{"id":204480,"date":"2015-12-02T15:20:33","date_gmt":"2015-12-02T17:20:33","guid":{"rendered":"http:\/\/revistapesquisa.fapesp.br\/?p=204480"},"modified":"2015-12-02T17:36:32","modified_gmt":"2015-12-02T19:36:32","slug":"echos-of-separation","status":"publish","type":"post","link":"https:\/\/revistapesquisa.fapesp.br\/en\/echos-of-separation\/","title":{"rendered":"Echos of separation"},"content":{"rendered":"
\"The

JAMSTEC <\/span>The Rio Grande Rise: now, fragments of a continentJAMSTEC <\/span><\/p><\/div>\n

Published in October 2014<\/em><\/p>\n

No, it wasn\u2019t a mistake. In 2011, geologists collected samples of granite, a type of continental rock, from the Rio Grande Rise, a chain of submerged mountains approximately 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 formation of the ocean floor and volcanic eruptions and that they therefore were composed of a different type of rock. Two years later, using a submersible craft, researchers collected additional samples of continental rock, and the resulting analysis supported the hypothesis that this region of the South Atlantic was actually a piece of a continent submerged when South America and Africa separated\u2014a process that began 120 million years ago.<\/p>\n

This finding is economically valuable for 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 lie in international waters; hence, finding reserves of other minerals, such as nickel, manganese and rare earths, has become more likely. The scientific value of the formation has also increased inasmuch as it offers additional arguments supporting the hypothesis that the separation of South America from Africa is a more fascinating and complicated process than earlier thought. Geologists from Brazil, the United States, Germany and France met in Rio de Janeiro in April 2014 and concluded that the great blocks of rock, or microplates, that composed the two continents and seabed did not separate like two parts of a torn sheet of paper; rather, the continents stretched out, broke apart and were chaotically repositioned. Certain portions may have been repositioned in the middle and submerged, while others separated and combined, forming an immense mosaic that is now becoming clearer.<\/p>\n

Researchers believe that rocks collected from the Rio Grande Rise\u2014granites, granulites, gneisses and pegmatites\u2014are 500 million to 2.2 billion years old, according to analyses conducted by teams from the University of Bras\u00edlia and the Geological Survey of Brazil (CPRM). \u201cThe ages are not outside what we usually find in South America and Africa,\u201d says Roberto Ventura Santos, director of geology at CPRM. Ventura says that the seismic uplifts indicate that the thickness of the crust in this area is nearly 30 km, \u201ctypical of continental rather than oceanic crust,\u201d further supporting the theory that it is a part of a continent.<\/p>\n

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\u2014the Rio Grande and the Mid-Atlantic Ridge\u2014formed during the same period, but now scientists think that this may not be the case. In addition, 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 is only 800 meters beneath the surface of the ocean, should create barriers to ocean circulation, but researchers do not understand the underlying mechanism. Ventura believes that analysis of a 70-meter column of sediment from the seabed will provide some answers, which he hopes will enable reconstruction of the climatic and geological phenomena of the past seven million years.<\/p>\n

\"042-045_Cien_Geologia_Eng_2015-02\"<\/a>\u201cThe 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,\u201d explains geologist Peter Christian Hackspacher, a professor at S\u00e3o Paulo State University (Unesp) in Rio Claro. Some 20 years ago, when 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 support the traditional model, in which the coastlines of the two continents, representing the blocks of rock that formed them, fit together. The coastline of northeastern Brazil fits with that of West Africa, but in other regions, such as the coast of Rio de Janeiro State, parts appear to be missing from the jigsaw puzzle of rocks.<\/p>\n

Serra do Mar rejuvenated<\/strong>
\nThe blocks of rock that once made up a single continent fragmented and aligned with older or newer blocks to form the mountainous region of southeastern Brazil and West Africa, concludes Hackspacher, whose research is carried out in collaboration with teams headed by Ulrich Glasmacher in Germany, Antonio Ol\u00edmpio Gon\u00e7alves in Angola and Ana Ol\u00edvia Magalh\u00e3es at the Federal University of Alfenas in the state of Minas Gerais. Contrary to expectations, older blocks\u2014such as the Mantiqueira and Bocaina mountain ranges, which were uplifted 120 million years ago\u2014are in the continental interior, and more recent blocks that are 35 to 20 million years old are on the margins, such as the coastline between the states of Paran\u00e1 and Rio Grande do Sul (see map<\/em><\/a>).<\/p>\n

