The origin of the Andes unravelled

December 11, 2017, University of Amsterdam

Why do the Andes exist? Why is it not a place of lowlands or narrow seas? Wouter Schellart, a geophysicist at the Vrije Universiteit Amsterdam, has been pondering these questions for more than a decade. Now, he has found the answers using an advanced computer model. "It's a matter of enormous size, longevity and great depth", he said. "These aspects made the Andes the longest and second-highest mountain belt in the world."

All the other major belts on Earth, such as the Himalaya and the Alps, were formed due to colliding continents. But there are no colliding continents in the Andes; rather, the Andes are located at a so-called subduction zone, a place where an oceanic tectonic plate sinks below another plate (in this case the Nazca plate sinking below the South American plate) into the Earth's interior, the mantle. There are numerous other on Earth, such as in Greece and Indonesia, but these locations are characterized by small seas (such as the Aegean Sea) and tropical lowlands, not massive mountain chains. So the big question is: Why did a massive mountain chain form in South America?

Andean evolution

Schellart's model, which took more than two years to complete on Australia's supercomputer Raijin, has reproduced the evolution of the South American subduction zone, from start to present (initiating some 200 million years ago and thereby the oldest subduction zone in the world), to investigate the origin of the Andes. What came out? The size of the subduction zone, some 7000 km and thereby the largest in the world, is crucial for mountain building. What else came out? The first signs of crustal shortening and mountain formation started already in the mid Cretaceous, some 120-80 million years ago. Before this time there were elongated narrow seas at the western edge of South America rather than mountains. Form the mid Cretaceous onwards the subduction zone was deep enough to induce large-scale flow in the deep mantle, down to 2900 km, the boundary between the Earth's mantle and core.

These flows dragged South America westward, causing the continent to collide with the subduction zone and thereby forming the Andes. Because the South American subduction zone is so wide, it provides much resistance to migrate laterally, in particular in the centre. This is why the collisional forces between the South American continent and the subduction zone are largest in the centre, resulting in the highest mountains in the Central Andes and formation of the Altiplano, a high plateau at 4 km above sea level, but much lower mountains in the north and south.

Read more about this research in Nature Communications

Explore further: Australian discovery solves mystery of the Andes

More information: W. P. Schellart, Andean mountain building and magmatic arc migration driven by subduction-induced whole mantle flow, Nature Communications (2017). DOI: 10.1038/s41467-017-01847-z

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Nik_2213
not rated yet Dec 11, 2017
Hey, the referenced article is currently open access !!
==
Okay, that's the High Andes explained. But why does the East coast have a large area below sea level ?? http://geology.co...a-level/
quote:
San Julían's Great Depression
Elevation: 105 meters below sea level { Yes, lower than Death Valley !! }
Country: Argentina Latitude/Longitude: 49°35′S 68°20′W
San Julian's Great Depression is located in southeastern Argentina. It is the lowest land location in both the Western and Southern Hemispheres. The deepest part is Laguna del Carbón, at approximately 105 meters below sea level. The area has abundant salt marshes.
/

Back-arc spreading ? Unlikely. Seems to have been an arm of the sea at an inter-glacial high-stand, which was cut off when sea levels fell and just dried out...
leetennant
not rated yet Dec 11, 2017
When I was in South America at the beginning of the year, this was the main thing I couldn't understand. There are trenches in Peru and Chile caused by the subduction but nobody could tell me how that same subduction could also cause a huge mountain range. I can't say I still completely understand it but it's nice to say I wasn't the only one confused.
Caliban
not rated yet Dec 11, 2017
To answer you both-

The implication that SA was somehow sucked or dragged towards a spontaneously created, 7,000 km subduction zone is plainly ludicrous. Most of this article appears to be some tyro's misinterpretation of the ill-understood truth.

There is no real difference between the Andes and the Rockies, including the back-arc lowlands, of which Death Valley is the NA counterpart.

Both featured, in earlier epochs, shallow inland seas and island arc accretion.

The Rockies and its subsidiary mountain chains being much more ancient, overall, than the Andes, is the main difference.

And, for anyone who cares to know, the mountain system exemplified by the Appalachians of NA is the largest, longest, and most extensive mountain system ever identified in the world.

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