Imaging the seismic velocity structure of the crust and upper mantle in the northern East African Rift using Rayleigh wave tomography

Abstract

Understanding the dynamics and evolution of continental rifting is broadly important for our understanding of plate tectonics. The northern East African Rift offers an excellent opportunity to study these processes at an active rift that was initiated by a large magmatic event. Multiple seismic models have been produced to understand the evolution of magmatism which image punctuated slow velocity zones in the asthenosphere. However, the depth extent of the slow velocity bodies has been less well constrained leading to much debate regarding the primary controls on melt generation. Variations between methods, resolution, and scale of the seismic models make direct quantitative comparisons challenging. The lack of instrumentation off-rift further limits our understanding of the spatial extent of tectonic and magmatic processes, which is crucial to understanding magmatic continental rifting. In this paper, we jointly invert Rayleigh wave dispersion curves from ambient noise and teleseisms to obtain absolute shear velocity maps at 10–150 km depth. This includes data from a new seismic network located on the Ethiopian Plateau and enhanced resolution at Moho and upper mantle depths from the joint inversion. At crustal depths, velocities are slowest beneath the Main Ethiopian Rift and the off-rift Ethiopian Plateau (<3.00-3.75 ±0.04 km/s, 10–40 km depth) and ongoing magmatic emplacement is required. At 60–80 km depth off-rift, we observe a fast velocity lid (>0.1 km/s faster than surroundings), in agreement with previous estimates of lithospheric thickness from receiver functions. The fast lid is not observed within the Main Ethiopian Rift or central Afar which instead are underlain by asthenospheric slow velocity anomalies (<4.05 ±0.04 km/s at 60–120 km depth). This suggests melt is infiltrating the lithosphere within the rift. Furthermore, punctuated asthenospheric slow velocity anomalies (∼110×80 km wide) exist in areas that have not undergone significant crustal and plate thinning, potentially indicating melt infiltration may start prior to significant plate deformation. Finally, the punctuated asthenospheric slow velocity zones are not located directly beneath melt-rich crustal regions including those off-rift, suggesting melt migration processes are dynamic and/or may occur laterally.

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ORCID for Derek Keir: orcid.org/0000-0001-8787-8446

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