Variations in melt emplacement beneath the northern East African Rift from radial anisotropy

Abstract

Where and how melt is stored in the crust and uppermost mantle is important for understanding the dynamics of magmatic plumbing systems and the evolution of rifting. We determine shear velocity and radial anisotropy in the magmatically rifting northern East African Rift to determine the locus and orientation of melt, both on and off-rift. Love and Rayleigh fundamental modes are extracted from ambient noise data from 9-26 s period and then inverted for shear velocity. VSV is 0.15 ± 0.03 km/s lower than VSH from 5-30 km depth on average. VSH>VSV across most of the study region suggests the crust is inherently horizontally layered, with the largest anisotropy in the upper 5-15 km. Effective medium theory suggests thin compositional layering of felsic and mafic intrusions can account for anisotropy up to 4%. However, to reconcile the largest observed anisotropy (6.5%), and lowest velocities, we require 2-4% partial melt oriented in sills. Along the rift, horizontally aligned radial anisotropy gets weaker north-eastwards, suggesting sills become less dominant with progressive rifting. The Erta Ale magmatic segment is the only location where VSV>VSH, suggesting the crust contains vertical micro-cracks and dykes. Overall, the results suggest during early continental breakup when the rift is narrow, sill formation is the dominant storage mechanism. As a rift widens, vertical dyke intrusion becomes dominant and is likely controlled by variations in crustal thickness and stress state.