Improved Joint Seismic-Geodynamic Tomography: Effects of Topography at Discontinuities in the Transition Zone on Mantle Heterogeneity and Convection-Related Observables

Abstract:

Despite the progress in tomographic imaging of Earth’s interior, a number of critical questions regarding the large-scale structure and dynamics of the mantle remain outstanding. One of these questions is the impact of phase-boundary undulations on global mantle heterogeneity and convection-related observables, such as the gravity field and dynamic surface topography. To address this issue, we developed a new generation of joint tomography inversions that incorporate 410- and 660-topography. These inversions include about 300,000 S-wave travel time data, SS-wave precursor data, geodynamic data and mineral physical relations between seismic and density heterogeneity. Our results confirm the sensitivity of gravity and dynamic surface topography to topography at the upper-mantle discontinuities (Le Stunff & Ricard 1997; Forte & Woodward 1997). In particular, our new thermal models (which assume that mantle heterogeneity is only due to thermal anomalies) that include discontinuity topography provide notably improved fits to gravity and dynamic surface topography observations compared to the previous generation of models (Lu et al. 2020). Consequently, the amplitude of non-thermal density anomalies required to explain the geodynamic data decreases in both the upper and lower mantle, demonstrating the geodynamic importance of phase-boundary undulations and their impact on inferences of global mantle heterogeneity.