A new study led by researchers at the University of Texas at Austin has revealed that an ancient piece of Earth’s crust buried deep beneath the Midwest is actively drawing down large sections of today’s North American crust into the mantle – a process scientists are calling “cratonic thinning.”
Using seismic imaging, scientists observed massive, drip-like formations beneath the central US, suggesting that the lithosphere (Earth’s rigid outer layer) is thinning as materials from the base of the continent’s ancient core, or craton, sink into the mantle transition zone.
This is the first time researchers have potentially captured cratonic thinning in action, as detailed in a paper published in Nature Geoscience.
Why is the dripping happening?
Geodynamic models suggest that the massive underground “drips” beneath North America are likely caused by large-scale mantle flow, set in motion by the long-subducted Farallon slab now deep in the lower mantle. This ancient slab may be weakening the overlying lithosphere, making it prone to dripping into the mantle.
Researchers found that these drip-like structures extend about 640 kilometres (400 miles) underground and are located beneath a wide region including Michigan, Nebraska, and Alabama. Though concentrated in the Midwest, their geological impact could extend across much of North America.
The study links the formation of these drips to the Farallon plate — a once-active oceanic plate that subducted beneath North America along its western edge. Around 20 million years ago, as the Pacific plate moved forward, the Farallon plate fractured. The leftover slabs continued their descent into the mantle, gradually pulling chunks of the North American lithosphere with them.
What does the dripping area look like?
Researchers found that a large funnel-shaped zone beneath the Midwest is pulling rocks from across North America toward it horizontally before they sink into the Earth’s mantle. This process is causing parts of the continent to lose material from the underside of their crust.
“We made the observation that there could be something beneath the craton,” said lead author Junlin Hua, who conducted the research during a postdoctoral fellowship at UT’s Jackson School of Geosciences. “Luckily, we also got the new idea about what drives this thinning.”
Though the phenomenon is significant, the researchers emphasized there’s no immediate cause for concern—the dripping process occurs slowly and won’t visibly affect the landscape anytime soon. It is expected to stop eventually as the ancient tectonic plate fragment sinks deeper and loses its influence on the craton.
The study is especially valuable for geoscientists exploring the long-term evolution of continents. “This sort of thing is important if we want to understand how a planet has evolved over a long time,” said co-author Thorsten Becker, professor at the Jackson School. “It helps us understand how do you make continents, how do you break them, and how do you recycle them [into the Earth.]”
The finding emerged from a broader project led by Hua, now a professor at the University of Science and Technology of China. Using a full-waveform seismic tomographic model built with data from the EarthScope project, developed by co-author Stephen Grand and his team, the researchers gained new insights into the crust and mantle beneath North America.