December 12 2019
Something was nagging at William Hawley, and it was more about what was missing than what was there.
As a Ph.D. student at the University of California, Berkeley, Hawley is fascinated with the geologic complexities of the Cascadia subduction zone, a giant fault off the coast of the Pacific Northwest. There, the Juan de Fuca tectonic plate plunges under the North American plate, building strain throughout the region and prompting fears of the massive earthquake that could strike when it releases.
Hawley, however, was distracted by a peculiar region below central Oregon where it appears that a chunk of the Juan de Fuca plate is missing. He wasn't the first to notice this gap, but it nagged at the back of his brain. Such a feature must be affecting the surface—and he wanted to know how.
Now, he might finally have an answer. In a study recently published in Geophysical Research Letters, Hawley and his coauthor Richard Allen suggest that the missing piece is not just a hole, but a tear that's slowly splitting the plate apart at least 93 miles beneath the surface.
"I know that this seems like this little tear is a long way down," Hawley says. But the presence of such a feature could be the root of a number of hazards at the surface, helping scientists more accurately identify future dangers. For instance, as the southern limb of the split rotates away, its motion could be the cause of strong earthquakes that rattle off the coast of southern Oregon and northern California, the new model suggests. Such a tear could also explain a string of curious volcanism that swoops across a broad swath of Oregon.
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What's more, the study is giving scientists a peek into the final moments of a tectonic plate's life. The Juan de Fuca is one of the few remaining fragments of the once mighty Farallon plate, which North America began languidly consuming some 180 million years ago as the supercontinent Pangea broke apart. What happens when such plates are swallowed up is still unknown—but it's a fate that awaits all of the planet's oceanic plates.
"What we are looking at right now is the death of an oceanic plate," Hawley says. (Find our what might happen when Earth's tectonic plates grind to a halt.)
Other scientists are greeting the new model with curiosity and excitement, but they also caution that it needs more testing before it can become geologic canon.
"I think there are certainly good ideas," says Martin Streck, a volcanologist at Portland State University who specializes in the geologic activity of the Pacific Northwest. "But we may discuss this for some years to come."
Earth's crust is fractured into an interlocking network of tectonic plates, whose slow-motion dance has shaped the surface of our planet. They grind against one another, pulling apart at some edges and colliding in others. These collisions often form what are known as subduction zones, where oceanic plates take the plunge while continental plates ride high.
Researchers once thought that at any given subduction zone, the diving plate simply curls down into the deep, dropping in a sheet like a curtain, Hawley explains. But scientists have realized that just isn't the case, based on the data gathered from an increasing number of instruments imaging Earth's innards—including the thousands of seismometers of the National Science Foundation's EarthScope project, as well as the onshore and underwater arrays of the Cascadia Initiative, all of which were used in this latest study.
Now, we know that as one plate dips under another, it can deform, warping and breaking in unexpected ways. How this tortured fate affects the surface, however, is not always clear, particularly as a tectonic plate nears its end. The dying Juan de Fuca plate is a perfect opportunity to study such impacts, in part because it's not going quietly into the deep, so scientists are closely monitoring the system's every shiver and burp. (Here's how we found out that a powerful earthquake split a tectonic plate in two.)
What we are looking at right now is the death of an oceanic plate. WILLIAM HAWLEY, UNIVERSITY OF CALIFORNIA, BERKELEY To study the plate's subterranean gap, Hawley and Allen first needed to confirm that it really exists and was not just some artifact hiding in the data. The duo constructed a high-resolution look into the subsurface by mapping out the different speeds of seismic waves that 217 earthquakes sent rippling through the region. The speeds of these waves change depending on the temperature and composition of the rock, so they were able to "see" the colder, denser oceanic plate as it sinks into Earth and confirm that part of it is indeed missing.
With this new high-resolution data on the gap, the duo are effectively saying, "No, really, really. It's really there," quips seismologist Lara Wagner of the Carnegie Institution for Science, who has extensively studied the Farallon plate.
Another piece of the puzzle clicked into place when Hawley and Allen discovered that the position of the tear aligns with a previously identified zone of weakness known as a propagator wake, which cuts through the Juan de Fuca plate exposed at the seafloor and likely continues as it descends into the mantle. The researchers posit that the subducted plate is tearing along this zone of weakness as the southern section of the plate rotates in a clockwise direction, slowly splitting from the northern section of the plate. (Find out how a peeling tectonic plate may one day cause the Atlantic Ocean to shrink.)
"You're essentially unzipping these two plates that used to be together," Wagner says. "But it was a weak zipper—and that zipper is that propagator wake."
The motion of this split and twist could also explain the distortion seen—and felt—at the surface off the coast of southern Oregon and northern California, Hawley explains. This southern zone of the Juan de Fuca plate is riddled with earthquake-prone faults—exactly like what you'd expect from the newly proposed model.
The final piece of the puzzle is the volcanism. A string of volcanoes called the High Lava Plains stretches across southern Oregon and spouts an odd combination of magmas: Some are silica-rich, forming light-colored rocks known as rhyolite, while others are rich in magnesium and iron, forming jet-black rocks known as basalts. A tiny amount also has a composition in between these magma types.
Megadroughts could return to southwestern U.S. The new model presents another compelling possibility: Intriguingly, the youngest zone of the High Lava Plains sits just above the tip of the tectonic tear. As this tip propagated through the plate's weak zone toward the west, mantle upwelling might have melted the crust above, resulting in explosions of rhyolitic magma that follow the age progression seen at the surface.
"I think it's a really interesting paper, and it's really cool seeing another explanation for something that's anomalous," says Matthew Brueseke, an igneous petrologist at Kansas State University. But the model still needs to incorporate more information from the chemistry of the rocks, he says. Brueseke is particularly curious to see which trace elements are locked in the rocks, and how that might compare to other regions that sit on top of proposed slab tears. (Learn more about how volcanoes commonly form around subduction zones.)
For example, past studies hint that similar rips might lurk under the Caribbean island of Hispaniola, as well as near the juncture between the volcanoes of Russia's Kamchatka Peninsula and Alaska's Aleutian Islands. By comparing the chemistry of rocks in a range of regions, scientists might get a better grip on their magmatic origins.
The new model also can't explain everything about the High Lava Plains, Streck adds. Even if the tear can account for the rhyolitic rocks' decrease in age from east to west, the basaltic rocks don't follow a similar trend, with that type of magma seeming to have erupted willy-nilly across the region.
Still, the latest work presents a compelling possibility for what might be lurking beneath Oregon—and it probes vital questions about the basic processes behind Earth's geologic churn.
"In many ways, when we're looking at these things, we're looking back in time," Wagner says. "If we don't understand how those processes work[ed] in the past, where we can see the whole story and study it, then our chances of seeing what's happening today and understanding how that might evolve in the future are zero."
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Photo: Scientists on board the R/V Thomas G. Thompson recover a seismometer that had been recording earthquakes on the seafloor off the Pacific Northwest coast. Scientists used this information to confirm the presence of a tear in the Juan de Fuca tectonic plate under central Oregon. PHOTOGRAPH BY WILLIAM HAWLEY