Juan Carlos Afonso, Farshad Salajeghegh

Geoid anomalies offer crucial information on the internal density structure of the Earth, and thus, on its constitution and dynamic state. In order to interpret geoid undulations in terms of depth, magnitude and lateral extension of density anomalies in the lithosphere and upper mantle, the effects of lower mantle density anomalies need to be removed from the full geoid (thus obtaining a residual signal known as the 'upper mantle geoid'). However, how to achieve this seemingly simple filtering exercise has eluded consensus for decades in the solid Earth community. While there is wide agreement regarding the causative masses of degrees > 10 in spherical harmonic expansions of the upper mantle geoid, those contributing to degrees < 7-8 remain ambiguous. Here we use spherical harmonic analysis and recent tomography and density models from joint seismic-geodynamic inversions to derive a representative upper mantle geoid, including the contributions from low harmonic degrees. We show that the upper mantle geoid contains important contributions from degrees 5 and 6 and interpret the causative masses as arising from the coupling between the long-wavelength lithospheric structure and the sublithospheric upper mantle convection pattern, including subducted slabs. Importantly, the contributions from degrees 3 < l < 8 do not show a simple power-law behaviour (e.g. Kaula's rule), which precludes the use of standard filtering techniques in the spectral domain. Our model of the upper mantle geoid will be useful in a wide range of geodynamic and geophysical applications, including the study of i) the thermochemical structure of the lithosphere, ii) dynamic topography and mantle viscosity, iii) the nature of the mechanical coupling of the lithosphere-asthenosphere system and iv) the global state of stress within the lithosphere and its associated hazards.