Abstract
AbstractEthiopia is a region where continental rifting gives way to oceanic spreading. Yet the role that pre‐existing lithospheric structure, melt, mantle flow, or active upwellings may play in this process is debated. Measurements of seismic anisotropy are often used to attempt to understand the contribution that these mechanisms may play. In this study, we use new data in Afar, Ethiopia along with legacy data across Ethiopia, Djibouti, and Yemen to obtain estimates of mantle anisotropy using SKS‐wave splitting. We show that two layers of anisotropy exist, and we directly invert for these. We show that fossil anisotropy with fast directions oriented northeast‐southwest may be preserved in the lithosphere away from the rift. Beneath the Main Ethiopian Rift and parts of Afar, anisotropy due to shear segregated melt along sharp changes in lithospheric thickness dominates the shear‐wave splitting signal in the mantle. Beneath Afar, away from regions with significant lithospheric topography, melt pockets associated with the crustal and uppermost mantle magma storage dominate the signal in localized regions. In general, little anisotropy is seen in the uppermost mantle beneath Afar suggesting melt retains no preferential alignment. These results show the important role melt plays in weakening the lithosphere and imply that as rifting evolves passive upwelling sustains extension. A dominant northeast‐southwest anisotropic fast direction is observed in a deeper layer across all of Ethiopia. This suggests that a conduit like plume is lacking beneath Afar today, rather a broad flow from the southwest dominates flow in the upper mantle.
Highlights
Ethiopia is often cited as a natural laboratory for the study of the final stages of continental breakup [Ebinger and Casey, 2001; Nyblade and Langston, 2002; Maguire et al, 2003]
We show that many other stations throughout Afar show the signature of multiple layer anisotropy with systematic variation in fast direction and time lag with backazimuth (Figure 3 and supporting information Figures S1–S21)
The extensive focus of seismic research in Ethiopia over the last two decades, including new deployments in the Afar Depression means that we have access to exceptional data sets across the region
Summary
Ethiopia is often cited as a natural laboratory for the study of the final stages of continental breakup [Ebinger and Casey, 2001; Nyblade and Langston, 2002; Maguire et al, 2003]. A number of studies have attempted to unravel the anisotropic signature beneath Ethiopia [Ayele et al, 2004; Gashawbeza et al, 2004; Kendall et al, 2005; Sebai et al, 2006; Montagner et al, 2007; Sicilia et al, 2008; Bastow et al, 2010; Hammond et al, 2010; Gao et al, 2010; Obrebski et al, 2010; Keir et al, 2011; Hammond, 2014], yet, to date, no consensus exists on the mechanisms causing the observed anisotropy. Novel shear-wave splitting inversion techniques [Wookey, 2012] to unravel multiple layers of anisotropy beneath the rift, allowing us to place constraints on the dominant mechanisms of anisotropy beneath this complex tectonic setting
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