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GEOLOGY

Echos of separation

Great blocks of rock of different ages and origins combined to form the margins of the South Atlantic

The Rio Grande Rise: now, fragments of a continent

JAMSTECThe Rio Grande Rise: now, fragments of a continent JAMSTEC

No it wasn’t a mistake. In 2011, geologists collected samples of granite, a type of continental rock, from the Rio Grande Rise, a chain of submerged mountains about 1,300 kilometers (km) off the coast of the state of Rio Grande do Sul. Scientists had thought that these mountains were the result of the formation of the ocean floor and volcanic eruptions, and that therefore they were made of a different type of rock. Two years later, using a submersible craft, they collected other samples of continental rock, and the resulting analysis supported the hypothesis that this region of the South Atlantic was actually a piece of continent that had submerged during the period when South America and Africa separated—a process that began 120 million years ago.

This finding has given economic value to the Rio Grande Rise. In July 2014, the Brazilian government received the green light to implement its plan to explore cobalt deposits in the region, which lies in international waters, and hence the possibility of finding reserves of other minerals such as nickel, manganese and rare earths has become more real. The scientific value of the formation has also increased, inasmuch as it offers another argument in favor of the hypothesis that the separation of South America from Africa was a more fascinating and complicated process than was earlier thought. Geologists from Brazil, the United States, Germany and France, meeting in Rio de Janeiro in April 2014, concluded that the great blocks of rock, or microplates, that make up the two continents and the seabed did not separate like two parts of a torn sheet of paper, but rather stretched out, broke apart and positioned themselves chaotically. Some parts may have ended up in the middle and submerged, while others separated and came together, forming an immense mosaic that is now becoming a little clearer.

The rocks collected from the Rio Grande Rise—granites, granulites, gneisses and pegmatites—are thought to be 500 million to 2.2 billion years old, according to analyses conducted by teams from the University of Brasília and the Geological Survey of Brazil (CPRM). “The ages are not outside what we usually find in South America and Africa,” says Roberto Ventura Santos, director of geology at CPRM. Ventura says that the seismic uplifts indicated that the thickness of the crust there is nearly 30 km, “typical of continental rather than oceanic crust,” reiterating the conclusion that it is a piece of continent.This discovery, one of the most spectacular in Brazilian geology in recent times, has raised a number of questions. According to earlier thinking, the two mountain chains in the South Atlantic—the Rio Grande and the Mid-Atlantic Ridge—had formed during the same period, but now scientists think that this may not have been the case. And what are the effects of the Rio Grande Rise? A chain of mountains 3,200 meters high at the bottom of the South Atlantic, the top of which comes to only 800 meters deep, should create barriers to ocean circulation, but no one yet knows for sure how. Ventura believes that some answers may come to light after analysis of a 70-meter column of sediments from the seabed, which he hopes will make it possible to reconstruct the climatic and geological phenomena of the past seven million years.

“The identification of continental rocks from the Rio Grande Rise changes the evolutionary picture of the South Atlantic, which was formed by the break-up of the two continents,” comments geologist Peter Christian Hackspacher, a professor at São Paulo State University (Unesp) in Rio Claro. Some 20 years ago, doing field research in southeastern and southern Brazil, Namibia, and Angola, he studied signs of possible forces that led to the separation of South America and Africa. His findings bolster the defense of the traditional model, according to which the coastlines of the two continents, representing the blocks of rock that formed them, could fit together. The coastline of the Brazilian Northeast fits with that of West Africa, but in other regions, such as the coast of Rio de Janeiro State, there appear to be parts missing from the jigsaw puzzle of rocks.

Serra do Mar rejuvenated
The blocks of rock that once made up a single continent fragmented and aligned with other, older or newer, blocks to form the mountainous region of southeastern Brazil and West Africa, concludes Hackspacher, in collaboration with teams headed by Ulrich Glasmacher in Germany, Antonio Olímpio Gonçalves in Angola and Ana Olívia Magalhães at the Federal University of Alfenas in the state of Minas Gerais. Contrary to expectations, older blocks such as the Mantiqueira and Bocaina mountain ranges, which were uplifted 120 million years ago, are in the continental interior; and more recent blocks that are 35 to 20 million years old are found on the margins, such as the coastline between the states of Paraná and Rio Grande do Sul (see map).

