Geoscientists from the University of Calgary were in for a surprise when they took a closer look at plate tectonics beneath the East Kootenay. What seemed to be a routine measurement ended up confuting prior geological assumptions.
“We’re proposing that there was a mantle delamination event that took place about 50 million years ago, which had profound consequences for how the mountains developed afterwards,” said Dr. David Eaton, professor of geophysics at the University of Calgary.
As part of a team with two other researchers from the university’s Department of Geoscience, Dr. Eaton had collaborative access to a network of seismograph stations throughout the region of the Rocky Mountain Trench.
“In addition to our own instruments in Canada, we took advantage of data from a giant project in the US called Earthscope,” he said.
The seismograph stations were built mainly for other purposes such as monitoring local earthquake activity. Using seismogram recordings of earthquakes from around the world, Dr. Eaton and his team combined data from hundreds of stations to paint a picture of what’s underground.
“The technique we used to image the Earth’s mantle is similar to medical CAT scan technology,” he said.
The team was expecting to find that below the crust, the mantle beneath the older part of the North American continent extended continuously below the Rocky Mountain Trench, to gradually merge into younger mantle rocks to the west.
“We were quite surprised with the results we found,” he said. “Our images revealed a remarkably sharp edge in the lower part of the North American plate, located right underneath Invermere and following the Rocky Mountain Trench.”
That sharp edge in the North American plate may have formed by a tectonic event about 50 million years ago.
“We are proposing that what we’re seeing today as the edge of the lower part of the plate underneath the Rocky Mountain Trench represents a location where the ancient mantle rocks broke off and foundered into the deep Earth. In effect, there used to be a tapering wedge of the continent that continued west, but now it’s no longer there.”
Before their discovery, seismic information about the Rocky Mountain Trench was based upon data from the 1980s, which did not probe as far into the Earth.
“The LITHOPROBE program produced wonderful deep images going to the base of the crust; just not as deep as we’re looking,” he said. “There have been plenty of publications based on that data in which deep cross-sections have been constructed that show a gradually tapering continental lithosphere extending to the west.”
Their new findings help to make sense of a number of features, including the abundance of local hot springs.
“A series of hot springs line up with the edge, where we postulate that this delamination event started,” he said. “The hot springs are the present day expression of a deep-seated change in the geology underneath that area.”
When that change occurred, more than 50 million years ago, the present-day Columbia Mountains are thought to have been the highest plateau on earth, he said.
Professor Eaton said that the application of their findings has the potential to help understand more than just local geology.
“It gives us not only a new model for understanding the area of the Columbia Mountains, but a model we can apply to other mountain ranges around the world.”