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Mountains on a plate form the Andes

Curving down the western coast of South America is the world's longest mountain chain – and one of its greatest puzzles. The Andes run for about 7,000 kilometres with the highest peak, Mount Aconcagua in Argentina, rising nearly 7,000 metres above sea level.

An international team of geologists were intrigued by the Andes' geology and have finally solved a mystery that scientists had previously been unable to answer: the mountains were formed only 45 million years ago yet the massive forces that pushed them up have been at work for 140 million years as the Nazca tectonic plate is thrust under the South American continent.

"We couldn't understand why the Andes had only begun to form long after the subduction had started," says team leader Dr Fabio Capitanio, of Monash University's school of geosciences in Melbourne.

"This is the process by which the Nazca plate is pushing its way under the continent. We knew when the subduction began, but why did it take nearly 100 million years before the mountain chain started to rise and what was it that caused the chain to bend the way it does in Bolivia?"

In a paper published in the journal Nature, Capitanio and international colleagues in Italy, Spain and the US describe a new approach to plate tectonics using the first three-dimensional physical model to be developed that goes beyond explaining how the Earth's tectonic plates move to why they cause the effects they do.

The researchers say the traditional approach to plate tectonics, of working back from data, resulted in 2D models with strong descriptive but no predictive power.

"The existing models allow you to describe the movement of the plates as it is happening but you can't say when they will stop or whether they will speed up, and so on. I developed a three-dimensional, physical model – I used physics to predict the behaviour of tectonic plates – then I applied data tracing the Andes back 60 million years and it matched," Capitanio says.

The researcher essentially switched the traditional approach around: instead of being driven by the data and asking why, he took the predictive method of physics and started investigating what would happen to the physical model if he excluded some factors or added others.

He then cross-checked the results with the actual geological data and found that some physical processes explained certain aspects while others produced different results.

"The Andes has two prominent features: one is the topography, which is very high around the centre of the mountain chain but much lower at the northern and southern parts of the cordillera [this refers to a system of parallel ranges and plateaus].

“The second feature is the curvature – called the Bolivian orocline – and neither of the two features were there 45 million years ago."

It was not until they made the breakthrough that Capitanio and his colleagues realised other researchers had been oversimplifying the enormous geological processes that have shaped the surface of the Earth – that a tectonic plate is not necessarily of uniform age all the way across.

They concluded that some parts of the Nazca plate were older so had time to cool, become heavier and sink faster into the Earth's mantle, thereby forcing up parts of what was to become the high parts of the Central Andes.

"A very important aspect of my model is that it is the first, three-dimensional model of the Andes whereas before it was considered a 2D type of problem. I have shown that the three-dimensional variation in the age of the Nazca plate during subduction is the key to the formation of the modern Andes.

“Looking at reconstructions back in time, only around 45 million years ago were the conditions met when the configuration of the Nazca age was the one physically required to create the mountains.

"This is a breakthrough that relied on the demanding computational technology required, but mostly it demonstrated that the Andes are the result of a three-dimensional configuration and this is new. It is also the reason why subduction alone is not sufficient, rather that specific 3D configurations of the Nazca plate are required."

Capitanio says the most important discovery is the idea that as the plate becomes older and heavier, it sinks into the mantle and pushes up mountains. This theory can now be expanded to see how the forces vary along the plate margins because they create mountain chains that look the same yet in the middle can double in height and then become small again.

These changing forces can also produce a chain that is bent in the way the Andes is in Bolivia and these are the same forces that create earthquakes. In their paper in Nature, the researchers describe how the older Nazca plate thrusting below the Central Andes can explain the locally thickened crust and the higher elevations of the mountain chain.

The researchers say that although the theory has been applied so far only to one plate boundary, it has broader applications. Understanding the forces driving tectonic plates will allow researchers to predict shifts and their consequences, notably with earthquakes, themselves a product of moving tectonic plates.

"Before our discovery, it was impossible to understand what was happening simply on the basis of the data; now we have a different way to look at earthquakes. Usually the seismic risk of an earthquake is assessed on a statistical basis: if you have a record of when they have occurred, you can make predictions as to when another might happen,” Capitanio says.

"But in Sumatra, for example, there were no records and no one knew earthquakes were likely until a massive one occurred under the sea that caused the tsunami that hit Aceh and caused such devastation, including 230,000 deaths around the Indian Ocean in 2004. This is a limitation of the descriptive approach, whereas my model is predictive."

He admits that predicting earthquakes is still very difficult, although his model may make it possible to assess risk factors more accurately: "We are closer to saying that in some areas earthquakes could occur, but only a tiny bit closer to understanding why."

* Capitanio's collaborators on the Andes project were Dr Claudio Faccenna of Roma Tre University and former Monash colleagues Dr Sergio Zlotnik of UPC-Barcelona Tech and Dr David Stegman of University of California San Diego.
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