We present 3D2015_07Sv, an S wave model of the upper mantle based on the waveform modeling of 1,359,470 Rayleigh waves recorded since 1976. The use of approximate forward theory and modeling allows updating the model with new data on a regular basis. 3D2015_07Sv contains azimuthal anisotropy, achieves a lateral resolution of ∼600 km, and is consistent with other recent models up to degree 60 in the uppermost 200 km and degree 15 in the transition zone. Although radial anisotropy has been found to extend deeper beneath continents than beneath oceans, we find no such difference for azimuthal anisotropy, suggesting that beneath most continents, the alignment of olivine crystal is preferentially horizontal and azimuthally random at large scale. As most continents are located on slow moving plates, this supports the idea that azimuthal anisotropy aligns at large scale with the present plate motion only for plates faster than ∼4 cm yr−1.
The seismic low velocity zone (LVZ) of the upper mantle is generally associated with a low-viscosity asthenosphere that plays a key role for the dynamics of plate tectonics 1 . However, its origin remains enigmatic, some authors attributing the reduction in seismic velocity to a small amount of partial melt 2,3 , others invoking solid-state mechanisms near the solidus [4][5][6] , or the effect of volatile contents 6 . Observations of shear attenuation provide additional constraints to unravel the origin of the LVZ 7 . Here, we report the discovery of partial melt within the LVZ from the simultaneous interpretation of global 3D shear attenuation and velocity models. We observe that partial melting down to 150-200 km depth beneath mid-ocean ridges, major hotspots and back-arc regions feeds the asthenosphere. A small part of this melt (<0.30%) remains trapped within the oceanic LVZ. The amount of melt is related to plate velocities and increases significantly between 3 and 5 cm yr -1 , similar to previous observations of mantle crystal alignement underneath tectonic plates 8 . Our observations suggest that by reducing viscosity 9 , melt facilitates plate motion and large-scale crystal alignment in the asthenosphere. Melt is absent under most of the continents.Our finding results from the simultaneous analysis of two upper mantle tomographic models of shear wave velocity (Vs) and attenuation (parameterised
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