I conducted my investigation into the Afro-Arabian Rift System (AARS) around the Afar Triple Junction on three different scales: upper crust, bulk crust and upper mantle. For each scale I used a different seismological technique: local earthquake traveltime tomography, receiver function analysis, and shear-wave splitting.
Using shear-wave splitting of the SKS phase to measure seismic anisotropy, I showed that mantle flow is coherent across the three branches of the rift system. Hot material in the asthenosphere upwells from the core-mantle boundary beneath southern Africa, passes through the East African Rift System and reaches the base of the lithosphere beneath the Afar Triple Junction. My seismic anisotropy results demonstrated that mantle flow then bifurcates around the cold, thick lithosphere of the Arabian Plate, preferentially travelling down the thin lithospheric corridors of the Red Sea and Gulf of Aden rifts.
To investigate the bulk crustal properties across northern Afar, I used teleseismic receiver functions to calculate the depth of the crust and the average crustal Vp/Vs ratio. Heterogeneous crust with variable crustal thickness underlies the region, with considerable amounts of partial melt indicated by anomalously high Vp/Vs ratios. By investigating the relationship between crustal thickness and topographic elevation, I estimated that the regional crustal bulk density is elevated compared to the global average. Topography is higher than would be expected due to crustal isostasy alone, implying that dynamic support from the mantle contributes to the topography in the region.
On the upper crustal scale, I investigated magmatism in the regionāspecifically,Nabro volcano in Eritrea following its June 2011 eruption. I used local earthquake traveltime tomography to invert for the the seismic velocity structure (P-wave velocity, Vp; S-wave velocity, Vs; and their ratio, Vp/Vs) beneath Nabro. Key observations were an aseismic region of low Vp, low Vs and high Vp/Vs ratio at depths of 6ā10 km b.s.l., interpreted as the primary melt storage region that fed the eruption; a zone of high Vs, low Vp and low Vp/Vs ratio, representing an intrusive complex of fractured rocks partially-saturated with over-pressurised gases.
Overall, I found that the crustal structure throughout the region is highly complex and heterogeneous. Despite this, large-scale mantle dynamics underlying the AARS are remarkably simple. Extensional processes in the region have facilitated magmatism throughout the crust and upper mantle, producing volcanic episodes such as Nabro volcanoās June 2011 eruption.