Effect of viscosity structure on long wavelength convection and comparison with tomographic models

The viscosity structure of the Earth's mantle affects nearly all aspects of the dynamics of Earth's interior, including the ascent of mantle plumes, settling of subducted oceanic lithosphere, and mixing of compositional heterogeneities in the mantle. These processes shape the overall thermal and dynamic evolution of the Earth. Global seismic tomography has provided important insights into the viscosity structure of the Earth. For example, a tomography model has recently reported that plumes are frequently deflected laterally below the transition zone at 1000 km, a depth not coincident with known seismic discontinuities, but at which slabs stagnate in some regions. There is also a change in the long-wavelength radial correlation structure below the transition zone ( 650-1000km) in most Vs and Vp tomography models that employ diverse seismic observations. While the cause of decorrelation in heterogeneity remains unresolved, the reported deflection of plumes, stagnation of slabs, and changes in radial correlation structure could be explained by a change in mantle viscosity at 1000 km. Such a viscosity increase has been reported in some inversions constrained by the long-wavelength dynamic geoid.

We present a suite of forward thermochemical mantle convection calculations that explore the interplay between viscosity and temporal evolution of detected mantle heterogeneity. Our mantle convection simulations are performed in 3D spherical shell geometry using the finite element code ASPECT, and we explore different viscosity and compositional profiles, and boundary conditions. We calculate radial correlation functions and vertical coherence for the modeled temperature fields as well as synthetic Vs fields generated from the temperature fields using the resolution operator from mantle tomographic models. Our self-consistent seismic-geodynamic approach places additional dynamical constraints on the mantle viscosity structure through its effects on the development of Earth's long wavelength lateral and radial structure.