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Groundbreaking Study Reveals Cratonic Thinning Beneath North America
Recent investigations by researchers at the University of Texas at Austin have unveiled a striking geological phenomenon occurring beneath the North American continent, termed “cratonic thinning.” This process, previously undocumented in real time, concerns the gradual erosion of ancient rock formations, or cratons, that form the foundational components of the continents. The findings have been detailed in the journal Nature Geoscience.
Cratons are typically stable structures that can endure for billions of years. However, certain geological actions can lead to the thinning or even disappearance of extensive rock layers. This phenomenon has historical precedent, including an example noted in the deep root layers of the North China Craton millions of years ago. Yet, until now, the direct observation of cratonic thinning has remained elusive.
Seismic waves exhibit varying velocities as they traverse different geological materials. This seismic map indicates the speed of seismic waves in the Earth’s crust at a depth of 200 kilometers across the continental United States, highlighting the North American craton (indicated by black dashes) which shows significantly higher seismic velocity compared to adjacent areas. Image Credit: Hua et al., Nature Geoscience.
The research team, led by geoscientist Junlin Hua, was engaged in developing a full-waveform seismic tomographic model for North America when they identified unusual behaviors at the boundary between the Earth’s deep mantle and its lighter lithosphere.
Hua remarked on their discovery, stating, “We observed potential anomalies beneath the craton. Fortunately, this led us to propose a new mechanism driving the thinning.”
Their findings indicate that the Farallon Plate, an oceanic tectonic slab located beneath a significant portion of the Pacific Ocean, might play a role in this phenomenon. The remnant of the plate has been subducting beneath North America for approximately 200 million years, and while it lies about 370 miles below the craton, it appears to influence the flow of mantle materials, causing melting at the base of the craton. This melting process potentially releases volatile compounds that further weaken the structural integrity of the craton.
“Thinning is being observed across a broad geographical range,” Hua added.
A visual excerpt from the study demonstrates the dripping of rock material from the craton, attributed to the remnants of the subducting Farallon Plate. Image Credit: Hua et al., Nature Geoscience.
To validate their hypothesis, the researchers conducted simulations that included and excluded the Farallon Plate. The analysis revealed that cratonic dripping only occurred when the plate was factored into the model.
Study co-author Thorsten Becker explained the significance of their findings, stating, “When we analyze the models, we must consider whether our interpretations are valid or if they offer new insights into Earth’s underlying processes. The evidence suggests that these phenomena are genuine and observable.”
He elaborated on the broader implications of such discoveries, emphasizing their importance for understanding the geological history and evolution of Earth. “These insights elucidate the processes through which continents are formed, eroded, and recycled within the planet’s framework,” Becker stated.
While the main areas of activity associated with this cratonic thinning appear to be under the Midwest U.S., the influence of such geological processes extends over millions of years and could affect tectonic stability in distant locales. As the Farallon Plate’s remnants continue to descend into the mantle, its impact on the North American craton is expected to diminish, potentially halting the dripping process.
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