The seismic data analyzed by the researchers in this study reveal how a gap in the descending tectonic plate about 60 miles below Patagonia has allowed hotter, less viscous mantle material to flow under South America. When glaciers melt, a tremendous weight is lifted from the ground that once supported them. Huge amounts of water, previously stored as ice, flow into the oceans. The newly discharged earth bounces and rises. Geologists see evidence of this combination of ice mass changes and uplift in places around the world.
A seismic study reveals the key reason why Patagonia is growing as glaciers melt. Geologists have discovered a link between the recent loss of ice mass, rapid rock uplift and a gap between the tectonic plates that underlie Patagonia, as the ice melts the ground below is shifting and uplifting.
Scientists at Washington University in St. Louis, led by seismologist Douglas Wiens , the Robert S. Brookings Distinguished Professor of Arts and Sciences, recently completed one of the first seismic surveys of the Patagonian Andes. In this study, they describe and map a map of local subsurface dynamics.
” Variations in the size of glaciers , as they grow and shrink, combined with the mantle structure we’ve imaged in this study are driving rapid and spatially variable uplift in this region ,” said Hannah Mark, a former postdoctoral fellow. Steve Fossett in Earth and Planetary Sciences at the University of Washington.
The seismic data that Mark and Wiens analyzed reveal how a gap in the descending tectonic plate about 60 miles below Patagonia has allowed hotter, less viscous mantle material to flow under South America.
“The low viscosities mean that the mantle responds to deglaciation on the time scale of tens of years, rather than thousands of years, as we see in Canada, for example,” Wiens said. “This explains why GPS has measured a large uplift due to ice mass loss. “Another important thing is that the viscosity is higher under the southern part of the South Patagonian Ice Field compared to the North Patagonian Ice Field, which helps explain why uplift rates vary from north to south. south,” he said.
When glaciers melt, a tremendous weight is lifted from the ground that once supported them. Huge amounts of water, previously stored as ice, flow into the oceans. The newly discharged earth bounces and rises. Geologists see evidence of this combination of ice mass changes and uplift in places around the world.
The ongoing movement of the earth, known as glacial isostatic adjustment, is important for many reasons, but especially because it affects predictions of sea level rise in future climate warming scenarios .
“This suggests to us that perhaps the mantle dynamics associated with the slab window have intensified over time, or that the continental plate in the south started out thicker and cooler and was therefore less affected by the slab window than the farthest part of the plate”. north,” Mark said.
” Patagonia is a remote area that is not densely populated and earthquake risks are relatively low , which helps explain why few seismic studies have been done in this area in the past,” Wiens said. The data he and his team collected is already being used for purposes beyond this mantle-imaging effort.
“The beautiful glaciers are shrinking in size,” Wiens said. “Over the next few decades, ice fronts receded further up the mountains and further inland, which could make it more difficult to visit. I can easily see that the glaciers have shrunk since I first visited this area in 1996.”
The investigation was stopped due to the pandemic and the teams remained in Patagonia, 10 months later they were recovered. “Instruments were stuck in Chile and Argentina during COVID, so they were not returned in April 2020 as planned,” Wiens said. “Instead, they were returned in February 2021 thanks to the tremendous help of our colleagues in those countries. But the seismographs worked fine without any service during this time, so we collected about 10 more months of data than we had initially planned,” Wiens explained.
Knowing more about what’s going on below ground is important for monitoring future changes in places like the Patagonian ice fields . “One thing we can and will now do is incorporate the 3D mantle structure into a model for glacial isostatic adjustment in Patagonia, along with constraints on the extent of glaciation over time,” said Mark. “Plate tectonics and deep-earth properties are critically important to understanding how the earth responds to glaciation and deglaciation,” Wiens said. “With better Earth models, we can do a better job of reconstructing recent changes in the ice sheets.”
Reference:
Hannah F. Mark et al. Lithospheric Erosion in the Patagonian Slab Window, and Implications for Glacial Isostasy. Geophysical Research Letters 2022. https://doi.org/10.1029/2021GL096863
