STEEP Researchers Link Climate and Tectonics in St. Elias Mountains

November 2008

The St. Elias Mountains of southern Alaska and northwestern Canada make up the highest coastal mountain range on Earth. They have been thrust up over the past few million years as a result of colliding tectonic plates and, at the same time, worn down by the world’s largest mid-latitude glaciers that grow and shrink with climate change. Scientists are now investigating whether glaciers actually promote mountain growth too.

The St. Elias Erosion/tectonics Project (STEEP) project, a multi-disciplinary collaboration funded by the National Science Foundation, is designed to reconstruct the history of building up and wearing down of the St. Elias Mountains. The team, which hails from 10 different institutions including the University of Texas at Austin and resembles an earth science version of television’s CSI team, is made up of geologists, seismologists, geodesists, glaciologists, geochronologists, and geodynamic modelers. Team members from the University of Texas’s Institute for Geophysics (UTIG), part of the Jackson School of Geosciences, are Sean Gulick, Gail Christeson, Harm Van Avendonk, and Paul Mann, as well as graduate students Lindsay Worthington, Ryan Elmore, and Bobby Reece.

Like CSI, the STEEP team hopes to crack a few unsolved cases, including:

1. Did changes in climate control how fast the St. Elias Mountains rose and create new faults in the crust?

In the classic textbook description of how mountains rise and fall, tectonic forces are portrayed as the major players in building them up and climate is the major player in wearing them away. Until recently, climate’s role in helping grow mountains was not appreciated.

The STEEP team discovered that climate plays a role in where and how quickly mountains form.

This past October, the STEEP team reported in an advance online version of the journal Nature Geoscience that yes, as they suspected, climate is responsible for the fast rate of exhumation (rise of rock from the subsurface to the surface) in the St. Elias Mountains, as well as the creation of faults near the mountains. Read the article: Quaternary tectonic response to intensified glacial erosion in an orogenic wedge.

About a million years ago, Earth began to experience more frequent and more intense periods of glaciation. As glaciers came and went, they literally ripped the tops off the St. Elias Mountains and washed huge volumes of material down to the coast, where it settled as sediment on the continental slope. This rapid erosion made Earth’s crust lighter in places. What the team discovered, based on the ages of rocks at the surface and the thickness and composition of underwater sediments just off the coast, is that this lighter crust acted as a release valve for the vice-like pressure of nearby tectonic collisions, so the eroded mountains grew even faster. To accommodate this rapid uplift, new faults formed near the mountains while existing offshore faults shut down as they became buried with sediment. Thus, the researchers reason, climate can move mountains or at least the faults that build them.

2. What’s pushing up these mountains?

Researchers from the University of Texas at Austin’s Institute for Geophysics used the RV Marcus G. Langseth to gather geophysical data just off the coast of the St. Elias Mountains.

Scientists understand that there is a big chunk of rock called the Yakutat Block that is colliding with the North American Plate. The block is subducting, or diving beneath the continent. The pressure of that collision and the buoyancy of the subducting block, with help from climate change (as mentioned above), are pushing up the St. Elias Mountains. That part of the story seems pretty straightforward. But, scientists wonder, what exactly is this chunk of rock? Where did it come from? Is it a small renegade chunk of continental crust that was ripped off of the North American plate and then drug up the coastline? Is it an oceanic plateau, magma that gushed up the ocean floor and hardened into a thick pile of basalt? Or is it something entirely different?

After this past summer’s underwater geophysical data collection, team members from UTIG now have the data they need to start answering the question. They plan to publish their findings soon.

3. As the Yakutat Block crunches into the North American continent, where does all the collisional energy go?

Institute researchers deployed ocean bottom seismic instruments to study the thickness and composition of seafloor sediments.

In other words, how much energy goes into making mountains grow vertically and how much goes into pushing crust out of the way? During a collision like this, which is exemplified by the slamming of India into Asia and the formation of the Himalayas, the energy can go two places—straight ahead, in which case, the crust wrinkles up and mountains form—or, off to the side, where crust is pushed out of the way, sort of like squeezing Jell-O between your hands and some of it oozing out the sides. These two options are known as “lithospheric shortening and uplift” and “lateral extrusion of detached crust.” The latter process has been observed in southeast Asia where large strike-slip faults like the U.S. San Andreas Fault allow large blocks to slide out of the way of the forming mountain belt.

Apparently, both processes are also happening in the St. Elias Mountain range in Alaska. What the scientists want to know is, how much of the collisional energy goes into each process? It isn’t just an academic exercise. This information can be applied to other tectonic margins around the world to better evaluate earthquake risks to humans.

One part of the STEEP project involved placing GPS receivers in remote locations across the mountain range.

Of the three main research questions, this last one has proved the most elusive. Funding for the five year project will come to an end in about a year, so the team hopes to get new funding to continue working on this important question.

In addition to the University of Texas at Austin, the sixteen principal investigators and their students hail from University of Alaska Fairbanks, Indiana University, Lehigh University, University of Maine, Purdue University, University of Texas at El Paso, University of Utah, Virginia Polytechnic Institute and State University, and University of Washington.

by Marc Airhart

For more information about the Jackson School contact J.B. Bird at jbird@jsg.utexas.edu, 512-232-9623.