Normal fault network evolution in 3D numerical models

Understanding how normal fault networks initiate and evolve is important for quantifying plate boundary deformation, assessing seismic hazard and finding natural resources. In recent years, 3D numerical models have been developed that can simulate the entire process of normal fault formation, from the start of rifting to the creation of new ocean floor. However, state-of-the-art methods treat faults as finite-width shear zones not as discrete entities, so additional work is needed to isolate individual faults and their characteristics in order to better understand fault system dynamics over geological scales.

We present 3D numerical rift models of moderately oblique extension using the ASPECT software. These models reproduce the thermo-mechanical behavior of Earth's lithosphere and simulate fault system dynamics from inception to breakup accounting for visco-plastic rheology, strain softening and surface processes. We use a method that extracts surficial fault systems as 2D networks of nodes and edges to study the evolution of normal faulting. By applying data analysis techniques, we group nodes and edges into components that represent individual faults and track their geometry and movement over time.

We find that the initial fault network forms through rapid fault growth and linkage, followed by competition between neighboring faults that leads to their coalescence into a stable network. At this point, modelled normal faults continue to accumulate displacement but do not grow any longer. As deformation localizes towards the center of the rift, the initial border faults shrink and disintegrate, being replaced by new faults in the center of the rift. During that transition, we document strain partitioning between predominantly dip-slip border faults and oblique-slip or strike-slip intra-basin faults. The longevity of faulting is thereby controlled by crustal rheology and surface process efficiency. Quantitative analysis of fault evolution allows us to deduce fault growth and linkage as well as fault tip retreat and disintegration in unprecedented detail.