Dynamic topography and mantle convection induced by subduction of oceanic lithosphere: a numerical approach

• Ano de defesa: 2019
• Autor(a) principal: Assunção, Jamison Faustino Gomes de
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: convecção do manto; dynamic topography; finite element method; mantle convection; método dos elementos finitos; subdução da litosfera oceânica; subduction of oceanic lithosphere; topografia dinâmica
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/14/14132/tde-19022020-123910/

One of the main forces that drive plate tectonics is induced by the subduction of cold and dense oceanic lithosphere, creating a negative buoyancy relative to the adjacent hotter mantle. This downward movement perturbs the flow pattern mainly in the upper mantle, also affecting the vertical displacement of the Earth\'s surface in the geological time scale, phenomenon known as dynamic topography. Due to the physical complexity of these processes, a natural way to study subduction of lithospheric plates is through the use of numerical models. The aim of the present work is the development of numerical scenarios that reproduce some of the main aspects observed about geometry and kinematics of oceanic plates in subduction based on geophysical constraints. Specifically, I focused my attention on the subduction of the Nazca plate under the continental lithosphere of South America, at a latitude of 18ºS. On the first part of this project, several numerical simulations were made to study the buoyancy of the Nazca plate. The results showed that for a relatively thicker oceanic lithosphere, the slab tends to deflect downwards, but it resists more to bending due to its rigidity. An opposing buoyant force produced by the oceanic crust contributed to an upward deflection of the subducting slab even when the resulting density of the entire slab was greater than the surrounding asthenospheric mantle density. The best combination found for the smallest deflection was that of an 80 km thick oceanic lithosphere with an 8 km thick oceanic crust with a density of 2800 kg/m3. On the second part of this project, the Nazca plate was simulated for more than 50 Myr to study its stagnation on the upper-to-lower mantle boundary. It was noticed that increasing the viscosity alone cannot explain slab flattening at 660 km, as phase change reflected by the increase in density was critical when studying slab penetration in the lower mantle.