The Wilson Tectonic Cycle
More than 40 years ago, pioneering tectonic geophysicist J. Tuzo Wilson published an article in the journal Nature describing how,
over Earth's long history, ocean basins opened and closed along North America's eastern seaboard. His observations, dubbed "The
Wilson Tectonic Cycle," suggested this process had occurred many times; most recently causing giant Pangaea to split into today's
Wilson's ideas were central to the so-called Plate Tectonic Revolution, the foundation for contemporary theories of processes
underlying mountain-building and earthquakes. Since his 1967 article, additional studies have confirmed that large-scale deformation
of continents repeatedly occurs in some regions but not others, though the reasons why remain poorly understood. New findings by Utah
State University geophysicist Tony Lowry and colleague Marta Pérez-Gussinyé of Royal Holloway, University of London shed surprising
light on these restless rock cycles.
The Role of Quartz
"It all begins with quartz," says Lowry, who published results of the team's recent study in the March 17, 2011 issue of Nature. In
"The Role of Crustal Quartz in Controlling Cordilleran Deformation," the scientists describe a new approach to measuring properties of
the deep crust that reveal quartz's key role in initiating the churning chain of events that cause the Earth's surface to crack, wrinkle,
fold and stretch into mountains, plains and valleys.
How Rocks Flow in Response to Stress
"Earthquakes, mountain-building and other expressions of continental tectonics depend fundamentally on how rocks flow in response to
stress," says Lowry, assistant professor in USU's Department of Geology and 2010 recipient of a National Science Foundation CAREER
Award. "We know that all tectonics are a response to the effects of gravity, but we know less about rock flow properties and how they
change from one location to another."
Wilson's theories provide an important clue, he says, as scientists have long observed that mountain belts and rift zones have formed
again and again at the same locations over long periods of time.
"Over the last few decades, we've learned that high temperatures, water and abundant quartz are all critical factors in making rocks
flow more easily," Lowry says. "But until now, we haven't had the tools to measure these factors and answer long-standing questions."
The Earthscope Transportable Array
Since 2002, the NSF-funded Earthscope Transportable Array of seismic stations across the western United States has provided valuable
remote sensing data about the continental crust's rock properties.
"We've combined the Earthscope data with other geophysical measurements of gravity and surface heat flow in an entirely new way that
allows us to separate out the effects of temperature, water and quartz in the crust," Lowry says.
The Velocity of Sound and Shear Waves
The Earthscope measurements have enabled the team to estimate the thickness, along with the seismic velocity ratio, of continental crust
in the American West. Seismic velocity describes how quickly sound waves and shear waves travel through rock, he says, offering clues to its temperature and composition.
"By themselves, seismic velocities are sensitive to both temperature and rock type," Lowry says. "But if the velocities are combined as a ratio,
the temperature dependence drops out. We found that the velocity ratio was especially sensitive to quartz abundance."
Weak Quartz-Rich Crust
But even after separating out the effects of temperature, the scientists found that a low seismic velocity ratio, indicating weak,
quartz-rich crust, systematically occurred in the same place as high lower-crustal temperatures modeled independently from surface heat flow.
"That was a surprise," he says. "We think this indicates a feedback cycle, where quartz starts the ball rolling."
If temperature and water are the same, Lowry says, rock flow will focus where the quartz is located because that's the only weak link.
Once the flow starts, the movement of rock carries heat with it and that efficient movement of heat raises temperatures, resulting in weakening of crust.
"On top of that, rock, when it warms up, is forced to release water that's otherwise chemically bound in crystals," he says.
Water further weakens the crust, which increasingly focuses the deformation in a specific area.
Belts of Quartz and Mountain-Building
"If you've ever traveled westward from the Midwest's Great Plains toward the Rocky Mountains, you may have wondered why the flat plains
suddenly rise into steep peaks at a particular spot," Lowry says. "It turns out that the crust beneath the plains has almost no quartz
in it, whereas the Rockies are very quartz-rich. We think those belts of quartz could be the catalyst that sets the mountain-building rock cycle in motion."
| Mountain chains in the Western United States cover a broad zone of nearly 2,000 kilometers. Image by Tony Lowry, Utah State University.
|New measurements of quartz abundance from EarthScope data show that mountains are quartz-rich (red-orange colors). The Great Plains, Columbia Basin and Great Valley have little or no quartz (blue-green colors). Image by Tony Lowry and Marta Pérez-Gussinyé.
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