Upper mantle seismic structure beneath the Ethiopian hot spot: Rifting at the edge of the African low‐velocity anomaly

Abstract

[1] The Miocene-Recent East African Rift in Ethiopia subaerially exposes the transitional stage of rifting within a young continental flood basalt province. As such, it is an ideal study locale for continental breakup processes and hot spot tectonism. We combine teleseismic traveltime data from 108 seismic stations deployed during two spatially and temporally overlapping broadband networks to present detailed tomographic images of upper mantle P and S wave seismic velocity structure beneath Ethiopia. Tomographic images reveal a $500 km wide low P and S wave velocity zone at 75 to !400 km depth in the upper mantle that extends from close to the eastern edge of the Main Ethiopian Rift (MER) westward beneath the uplifted and flood basalt-capped NW Plateau. We interpret this broad low-velocity region (LVR) as the upper mantle continuation of the African Superplume. Within the broad LVR, zones of particularly low velocity are observed with absolute delay times (dt P $ 4 s) that indicate the mantle beneath this region is amongst the slowest worldwide. We interpret these low velocities as evidence for partial melt beneath the MER and adjacent NW Plateau. Surprisingly, the lowest-velocity region is not beneath Afar but beneath the central part of the study area at $9°N, 39°E. Whether this intense low-velocity zone is the result of focused mantle upwelling and/or enhanced decompressional melting at this latitude is unclear. The MER is located toward the eastern edge of the broad low-velocity structure, not above its center. This observation, along with strong correlations between low-velocity zones and lithospheric structures, suggests that preexisting structural trends and Miocene-to-Recent rift tectonics play an important role in melt migration at the base of the lithosphere in this magmatic rift zone.

Surface wave imaging of the weakly extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers

“…A similar broadening and westward shift in the location of the RVP anomaly is observed in the Rayleigh-wave phase-velocity maps of both Adams et al (2012) and O'Donnell et al (2013). The observation of low velocities at depth outside the rift valley and volcanic province is qualitatively similar to observations from the Main Ethiopian Rift (MER), where low velocities extend west beneath the Ethiopian Plateau at depths >75 km (Bastow et al 2008;Gallacher et al 2016). However, the absolute phase velocities are substantially lower (>5 per cent) within the MER (Gallacher et al 2016) at all periods compared to those in the Malawi Rift.…”

Volcanism in the Afar Rift sustained by decompression melting with minimal plume influence

“…One per cent melt retention assumes strong melt-buoyancy effects, which have been previously proposed for mid-ocean ridges 21 and are also probably required beneath Afar. Previous surface-wave, body-wave and receiver-function results have similarly interpreted very slow velocities and very high V p /V s values as significant amounts of shallow melting 9,11,12 .…”

“…Several recent seismic studies found evidence for melt in the crust and upper mantle beneath the rift based on high ratios of the P-wave velocity to the S-wave velocity (V p /V s ratios), velocities 5-10% slower than the global average and anisotropy [9][10][11][12][13] . However, precise determination of lithospheric thickness and depth of melting have proved challenging with existing seismic methodologies.…”

The development of extension and magmatism in the Red Sea rift of Afar

“…In the case of Ethiopia, transition zone thickness, determined by receiver function analysis, shows evidence for thinning compared to the global mean; Cornwell et al (2011) cited this as evidence for high temperatures at transition zone depths, and thus connectivity between shallow low velocities imaged tomographically in Ethiopia (e.g., Bastow et al, 2008;Benoit et al, 2006;Debayle et al, 2001;Hansen et al, 2012;Pasyanos and Nyblade, 2007) and the superplume in the lower mantle beneath.…”

“…The consensus emerging from recent studies in Ethiopia is that it is underlain by a broad low velocity anomaly, not a traditional narrow mantle plume (e.g., Bastow et al, 2008;Benoit et al, 2006;Cornwell et al, 2011;Hansen et al, 2012;Ritsema et al, 2010) (Fig. 5).…”