1.23 - Constraints on Seismic Models from Other Disciplines - Implications for Mantle Dynamics and Composition

Mantle flow models based on seismic tomographic images of 3-D mantle structure may be used to successfully explain a wide range of global surface data related to mantle convection. These tomography-based convection models place strong constraints on viscosity and lateral density variations in Earth’s mantle. The convection-related observables which can be modelled include the global free-air gravity anomalies, the dynamic surface and CMB topography, and the horizontal divergence of the present-day tectonic plate motions. The fundamental mantle flow theory needed to interpret these geodynamic observables is reviewed for the case of a self-gravitating, compressible mantle with coupled surface plates. The most important input to the flow models, namely the effective viscosity of the mantle, may be inferred from simultaneous inversions of glacial-isostatic adjustment and mantle convection data. These viscosity inferences are used in numerical calculations of the geodynamic response or kernel functions which provide the theoretical relationship between the surface observables and the mantle density anomalies. Understanding the present-day dynamics of the mantle is critically dependent on knowledge of these internal density perturbations and they are derived here on the basis of a wide selection of recent S-wave tomography models. A detailed review of the geodynamic observables predicted on the basis of these S-wave tomography models is presented and it is found that all tomography models provide very similar fits to the convection data. An outstanding issue in current efforts to understand mantle flow dynamics is the relative importance of the thermally and compositionally generated buoyancy in the mantle. Joint shear and bulk-sound tomography models are employed to resolve lateral variations in the theoretical scaling between mantle density and seismic velocity anomalies and thereby detect compositional heterogeneity in the mantle. A similar analysis of the fast/cold and slow/hot regions in the S-wave tomography models also provides indications of significant lateral variations in composition. The most anomalous regions appear to be the shallow subcontinental mantle, the mid-mantle region near 1500km depth, and the seismic D″ layer.