Published in Nature Geoscience, the study marks the first time geoscientists may be witnessing this type of thinning in the continent's cratonic root as it occurs. Cratons are ancient, stable blocks of the Earth's crust that typically remain unchanged for billions of years. However, under specific conditions, parts of these cratons can become unstable and begin to deteriorate.
"We made the observation that there could be something beneath the craton," explained lead author Junlin Hua, who conducted the research during his postdoctoral work at UT Austin's Jackson School of Geosciences. "Luckily, we also got the new idea about what drives this thinning."
The root of the North American craton appears to be fragmenting beneath the U.S. Midwest, although researchers emphasize that the process is too slow to produce any immediate surface changes. Instead, it sheds light on the gradual evolution of tectonic plates and continental interiors. The culprit behind the dripping, the team suggests, may be lingering remnants of the Farallon Plate, a long-subducted oceanic plate.
These descending rock blobs were revealed through a newly constructed full-waveform seismic tomography model, part of a larger study led by Hua. This model, incorporating data from the EarthScope project, allows researchers to visualize the dynamic interactions between the continent's rigid crust and the underlying mantle.
Co-author Thorsten Becker, a professor in the Department of Earth and Planetary Sciences and the Institute for Geophysics at the Jackson School, noted the significance: "This sort of thing is important if we want to understand how a planet has evolved over a long time. It helps us understand how do you make continents, how do you break them, and how do you recycle them [into the Earth.]"
The model also pointed to the ongoing influence of the Farallon Plate. Although it currently lies about 600 kilometers beneath the surface, it continues to affect the flow of mantle material. This redirected flow appears to shear the base of the craton while releasing volatiles that further destabilize it.
Despite being localized in the Midwest, the thinning process seems to have a broader reach. "A very broad range is experiencing some thinning," Hua said, referring to the extensive North American craton that spans much of the United States and Canada.
In simulations, cratonic dripping only occurred when the Farallon Plate was included. Its removal halted the phenomenon, supporting the hypothesis that the ancient slab is still actively influencing North America's deep structure.
Although computer models have their limitations, Becker remarked that the results align well with seismic observations: "You look at a model and say, 'Is it real, are we overinterpreting the data or is it telling us something new about the Earth?'," he said. "But it does look like in many places that these blobs come and go, that it's [showing us] a real thing."
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