From plate tectonic reconstructions to 4D geodynamic models of the Alpine Orogeny

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Most geological evolution scenarios for the Alps rely on plate tectonic reconstructions, geological data and geophysical observations of the present-day Alps. Such reconstructions give plausible scenarios on how the Alps may have formed, but they are purely based on kinematic rules. It is therefore unclear whether they are also consistent with the physics of the lithosphere and mantle. Moreover, uncertainties of the various constraints that are included in the reconstructions are usually not considered.To fully understand mountain building processes in 4D, we will address both aspects:1) We propose to investigate the physical consistency of tectonic scenarios using time-resolved 3D geodynamic models. They give insights in parameters such as the rheology of the lithosphere, the occurrence of phenomena like slab breakoff events, and show how the reorganization of the lithosphere affects mountain building. Dynamically consistent models also allow us to refine the plate tectonic reconstructions and guide the interpretations of seismic tomography models. 2) We will take uncertainties into account in the plate tectonic reconstructions. This will give a better understanding of which features are robust and which less so, a crucial piece of information when comparing them with dynamic models.We will first perform 3D thermo-mechanical geodynamic simulations of the Alps, starting from plate tectonic reconstructions and using a combined inverse and forward modelling approach. To make the problem tractable, we start with reconstructions at 20 Ma and perform systematic forward simulations to explore alternative evolution scenarios for different rheologies, initial geometries and thermal structures. Our simulations will include nonlinear visco-elasto-plastic rheologies and a free surface and will thus be able to simulate the spontaneous occurrence of shear zones, slab breakoff events and the generation of new oceanic crust in a self-consistent manner. Erosion is implemented in a simplified manner, which allows testing the interaction between surface and deep lithospheric processes. In a next step, we will extract plate kinematic rules and uncertainties from the geodynamic simulations by performing a sensitivity analysis on quasi-instantaneous models for different snapshots of the time-dependent models, to constrain how small changes in the model parameters affect plate velocities. This is an important ingredient for a new inverse modelling approach, developed in the third part of this project, that employs data assimilation techniques to formulate plate tectonic reconstructions as an inverse problem. In this, we take uncertainties in the data, as well as physics-based constraints on plate velocities into account which will result in a range of plate tectonic reconstructions that are all consistent with the data. Combined with new and existing data of the 4DMB project, this will give new insights in the physics of mountain building processes.