Mantle response to continental breakup and elevated potential temperatures beneath the afar triple junction

Abstract

Continental breakup within the Afar triple junction makes it an ideal location to investigate the response of the mantle and lithosphere to rifting and elevated mantle potential temperatures. Many factors have been proposed as being important to the initiation and development of rifting including mantle potential temperature, mantle composition, melt focusing, prior rift history and active upwelling. Rayleigh-wave tomography is used to construct a 3-dimensional shear wave velocity model and to produce a model of azimuthal anisotropy divided into 6 areas for the upper 250 kilometres beneath the Afar triple junction. Also presented are shear wave splitting results from along the northern and southern margins of the Gulf of Aden. Low shear wave velocities are interpreted as partial melt segmented beneath the rifts. The distribution of melt is indicative of buoyancy driven active upwelling suggesting that mantle segmentation observed at oceanic spreading centres initiates early during continental rifting. Across the majority of the continental areas, crustal anisotropy is orientated NE-SW and attributed to pre-existing Pan African basement foliation. Lithospheric anisotropy for rifted areas is rift parallel and likely due to magmatic intrusion and aligned partial melt with negligible lithospheric anisotropy in Afar and the plateaux. Asthenospheric anisotropy throughout the whole region ranges from N-S to NE-SW which is interpreted as being due to mantle flow associated with the African Superplume. Shear wave splitting results from the Gulf of Aden margins are generally null which can be explained by widespread vertical mantle flow due to small scale convection. Limited shear wave splitting is recorded and likely occurs at the edge of the African LLSVP in the lower mantle. Thus the style of rifting in Afar Triple Junction can be attributed to the elevated mantle potential temperatures of the African Superplume in addition to melt focusing and active upwelling due to the lithospheric topography of the rifts.

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ORCID for Derek Keir: orcid.org/0000-0001-8787-8446

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