Confirmation of a change in the global shear velocity pattern at around 1000 km depth

International audienceIn this study, we confirm the existence of a change in the shear velocity spectrum around1000 km depth based on a new shear velocity tomographic model of the Earth’s mantle,SEISGLOB2. This model is based on Rayleigh surface wave phase velocities, self- and crosscouplingstructure coefficients of spheroidal normal modes and body wave traveltimes whichare, for the first time, combined in a tomographic inversion. SEISGLOB2 is developed up tospherical harmonic degree 40 and in 21 radial spline functions. The spectrum of SEISGLOB2is the flattest (i.e. richest in ‘short’ wavelengths corresponding to spherical harmonic degreesgreater than 10) around 1000 km depth and this flattening occurs between 670 and 1500 kmdepth.We also confirmvarious changes in the continuity of slabs and mantle plumes all around1000 km depth where we also observed the upper boundary of Large Low Shear VelocityProvinces. The existence of a flatter spectrum, richer in short-wavelength heterogeneities, in aregion of the mid-mantle can have great impacts on our understanding of the mantle dynamicsand should thus be better understood in the future. Although a viscosity increase, a phasechange or a compositional change can all concur to induce this change of pattern, its preciseorigin is still very uncertain