Intense glacial erosion of the St. Elias range in Alaska has led to a structural response from deep within the mountain, according to recent real-world data collected in part by researchers at Virginia Tech. The findings support decades of model simulations of mountain evolution and a climate's impact on the distribution of deformation associated with plate tectonics.
The St. Elias range is formed by the North American plate pushing material up as it overrides the Pacific plate. Glaciers then wear down the material. One million years ago, glacial conditions became more intense, creating larger and more erosive ice streams that changed the shape and evolution of the mountains. The process continues today, resulting in the particularly active and dramatic St. Elias “orogen” — geologists’ word for mountains that grow from the collision of tectonic plates.
James Spotila, associate professor of geology in the Department of Geosciences in Virginia Tech's College of Science, and Aaron L. Berger, a Ph.D. graduate of the geosciences department, collaborated with researchers as part of the National Science Foundation-funded St. Elias Erosion-Tectonics Project (STEEP).
Berger and Spotila led the land-based erosion research team and braved the St. Elias mountain range over many years to collect physical evidence. They have been dropped in remote and dangerous locations by helicopter to place instruments and collect samples to determine bedrock cooling rates and sedimentation. Their findings were published in Geology, an earth science journal, in July 2008. Their research was also published, with co-authors from STEEP, in the journal Nature Geosciences in October 2008.
Spotila explains, “If you push snow with a plow, it will always pile up in front of the plow with the same shape,” called the Coulomb wedge. The increased glacial erosion across the top of the wedge of the past million years pushed the orogen to a tipping point as it struggled to maintain its wedge shape.
“Rock faulting and folding has become more intense beneath the large glaciers as the orogen internally deforms to adjust to the focused erosion," Spotila said. "Some faults, which previously responded to the push of the plow or tectonic plate, relocated to respond to the accelerated erosion.”
The research showed how a change in climate led to a change in the way the motion of tectonic plates is accommodated by structural deformation within the orogen, Spotila said.
“It is remarkable that climate can have such a profound impact on tectonics and the behavior of the solid earth,” Spotila said.
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