Scientists have discovered the source of unusual deformation along Earth’s largest continental rift

Computer simulations confirm that the African superplume causes the unusual deformations and rift-parallel seismic anisotropy detected below the East African Rift System.

According to geophysicist D. Sarah Stamps explains. This process is associated with the stretching of the lithosphere, the Earth’s solid outer layer. Because it is so tight, the upper parts of the lithosphere undergo brittle changes, leading to rock fractures and earthquakes.

Seals will study these processes using computer modeling and GPS Playing with Silly Putty compares the different deformation styles of a rifting continent to map surface movements with millimeter precision.

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

In extension or rupture, deformation accompanying continental rifting generally follows predictable directional patterns relative to the rift: deformation is perpendicular to the rift. The East African Rift System, the largest continental rift system on Earth, contains those rift-perpendicular faults. But after more than 12 years of measuring the rift system with GPS instruments, Seals also noticed deformation running in the opposite direction parallel to the system’s rifts. His team at the Geodesy and Tectonophysics Laboratory worked to find out why.

Assistant Professor D. 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 through 3D thermomechanical modeling developed by first author Tahiry Rajaonarison, a postdoctoral researcher at New Mexico Tech who received his Ph.D. He is a member of the Stamps Laboratory at Virginia Tech. His models show that the unusual, rift-parallel deformation of the rift system is driven by northward mantle flow associated with the African superplume, which rises from deep within the Earth beneath southwest Africa and moves northeast across the continent to become more shallow. It stretches towards the north.

Their findings, combined with insights from a 2021 study published by researchers using Rajaonarisan’s modeling techniques, will help clarify the scientific debate about which plate-driving forces dominate the East African Rift System: lithospheric buoyancy forces, mantle drag forces, or both.

As a postdoctoral researcher, Seals studied the unusual, rift-parallel deformation of the East African Rift System using data from GPS stations and measuring signals from more than 30 Earth-orbiting satellites from 25,000 kilometers away. His observations add a layer of complexity to the debate about what drives the rift system.

Some scientists see the rifting in East Africa as being driven primarily by lithospheric buoyancy forces, which are relatively shallow forces mainly due to the high topography of the rift system known as the African superswell and density variations in the lithosphere. Others point to horizontal mantle drag forces, deep forces arising from contact with the horizontally flowing mantle beneath East Africa, as the primary driver.

of the team 2021 study It was found through 3D computational simulations that the fission and its rupture may be driven by a combination of two forces. Their models showed that lithospheric buoyancy forces were responsible for the more predictable, rift-perpendicular deformation, but those forces could not account for the anomalous, rift-parallel deformation picked up by GPS measurements of the seals.

In the newly published study, Rajonarison again used 3D thermomechanical modeling, this time focusing on the source of cleavage-parallel deformations. His models confirm that the African superplume is responsible for the unusual deformations and rift-parallel seismic anisotropy observed beneath the East African Rift System.

Seismic anisotropy is the orientation or alignment of rocks in a particular direction in response to mantle flow, melt pockets or preexisting structural fabrics in the lithosphere, Stamps said. In this case, the alignment of the rocks followed the direction of the northward mantle flow of the African superplume, suggesting mantle flow as their source.

„We suggest that mantle flow is not driving the east-west, rift-perpendicular direction of some of the deformation, but it may be causing irregular northward deformation parallel to the rift,” Rajonarisen said. „We confirmed previous ideas that lithospheric buoyancy forces drove rifting, but we bring new insight that irregular metamorphism may have occurred in East Africa.”

Learning more about the processes involved in continental rifting, including these paradoxes, will help scientists think through the problem behind the breakup of a continent that scientists have been trying to figure out for decades. „We are excited about this result from Dr. Rajaonarisan’s numerical modeling because it provides new information about the complex processes that shape the Earth’s surface through continental rifting,” said Stamps.

Note: Tahiri A. Rajaonarisan, D. Sarah Stamps, John Nalipoff, Andrew Nyblade, and Emmanuel A. Njinju, 27 March 2023, “A Geodynamic Study of Plume-Lithosphere Interactions Beneath the East African Rift” Journal of Geophysical Research Solid Earth.
DOI: 10.1029/2022JB025800