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International audience; Several seismic and numerical studies proposed that below, some hotspots upper-mantle plumelets rise from a thermal boundary layer below 660 km depth, fed by a deeper plume source. We recently found tomographic evidence of multiple upper-mantle upwellings, spaced by several 100 km, rising through the transition zone below the northern East African Rift. To better test this interpretation, we run 3-D numerical simulations of mantle convection for Newtonian and non-Newtonian rheologies, for both thermal instabilities rising from a lower boundary layer, and the destabilization of a thermal anomaly placed at the base of the box (700-800 km depth). The thermal structures are converted to seismic velocities using a thermodynamic approach. Resolution tests are then conducted for the same P and S data distribution and inversion parameters as our traveltime tomography. The Rayleigh Taylor models predict simultaneous plumelets in different stages of evolution rising from a hot layer located below the transition zone, resulting in seismic structure that looks more complex than the simple vertical cylinders that are often anticipated. From the wide selection of models tested, we find that the destabilization of a 200 degrees C, 100 km thick thermal anomaly with a non-Newtonian rheology, most closely matches the magnitude and the spatial and temporal distribution of the anomalies below the rift. Finally, we find that for reasonable upper-mantle viscosities, the synthetic plume structures are similar in scale and shape to the actual low-velocity anomalies, providing further support for the existence of upper-mantle plumelets below the northern East African Rift.

Various changes in seismic structures across the mantle transition zone (MTZ) indicate that it may hamper thermal and chemical circulation. Here we show how thermal elevation of the postspinel phase transition at 660 km depth plus harzburgite segregation below this depth can project as narrow high-velocity anomalies in tomographic images of continuous thermochemical mantle upwellings. Model S40RTS features a narrow high-velocity anomaly of +0.8% near 660 km depth within the broad low-velocity structure beneath the Samoa hot spot. Our analyses indicate that elevation of the 660 phase boundary in a hot pyrolitic plume alone is insufficient to explain this anomaly. An additional effect of harzburgite enrichment is required and consistent with geodynamic simulations that predict compositional segregation in the MTZ, especially within thermochemical upwellings. The Samoa anomaly can be modeled with a 125–175°C excess temperature and a harzburgite enrichment below 660 of least 60% compared to a pyrolitic mantle.

It is strongly debated whether the interface between the lithosphere and underlying asthenosphere is a temperature-dependent rheological transition, as expected in a thermal convection system, or additionally affected by the presence of melts and/or fluids. Previous surface wave studies of Pacific oceanic lithosphere have found that shear velocity and azimuthal anisotropy vary with seafloor crustal age as expected for a thermal control; however radial anisotropy does not. Various thermomechanical models have been proposed to explain this disparate behaviour. Nonetheless, it is unclear how robust the surface wave constraints are, and this is what we test in this study. We apply a Bayesian model space search approach to three published Pacific surface wave dispersion data sets, two phase-velocity and one combined phase- and group-velocity set, and determine various proxies for the depth of the lithosphere–asthenosphere boundary (LAB) and their uncertainties based on the velocity and radial anisotropy model distributions obtained. In their overall character and pattern with age, the velocity models from different data sets are consistent with each other, although they differ in their values of LAB depths. Uncertainties are substantial (as much as 20 km on LAB depths) and the addition of group-velocity data does not reduce them. Radial anisotropy structures differ even in pattern and display no obvious age dependence. However, given the uncertainties, we cannot exclude that radial anisotropy, azimuthal anisotropy and velocity models actually reflect compatible, age-dependent, LAB depth estimates. The velocity LAB trends are most like those expected for half-space cooling, because velocity differences persist at old ages, below the depth of common plate cooling models. Any direct signature of sub-ridge melt would be too small-scale to be resolved by these data. However, the velocity-increasing effects of dehydration and depletion due to melting below the ridge could explain why LAB proxy depths tend to a minimum of ∼60 km below young ocean floor.