Thesis Defense
Student: Edgar Bueno dos Santos
Program: Geophysics
Title: “Modelagem numérica da dinâmica do manto e sua interação com quilhas cratônicas da litosfera continental”
Advisor: Prof. Dr. Victor Sacek
Judging Committee
- Prof. Dr. Victor Sacek - IAG/USP
- Dr. Anna Eliza Svartman Dias – PETROBRAS (videoconferência)
- Dr. Rafael Monteiro da Silva – CPRM (videoconferência)
- Dr. Anderson Moraes – PETROBRAS (videoconferência)
- Prof. Dr. Claudio Alejandro Salazar Mora - IGc/USP
Abstract
Different lithosphere thickness models based on seismic tomography indicate that lateral variations of lithospheric thickness can be abrupt, especially along the borders of cratons, where the thickness can vary more than 100 km around the limits of the cratonic keel. These lateral variations can affect the flow of the asthenospheric mantle during the movement of the lithospheric plates which can eventually impact the stress field in the interior of the mantle and crust and affect the topographic evolution of continental margins. The correct quantification of the geodynamic interaction between asthenosphere and cratonic keels involves non-linear flow and scenarios with complex geometric configurations. For this reason, the use of numerical codes is a natural approach to study these geodynamic problems. In the present work, thermo-mechanical numerical models were used with realistic rheology for the crust and mantle to assess how the asthenospheric flow under cratonic keels affected the topography and intraplate stress field. Different thickness values for the cratonic keel and the relative speed between the lithosphere and the base of the upper mantle were tested. It was possible to observe that the horizontal flow of the asthenosphere under the cratonic keel induces extensional stresses in the crust when the asthenospheric flow occurs from the thinner lithosphere towards the craton, defined here as the ``cratonic bow''. On the other hand, compressional stresses in the crust are observed in the region where the asthenospheric flow occurs from the craton towards the thinner lithosphere, a portion defined here as the ``cratonic stern''. The magnitude of the stresses increases with higher speeds and a thicker cratonic keel, reaching a magnitude of $\pm8 - 10$ MPa in the cratonic crust in the scenarios with a cratonic keel with 200 km in thickness. The asthenospheric flow under the cratonic keel induces edge-driven convection with larger vigor adjacent to the cratonic stern, where topographic perturbations are observed especially in scenarios with thick cratonic keel, resulting in negative dynamic topography of hundreds of meters. I propose that this mechanism of dynamic subsidence can explain part of the negative residual topography observed along the southern Australian margin, induced by the fast ($\sim7.4$ cm/year) northward movement of the plate combined with the presence of a thick lithospheric keel in the continent. Furthermore, the north-south continental tilt observed in Australia during the Miocene can be partially explained as the slowdown of the plate and consequent reduction of the dynamic topography amplitude in the continent and marginal regions during the last 30 Myr.
Keywords: intraplate stress,numerical modeling,cratonic keels,mantle convection.