Life and Death of Normal Faults: Quantitative Analysis of Fault Network Evolution in 3D Rift Models

Quantifying the spatial and temporal evolution of fault systems is crucial in understanding plate boundary deformation and the associated seismic hazard, as well as to help georesources exploration in sedimentary basins. During the last decade, 3D lithospheric-scale geodynamic models have become capable of simulating the evolution of complex fault systems, from the onset of rifting to sea-floor spreading. But since these models describe faults as finite-width shear zones within a deforming continuum, additional efforts are needed to isolate and analyse individual faults, so we can understand the entire life span of normal fault networks.

Here we present 3D numerical forward models using the open-source community software ASPECT. Our thermo-mechanical models include visco-plastic rheology, strain softening as well as lithospheric and asthenospheric layers to capture rift evolution from inception to continental break-up. We quantify normal fault evolution at the surface of the model with a method that describes fault systems as 2D networks consisting of nodes and edges. Building on standard image analysis tools such as skeletonization and edge detection, we establish a hierarchical network structure that groups nodes and edges into components that make up individual evolving faults. This allows us to track fault geometries and kinematics through time enabling us to analyse the growth, linkage and disintegration of faults.

We find that the initial fault network is formed by rapid fault growth and linkage, followed by competition between neighbouring faults and coalescence into a mature fault network. At this stage, faults accumulate displacement without a further increase in length. Upon necking and basin-ward localisation, the first generation of faults shrink and disintegrate successively while being replaced by newly emerging faults in the rift centre. These new faults undergo a localisation process similar to the initial rift stage. We identify several of these basin-ward localisation phases, which all feature this pattern. In oblique rift models, where the extension direction is not parallel to the rift trend, we observe strain partitioning between the rift borders and the centre, with strike-slip faults emerging in the centre even at moderate obliquity. Analysing the spatio-temporal evolution of modelled faults thus allows us to map their entire life span to observed stages of rift system evolution.