Computer models confirm that the African Superplume is responsible for
the unusual deformations as well as rift-parallel seismic anisotropy observed beneath the East African Rift System

Date:
June 13, 2023
Source:
Virginia Tech
Summary:
Computer models confirm that the African Superplume is responsible
for the unusual deformations, as well as rift-parallel seismic
anisotropy observed beneath the East African Rift System.


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==========================================================================
FULL STORY ========================================================================== Computer models confirm that the African Superplume is responsible for
the unusual deformations as well as rift-parallel seismic anisotropy
observed beneath the East African Rift System.

In continental rifting, there's a mix of stretching and breaking
that reaches deep into the Earth, said geophysicist D. Sarah
Stamps. Continental rifting involves the stretching of the lithosphere --
the outermost, rigid layer of the Earth. As the lithosphere stretches
thin, its shallow regions experience brittle deformation, with the
breaking of rock and earthquakes.

Stamps, who studies these processes by using computer modeling and GPS
to map surface motions with millimeter precision, compares a rifting continent's different deformation styles with playing with 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." Whether in stretching
or breaking, the deformation that comes with continental rifting usually follows predictable directional patterns in relation to the rift: The deformation tends to be perpendicular to the rift. The East African Rift System, the Earth's largest continental rift system, has those rift- perpendicular deformations. But after measuring the rift system with GPS instruments for more than 12 years, Stamps also observed deformation that
went in the opposite direction, parallel to the system's rifts. Her team
at the Geodesy and Tectonophysics Labhas worked to find out why.

In a recent study published in theJournal of Geophysical Research, the
team explored the processes behind the East African Rift System using
3D thermomechanical modeling developed by the study's first author,
Tahiry Rajaonarison, a postdoctoral researcher at New Mexico Tech who
earned his Ph.D.

at Virginia Tech as a member of Stamps's lab. His models showed that the
rift system's unusual, rift-parallel deformation is driven by northward
mantle flow associated with the African Superplume, a massive upwelling
of mantle that rises from deep within the Earth beneath southwest Africa
and goes northeast across the continent, becoming more shallow as it
extends northward.

Their findings, combined with insights from a study the researchers
published in 2021 using Rajaonarison's modeling techniques, could help
clear up scientific debate on which plate-driving forces dominate the
East African Rift System, accounting for both its rift-perpendicular and rift-parallel deformation: lithospheric buoyancy forces, mantle traction forces, or both.

As a postdoctoral researcher, Stamps began observing the East African
Rift System's unusual, rift-parallel deformation using data from GPS
stations that measured 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 around what drives the rift system.

Some scientists see the rifting in East Africa as driven primarily
by lithospheric buoyancy forces, which are relatively shallow forces
attributed mainly to the rift system's high topography, known as the
African Superswell, and to density variations in the lithosphere. Others
point to horizontal mantle traction forces, the deeper forces arising
from interactions with mantle flowing horizontally beneath East Africa,
as the primary driver.

The team's 2021 study found through 3D computational simulations that
the rift and its deformation could be driven by a combination of the
two forces. Their models showed that lithospheric buoyancy forces were responsible for the more predictable, rift-perpendicular deformation,
but those forces couldn't account for the anomalous, rift-parallel
deformation picked up by Stamps's GPS measurements.

In their newly published study, Rajaonarison again used 3D
thermomechanical modeling, this time to focus on the source of the rift-parallel deformations.

His models confirm that the African Superplume is responsible for the
unusual deformations as well as 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
pre-existing structural fabrics in the lithosphere, Stamps said. In
this case, the rocks' alignment followed the direction of the African Superplume's northward mantle flow, which suggests mantle flow as
their source.

"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." Learning more about the processes involved in continental rifting, including these anomalous ones, will help scientists
chip away at the complexity behind the breaking of a continent, which
they've been attempting for decades. "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.

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========================================================================== Story Source: Materials provided by Virginia_Tech. Original written by
Suzanne Irby. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Tahiry A. Rajaonarison, D. Sarah Stamps, John Naliboff, Andrew
Nyblade,
Emmanuel A. Njinju. A Geodynamic Investigation of
Plume‐Lithosphere Interactions Beneath the East African
Rift. Journal of Geophysical Research: Solid Earth, 2023; 128 (4)
DOI: 10.1029/2022JB025800 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/06/230613190839.htm

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