Mantle heterogeneity and flow from seismic and geodynamic constraints
Simmons, Nathan Alan
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I have developed 3-D models of mantle heterogeneity that satisfy seismic and geodynamic observations. The seismic constraints include body wave travel times of multi-bounce mantle and core shear waves. The geodynamic constraints include the Earth’s gravity field, dynamic surface topography, tectonic plate motions and the excess ellipticity of the core-mantle boundary. These geodynamic observations are directly dependent upon density perturbations in the mantle which are the driving force of mantle flow. Furthermore, the effect of density anomalies on these geodynamic observables is dependent upon the viscosity of the mantle and whether or not there are boundaries to vertical flow within the mantle. Assuming a linear relation between seismic velocity and density, I tested several hypotheses for how the mantle convects by jointly inverting the seismic and geodynamic data. The data were best fit by a model assuming whole mantle convection with no internal layers that strongly inhibit vertical flow. The simultaneous inversion of seismic and geodynamic observations requires knowledge of the link between seismic velocity and density perturbations in the mantle. Therefore, I have tested several radially-symmetric profiles of densityvelocity scaling from mineral physics studies which assume that all lateral heterogeneity is generated by lateral temperature variations. Integration of the optimum density-velocity conversion profile into the joint modeling framework has yielded mantle-scale models of seismic velocity and thermally-induced density perturbations. These models satisfy the combined dataset to a reasonably high degree implying that variations of temperature are the primary cause of mantle seismic heterogeneity outside of the roots of continental cratons. Using inversion techniques, I have also found a 3D density-velocity relationship in the mantle thereby revealing density perturbations associated with compositional variability. Compositional buoyancy of the cratons is clearly detected and intrinsically-dense material is found within the mid-mantle extension of the rising African superplume structure. This high-density component within the superplume hinders the buoyancy of the structure and possibly redirects the flow within. Collectively, these models yield a better understanding of the dynamics of the mantle.