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Abstract:
The Ethiopian Rift is a unique natural environment to study the different stages of evolution from initial continental rifting to embryonic seafloor spreading. We used transdimensional hierarchical Bayesian seismic ambient noise tomography to first construct group velocity maps across the Afar, Main Ethiopian Rift, and the adjoining plateaus, and then inverted these for a shear wave velocity model. The uppermost mantle shear wave velocity ranges between 3.9 and 4.3 km/s, 5–15% lower than the upper mantle velocity in the PREM model. The combined effect of temperature and partial melt is needed to explain a 15% shear wave velocity reduction in the uppermost mantle. Tectonic and magmatic activities are not limited to the rift center, but instead are widespread within the upper crust beneath the Main Ethiopian Rift and Afar. The Main Ethiopian Rift is dominated by two velocity belts, the Wonji Fault Belt along the rift axis and the Silti-Debre Zeit Fault Zone on the western side of the Central Main Ethiopian Rift; the Boru-Toru structural high appears to serve as a transfer zone between them, exhibiting relatively high crustal velocities (3.6 km/s) at 14 km depth. Low velocities persist in the crust beneath the rift flanks and border faults, indicating that they are still tectonically and magmatically active. The crust beneath the western plateau is characterized by a low-velocity anomaly, implying that the plateau is also active. Low-velocity linear belts are further imaged beneath the western and eastern plateaus, away from the active rift axes. These off-axis belts could represent failed or buried rifts.
