Structure and Dynamics of the Continental Tectosphere

We examine, as a function of depth, the relationship between a tectonic regionalization and upper-mantle shear-wave heterogeneity represented by a recent seismic tomographic model. We perform Monte Carlo simulations that incorporate the spectral properties of both the regions and the seismic signal. Our results indicate that ridges can be readily distinguished from older oceans to a depth of about 200 km. The corresponding platform and shield signature differs significantly (> 99% confidence) from that under oceans and orogenic zones to at least 400 km depth. Results from analogous Monte Carlo simulations reveal that the earth's gravity variations correlate with surface tectonics no better than they would were the geoid (or gravity field) randomly oriented with respect to the surface. We estimate for the upper mantle a platform and shield signal of -8 ±5 m and thus conclude that there is little contribution of platforms and shields to the gravity field, consistent with their keels having small density contrasts. We estimate an average value for dln p/dln v, within the 140-440 km depth range beneath platforms and shields of 0.035 ± 0.025, consistent, at the 1.5level, with Jordan's [1988] isopycnic hypothesis. Through a suite of numerical finite element experiments, we evaluate the relative importance of (1) activation energy (used to define the temperature-dependence of viscosity), (2) compositional buoyancy, and (3) linear or nonlinear rheology in achieving the long-term stability of the continental tectosphere. Stability is assured with a realistic activation energy regardless of the chemical concentration. With lower values of activation energy, compositional buoyancy can significantly influence stability. Compositional buoyancy plays a dual role: It (1) counteracts the thermally-induced density increase and, relatedly, (2) reduces the stress within the boundary layer. With a stress-dependent rheology, this reduction in stress results in an increase in effective viscosity which, in turn, inhibits a greater region of the boundary layer from deforming. The joint application of longevity and gravity constraints allows us to reject all models containing no compositional buoyancy and to predict that the ratio of compositional to thermal buoyancy within the continental tectosphere is approximately unity. Thesis supervisor: Bradford H. Hager, Professor of Geophysics Thesis co-supervisor: Thomas H. Jordan; Professor of Geophysics