Mantle flow and the geological record, dynamical mechanisms for continental epeirogeny

grantor: University of TorontoThe geologic record shows that continents have periodically experienced episodes of large-scale subsidence and uplift which result in widespread marine transgressions. The focus of my thesis is to consider the dynamic topography caused by fluid convection in the mantle as a mechanism for these enigmatic epeirogenic motions. The first part of the thesis examines subsidence at continental margins and the role of subduction-induced flow in producing the surface deflections. Numerical simulations of mantle convection in a Cartesian geometry are adopted to model the flow dynamics associated with stiff subducting slabs. An analysis of the stratigraphy and paleotectonics of the Russian Platform during the Paleozoic reveals a correlation between subsidence of the platform and subduction events at the continental margin. Numerical models of mantle flow and dynamic topography are computed and the predictions are able to reconcile the observed sedimentation history. The Late Paleozoic subsidence of the Karoo Basin is also considered in this context. Numerical simulations indicate that dynamic tilting caused by subduction-induced mantle flow works in tandem with flexural loading of the lithosphere to explain the subsidence and uplift history of the basin. The latter half of the thesis focuses on the interaction of various classes of mantle flow with the endothermic phase change at 660 km depth and investigates the associated surface manifestations. A model of fluid convection in axisymmetric spherical geometry is introduced to simulate flow regimes in a multi-phase mantle. My numerical simulations show that 'mantle avalanche' events are capable of causing long-wavelength lithospheric subsidence with vertical amplitudes of ~2500 m over timescales of 150-200 m.y. Based on these results, it is speculated that an avalanche event may provide a plausible explanation for the Devonian evolution of the Laurussian supercontinent. I also consider the dynamic topography associated with more 'generic' mantle flow regimes, namely 'stiff' descending plumes and slabs. The plume/slab flow models are found to induce wide (~1000 km) topographic deflections of amplitude ~1 km which persist over timescales of 100-150 m.y. Simple models of sedimentation in these dynamic depressions suggest that these classes of mantle flow may be a plausible mechanism for the enigmatic development of transient intracratonic basins.Ph.D