Unusual deformation in Earth’s largest fissure could give clues to a continent breaking up

30 million years ago, a fault called the Great East African Rift Valley began to form in Africa. It stretches from the Afar region of Ethiopia to Mozambique, spanning over 6,000 kilometres, making it one of the largest cracks on the surface.

The Rift Valley was formed by the fissure that exists between two African tectonic plates: Somalia and Nubia. This rift is currently an incipient plate boundary, is mostly subterranean, and is over 3,500 kilometers long where the African continent is slowly disintegrating, creating new ocean basins and volcanic mountains.

The crack system It is a kind of natural laboratory to study the processes and mechanisms of continental break (continental rift), which involve the stretching and thinning of the lithosphere, which is the solid, rigid layer of the Earth that includes the crust and the upper part of the mantle. When this happens, shallow regions of the lithosphere experience brittle deformation, with rock breakage and earthquakes.

A recent study published in the Journal of Geophysical Research: Solid Earth , conducted by researchers at Virginia Tech and other institutions, explained the origin and significance of this unusual deformation in the East African Rift System using 3D thermomechanical models (developed by the study’s first author, Tahiry Rajaonarison) that simulate the effects of mantle flow on the stretching of the lithosphere.

In general, the deformation that accompanies continental rifting tends to be perpendicular to the rift. But after measuring the crack system with GPS instruments for more than 12 years, the team observed a deformation going in the opposite direction, parallel to the cracks in the system, which is quite unusual.

Computer models in the study showed that The African Super Plume is responsible for the unusual deformations as well as the rift-parallel seismic anisotropy observed beneath the East African Rift System.

The African Super Plume is a massive outcrop of the warm, floating mantle that rises from deep within the Earth beneath southwestern Africa and heads northeast across the continent, becoming shallower as it extends north.

On the other hand, seismic anisotropy is the alignment of rocks in a particular direction in response to mantle flow. This can reveal the orientation and shape of rock structures and minerals underground and thus provide clues to the history and dynamics of Earth’s deformation.

With its study, geophysics Mrs. Sarah Stamps associate professor at the Faculty of Sciences of Virginia Tech and postdoctoral researcher, added a layer of complexity to the debate about what drives the breaking system.

Stamps studies these processes using computer models and GPS to map surface movements with pinpoint accuracy. He began studying the unusual rifting-parallel deformation of the East African Rift System using data from GPS stations that measured signals from more than 30 satellites orbiting the Earth, about 25,000 kilometers away.

Their findings, combined with findings from a study that researchers published in 2021 using Rajaonarison’s 3D modeling techniques, could help clarify the scientific debate about the driving forces of the plates that dominate the East African Rift system, explaining both its perpendicular and parallel to the rift deformation: lithospheric buoyancy forces, mantle pulling forces, or both.

Lithospheric buoyancy forces are relatively shallow forces attributed primarily to the high topography of the rift system, known as the African Superswell, and density variations in the lithosphere. Some scientists believe that rifting in East Africa is primarily driven by these forces.

Others point to horizontal mantle pulls, the deeper forces resulting from interactions with the horizontally flowing mantle beneath East Africa, as the main driver, he says. Physical .

However, the 2021 study found that the crack and its deformation could be driven by a combination of the two forces. Their models suggested that lithospheric buoyancy forces were responsible for the more predictable deformation perpendicular to the fissure, but these forces could not explain the anomalous deformation parallel to the fissure detected by Stamps’ GPS measurements.

This time, the new study focused on the origin of the crack’s parallel deformations, confirming that the African Superpanache is responsible for the unusual deformations, as well as the observed seismic anisotropy parallel to the fissures.

In this case, the models showed that the northward mantle flow associated with the African super plume can generate parallel deformation in parts of the East African rift system by inducing shear stress and stress at the base of the lithosphere, according to Phys.

This shear stress and strain can rotate or align rock structures and minerals along the direction of mantle flow, creating seismic anisotropy parallel to the rift axis.

“We say that the mantle flow is not directing the direction perpendicular to the east-west rift of some of the deformations, but rather may be the cause of the anomalous deformation to the north parallel to the fissure.” “, Rajaonarison told Phys. “We confirm previous ideas that lithospheric buoyancy forces are driving the fault, but we offer a new perspective on the fact that anomalous deformations can occur in East Africa.

This deformation has important implications for understanding and predicting the dynamics and diversity of continental rifting. The results also showed that this parallel deformation affects the evolution and outcome of the continental breakup and has implications for improving climate predictions. .

“We are excited about this result of Dr. Rajaonarison’s numerical modeling because provides new insights into the complex processes shaping the Earth’s surface across the continental rift.” Stamps said Phys.

Discoveries about the processes involved in the break-up of continents, including anomalous processes, will provide clues for scientists to unravel the complexity behind the break-up of a continent, something they have been trying for decades.