Geophysical constraints on mantle dynamics

grantor: University of TorontoThis thesis contains a study of four geophysical fields intimately related to the general convective circulation within Earth's mantle. Detailed investigations of the geoid, the free-air gravity anomaly, and for the first time, of the radial component of heat advection and the pattern of surface heat flux anomalies, are presented within the framework of an anelastically compressible internal loading theory. Recent global tomographic models of shear-wave velocity heterogeneity are employed to define the three-dimensional field of mantle buoyancy sources which drive the flow. Theoretical predictions of the longest wavelength components of these geodynamical fields are compared to observations in order to constrain mantle properties such as viscosity, as well as to interpret large scale features of the convective circulation. The nonhydrostatic geoid is found to provide useful constraints on the depth-dependence of mantle viscosity. The class of viscosity profiles that is inferred on this basis is shown to also allow a description of postglacial rebound data and the average heat flow at Earth's surface. A detailed investigation of the depth profile of heat advection is presented and the usefulness of this profile to probe for thermal boundary layers is discussed. A method to compute the pattern of surface heat flux anomalies within the internal loading formalism is proposed. It is shown that optimal descriptions of the long-wavelength components of this field are obtained within the framework of a layered-mantle theory. This is contrasted with the gravity field which is shown to be best reconciled within the context of the whole-mantle circulation theory. A synthesis of these results suggests that it is not currently possible to deduce whether the radial style of the mantle general circulation is whole-mantle or layered at the depth of 670 km seismic discontinuity. The free-air gravity field is employed to constrain the nature of the deep, subcontinental mantle. On this basis, it is demonstrated that the hypothesis that seismically fast, subcontinental features may be the reflection of chemically distinct and anomalously light material is rejected in favor of the alternative hypothesis in which the subcontinental mantle is composed of anomalous dense, downwelling material.Ph.D