Using Radial Anisotropy to Image Melt Migration and Storage in the Crust and Uppermost Mantle of the northern East African Rift

Abstract:

Where and how melt is stored in the crust and uppermost mantle is important for understanding magmatic plumbing systems, their eruptive potential and also the general dynamics and evolution of rifting. We determine shear velocity and radial anisotropy in the magmatically rifting northern East African Rift to determine the location and shape of melt both on and off rift. Love and Rayleigh surface waves are extracted from ambient noise data from 9 – 26 s period. The Rayleigh - Love wave phase speed difference cannot be accounted for by an isotropic model, with Love waves 0.40 ±0.03 km/s faster than Rayleigh waves. After inverting for shear velocity, we require radial anisotropy in the crust from 5 - 30 km depth, with our 1-D average V_SV 0.15 ±0.03 km/s slower than V_SH. Below 30 km depth, radial anisotropy is not required. V_SH>V_SV across most of the area and is strongest in the upper 5 - 15 km, suggesting the crust is inherently horizontally layered. Anisotropy is weaker at 16 - 30 km depth but velocities are low enough to require melt suggesting melt is stored as sills and more homogenous bodies. Effective medium theory suggests thin compositional layering of felsic and mafic intrusions can account for anisotropy up to 4%, however to reconcile the highest observed anisotropy (6.5 ± 0.5%) and lowest velocities, we require 2 - 4% partial melt oriented in sills. Along rift horizontally aligned radial anisotropy gets progressively weaker towards areas at more advanced rifting, suggesting sills become less dominant with progressive rifting. The Erta Ale magmatic segment is the only location where V_SV>V_SH, suggesting the crust is dominated by vertically aligned micro-cracks and dykes providing conduits for vertical flow of melt. The observed variations in melt storage likely reflect the changing stress with stage of rifting. At the younger, narrower rift with thicker crust, compression dominates favouring sill formation. In the more advanced, broader rift regions with thinner crust, extension in the centre of the rift promotes dyking, while compression on the margins promotes sill formation.