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Mystery behind mountain range beneath Antarctica's ice solved

London, Wed, 08 Feb 2012 ANI

London, Feb 8 (ANI): Geologists have found a new understanding to the mountain-building process, which had baffled them for decades now, a new study claims.


In 1958, geologists discovered a mountain range buried more than a kilometer beneath the East Antarctica Ice Sheet.


For more than half a century, the origins of the Gamburtsev Subglacial Mountains had proven to be a geological puzzle.


The Gamburtsevs lie under the highest point in Antarctica, the 4,000-meter-high Dome Argus Plateau. The mountain range, in the middle of an ancient continental craton, has a thick, crustal root and high topography, which has made the Gamburtsevs the least-understood tectonic feature on Earth for the past 50 years.


"The reason why the Gamburtsevs are so enigmatic is that stable cratons are generally low-lying features with no huge mountain ranges atop," Fausto Ferraccioli, a geophysicist at the British Antarctic Survey in Cambridge, England said.


The Gamburtsevs are completely covered by ice, preventing scientists from recovering physical rock samples. Earlier studies that determined the root - all that remains of an earlier mountain chain - was at least Precambrian in age relied on indirect evidence: outcrops exposed to the north of the mountains, detrital zircons retrieved from ice cores, and seismic measurements that revealed a crustal thickness of 45 to 60 kilometers. Elsewhere in East Antarctica, the average crustal thickness is about 35 to 40 kilometers.


Ferraccioli and his colleagues used two Twin Otter aircraft equipped with ice-penetrating radar, laser-ranging systems, gravity meters and magnetometers to collect data over a 3-million-square-kilometer area between the South Pole and the Lambert Glacier.


The data revealed a previously unknown extension of the East Antarctic Rift System - a 2,500-kilometer-long fracture that extends from East Antarctica across the ocean to India, which, when active, resembled today's East African Rift System - making it the longest fossil rift system on Earth.


Finding the rift extension led the team to propose that the mountains are actually the result of multiple orogenies, or mountain building episodes, involving a unique combination of continental collision, rifting and uplift processes not previously seen together in other mountain ranges.


"We infer that the root may have formed about 1 billion years ago during the assembly of the supercontinent Rodinia,


"However, sparse zircon data retrieved from small amounts of sediment inclusions entrained in accreted ice at Lake Vostok suggest that 1.8-billion to 1.6-billion-year-old rocks may be present in the Gamburtsev Province," he said.


The root is the last remnant of the proto-Gamburtsev mountains, which would have been completely eroded by the time the East Antarctic Rift System became active 250 million years ago. Rifting did not destroy the crustal root, but rather reactivated it by magmatic activity.


With crustal thinning and stretching, the crustal root became more buoyant, isostatically rebounding out of the mantle, and uplifting the flanks of the rift zone to form the modern Gamburtsevs about 100 million years ago, the authors claimed.


Steep downcutting and erosion of the rift flanks - first by rivers and then, about 34 million years ago, by the first glaciers of the East Antarctic Ice Sheet - further unloaded the crust, which buoyed the range higher, the authors suggested. Once a more stable continental-scale ice sheet formed about 14 million years ago, Ferraccioli says, the landscape became permanently entombed by ice and the peaks were preserved.


The study has been published in Nature. (ANI)


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