Small-scale thermal upwellings under the northern East African Rift from S travel time tomography

Chiara Civiero (1) , Saskia Goes (1) , James O S Hammond , Stewart Fishwick (2) , Abdulhakim Ahmed (3) , Atalay Ayele (4) , Cécile Doubre (5) , Berhe Goitom (6) , Derek Keir (7) , J. Michael Michael Kendall , Sylvie Leroy (3) , Ghebrebrhan Ogubazghi (8) , Georg Rümpker (9) , Graham Stuart (10)

1 Department of Earth Science and Technology [Imperial College London]
2 Department of Geology [Leicester]
3 iSTeP - Institut des Sciences de la Terre de Paris
4 AAU - Addis Ababa University
5 IPGS - Institut de physique du globe de Strasbourg
6 University of Bristol [Bristol]
7 National Ocenalography Centre Southampton
8 Department of Earth Sciences, University of Asmara
9 Goethe University Frankfurt
10 SEE - School of Earth and Environment [Leeds]

There is a long-standing debate over how many and what types of plumes underlie the East African Rift and whether they do or do not drive its extension and consequent magmatism and seismicity. Here we present a new tomographic study of relative teleseismic S and SKS residuals that expands the resolution from previous regional studies below the northern East African Rift to image structure from the surface to the base of the transition zone. The images reveal two low-velocity clusters, below Afar and west of the Main Ethiopian Rift, that extend throughout the upper mantle and comprise several smaller-scale (about 100 km diameter), low-velocity features. These structures support those of our recent P tomographic study below the region. The relative magnitude of S to P residuals is around 3.5, which is consistent with a predominantly thermal nature of the anomalies. The S and P velocity anomalies in the low-velocity clusters can be explained by similar excess temperatures in the range of 100–200°C, consistent with temperatures inferred from other seismic, geochemical, and petrological studies. Somewhat stronger V S anomalies below Afar than west of the Main Ethiopian Rift may include an expression of volatiles and/or melt in this region. These results, together with a comparison with previous larger-scale tomographic models, indicate that these structures are likely small-scale upwellings with mild excess temperatures, rising from a regional thermal boundary layer at the base of the upper mantle.

Domaines: Géophysique [physics.geo-ph]

Chiara Civiero, Saskia Goes, James O S Hammond, Stewart Fishwick, Abdulhakim Ahmed, et al.. Small-scale thermal upwellings under the northern East African Rift from S travel time tomography. Journal of Geophysical Research : Solid Earth, 2016, 121 (10), pp.7395 - 7408. ⟨10.1002/2016JB013070⟩. ⟨hal-01862645⟩