A comprehensive survey of the ice-covered region has revealed the base of a one-billion-year-old mountain range that forms the foundation of the present-day mountains. "We have discovered the largest rift system in the world," says Fausto Ferraccioli, lead author of the study from the British Antarctic Survey. "The root under the Gamburtsevs is very old and might be related to the original process that brought together East Antarctica – it's potentially the key to the stability of this region."

Ghostly mountains

Discovered by Soviet researchers in 1958, the Gamburtsev mountains are the most enigmatic of the Earth's tectonic features as they are hidden underneath the immense East Antarctic Ice Sheet (EAIS). This mountain range, buried under as much as 4 km of ice and snow, has an estimated length of 1200 km and a maximum elevation of more than 3000 m, making it comparable to the Alps.
Since their discovery, these mountains have been a geological mystery. The last large-scale plate tectonic activity in this region occurred more than 500 million years (Myr) ago, and a mountain range that formed this long ago should have eroded by now. One explanation is that the ice sheet has protected the mountains from the harsh Antarctic elements. However, the ice sheet itself is only 35 Myr old; so either the mountains are much younger than previously thought, or there has been some unexpected geological activity in the interior of Antarctica.

Looking beneath the ice

Understanding the formation of the Gamburtsev mountains was a key goal of the seven-nation Antarctica's Gamburtsev Province Project (AGAP), a flagship initiative of the 2007/2008 International Polar Year. In late 2008 members of the AGAP team travelled to East Antarctica to collect remote-sensing data using two Twin Otter aircraft equipped with ice-penetrating gravimeters, radars and magnetometers. In total, the aircraft travelled more than 120,000 km back and forth across the range, providing a detailed cross-section of the ice sheet and underlying mountains.
Analysing these data, Ferraccioli and colleagues discovered that a rift-valley system 2500 km in length surrounds the Gamburtsevs, below which extends a deep crustal base, or "root" – the remnants of an ancient mountain range.
The researchers propose that this root formed through continental collisions around one billion years ago. Although the ancestral mountains created during this event subsequently collapsed and eroded away, their dense root was preserved.

A rejuvenated root

This crustal root was later rejuvenated by rifting – the process whereby part of the Earth's crust is pulled apart – during the Permian (around 250 Myr ago) and Cretaceous (100 Myr ago) periods. This, the researchers believe, reduced the density of the root, increasing its buoyancy and triggering uplift.
The result was a rift-valley system, similar in length to the East African Rift, which today stretches from East Africa across the ocean to India. The steep peaks and valleys that characterize the Gamburtsev mountains were carved by rivers and, from 34 to 14 Myr ago, by glaciers. The subsequent growth of the EAIS immaculately preserved this rugged topography, freezing the mountains within their subglacial crypt.
"This is the first extensive, detailed mapping of the bedrock surface and subsurface of this region," says John Veevers, a geologist at Macquarie University in Sydney, Australia. "The next step is to drill through the ice to sample the bedrock itself. This would constrain the modelled origin of the Gamburtsevs."

The birth of an ice sheet

This study also has important implications for understanding how the EAIS formed, as the Gamburtsevs are thought to have been a key nucleation site for early ice-sheet growth.
"The Gamburtsevs really formed in the right place at the right time," says Ferraccioli. "Their height and position in the interior of the continent made them an ideal site to form local ice caps, from which the ice sheet formed. A future project is to try to understand with the modellers if we can develop improved views of ice-sheet growth, perhaps to explain how resilient these ice sheets are to a warmer climate."
The research is described in Nature.