No abstract

Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuípe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust. Highlights ► The transitional domain in the Jequitinhonha basin is, at least, 150 km wide. ► The transitional crust is composed of exhumed lower continental crust. ► Necking occurs within less than 100 km. ► An anomalous velocity zone is imaged at the base of the crust.

The Parnaíba Basin is a Phanerozoic intracontinental basin within the South America plate, lying on top of and within Precambrian terranes. The Parnaíba Basin Analysis Program wide-angle reflection–refraction (WARR) lies east–west and is 1150 km long profile crossing the basin and its margins. The WARR results show that the crust and uppermost mantle along the profile consist of the Amazonian Craton and Borborema Province, and the Grajaú and Teresina domains comprising the Parnaíba block hidden below the sedimentary cover of the basin itself. The lithospheric characteristics of the Parnaíba block and their differences from the adjacent Precambrian Amazonian Craton and Borborema Province elucidate some aspects of the present day existence of the sedimentary basin covering it. Important elements include the presence of a high mantle velocity and high-velocity lowermost crustal region, interpreted as linked to the intrusion of mafic material into the crust underlying the Grajaú domain, and indications that the crust in this area has been intruded since its consolidation in the Neoproterozoic. It is tentatively proposed that magmatism is related to the inferred thinning of the lower crust of the Teresina and Borborema segments of the profile, with this, in turn, linked to Cretaceous extensional tectonics and the opening of the South Atlantic Ocean.

RESUMO. O objetivo deste estudo foi implementar um algoritmo de inversão para estimar as velocidades da onda cisalhante (S) assim como as densidades das camadas acima e abaixo do refletor em questão, explorando-se a informação contida nas mudanças de fase no pulso da onda compressional (P), refletida acima doângulo crítico de incidência. Inicialmente, um estudo do comportamento da função objetivo, com dados sintéticos, foi realizado, no qual procurou-se avaliar a unicidade e a estabilidade da solução do problema inverso como, também, o efeito, sobre os valores da função objetivo, de vários aspectos envolvidos no processo de inversão. O problema proposto visa determinar quatro parâmetros, de modo que o comportamento da função objetivo foi analisado pelas seções transversais, variando dois dos parâmetros e mantendo os outros dois fixos em seus valores corretos. As análises, usando os dados sintéticos, apresentaram resultados promissores quantoà viabilidade da metodologia proposta e permitiram identificar os fatores que, na prática, poderiam inviabilizar a convergência do processo de inversão (normalização das amplitudes e escolha da wavelet para o cálculo dos sismogramas). Os testes, com dados reais, não apresentaram resultados satisfatórios. Acredita-se que o principal motivo decorra do ajuste inadequado entre as wavelets dos dados reais e a utilizada para gerar os dados calculados, sendo, portanto, necessário investigar qual o método mais adequado para se obter um pulso mais representativo daquele dos dados reais.ABSTRACT. The objective of this study was to implement an inversion algorithm in order to estimate shear wave velocities (S) as well as layer densities above and below the reflector by using information from phase changes in compressional wave pulse (P), reflected post-critical angle of incidence. At first, a study of objective function behaviour, with synthetic data, was accomplished in which (1) unicity and (2) stability of inverse problem solution as well as (3) effect of several aspects involved in the inversion process, with the objective function values, were evaluated. The proposed problem aims at determining four parameters for that objective function behaviour was analyzed by transversal sections, varying just two of the four parameters, keeping the other two fixed in their correct values. Analysis by using synthetic data presented good results related to viability of proposed methodology; these results allowed identifying factors that could disturb the convergence of inversion process (amplitude normalization and wavelet choice to seismogram calculation). Tests with real data did not presented good results. Apparently the main reason for that was the inadequate adjustment between wavelet of real data with that one used to generate calculated data, being necessary to investigate which method is more adequate to obtain a more representative pulse than that one from real data.339

Deep seismic refraction experiment carried out in Borborema Province in November 2008 (INCT-ET/CNPq) permits to determine the seismic structure of the lithosphere along a profile 880 km long, crossing the main tectonic domains. It reveals a very thin crust probably reworked and thinned by recent tectonic process. Vp/Vs results for the upper and lower crust are being determined to better constrain crustal composition and better understand the crustal deformation process, particularly in the Borborema Province.