P-wave travel-time tomography reveals multiple mantle upwellings beneath the northern East-Africa Rift

Thursday, 18 December 2014
James O S Hammond1, Chiara Civiero2, Saskia D B Goes1, Abdulhakim Ahmed3, Atalay Ayele4, Cecile Doubre5, Berhe Goitom6, Derek Keir7, Michael Kendall8, Sylvie D Leroy3, Ghebrebrhan Ogubazghi9, Georg Rumpker10 and Graham William Stuart11, (1)Imperial College London, London, SW7, United Kingdom, (2)Imperial College London, London, United Kingdom, (3)University Pierre and Marie Curie Paris VI, Paris, France, (4)Addis Ababa University, Addis Ababa, Ethiopia, (5)University of Strasbourg, Strasbourg Cedex, France, (6)University of Bristol, Bristol, United Kingdom, (7)University of Southampton, Southampton, United Kingdom, (8)University of Bristol, School of Earth Sciences, Bristol, United Kingdom, (9)Eritrea Institute of Technology, Asmara, Eritrea, (10)Goethe University Frankfurt, Frankfurt, Germany, (11)University of Leeds, Leeds, United Kingdom
The East African Rift (EAR) shows evidence for active magmatism from the eruption of flood basalts 30 Ma to active volcanism associated with rifting today. Mantle plumes have been invoked as the likely cause. However, the nature of mantle upwelling is debated, with proposed models ranging from a single broad plume, the African Superplume, connected to the LLSVP beneath Southern Africa, to multiple distinct sources of upwelling along the East-Africa Rift.

We present a new relative travel-time tomography model that images detailed P-wave velocities below the northern East-African rift from the surface to lower mantle depths. Data comes from 439 stations that cover the area from Tanzania to Saudi Arabia. The aperture of the integrated dataset allows us to image for the first time low-velocity structures of ~ 100-km length scales down to depths of 900 km beneath this region.

Our images provide evidence of at least two separate low-velocity structures with a diameter of ~200 km that continue through the transition zone and into the lower mantle: the first, and most pronounced, is beneath the Afar Depression, which extends to at least 900 km depth and a second is located beneath the Main Ethiopian Rift that extends to at least 750 km.

Taking into account seismic sensitivity to temperature and thermally controlled phase boundary topography, we interpret these features as multiple focused upwellings from below the transition zone with excess temperatures of ~ 100-150 K. Such temperatures are also fully consistent with previous petrological and other geophysical estimates. Furthermore, the separate structures could explain differences in geochemistry of erupted magmas along the rift zone, as well as the dynamic topography seen at the surface. Our findings thus support the involvement of multiple plumes in the evolution of the EAR and a direct connection between lower mantle features and the volcanism at the surface.