Geosciences graduate student Chad Martin collected rocks in Utah to understand how the Willard Thrust Fault formed. Photo courtesy of Chad Martin.When rocks move along a fault plane, they often grind along each other as solids in what’s known as “brittle deformation.” When they’re moving against each other deep beneath the Earth’s surface, however, the enormous pressure and heat causes the rocks to squish together instead. It’s called a “shear zone.”
But along the Willard Thrust Fault in Utah, there’s evidence of rock shearing even though the rocks weren’t deep enough to be in a shear zone.
Chad Martin wants to know why, and he thinks the answer lies in water and microscopic grains of quartz.
“What we’re thinking is that microfractures formed in these grains. There was normal brittle deformation. There was enough pressure for some these grains to start cracking and fracturing,” he said. “That allowed water to get into these grains and become part of their crystal structure.”
When minute amounts of water – on the scale of parts per billion – enters those cracks, it can cause hydrolytic weakening. Instead of cracking, the grains of quarts deform, acting like Play- Doh as they move.
Martin is working toward his master’s degree in geosciences at UWM. As part of his research, he’s spent the last year collecting quartz samples from the Willard Thrust Fault and examining them for evidence of microfractures. If he can find them, and find signs of water, he’ll be well on his way to proving his theory.
There are three steps Martin follows to look for fractures. First, he grinds the rock down until it’s 30 microns thick so he can look at it under a petrographic microscope to identify strain features which show how the quartz grains have deformed. The second step is to look at those grains under a scanning electron microscope with a cathodoluminescence attachment.
“It allows you to see the microfractures that you couldn’t see any other way,” Martin says as he points to an image of a quartz grain on his computer. It’s crisscrossed by tiny black lines, which are microfractures that opened in the rock but subsequently “healed” and disappeared from normal view.
The last step is a doozy: Martin has to travel to California to use a Fourier-transform infrared (FTIR) beam microscope at Berkeley National Lab’s Advanced Light Source (ALS). Berkeley’s FTIR microscope is extremely powerful because it uses light from a synchrotron.
“A synchrotron, the way it has been described to me, is basically a particle accelerator but for light,” Martin said. “So they have a center ring where they’re shooting light around it and they can peel off individual wave lengths and send it to instruments at very high power.
What it means for my work is very high precision. I can get a very fine beam out of it so I can map very small things.”
When those individual wavelengths of light hit certain anomalies along the quartz microfractures – like, say, the water molecules that Martin is searching for – it registers as a “peak” that tells Martin he’s been successful.
“What we’re trying to do is show evidence of water along those healed microfractures that you can’t see any other way, and see if these lines that I get on the FTIR can match up with the fractures,” Martin said.
So far, his research has yielded promising results, but it is slow going.
Creating a map of just a quarter of a single quartz grain can take up to 12 hours, and Martin can only use Berkeley’s FTIR for 72 hours at a time. Even so, he’s found evidence of water along microfracture lines that lend credence to his hypothesis that water caused the shearing in the Willard thrust fault.
That’s important because the Earth is still moving. By understanding how geologic structures formed in the past, scientists might gain a better understanding of how the Earth might change in the future.
Martin’s research is also important because he’s pioneered a new technique for examining rock samples. Usually, geologists use a mirror-finish and an epoxy to fix rock samples to microscope slides, but the epoxy would have thrown off the readings of the FTIR microscope. Instead, Martin used a heat-removable epoxy and, after he warmed up his rock samples on slides on a hot plate, took them off the slides and cleaned them in an acetone bath.
“This is a viable method,” Martin said. “I showed it was possible.”
Martin will present his findings at a conference for the Geological Society of America in Phoenix, Arizona, later this month.
Originally appeared in College of Letters & Science In Focus magazine, September 2019.