Investigation into the Localization of Rifting within the Northern Main Ethiopian Rift using Rayleigh Waves and Receiver Functions

Abstract:

The East African Rift System is an intracontinental rift zone that exhibits continental thinning through extensional rifting. Rift-driven tectonic and magmatic processes create complex subsurface structure. We measure Rayleigh wave phase velocities from 20-180 seconds, and P-s converted phase Receiver Functions (RF’s) to interrogate the crust and upper mantle structure of this region. We incorporate 354 seismic stations deployed from 2000-2016 to generate spatially complete phase velocity maps from rift to flank. We develop P-s RF’s for 138 stations using a density-based clustering algorithm, and use these RF’s to calculate Moho depths and Vp/Vs ratios through H-Kappa stacking.

Slow phase velocities are observed in the Main Ethiopian Rift (MER), Afar, and the northwestern rift flank near Lake Tana. Phase velocity reduction may result from elevated temperatures or the presence of melt. Subsurface melt is unsurprising given the presence of Holocene surficial volcanic features and upwards of 94 km of extension across Ethiopia since MER initiation 18 Ma, at current extension rates of 5.2 mm/yr. The northwestern rift flank exhibits the fastest phase velocities in the region, but with notable heterogeneity. We find that crust is thinnest under Afar and the MER, but thickens towards the Ethiopian Highlands to a maximum of 37 km. Moho depth does not linearly increase with distance from the rift axis, potentially due to differential underplating and thick flood basalts in the Ethiopian Plateau. Thinner crust (<32 km) is seen at the most northwestern edge of Ethiopia.

We suggest that while crustal thinning is dominantly controlled by closeness to the MER axis, off -axis deformation continues to be significant in controlling the extensional evolution of the rift. We further support these ideas by jointly inverting for shear velocity structure, with preliminary results closely agreeing with our phase velocity and receiver function based conclusions.