The ice fields that stretch for hundreds of kilometers atop the Andes Mountains in Chile and Argentina are melting at some of the fastest rates on the planet. The ground that was under this ice also shifts and rises as these glaciers disappear. Geologists have discovered a link between recent ice mass loss, rapid rock uplift, and a gap between the tectonic plates that underlie Patagonia.
Scientists at Washington University in St. Louis, led by seismologist Douglas Wiens, Robert S. Brookings Professor Emeritus of Arts and Sciences, recently completed one of the first seismic surveys of the Patagonian Andes. In a new issue in the journal Geophysical Research Lettersthey describe and map the local dynamics of the subsoil.
“Variations in the size of glaciers as they grow and shrink, combined with the mantle structure we imaged in this study, results in rapid and spatially variable uplift in this region,” said Hannah Mark, former fellow. Steve Fossett Postdoctoral Fellow in Earth and Planetary Sciences at the University of Washington, the publication’s first author. Mark is now a postdoctoral researcher at the Woods Hole Oceanographic Institution.
Seismic data analyzed by Mark and Wiens reveals how a void in the descending tectonic plate about 60 miles below Patagonia allowed warmer, less viscous mantle material to flow beneath South America.
Above this gap, the ice fields have shrunk, removing the weight that previously caused the continent to flex downward. Scientists have found very low seismic velocity in and around space, as well as a thinning of the rigid lithosphere covering space.
These particular mantle conditions are driving many of the recent changes that have been observed in Patagonia, including the rapid uplift of some areas that were once ice-covered.
“Low viscosities mean that the mantle responds to deglaciation on a time scale of tens of years, rather than thousands of years, as we observe in Canada for example,” Wiens said. “This explains why the GPS measured a large uplift due to the loss of ice mass.
“Another important thing is that the viscosity is higher under the southern part of the Southern Patagonian Icefield compared to the Northern Patagonian Icefield, which helps explain why uplift rates vary from north to south. “, did he declare.
Bounce and rise
When glaciers melt, an enormous weight is lifted from the ground that once supported them. Huge amounts of water, previously stored as ice, are flowing into the oceans. The newly discharged earth bounces and rises.
Geologists are seeing evidence of this combination of ice mass shifts and uplift in places around the world.
The continuous movement of land – the so-called “glacial isostatic adjustment” – is important for many reasons, but most importantly because it affects predictions of sea level rise in global warming scenarios. future.
Mark said one of the most interesting things they found in this study was that the hottest, least viscous parts of the mantle were in the gap region, or slab window, below the part of the Patagonian ice fields that had opened up the most. recently.
“This suggests to us that perhaps the mantle dynamics associated with the slab window may have intensified over time, or that the continental plate to the south started thicker and cooler and was therefore less affected by the slab window than the part of the slab furthest north,” Mark said.
Mark and Wiens worked with colleagues from the California Institute of Technology/Jet Propulsion Laboratory, Universidad Nacional de La Plata, Southern Methodist University and Universidad de Chile to complete the seismic survey, which was funded by the National Science Foundation.
Patagonia is a remote region that is not densely populated and earthquake risks are relatively low, which is why few seismic surveys have been conducted in this region in the past, Wiens said. The data he and his team collected are already being used for purposes other than this mantle imaging effort.
Wiens first visited Patagonia over 25 years ago. He said he was shocked by the changes he has seen in his lifetime.
“The magnificent glaciers are shrinking,” Wiens said. “Over the next few decades, the ice fronts will retreat higher into the mountains and further inland, potentially making them harder to visit. I can easily see that the glaciers have shrunk since I visited. this region for the first time in 1996.”
The highs and lows of fieldwork in Patagonia
A group of University of Washington students helped Wiens and his team maintain and collect data from the seismographs that were installed for this study as part of a field trip to the geology class at undergraduate in 2019, led by Phil Skemer and Alex Bradley of the Department of Earth and Planetary. Sciences. The students had the opportunity to spend their spring break gaining first-hand experience of the geology of Patagonia – exploring the tectonics, sediment accumulations and geomorphological effects of Alpine glaciation in the region.
Then the coronavirus pandemic hit and international travel came to a halt.
“The instruments were trapped in Chile and Argentina during COVID, so they weren’t returned in April 2020 as planned,” Wiens said. “Instead, they were returned in February 2021 thanks to the tremendous help of our colleagues in these countries.
“But the seismographs worked fine without any maintenance during that time, so we collected about 10 months more data than originally planned,” he said.
Knowing more about what’s happening below ground is important for monitoring future changes in places like the Patagonian Icefields.
“One thing we can and will do now is incorporate 3D mantle structure into a model of glacial isostatic adjustment in Patagonia, along with constraints on the extent of glaciation over time,” Mark said. .
“Plate tectonics and deep earth properties are vitally important to understanding how the earth responds to glaciation. [and deglaciation]”Wiens said. “With better Earth models, we can better reconstruct recent changes in the ice sheets.