According to new research from the University of Illinois at Urbana-Champaign, seemingly stable regions of Earth’s continental plates — the so-called stationary cratons — have repeatedly deformed beneath the crust since they formed in the distant past. This hypothesis contradicts decades of traditional plate tectonics theory and attempts to answer why most cratons remain structurally stable while their lower cores undergo significant changes.

In the study, led by Illinois geology professor Lijun Liu, the researchers used previously collected data on the density of the uppermost solid layers of the Earth’s crust and mantle, known as the lithosphere, to examine the relationship between the surface topography of cratons and thickness. from the lower layer of the lithosphere.

The results of the research were published in the journal Natural Geosciences .

The lack of deformation in cratons since their formation makes them the longest-lived tectonic units on Earth — they have survived supercontinent cycles such as the formation and breakup of the supercontinent Pangea and the lesser-known and older supercontinent Rodina, the study reports. .

“It is generally accepted that cratons are protected by buoyancy and thick subsoil mantle roots or spines, which are thought to be strong and therefore stable over time,” Lui said. Said.

Several recent papers from Liu’s research group directly challenge this wisdom by showing that these mantle wedges are actually quite dense.

In a 2022 study, the team showed that the conventional view of floating craton releases suggests that most cratons on Earth are about 3 kilometers above sea level, whereas in reality they are only a few 100 meters high. This would require the lithosphere mantle beneath the crust to be dense enough to sink the surface by about 2 kilometers, Liu said.

In another study, the team used measurements of the gravitational field to determine the density structure of craton wedges and found that the bottom of the mantle wedge is where high-density material is located, meaning a density profile that increases with depth below cratons.

In the new paper, the team shows that the lower part of the mantle spine tends to be repeatedly stripped from the highly dense and upper lithosphere, with uplifts of the mantle called plumes initiating the disintegration of the supercontinent. Exfoliated or exfoliated spines can return to the bottom of the lithosphere after being heated in the hot mantle.

“The whole process is similar to what happens in a lava lamp where cold material near the surface sinks and hot material near the bottom rises,” Liu said. Said.

This date of deformation is expressed in some of the more mysterious geophysical features observed in the lithosphere, the study reports.

“For example, repeated vertical deformation of the lower half of the mantle spine allows seismic waves that vibrate the rock vertically to travel faster compared to the upper half of the spine, which experiences less vertical deformation,” Liu said. Said.

The team also concluded that mantle delamination would lead to uplift and erosion of the craton surface.

“This is reflected in the strong dependence of crustal thickness on lithosphere thickness, an observation that had never been made before this study,” said Liu. “In particular, two major uplifts and erosions occurred when the supercontinents Rodinia and Pangea broke apart in the past, the first causing the so-called Great Discord; craton erosion. This is why we see fragments of ancient lower crust exposed on the surface of the craton today.”

Using numerical simulations, the team said how this mode of episodic deformation of the craton’s lower wedges survived the long geological history of the craton’s crust.

“We believe this new hypothesis of a craton lifestyle will fundamentally change people’s understanding of how continents develop and how plate tectonics works on Earth,” Liu said. Said. Source