BEN MCGEERegiao de Nobres, in Mato Grosso: one of the areas studiedBEN MCGEE
The chronological history of Gondwana, the ancient southern super-continent that included most of the land now located in the Southern Hemisphere, is being rewritten by Brazilian and North American researchers. According to new rock dating and analyses of the magnetic field in segments of a chain of mountains in Central Brazil, the final event that created the super-continent occurred 100 million years later than was thought. The disappearance of an ocean, Clymene, that separated the Amazon from the other blocks of the future South America, took place 520 million years ago.
“Before, we worked with the idea that Clymene had closed about 620 million years ago,” said the geologist Ricardo Trindade, from the Institute of Astronomy, Geophysics and Atmospheric Sciences of the University of São Paulo (IAG-USP), one of the authors of the article published in the March issue of the journal Geology. “Now we know that the Amazon Region spent a lot of time separated from the other segments of South America and from the rest of Gondwana by this ocean.” The existence of this inland sea, splitting the heart of ancient Brazil, is one of the proposals put forth by this group of researchers. Its name was taken from Greek mythology. Clymene was the wife of Iapetus, a Titan. The ocean was so called to emphasize its connection with another ocean, the great Lapetus, which bathed the Southern Hemisphere at the same time.
The junction of the original fragments of South America left marks on the relief of the land in the form of elevations that are visible to this day. The detailed study of the features of one of these geological scars, the Paraguay Fold Belt, led the researchers to establish a new date for the end of the assembly process of Gondwana. The Paraguay Fold Belt is a group of elevations that marks the collision zone, or suture, in the jargon of geologists, between the biggest of the South American blocks, the Amazon, and the other parts of the continent. Strictly speaking, it is part of an enormous mountain chain that extends from the border of Maranhão and Pará, passing through Mato Grosso and Mato Grosso do Sul, right to the south of Argentina. “It’s striking that these mountains form a curve,” comments the North American geologist Eric Tohver, from the University of Western Australia, another person responsible for the work. “In Mato Grosso, the direction of the chain is west to east. From Mato Grosso do Sul to Paraguay it’s north to south.” With funding from FAPESP and the National Science Foundation in the United States, Tohver took part in the study for three years while he was doing post-doctoral studies at USP with Trindade and Cláudio Riccomini’s team.
The age of the mountains in the Paraguay Fold Belt, of which the Serra das Araras in Mato Grosso forms part, was determined by dating the clay at the bottom of the ancient ocean, Clymene. Using a variation of the technique normally employed for calculating the age of certain types of geological faults and earthquakes, the method measures the quantity of isotopes of argon (40Ar and 39Ar) in the rocks. The results suggest that the formation of the mountains – and, therefore, the closing off of the Clymene ocean – having occurred about 520 million years ago, i.e., 100 million years later than was thought.
The second analysis used for justifying this conclusion concerns the direction and intensity of the earth’s magnetic field stored in the minerals found in the rocks and sediment of these mountains. These materials have something like a compass embedded inside them, a sign that allows one to deduce where the magnetic poles were at a given moment in the evolutionary history of the planet and supplies clues to the movement of the continents in the remote past. In the case of the sample from the Paraguay Fold Belt, studies indicate that the non-rectilinear conformation of the mountain chain is compatible with the paleomagnetic records filed away in its minerals. “The magnetic vectors follow the curve of the mountains,” says Tohver. Analyses also show that once the chain was, in fact, straight, but that then it was folded by a rotating movement around a vertical axis. This type of adjustment commonly occurs in places where there was a shock and a settlement of ancient blocks of earth, as happened in Central Brazil when Clymene disappeared.
Until the 1980’s, the dominant idea was that Gondwana acquired its final outlines at one go. All the constituent pieces of the southern super-continent, ancient and relatively stable parts of the continental crust, called cratons by geologists, were assumed to have slotted into one another at approximately the same time. Over the last few decades, the hypothesis has gained strength that the birth of the super-continent was less of a one-off process and that its final act occurred precisely in the center of Brazil, where not all the pieces of this geological jigsaw puzzle had found the right point to fit in.
According to this new model, unlike the rest of Gondwana, whose parts were already joined and adjusted, a little more than a half billion years ago South America was still fractioned into blocks. The Amazon, São Francisco (linked to Africa), Rio Apa, Paraná, Luís Alves and Rio da Prata cratons existed. With the exception of the Andes, which were yet to be formed, the main parts of our continent were close to each other, but still separated by Clymene (see table). The ocean had to close up so that the blocks of land could finally fit together. This was the final movement in assembling Gondwana, which included pieces of the current South America, Africa, Oceania, Antarctica, India and the Arabian Peninsula.
The Project
Sedimentation after the glaciations of the Neoproterozoic Era: an integrated study of the carbonatic mantles of Brazil and Africa (nº 05/53521-1); Type Regular Research Awards; Coordinator Ricardo Trindade – IAG/USP; Investment R$ 127,962.86 and US$ 13,669.00
Scientific Article
TOHVER, E. et al. Closing the Clymene ocean and bending a Brasiliano belt: Evidence for the Cambrian formation of Gondwana, southeast Amazon craton. Geology. v. 38. n. 3, p. 267-70. Mar. 2010.
