Modelagem numerica conjunta de processos sedimentares e tectônicos em bacias sedimentares
Resumen Abstract Índice Conclusiones
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The Earth’s surface is shaped by different processes that act in different scales of time and space and evolves through the combined effect of tectonic movements and surface processes. While tectonic movements modify the topography as a result of the Earth’s interior dynamics, the surface processes sculpt the landscape through erosion and deposition, resulting in horizontal remobilization of material. These surface processes are dependent on the climatic conditions, local relief and lithological controls, among other factors.
On the other hand, the evolution of the Earth’s surface also affects the interior of the Earth. For example, the primary direct influence of surface processes on the lithosphere is the isostatic effect due to erosion of continents and deposition of sediments in the sedimentary basins. These processes related to (un)loading at the surface can result in many kilometers of vertical lithospheric displacement (Reiners and Brandon, 2006).
Therefore, the surface and the interior of the Earth must be seen as a coupled system, each one modifying the evolution of the other. Due to the complexity of the coupling of the many processes acting in the evolution of the Earth’s surface, the use of computer models is a natural procedure to quantify the relative importance of each of them.
In spite of the great number of numerical models created in the last decades to study the coupling between surface and tectonic processes (see Beaumont et al., 2000 and Bishop, 2007 for reviews), there is a lack of models that takes into account the onshore and offshore parts of rifted margins, with a few exceptions (e.g. van Balen et al., 1995). Furthermore, previous models developed to study the evolution of passive margins and the coupling of surface processes and flexural response do not include the rifting during the simulation and treat the rifting conditions as a “black box”.
Aiming to further contribute to the study of divergent margins, this work presents a numerical model developed by Victor Sacek during his doctorate that tries to simulate the evolution of divergent margins since the onset of rifting, incorporating onshore and offshore processes of erosion and sedimentation, flexural compensation and thermal processes in the lithosphere. The purpose of the thesis was to study how the combination of models for different geological processes contributes to the emergence of new scenarios for the evolution of divergent margins, that otherwise could not be predicted through the analysis of isolated processes.
The numerical model developed consists of four main physical/geological processes: 1 -‐ stretching of the lithosphere; 2 -‐ thermal effects in the mantle; 3 -‐ flexural isostasy and 4 -‐ surface processes.
In this model (Sacek et al., 2012) the stretching of the lithosphere reshapes the surface by faulting and alters the flexural isostatic state of the lithosphere due to horizontal movement of density interfaces, creating stresses related to faulting of the upper crust and thinning of the lower crust during stretching. Stretching also results in advection of material that perturbs the thermal state of the lithosphere, which, in turn, changes its density, resulting in additional vertical stress. Surface processes also induce stresses in the lithosphere by redistribution of mass associated with erosion, transport and deposition of sediments. The flexural response of the lithosphere to these loads results in vertical movement of the surface, perturbing the topography and consequently the evolution of surface processes.
In the numerical model, the lithospheric stretching occurs by faulting in the upper crust and plastic deformation in the lower crust and mantle (Kusznir et al. 1991; Fletcher et al., 2009). The 3D thermal evolution of the mantle was numerically solved through the finite element method (Bathe, 1982). The flexural response of the lithosphere to vertical loads was approximated by a thin elastic plate and solved
numerically by the finite element method (Sacek & Ussami, 2009). The surface processes model describes how the landscape is eroded and how the sediments are transported and deposited in the sedimentary basins and was based in the formulation developed by Braun & Sambridge (1997).
With this model, different scenarios for the evolution of divergent margins were studied and new insights were obtained with the coupling of the different processes. The numerical results show that the amount of lithospheric stretching has a profound influence on the evolution of escarpment migration in divergent margins (Sacek et al., 2012). These results suggest that fault-‐bounded escarpments created at rift flanks by mechanical unloading and flexural rebound have little potential to “survive” as retreating escarpments if the lower crust under the rift flank is substantially stretched. In this configuration, a drainage divide that persists through time is created landward in a position that depends on the flexural rigidity of the upper crust. This scenario occurs when the pre-‐rift topography dips landward, otherwise the evolution of the escarpment is guided by the pre-‐existing inland drainage divide. These concepts were applied to study the margins of Southeastern Australia and Southeastern Brazil, where the retreating escarpment scenario showed to be unlikely. The same numerical model was used to study how the passage of a thermal anomaly under the lithosphere can affect the post-‐rift evolution of sedimentary basins in divergent margins. The numerical results show that the velocity of the lithosphere relative to the thermal anomaly and the flexural rigidity of the continental and oceanic lithospheres affect the evolution of sedimentary basins due to surface uplift related to thermal expansion of the lithosphere. As an example, the model is applied to assess the possible influence of a thermal anomaly (Trindade Plume?) on the evolution of the Campos and Espírito Santo Basins, in Southeastern Brazilian margin.
Other scenarios can be tested in the future to quantify the relative importance of the different surface and tectonic processes. Together with geological and geophysical data, this numerical model can give important contributions to the understanding of rifted margins and help the research in hydrocarbon exploration.