On a recent 11-day rafting trip through the Grand Canyon, geologists gathered evidence in hopes that it would allow them to see billions of years into the past. Using a relatively new technique, they were able to assess the age of a mysterious part of the national landmark.
The great discrepancy of the Grand Canyon arises where sedimentary rock layers rest on basement stones that are as much as 1.3 billion years older. Researchers have long asked: What happened in the long period between? Olivia Thurston, a former graduate student at UIUC who now works as a post-doc researcher at Indiana University, led the research project. She said this gap in the rock record has been the source of debate for many years.
Now, with the team’s research findings published in the journal Geology, geologists are getting closer to understanding what happened millions of years ago in what is now the eastern Grand Canyon.
“I was aware that they had tried different techniques to date erosion surfaces in the Grand Canyon before,” Thurston said. “When (Professor at the University of New Mexico) Dr. Karl Karlstrøm came to visit, he showed us his work, and I said,” Why haven’t we used zircon helium yet? We are discussing these erosion ages, but we have not used all the available techniques. ‘”
Karlstrom, a professor at the University of New Mexico and co-author of the paper, has been a key figure in Grand Canyon research for decades. He also led the tour through the Grand Canyon, where Thurston and 25 other scientists gathered to study the gorge, each with their own research in mind.
Thurston traveled to the Grand Canyon to test the range of zircon-helium thermochronology, a technique that measures when a mineral cools when erosion brings it closer to the surface. The technique looks at the ratio of helium to uranium inside the mineral zircon to determine its cooling history. Zircon is a mineral commonly found in microscopic form throughout the gorge, Thurston said. She was made aware of the technique by her former graduate advisor, William Guenthner, a professor of geology at the University of Illinois and another co-author on paper. Guenthner has spearheaded efforts to expand the potential of the technique to deep geological time.
“This promising new ZHe thermochronology technique, which Dr. Guenther helped pioneer, is being tested in the Grand Canyon because of the richness of previous studies by UNM researchers and others,” Karlstrom said. “It supports the ‘young (
“You measure uranium and you measure helium [inside the zircon] and it gives you a date because you know how fast uranium decays to helium, “said Guenthner,” what Olivia has demonstrated is that this particular technique can describe both a really old event – erosion to create the great discrepancy of 1 , 2 billion years ago – and a relatively young event – the very carving of the gorge itself in the last 6 million years. “
The process is far from simple. Despite being crucial to the research, uranium complicates the calculations. One of the reasons zircon is used is its resistance to physical and chemical weathering, but its crystalline structure is damaged by the radiation emitted from uranium. This radiation damage affects how helium moves around the crystal.
“If the crystal is hot enough, helium can actively diffuse out of the crystal because it is not actually part of the crystalline structure,” Thurston said. that is, when the clock starts – and we can use the uranium / helium (parent / daughter) ratio to calculate the time in the past when it cooled through this temperature. “
When the zircon is deposited further below the earth’s surface, where temperatures are higher, helium is released quickly, Thurston said. As the minerals begin the excavation process, they are lifted to the surface, cooled, and the helium is captured. By looking at how much helium is retained in the crystalline structure of the zircon, Thurston and her team can determine when the cooling process began and when the rock reached the surface, where it remains to this day.
In terms of better understanding the sensitivity of this new method of reconstructing past, now eroded, landscapes, it is remarkable that these zircon grains have a “memory” of their earliest and most recent cooling episodes, but less memory of the intermediate billions years “said Karlstrøm. “What binds these results together is that large erosion surfaces that form today on the Colorado Plateau, and which also sloped the North American continent in the distant past to a flat continent near the sea surface as Australia is today – take place in pulses of Continental lift driven by ups and downs caused by distant plate tectonic events. “
According to Thurston’s research, the major excavation and erosion of the Great Unconformity in the eastern Grand Canyon took place about 1.25-1.35 billion years ago. This time frame precedes the dissolution of the supercontinent Rodinia (550-800 million years ago), an event that some theorists created the discrepancy.
But their findings suggest that the major discrepancy is “broadly related to” the Rodinia collection, which took place about 900 million to 1.3 billion years ago. Still, “a broader deep thermochronological transect across Laurentia is needed to fully understand the many mechanisms” that created the great discrepancy, according to Thurston.
“I think the exciting thing for the general public about this study is to think about how indifferent we are in geological time,” Thurston said. “Rocks in the Grand Canyon stretch back 1.8 billion years, and the Earth itself is 4.5 billion years old. It’s a very powerful and humiliating perspective to have when we think about the meaning or lifespan of humans in relation to The earth.”
“The great discrepancy, and how it was formed, are hot topics, and these results help us understand that the Cambrian explosion of hard-bodied wildlife (the trilobites) and the advance of oceans (called transgression) across North America for about “500 million years ago, a previous slope episode followed about 1.2 billion years ago. Together, re-tools and these events shaped this famous composite erosion surface,” Karlstrom added.
Geologists are digging into the Grand Canyon’s mysterious gorge in time
Olivia G. Thurston et al., Zircon (U-Th) / He thermochronology of Grand Canyon solves 1250 Ma detachment at Great Unconformity and Geology (2021). DOI: 10.1130 / G48699.1
Provided by the University of New Mexico
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