Led by geoscientists at the University of Arizona, an international research team will use data from earthquakes, geology and geochemistry to study, in greater detail than ever before, how mountain ranges are built.

Supported by a $3 million grant from the National Science Foundation, the project will shed light on how the Andes in South America formed, and produce a 3D model of mountain-building based on the Andes as a natural laboratory.

The project, which is part of the NSF Frontier Research in Earth Science program, is dubbed TANGO, which stands for Trans Andean Great Orogeny. At the heart of the project is one of the most extensive network of earthquake sensors, or seismometers, to ever be installed in the Andes region of South America. Scientists will use seismic waves traveling through Earth's interior from quakes around the globe to better understand the geologic processes underlying the formation of mountain ranges.  

TANGO will focus specifically on the Andes from northern to southern Chile and in Argentina.

"TANGO is an excellent example of the type of international collaboration that characterizes the University of Arizona's unique capacity to tackle the grand challenges of our time," said University of Arizona President Robert C. Robbins. "Building on our strengths and ongoing research in the geosciences, our faculty laid the groundwork that allowed them to successfully assemble an international team to help us gain a better understanding of a natural process where there is still a lot to learn."

Susan Beck, a UArizona professor of geosciences, will serve as TANGO's lead principal investigator, with co-principal investigators Barbara Carrapa, Peter DeCelles, Mihai Ducea and Eric Kiser of the UArizona Department of Geosciences.

A major part of the TANGO project centers around seismic imaging, which works much like medical imaging such as CT scans, which use X-ray images to make tissues visible based on their densities. Just like bone and soft tissue show up as different features, geologic features beneath the Earth's surface show up distinctly when geologists "X-ray" them by recording shockwaves from earthquakes as they travel through the Andes.

"Instead of sending X-rays through your head, we use seismic waves," Beck said. "We deploy our instruments across a large area, and we wait for earthquakes to happen. We might take a year's worth of data, from which we then assemble a tomographic image of what's down there." 

While many of the processes involved in mountain-building — known as orogeny — are known to take place at the surface, other processes take place very deep inside the Earth, hidden from view. Seismic imaging allows researchers to probe the Earth's interior down to about 700 miles, Beck said.

"Combined with geologic and geochemistry data from the rocks, we can understand how the Andes formed over the last 90 million years," she said.  

Along the western edge of South America, a chunk of ocean floor known as the Nazca plate pushes against its neighbor — the plate that contains the South American continent — at a rate of a little over 2 inches per year. This process, known as subduction, causes Earth's crust to fold up, pushing up mountain peaks up to 20,000 feet in elevation.

"Subduction affects almost every aspect of our lives," Beck said. "Think of it as a recycling program for Earth's crust; it affects where mountains will rise up, where minerals and ores are formed, where tension is released as earthquakes and where the largest volcanic eruptions occur."

Piecing Together 'A Giant Puzzle'

Geologists still only have a vague idea of the details of mountain-building processes, Beck said, and TANGO is poised to fill some of the gaps.

"For example, we know that as one plate goes under the other, it causes earthquakes, it drags layers of rock down with it and causes volcanoes to erupt," she said. "But what happens with that molten rock before it gets to the surface? How deep does the Nazca plate go before it gets assimilated into the mantle?"

The Andes serve as a giant natural laboratory to study the complex process involved in building a mountain range, Beck said.

"When you make mountains, rocks erode, and all that eroded rock has to go somewhere," Beck said. "In a large mountain range like the Andes, that eroded material adds up."

As debris from the eroding mountains accumulates in basins on the east side of the Andes, it creates a layered archive of time that "is amazing to unravel," Beck said, but also presents geologists with head-scratchers.

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"We have a decent understanding of the big picture, but we don't really understand the dynamics of it in detail," Beck said. "For example, we find deposits from those basins high up in the mountains, and we don't really know how they ended up there, so it's like a giant puzzle."  

Beck said she is excited about the seismic imaging component of TANGO.

"Each seismic wave has a travel time that we can measure," she said. "The time it takes a seismic wave to get from the epicenter of an earthquake to our station depends on the materials it travels through at different speeds, and we can unravel that. For example, a seismic wave that goes through a magma body really slows down compared to a wave that doesn't, and we will see that difference."

To record thousands of earthquakes occurring in South America and around the globe, the team will install seismic stations across an area measuring about 800 miles by 400 miles. Deploying the technology in the field will involve many students from UArizona and partner institutions.

"Some stations are easy, as they are in readily accessible locations and we just need to dig a hole and insert the sensors," Beck said, "but others are in very remote locations, at high elevations. Some seismic stations require building a vault, mounting solar panels and batteries so the seismic station can run for years."

TANGO differs from similar efforts in scope and scale, Beck said.

"In a typical scenario, people would put these stations out for a month, pull them up and call it good, but we will be going into very remote areas, and we will have to deploy our instruments over many months to years. We look at this as our one-time chance to get the data that could help us answer these fundamental questions. It's going to be a huge field effort."

Since orogenic mechanisms are not unique to the Andes, TANGO will help scientists better understand tectonic processes in other areas as well. Beck said the Andes are a modern analog for what the western margin of North America looked like between 70 and 90 million years ago.

"Similar processes have happened through geologic time in many places throughout the world," she said.