The dynamic crustal and mantle structure around the afar triple junction

Thesis

The Afar Triple Junction marks the confluence of the Red Sea rift, the Gulf of Aden and the East African Rift System. The Afro-Arabian Rift System (AARS) encompasses these three rifts, each in a different stage of continental breakup. I investigate the crust and mantle beneath this complex tectonic region using three different seismological techniques.

I explore mantle flow using measurements of seismic anisotropy, and show that mantle flow is coherent across the three branches of the AARS. Shear-wave splitting of the SKS phase reveals patterns of density-driven flow in the upper mantle beneath the Afar Triple Junction. Fast shear-wave polarisation directions in northeastern Afar are directed NE-SW, coherent with observations further south. Rotation of the fast direction in southwestern Yemen suggests that the flow then bifurcates around the thick, cold lithosphere of Arabia and is channeled along both the Gulf of Aden and Red Sea spreading centres. The magnitude of splitting is smaller in the Gulf of Aden, indicating that mantle flow is less persistent beneath this branch of the AARS.

I interrogate crustal thickness and structure by calculating teleseismic receiver functions across northeastern Afar and the Danakil microplate. I use the H-κ stacking method to estimate the Moho depth and bulk crustal VP/VS ratio. My results reveal a heterogeneous crust with variable crustal thickness, and regions of partial melt indicated by anomalously high VP/VS ratios. These variations are on a shorter length scale than the mantle flow patterns, which suggests the influence of pre-existing lithospheric structures on crustal properties. By investigating the relationship between crustal thickness and topographic elevation, I estimate that the regional crustal bulk density is elevated compared to the global average. I show that 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.

Extensional processes in the region have facilitated magmatism throughout the crust, producing volcanic episodes such as the 2011 eruption of Nabro volcano in Eritrea. I use local earthquake traveltime tomography to invert for the the seismic velocity structure (VP, VS, and VP/VS) beneath Nabro. Key observations are an aseismic region of low VP, low VS and high VP/VS ratio at depths of 6–10 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.

Authors: Gauntlett, M

• DOI: 10.5287/ora-xm1yzgb6z
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: rifting; mantle structure; crustal structure; volcano seismology; seismic anisotropy; receiver functions; seismology; geophysics; afro-arabian rift system
• Subjects: Geophysics; Earth Sciences; Seismology