Ambient Noise Tomography of the Mantle and Upper Crust in the Northern East African Rift

The Northern East African Rift (EAR) is a unique location where we can observe different stages of rifting from continental to incipient seafloor spreading. Here we present a 3-D absolute shear wave velocity model of the crust and uppermost mantle of the Northern EAR generated from ambient noise tomography, utilising 12 years of seismic data. The model helps us to interpret crustal structure and how rifting modifies the lithosphere by tectonic and magmatic processes. We generate 4820 station pair correlation functions, from 170 stations, which were then inverted for phase velocity maps from 8 - 40 s period and finally for 3-D absolute shear velocity structure. In our model we infer the Moho as the 3.75 km/s contour and find the thinnest crust of 14 km in the Danakil depression, and the thickest crust of >40 km is found in the Ethiopian Plateau. We find the average shear velocity in the crust is higher in the Afar region (3.83 km/s ± 0.040) than the MER (3.60 km/s ± 0.035), although the Afar does have localised low velocity zones beneath active volcanic centres down to 3.4 km/s. We interpret these low velocity regions beneath volcanic centres (including the MER) as being due to magmatic intrusion and heating of the crust. In the mantle, shear velocity is everywhere lower than expected for a mantle peridotite composition (<4.1 km/s). We infer pervasive partial melt in the mantle across the region, with focused upwelling and melting beneath the MER, where the lowest velocities in the region (3.20 km/s ± 0.035) are found. Beneath the Ethiopian Plateau, the crustal velocities are laterally heterogeneous (variations > 0.50 km/s ± 0.045), suggesting a complex geological history and an inhomogeneous magma distribution during evolution. Directly comparing the MER and Afar allows us to draw conclusions between different stages of rifting. In particular, the MER shows the highest amplitude slow velocities early during the breakup process, suggesting magmatism peaks well before the transition to seafloor spreading.