Scientists finally crack the reason behind unusual deformation in Earth's crust

“If you hit Silly Putty with a hammer, it can actually crack and break,” said Sarah Stamps, associate professor in the Department of Geosciences, in an official release. “But if you slowly pull it apart, the Silly Putty stretches. So on different time scales, Earth’s lithosphere behaves in different ways.”

However, the EAR displays strange deformity. The study finds that EAR has both perpendicular and parallel deformation to plate movement. 

It was previously considered that the East African Rift System had solely rift-perpendicular deformations. However, after analyzing the rift system with GPS sensors for almost a decade, the geologists discovered deformation that ran in the other way, parallel to the rifts. 

“The rift system’s unusual, rift-parallel deformation is driven by northward mantle flow associated with the African Superplume, a massive upwelling of the mantle that rises from deep within the Earth beneath southwest Africa and goes northeast across the continent, becoming more shallow as it extends northward,” explains the official release. 

3D thermomechanical modeling helped crack this mystery 

The scientists used 3D thermomechanical simulation to investigate the fundamental dynamics of the East African Rift System. 3D thermomechanical modeling helped to pinpoint the source of rift-parallel deformations. 

Moreover, the unusual rift-parallel deformation was also analyzed using data collected from GPS stations on more than 30 satellites circling Earth from around 25,000 kilometers.

The authors highlight that the finding is significant because it adds to our understanding of the intricate processes that change the Earth's surface through continental rifting.

The results have been reported in the Journal of Geophysical Research. 

Study abstract:

The force balance that drives and maintains continental rifting to breakup is poorly understood. The East African Rift (EAR) provides an ideal natural laboratory to elucidate the relative role of plate driving forces as only lithospheric buoyancy forces and horizontal mantle tractions act on the system. Here, we employ high-resolution 3D thermomechanical models to test whether: (a) the anomalous, rift-parallel surface deformation observed by Global Navigation Satellite System (GNSS) data in the EAR are driven by viscous coupling to northward mantle flow associated with the African Superplume, and (b) the African Superplume is the dominant source mechanism of anomalous rift-parallel seismic anisotropy beneath the EAR. We calculate Lattice Preferred Orientations (LPO) and surface deformation from two types of mantle flow: (a) a scenario with multiple plumes constrained by shear wave tomography and (b) a single superplume model with northward boundary condition to simulate large-scale flow. Comparison of calculated LPO with observed seismic anisotropy, and surface velocities with GNSS and plate kinematics reveal that there is a better fit with the superplume mantle flow model, rather than the tomography-based (multiple plumes) model. We also find a relatively better fit spatially between observed seismic anisotropy and calculated LPO with the superplume model beneath northern and central EAR, where the superplume is proposed to be shallowest. Our results suggest that the viscous coupling of the lithosphere to northward mantle flow associated with the African Superplume drives most of the rift-parallel deformation and is the dominant source of the first-order pattern of the observed seismic anisotropy in the EAR.