Effects of uplift after the opening of the Atlantic: valley of a river in West Central Angola with rocks formed 2 billion years ago...

PETER C. HACKS PACHER/UNESPEffects of uplift after the opening of the Atlantic: valley of a river in West Central Angola with rocks formed 2 billion years ago…PETER C. HACKS PACHER/UNESP

“I’m not discovering the wheel, I’m just measuring by using other techniques,” he says, acknowledging the conceptual baselines offered by professors at the University of São Paulo (USP) such as Fernando Almeida, Umberto Cordani and Benjamim Bley Brito Neves, who had already recognized that South America was formed of microplates of rocks of different ages and varied origins (see Pesquisa FAPESP Issue No. 188). Claudio Ricommini, also from USP, challenged the customary view of the continent’s formation somewhat further when he ascertained that the age of the rocks of the Taubaté sedimentary basin was between 33 million and 55 million years, far from the 120 million years that had been hypothesized because of their proximity to the coastline.

Almost 10 years ago, with equipment on hand to measure the age and temperature variation of rocks according to depth—the lower the temperature, the closer to the surface and the more recent the rock is—Magalhães proposed to Hackspacher, her thesis advisor at the time, that they examine the age of the rocks distant from the coast in southern and southeastern Brazil. They started from the assumption that older and more recent blocks of rock rise and sink, becoming exposed on the surface in alternating fashion. From there, “it was possible to develop very good, geologically consistent findings with a reasonable degree of statistical reliability about the processes responsible for the crustal uplift of the Serra do Mar and Mantiqueira ranges,” she says. In a series of “spectacular discoveries,” to use Hackspacher’s description, they found blocks of rock, uplifted 60 to 90 million years ago, that did not fit the classic model of the formation of South America referenced against the separation from Africa.

... and the Florianópolis-São Joaquim road, which crosses the Rio do Rastro mountain range in the state of Santa Catarina, whose volcanic rocks were formed 134 million years ago

PETER C. HACKS PACHER/UNESP… and the Florianópolis-São Joaquim road, which crosses the Rio do Rastro mountain range in the state of Santa Catarina, whose volcanic rocks were formed 134 million years agoPETER C. HACKS PACHER/UNESP

Hundreds of measurements led to conclusions that are helping scientists break down old notions. One example is the probable age of the Serra do Mar, a mountain chain that stretches for nearly 1,500 km along the Brazilian coast, between the states of Espírito Santo and Santa Catarina. “Until 10 years ago, when we began to zone in on the problem and challenge some of the assumptions about the geological evolution of the South Atlantic,” Hackspacher says, “everyone understood that the Serra do Mar was formed 120 million years ago. But now we are seeing that the chain is only 35 million years old, and is not a consequence of the continental separation.”

The fact that the Tietê River flows westward is one indication of more recent geological phenomena. According to Hackspacher, if the range had formed 120 million years ago, the river would probably be flowing towards the ocean rather than inland. Today, the most widely probed hypothesis is that this mountain chain is an effect of the formation of the Andes, beginning about 60 million years ago, which may have generated large waves that affected the topographical relief, creating depressions such as the Pantanal wetlands in the state of Mato Grosso, and peaks such as the Mantiqueira and Serra do Mar. “I don’t find it hard to accept that possibility, but there is not yet sufficient evidence,” he says.

Hackspacher and his colleagues are seeing similar phenomena in Namibia and Angola. In research conducted in June 2014 that complements the land-based surveys, a German oceanographic vessel recorded signs of rock plates near the coast of Namibia that are of an age similar to that of the Rio Grande Rise.

Project
History of exhumation of the South American platform in Southeastern Brazil: thermochronology by fission track analysis and (No. 2000/03960-5); Grant mechanism Thematic project; Principal investigator Peter C. Hackspacher (Unesp); Investment R$1,282,335.65 (FAPESP).

Scientific articles
KARL, M. et al. Evolution of the South Atlantic passive continental margin in southern Brazil derived from zircon and apatite (U–Th–Sm)/He and fission-track data.Tectonophysics. v. 604, p. 224-44. 2013.
SALOMON, F. et al. Major paleostress field differences on complementary margins of the South Atlantic. EGU 13, p. 10894. 2013.

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