\"Effects

Peter C. Hackspacher\/UNESP<\/span>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. Hackspacher\/UNESP<\/span><\/p><\/div>\n

\u201cI\u2019m not discovering the wheel, I\u2019m just measuring by using other techniques,\u201d he says, acknowledging the conceptual baselines offered by professors at the University of S\u00e3o Paulo (USP), such as Fernando Almeida, Umberto Cordani and Benjamim Bley Brito Neves, who had already recognized that South America was formed from microplates of rocks of different ages and varied origins (see Pesquisa FAPESP<\/em> Issue n\u00ba 188<\/a>). Claudio Ricommini, also from USP, challenged the customary view of the continent\u2019s formation somewhat further when he ascertained that the age of rocks from the Taubat\u00e9 sedimentary basin was between 33 million and 55 million years\u2014far from the 120 million years that had been hypothesized because of their proximity to the coastline.<\/p>\n

Almost 10 years ago, with equipment on hand to measure the age and temperature variation of rocks according to depth\u2014the lower the temperature, the closer to the surface and the more recent the rock is\u2014Magalh\u00e3es proposed to Hackspacher, her thesis advisor at the time, that they examine the ages of 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 and become exposed on the surface in an alternating manner. From there, \u201cIt 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,\u201d she says. In a series of \u201cspectacular discoveries\u201d (Hackspacher\u2019s 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 separation from Africa.<\/p>\n

\"...and

Peter C. Hackspacher\/UNESP<\/span>…and the Florian\u00f3polis-S\u00e3o Joaquim road, which crosses the Rio do Rastro mountain range in the state of Santa Catarina and consists of volcanic rocks that were formed 134 million years agoPeter C. Hackspacher\/UNESP<\/span><\/p><\/div>\n

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 Esp\u00edrito Santo and Santa Catarina. \u201cUntil 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,\u201d Hackspacher says, \u201ceveryone 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.\u201d<\/p>\n

The fact that the Tiet\u00ea River flows westward is an indication of a more recent geological phenomena. According to Hackspacher, if the range had formed 120 million years ago, the river would probably flow toward the ocean rather than inland. Today, the most widely probed hypothesis is that this mountain chain resulted from the formation of the Andes, beginning about 60 million years ago, which may have generated large waves that affected the topography, creating depressions such as the Pantanal wetlands in the state of Mato Grosso and peaks such as the Mantiqueira and Serra do Mar. \u201cI don\u2019t find it hard to accept that possibility, but there is not yet sufficient evidence,\u201d he says.<\/p>\n

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 similar in age to the Rio Grande Rise.<\/p>\n

Project<\/strong>
\nHistory of exhumation of the South American platform in Southeastern Brazil: thermochronology by fission track analysis and ar\/ar and sm\/nd systematics (n\u00ba 2000\/03960-5<\/a>); Grant mechanism<\/strong> Thematic project; Principal investigator<\/strong> Peter C. Hackspacher (Unesp); Investment<\/strong> R$1,282,335.65 (FAPESP).<\/p>\n

Scientific articles<\/em>
\nKARL, M. et al.<\/em> Evolution of the South Atlantic passive continental margin in southern Brazil derived from zircon and apatite (U\u2013Th\u2013Sm)\/He and fission-track data.<\/a> Tectonophysics<\/strong>. V. 604, pp. 224-44. 2013.
\nSALOMON, F. et al<\/em>. Major paleostress field differences on complementary margins of the South Atlantic<\/a>. EGU<\/strong> 13, p. 10894. 2013.<\/p>\n","protected":false},"excerpt":{"rendered":"Blocks of rock combined to form the margins of the South Atlantic","protected":false},"author":17,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"footnotes":""},"categories":[159],"tags":[239,240],"coauthors":[5968],"class_list":["post-204480","post","type-post","status-publish","format-standard","hentry","category-science","tag-geography","tag-geology"],"acf":[],"_links":{"self":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/204480","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/users\/17"}],"replies":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/comments?post=204480"}],"version-history":[{"count":0,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/204480\/revisions"}],"wp:attachment":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media?parent=204480"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/categories?post=204480"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/tags?post=204480"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/coauthors?post=204480"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}