東アフリカ地溝下におけるプルーム-リソスフェア相互作用の地質学的検討 A Geodynamic Investigation of Plume-Lithosphere Interactions Beneath the East African Rift
Tahiry A. Rajaonarison, D. Sarah Stamps, John Naliboff, Andrew Nyblade, Emmanuel A. Njinju
Journal of Geophysical Research Published: 27 March 2023
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.
- Geodynamic modeling suggests observed rift parallel motions influenced by the African Superplume
- Asthenospheric seismic anisotropy induced by northward flow associated with the African Superplume
- Horizontal mantle tractions driving surface deformation
Plain Language Summary
Geodynamicists are interested in the tectonic forces that cause continents to break-up. During continental rifting, surface deformation and the underneath mantle flow are usually perpendicular to the rift. In the case of the East African Rift (EAR), the largest continental rift on Earth, it has been demonstrated that its E-W extension is mostly driven by forces due to its high-topography, but some deformations are parallel northward to the rift. Here, we use 3-dimensional computer simulation to test if these anomalous deformations are driven by hot buoyant upwelling known as the African Superplume. Comparison of model results with measured surface deformation, measured with highly accurate Global Navigation Satellite System measurements, show that mantle northward flow associated with the African Superplume drive some of the rift parallel deformations in the EAR. Our results also suggest that most of the enigmatic rift parallel seismic anisotropy, nowadays observed by geophysicist beneath the EAR, can be explained by the large northward flow of the upper mantle from the African Superplume.