Observations of Dynamic Topography from South American Atlantic Margin

Topography of the Earth is mainly controlled by its crustal and lithospheric architecture. Variations in thickness and density of the crust and of the lithospheric mantle create the most noticeable topography. However, it is also widely recognized that mantle convective processes dynamically support topography at long wavelengths (i.e. longer than ∼ 1,000km). Observations of dynamic topography, varying in space and time, are more straightforward in the oceans since less complex lithosphere facilitates necessary corrections for topography arising from density and thickness variations of the crust, sediments and flexure. The aim of this dissertation is to investigate the spatial and temporal variation of dynamic topography and its expressions along the South American Atlantic margin. First, the spatial pattern of dynamic topography is examined through the study of residual depth anomalies on oceanic crust abutting this margin. An extensive seismic reflection and refraction database is compiled, and a revised and augmented residual bathymetric map is presented. This map defines the spatial distribution of dynamic topography along South American Atlantic margin. Results are compared to a range of independent geophysical and geologic observations. Secondly, the temporal evolution of dynamic topography is studied by focusing on the southern South Atlantic Ocean. A significant oceanic depression (∼ 2000km along its axis) occurs in the region known as Argentine Basin. This anomaly has a strong negative residual depth signal observed in different dynamic topography models. This study defines the amplitude and wavelength of the Argentine Basin depression and investigates its temporal evolution by looking into the stratigraphic record of sedimentary basins fringing this oceanic depression. To further investigate the temporal evolution of dynamic topography, Cenozoic epeirogeny of Argentine Patagonia is explored. Regional uplift is identified in this region which is contemporaneous with the evolution of the nearby Argentine Basin. Patagonia uplift is studied by inverse modeling of drainage networks, which provides spatial and temporal uplift patterns. The temporal evolution of this region is also analysed by geochemistry of intraplate magmatic rocks and earthquake tomography. Relationships between long-wavelength topography, spatial distribution of intraplate magmatism and uplifted marine terraces are discussed. Finally, constraints on the sub-lithospheric structure of Southern South America are assembled and an empirical conversion for shear-wave velocities to temperature is exploited with a view to investigating possible driving mechanisms for uplift and subsidence across Patagonia and the Argentine Basin.