Surface wave tomography across Afar, Ethiopia: Crustal structure at a rift triple-junction zone

Abstract

The Afar Depression in northeast Africa contains the rift triple‐junction between the Nubia, Arabia and Somalia plates. We analyze Rayleigh wave group velocity from 250 regional earthquakes recorded by 40 broadband stations to study the crustal structure across Afar and adjacent plateau regions in northern Ethiopia. The dispersion velocities are inverted to obtain surface wave tomographic maps for periods between 5 and 25 seconds, sensitive to approximately the top 30 km of the lithosphere. The tomographic maps show a significant low dispersion velocity anomaly (>20%) within the upper crust, below the site of recent dyke intrusions (2005–present) in the Dabbahu and Manda‐Hararo magmatic segments. Similar low velocity regions are imaged where magma intrusion in the Afar crust has been inferred over the last decade from seismicity or volcanic eruptions. We invert two group velocity curves to compare the S‐wave velocity structure of the crust within an active magmatic segment with that of adjacent areas; the active region has a low velocity zone (Vs ∼ 3.2 km/s), between about 6–12 km, which we infer to be due to the presence of partial melt within the lower crust.

Magma imaged magnetotellurically beneath an active and an inactive magmatic segment in Afar, Ethiopia

“…There are broad pronounced low velocity features over the area of our profiles from both surface waves of up to 12 s period (Guidarelli et al 2011) and P n studies (Stork et al 2013). High upper crustal seismic anisotropy values are attributed to melt pockets in cracks orientated parallel to the rift segment axes .…”

“…It was thus interpreted as a porous melt channel rapidly delivering magma to the crust (Key et al 2013). In the part of Afar we investigated, the highest mantle conductivities were also offset from the rift axes and the base of the crust and uppermost mantle were seismically slow (Guidarelli et al 2011;Stork et al 2013); we did not have the depth penetration to determine whether these broad conductors become more laterally confined at depth. Magma is stored in the crust in both axial and off-axis (4-8 km from the axis) locations beneath the East Pacific Rise; in the latter, the magma is stored in partially molten lower crustal sills (Canales et al 2012).…”

Mantle upwelling and initiation of rift segmentation beneath the Afar Depression

“…S-wave velocities are more sensitive to melt than P-waves. The lower crust and upper mantle beneath Afar contain appreciable amounts of melt, giving rise to high V p /V s (>2.0) deduced from receiver function studies , and low S-wave velocities derived from surface-wave studies (Guidarelli et al, 2011). The differences in P-wave and S-wave anomalies on June 2, 2015 geology.gsapubs.org Downloaded from in the presence of melt are even more pronounced when melt retains a preferential orientation, as has been proposed to explain mantle seismic anisotropy beneath the Main Ethiopian Rift and Red Sea Rift (Kendall et al, 2006;Gao et al, 2010).…”

Why is Africa rifting?

“…They saw a stratified anisotropic structure, but noted that their lateral resolution was of the order of 500 km. Guidarelli et al (2011) used regional Rayleigh waves to image the Afar region in finer detail, revealing a patchwork of stranded continental slivers surrounded by regions of low velocity, melt-rich material, a pattern not unlike that reported by Stork et al (2013). Using ambient noise imaging, Korostelev et al (2015) showed how magmatic processes are focused at the axial magmatic segments and also at the rift flanks.…”