Rifted margin architecture and the interplay between mantle, crustal and surface processes from geodynamic numerical experiments

Divergent margin development is a fundamental aspect of plate tectonics, yet it remains poorly understood. Key issues like the formation of tectonic asymmetry between conjugate margins, the detailed history of vertical movements, and the influence of sedimentation on margin architecture remain unresolved. In this PhD I developed accurate numerical tools essential to understand margins and their sedimentary response.

I placed particular emphasis in simulating uplift and subsidence for which I have developed a free-surface algorithm. Stress-free surfaces typically suffer from instabilities when the time step is bigger than the viscous relaxation time. The new free-surface algorithm improves performance of the models avoiding instability, so that the code yields stable and accurate dynamic topographies.

Subsequently, I explored the factors influencing the polarity of the asymmetry between conjugate margin pairs. Along the magma-poor stretch of the South Atlantic margins the polarity correlates with the distance of the rift to nearby cratons. Numerical experiments of extension show that the presence of a thick cratonic lithosphere inhibits asthenospheric flow from underneath the craton towards the fold belt, while flow from underneath the fold belt towards the craton is favoured thereby enhancing craton-ward faulting. These faults become dominant, resulting in a wide faulted margin in the fold belt, and a narrow conjugate margin in the craton side, as observed in nature.

Finally, I implemented surface processes into the models to study the feedbacks between tectonics and sedimentation. Models show that different rates in erosion/deposition have an important impact on margin subsidence and architecture. This influence is modulated by lower crust rheology. Furthermore, models showcase varying-in-time break-up unconformities along the margins, which are explained as a result of rift migration.

Although numerical models do not represent nature in its full complexity, they are an excellent testing tool for studying interplays between geological processes. Future work will include further addition of complexity into the codes to understand a variety of problems including oceanization and feedbacks between climate and tectonics.