003;Assumpção et al 2004;Rocha et al 2011) had all shown generally higher velocities beneath the SFC and lower velocities beneath the Cretaceous igneous provinces (Iporá, Alto Paranaíba, and Serra do Mar) emplaced along the orogenic belts that fringe the craton to the south. High velocities have also been observed beneath the northern part of the Paraná basin, not in a single block, but in separate patches.…”

Mathiasite-loveringite and priderite in mantle xenoliths from the Alto Paranaíba Igneous Province, Brazil: genesis and constraints on mantle metasomatism

“…Of particular relevance, a significant low velocity anomaly of P and S seismic waves was identified in the northeastern border of Paraná Basin, where it forms a vertical cylinder in the upper mantle. This was initially interpreted as a fossil plume [19], but later work suggested that it had a chemical rather than thermal origin [20].…”

Modelo de velocidade sísmica de ondas P da crosta e manto superior ao longo do perfil PABBRISE, estado de São Paulo

“…Estudos regionais recentes envolvendo dados sismológicos (Julià et al, 2008;Rocha et al, 2011) favorecem a interpretação de Milani & Ramos (1998).…”

“…Esse cenário impossibilitou a definição de um limite crustal lateral sob a Bacia do Manto Litosférico Paraná Norte como relativamente veloz Feng et al, 2004;Rocha et al, 2011;Chaves, 2014), condutivo (Minei, 2015;Padilha et al, 2015) e denso (Chaves, 2014 Quintas et al, 1999), que provavelmente aproveitaram transtracionalmente as estruturas brasilianas NE-SW, e foram impulsionados pelas diacrônicas fases compressivas da Orogenia Oclóyica, ocorridas na borda sul-ocidental do Gondwana (Milani & Ramos, 1998). Neste contexto, seria improvável a existência de uma litosfera com estrutura termal de baixa temperatura sob a Bacia do Paraná àquela época.…”

“…Nos últimos anos, no Brasil, vêm sendo envidados esforços no sentido de utilizar métodos sísmicos (Bassini, 1986 e referências ali contidas; Soares, 2005;Soares et al, 2006;Tavares, 2014;Evain et al, 2015;Lima et al, 2015) e sismológicos (Assumpção et al, 2002;França & Assumpção, 2004;Julià et al, 2008;Rocha et al, 2011;Trindade, 2014;Azevedo et al, 2015) para mapear a estrutura profunda da crosta e manto superior e, com isso, aprimorar o entendimento da compartimentação tectônica e evolução crustal do território brasileiro.…”