Scientists have uncovered the source of unusual deformation in Earth’s largest continental rift

Computer simulations confirm that the African superelement causes unusual deformations and seismic anisotropy parallel to the fault detected beneath the East African Rift System.

Geophysicist Dr. This process relates to the elongation of the lithosphere, the hard outer layer of the Earth. As the lithosphere becomes tighter, the upper parts of the lithosphere undergo brittle changes, which leads to fractures in the rocks and earthquakes.

Stamp, who studies these processes using computer modeling and GPS To map surface motions with millimeter precision, he compares the different deformation patterns of the rifting continent by playing with silly putty.

“If you hit cellulite putty with a hammer, it can crack and break,” said Stamps, an associate professor in the Department of Earth Sciences, part of Virginia Tech’s College of Science. “But if you loosen it slowly, the silly putty expands. So on different time scales, the Earth’s lithosphere behaves in different ways.”

Whether expanding or breaking, the deformation that comes with a continental rift usually follows predictable directional patterns with respect to the rift: the deformation tends to be perpendicular to the rift. The East African Rift System, the largest continental rift system on Earth, has those vertical deformations. But after measuring the fault system with GPS instruments for more than 12 years, Stamps also noticed a deformation going in the opposite direction, parallel to the system’s faults. Her team at the Laboratory of Geodesy and Tectonic Physics worked to find out why.

Assistant Professor Dr. Sarah Stamps. Credit: Virginia Tech

In a recent study published in Journal of Geophysical Research, the team explored the processes behind the East African Rift System using 3D thermal modeling developed by the study’s first author, Tahiri Rajaonarisson, a postdoctoral researcher at New Mexico Tech who received a Ph.D. at Virginia Tech as a member of the Stamps Lab. His models showed that the parallel-rift deformation of the rift system is driven by the northward mantle flow associated with the Great African Surface, a massive mantle uplift rising from the depths of the Earth below southwest Africa and extending northeast across the continent, becoming shallower. Because it extends north.

Their findings, combined with insights from a study the researchers published in 2021 using Rajaonarisson modeling techniques, could help clarify the scientific debate about which plate driving forces dominate the East African Rift System, which explains both perpendicular deformation and parallel rifting. . The buoyancy forces in the lithosphere, the drag forces in the mantle, or both.

As a postdoctoral researcher, Stamps began observing the unusual, parallel rifting deformation of the East African System using data from GPS stations that measure signals from more than 30 satellites orbiting Earth, from about 25,000 kilometers away. Her observations have added a layer of complexity to the debate about what drives the fault system.

Some scientists theorize that the East African Rift is primarily driven by buoyancy forces in the lithosphere, which are relatively shallow forces attributed mainly to the high topography of the rift system, known as the African Superwell, and to density differences in the lithosphere. Others point to horizontal mantle drag forces, the deeper forces arising from interactions with the horizontally flowing mantle beneath East Africa, as the underlying driver.

the team Study 2021 He found through 3D computer simulations that the crack and its deformation could be driven by a combination of the two forces. Their models showed that buoyancy forces in the lithosphere were responsible for the more predictable vertical crack deformation, but these forces cannot explain the anomalous deformation parallel to the fault captured by Stamps’ GPS measurements.

In their newly published study, Rajaonarison once again used 3D thermomechanical modeling, this time to focus on the source of the deformations parallel to the crack. His models confirm that the African super plume is responsible for the unusual deformations as well as the seismic anisotropy parallel to the faulting observed under the East African Rift System.

Seismic anisotropy is the orientation or alignment of rocks in a specific direction in response to mantle flow, melt pockets, or pre-existing structural fabrics in the lithosphere, Stamps said. In this case, the rock alignment followed the northward direction of the large African mantle flow, indicating mantle flow as its source.

“We’re saying that mantle flow is not driving the east-west, orthogonal rift of some deformation, but may cause anomalous northward deformation parallel to the rift,” said Rajaonarisson. “We have confirmed previous ideas that buoyancy forces in the lithosphere are driving the rift, but we provide new insight that abnormal deformation can occur in East Africa.”

Knowing more about the processes involved in continental rifting, including the anomalous ones, will help scientists untangle the complexity behind continent breakup, something they’ve been trying for decades. “We are 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 rift,” Stamps said.

Reference: “A Geodynamic Investigation of Lithospheric and Feather Interactions Below the East African Rift” by Taheri A. Rajaonarison and Dr. Sarah Stamps, John Nalipov, Andrew Nibbled, and Emmanuelle A. Journal of Geophysical Research, Solid Earth.
doi: 10.1029/2022JB025800