Insights on mantle convection from global tomography

In this presentation, I will review some of our recent global tomography results, that provide constraints on the Earth mantle structure and mantle convection.

In the upper mantle, we have recently constructed global tomographic models of SV wave velocity, 𝑉𝑠𝑣, and radial anisotropy, 𝜉, using the same tomographic approach, with similar regularization and smoothing for the Rayleigh and Love wave data. We also use Rayleigh waves to constrain the azimuthal anisotropy, the quality factor 𝑄 and the melt content. We find that a 1-D model of radial anisotropy, close to PREM, but including a 3D crustal structure, explains the Love/Rayleigh differences almost everywhere, except in oldest parts of the continents and youngest parts of the Pacific ridge. No age dependence of the radial anisotropy 𝜉 in the oceanic upper mantle is required, while age is the main parameter controlling 𝑉𝑠𝑣, melt content and azimuthal anisotropy. In the asthenosphere, azimuthal anisotropy aligns on a large scale with present  plate motion only for fast plates (> ∼4 cmyr−1), suggesting that only fast-moving plates produce sufficient shearing at their base, to organize the flow on the scale of the entire tectonic plates. Part of the azimuthal anisotropy is also frozen in the shallow oceanic lithosphere. The presence of a small amount of partial melt, by reducing mantle viscosity, facilitates plate motion and large-scale crystal alignment in the asthenosphere.

We have also built global shear tomographic models of the whole mantle for the shear velocity (SEISGLOB2) and attenuation (Q_L3D). In the lower mantle, SEISGLOB2 has revealed a change in the shear velocity spectrum at around 1000 km depth. The spectrum is the flattest (i.e. richest in "short" wavelengths corresponding to spherical harmonic degrees greater than 10) around 1000 km depth and this flattening occurs between 670 and 1500 km depth. Q_L3D combines various S-phase measurements, including surface waves, direct (S, SS, SSS, SSSS), core-reflected (ScS, ScSScS, ScSScSScS), diffracted (S_{𝑑𝑖𝑓𝑓}) and their depth phases (e.g., sS, sScS, sS_{𝑑𝑖𝑓𝑓}), providing extensive depth and spatial coverage. A high attenuation zone highlights the peculiar nature of the mantle around 1000 km depth. This may indicate the presence of a global low-viscosity layer, in a region that roughly corresponds to the upper boundary of the Large Low Shear Velocity Provinces (LLSVPs), and where various changes in the continuity of slabs and mantle plumes have been observed. Our 3D shear quality factor model also confirms that the LLSVPs are attenuating, at least for body waves with periods near  35 s. The correlation between strong attenuation and low shear velocities within these regions suggests that the shear quality factor mostly captures the thermal signature of the LLSVP.