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GEOLOGY

The fault that started it all

Research studies identify how a large fault in Brazil’s crustal framework influenced the formation of the sedimentary basins of the Paraná and Parnaíba Rivers

Sete Cidades National Park, in the state of Piauí: Transbrasiliano Lineament cuts through the region

ICMBio / MMASete Cidades National Park, in the state of Piauí: Transbrasiliano Lineament cuts through the regionICMBio / MMA

The Transbrasiliano Lineament—an enormous scar in the Earth’s crust that cuts across Brazil—had an impact on the formation of the sedimentary basins of the Paraná and Parnaíba rivers. Although there has been debate on this theory over the past 40 years, ever since that geological structure was discovered, only now has a group of researchers from the University of Brasília (UnB), the Federal University of Rio Grande do Norte (UFRN) and the University of Campinas (Unicamp) been able to advance our understanding of the role that the lineament played in the formation of the two river basins. These geologists have produced a more precise picture of the substrate in the areas traversed by the lineament, which is almost entirely covered by sediments.

Measurements taken in the crust and the mantle (the geological layer beneath the crust) show that fracturing along the lineament formed the first deposition centers—points where sediments accumulate and culminate in the formation of river basins. “Many other places in the world have basins whose deposition centers were related to the reactivation of geological faults,” explains Julia Curto, a researcher at UnB and first author of a paper published in the journal Tectonophysics in August 2015. Lineaments do not always give rise to sediment deposition, however. A fault must be reactivated—i.e., move —from time to time, “thus creating space to accommodate the sediments,” she says.

The new analyses have also created a more precise picture of the basement relief of the Paraná and Parnaíba basins. The basement is the deepest and oldest layer, composed of denser rocks. The sediments resulting from the erosion process are deposited on top of these rocks, forming the sedimentary basins.

In order to get a picture of what lies beneath the basin, the geophysicists cross-referenced magnetic and gravimetric data, which are obtained by instruments aboard aircraft that fly over the study area and detect small changes in the Earth’s gravitational and magnetic fields. The two fields vary according to the density and magnetic properties of the rocks. The instruments measure the contrasts between rocks of different densities and different intensities of magnetization, forming detailed maps of the substrate.

It was the first time the two methods were used simultaneously to study the lineament. “There was previously only a gravimetry estimate, which can lead to huge inaccuracies,” says Reinhardt Fuck, a researcher at UnB. Some of the surveys used by the researchers were done in the 1970s on the Radam Brasil project, which mapped the Brazilian substrate for the first time. It was while compiling the data from these flights that geologist Carlos Schobbenhaus, then with the National Department of Mineral Production (DNPM), discovered the lineament. The more recent data were obtained from flyovers conducted by the National Agency of Petroleum, Natural Gas and Biofuels (ANP). “After a long period when oil exploration practically only focused on the ocean, Brazil is beginning to look to the continent,” says Hilário Bezerra, a professor at UFRN and co-author of the paper. The group’s research is part of a Petrobras-funded project that ends in 2015.

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“The work is turning up very interesting findings because, before then, some authors had even disputed whether the lineament really crossed beneath the Paraná Basin,” says Marcelo Assumpção, a professor at the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP), who was not involved in the project. In the Parnaíba Basin, he says, the influence of the Transbrasiliano is more obvious, since the lineament reappears on the other side, in the state of Ceará. “Now we’ve been able to see exactly where the lineament passes beneath the basin, and we even discovered several subterranean regions that we didn’t know existed,” says David Castro, a researcher at UFRN who published a paper on the subject in 2014.

The lineament divides the Brazilian landmass into two large regions. On one side lies Amazonia, a portion of the Central-West region and small sections of the states of Ceará and Piauí; on the other side are the South, the Southeast, and all the rest of the Northeast. It begins in Argentina, crosses through Paraguay and runs towards the coast of Ceará, for a total length of 5,000 kilometers (km). It reaches depths of up to 40 km, and in some sections it can be as much as 200 km wide. Since it was formed when South America and Africa were still part of the same supercontinent, Gondwana, it continues on the African continent, there known as the Kandi Lineament, which traverses the Sahara for some 4,000 km.

This crustal fault originated in the geological period known as the Brazilian cycle, between 750 million and 540 million years ago, when the São Francisco Craton collided with the Amazonian Craton. Cratons are old, relatively stable pieces of tectonic plates. A collision between two such blocks moved rocks around, mixed more recent ones with older ones and joined together rocks with both high and low magnetic mineral content. Other geological faults were also generated. Some of these were filled in by sediments that were deposited and began to form the basins.

After the lineament stabilized, it again began to move, first during the Cambrian period about 540 million years ago, and then during the Mesozoic, between 252 million and 65 million years ago. These movements shook up the structure of the lineament even more, further mixing the rocks and sediments around it. Today there are no collisions between the edges of the cratons. There could potentially be small movements on the edges over intervals of millions of years, but the block as a whole is stable.

The data obtained in the Transbrasiliano Lineament project will continue to be analyzed. One idea is to map some areas in greater detail, which could enable geologists to more precisely identify the origin of the sedimentary basins that the lineament traverses. “The natural thing now is to continue to zoom in,” says Curto.

Scientific article
CURTO, J. B. et al. Crustal framework of the northwest Paraná Basin, Brazil: Insights from joint modeling of magnetic and gravity data. Tectonophysics. V. 655, p. 58-72. August 1, 2015.

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