Continental rifting happens when Earth's rigid outer shell begins to stretch and thin. That outer shell, called the lithosphere, includes the crust and the uppermost part of the mantle.

As the lithosphere is pulled apart, it can behave differently depending on how quickly and where the stress is applied. Near the surface, rocks can fracture, producing faults and earthquakes. Deeper down, hotter material can deform more gradually.

Geophysicist D. Sarah Stamps compares those different behaviors to Silly Putty.

"If you hit Silly Putty with a hammer, it can actually crack and break," said Stamps, associate professor in the Department of Geosciences, part of the Virginia Tech College of Science. "But if you slowly pull it apart, the Silly Putty stretches. So on different time scales, Earth's lithosphere behaves in different ways."

In most continental rifts, deformation follows a relatively predictable pattern. The strongest movement generally occurs perpendicular to the rift, in the direction the crust is being pulled apart.

The East African Rift System, the largest continental rift system on Earth, does show that expected rift perpendicular deformation. But Stamps found something else after more than 12 years of GPS measurements. Parts of the region were also deforming in a direction parallel to the rift itself.

That unexpected motion became a central mystery for her team at the Geodesy and Tectonophysics Lab.

A Giant Superplume Beneath Africa

In a study published in the Journal of Geophysical Research, the researchers used 3D thermomechanical models to investigate what could be producing the unusual deformation.

The modeling was developed by first author Tahiry Rajaonarison, a postdoctoral researcher at New Mexico Tech who earned his Ph.D. at Virginia Tech while working in Stamps's lab.

The simulations indicate that the rift parallel motion is linked to northward mantle flow associated with the African Superplume.

The African Superplume is an enormous zone of rising mantle material that begins deep beneath southwest Africa and extends northeastward across the continent. As it travels north, it becomes progressively shallower beneath the surface.

According to the models, this deep mantle flow can account for deformation that does not fit the simpler pattern expected from a continent being stretched apart.

Two Forces May Be Shaping the Rift

The findings also add new evidence to a long-running debate over what is driving the East African Rift System.

Scientists have generally focused on two main possibilities: lithospheric buoyancy forces, mantle traction forces, or some combination of the two.

Lithospheric buoyancy forces act relatively close to the surface. They are influenced by differences in elevation and density within the lithosphere. In East Africa, one important feature is the African Superswell, a broad region of unusually high topography.

Mantle traction forces originate deeper inside Earth. They result from the movement of mantle material beneath the lithosphere and the forces that flowing mantle can exert on the rigid plate above it.

Stamps first began documenting the unusual rift parallel motion while working as a postdoctoral researcher. She used GPS stations that received signals from more than 30 satellites orbiting Earth from roughly 25,000 kilometers away.

Those measurements were precise enough to track surface motion at the millimeter scale.

Her observations complicated the existing picture because shallow buoyancy forces could explain much of the expected motion across the rift, but not the movement running along it.

Earlier Models Pointed to a Combination

In a 2021 study, the team used 3D computational simulations to test how the two sets of forces might interact.

Those models suggested that both could be important.

Lithospheric buoyancy forces appeared to account for the more familiar deformation running perpendicular to the rift. However, they could not reproduce the anomalous deformation that Stamps had measured running parallel to it.

That left researchers searching for another mechanism.

In the newer study, Rajaonarison again used 3D thermomechanical modeling, this time concentrating specifically on the unexplained rift-parallel deformation.

The models showed that northward mantle flow connected to the African Superplume could produce the observed motion.

They also reproduced another important feature beneath the rift: rift-parallel seismic anisotropy.

What Seismic Anisotropy Reveals

Seismic anisotropy refers to a situation in which seismic waves travel differently depending on the direction they move through rock.

That can happen when minerals and rock structures become aligned. The alignment may be produced by mantle flow, pockets of melt, or older structures already present in the lithosphere.

In this case, the orientation of the rocks matches the direction of the African Superplume's northward mantle flow.

That agreement provides another clue that deep mantle movement is influencing the unusual deformation observed at the surface.

"We are saying that the mantle flow is not driving the east-west, rift-perpendicular direction of some of the deformations, but that it may be causing the anomalous northward deformation parallel to the rift," Rajaonarison said. "We confirmed previous ideas that lithospheric buoyancy forces are driving the rift, but we're bringing new insight that anomalous deformation can happen in East Africa."

A More Complex Picture of Continental Breakup

Taken together, the studies suggest that no single force is responsible for everything happening in the East African Rift System.

Shallower lithospheric buoyancy forces appear to play a major role in the more typical stretching of the rift. At the same time, deeper mantle flow associated with the African Superplume may be responsible for the unusual northward deformation and the seismic signature beneath it.

Understanding how those forces interact is important because the East African Rift provides scientists with a natural laboratory for studying how continents begin to break apart.

Researchers have spent decades trying to understand the full chain of processes involved in continental rifting, from deep-mantle motion to cracking and earthquakes at the surface.

"We're excited about this result from Dr. Rajaonarison's numerical modeling because it provides new information about the complex processes that shape the Earth's surface through continental rifting," Stamps said.