Mantle transition zone structure and upper mantle S velocity variations beneath Ethiopia: Evidence for a broad, deep‐seated thermal anomaly

Abstract

[1] Ethiopia has been subjected to widespread Cenozoic volcanism, rifting, and uplift associated with the Afar hot spot. The hot spot tectonism has been attributed to one or more thermal upwellings in the mantle, for example, starting thermal plumes and superplumes. We investigate the origin of the hot spot by imaging the S wave velocity structure of the upper mantle beneath Ethiopia using travel time tomography and by examining relief on transition zone discontinuities using receiver function stacks. The tomographic images reveal an elongated low-velocity region that is wide (>500 km) and extends deep into the upper mantle (>400 km). The anomaly is aligned with the Afar Depression and Main Ethiopian Rift in the uppermost mantle, but its center shifts westward with depth. The 410 km discontinuity is not well imaged, but the 660 km discontinuity is shallower than normal by $20-30 km beneath most of Ethiopia, but it is at a normal depth beneath Djibouti and the northwestern edge of the Ethiopian Plateau. The tomographic results combined with a shallow 660 km discontinuity indicate that upper mantle temperatures are elevated by $300 K and that the thermal anomaly is broad (>500 km wide) and extends to depths !660 km. The dimensions of the thermal anomaly are not consistent with a starting thermal plume but are consistent with a flux of excess heat coming from the lower mantle. Such a broad thermal upwelling could be part of the African Superplume found in the lower mantle beneath southern Africa.

Mantle transition zone variations beneath the Ethiopian Rift and Afar: Chemical heterogeneity within a hot mantle?

“…An elevated temperature is coherent with the geodynamic context and tomographic results [e.g., Benoit et al, 2006].…”

“…Tauzin et al [2008] observed both discontinuities to be depressed by 30-50 km, most likely caused by the use of a velocity model without slow upper mantle anomalies. A study using regional deployments [Benoit et al, 2006] found the MTZ beneath Ethiopia to be complex and reported no conclusive evidence for the '410' with P-to-S converted-waves, whereas the '660' is mapped with considerable relief (625-675 km).…”

“…Numerous regional seismological studies [e.g., Benoit et al, 2006;Bastow et al, 2008;Chang and Van der Lee, 2011] favor the hypothesis that this broad (>500 km-wide) low velocity body extends across the mantle transition zone (MTZ) somewhere in the region of Kenya/ Ethiopia but limited depth resolution usually renders this claim uncertain. Also unclear is the extent to which the low velocity zone beneath Ethiopia is a purely thermal anomaly, or one in which chemical heterogeneity also plays an important role.…”

“…[4] Previous MTZ thickness studies beneath Ethiopia/Afar either illuminated the MTZ beneath permanent seismological stations [e.g., Nyblade et al, 2000;Tauzin et al, 2008] or did not successfully image both the '410' and '660' beneath the region [Benoit et al, 2006] and therefore do not place constraints on the lateral changes in MTZ thickness that are required to assess whether the African Superplume intersects the MTZ beneath Ethiopia.…”

Nd, Pb, and Sr isotope composition of Late Mesozoic to Quaternary intra-plate magmatism in NE-Africa (Sudan, Egypt): high-μ signatures from the mantle lithosphere

“…1) is matter of an ongoing debate. Topographic elevation and gravity field (e.g., Ebinger and Hayward 1996;Woldetinsae and Götze 2005) and seismic velocity anomalies in the mantle (e.g., Ritsema and Allen 2003;Benoit et al 2006;Sebai et al 2006) indicate major thermal anomalies in the mantle, which occur according to the seismic velocities in the asthenosphere and in the deep mantle. The anomalies in the deep mantle are ascribed to mantle plume activity, but the relations between deep and shallow mantle's anomalies are still unresolved (e.g., Ritsema and Allen 2003;Benoit et al 2006).…”