Bridging spatiotemporal scales of normal fault growth using numerical models of continental extension

Continental extension is accommodated by the development of kilometre-scale normal faults, which grow by accumulating metre-scale earthquake slip over millions of years. Reconstructing the entire lifespan of a fault remains challenging due to a lack of observational data of appropriate scale, particularly over intermediate timescales (10^4-10^6 yrs). Using 3D numerical simulations of continental extension and novel automated image processing, we examine key factors controlling the growth of very large faults over their entire lifetime. Modelled faults quantitatively show key geometric and kinematic similarities with natural fault populations, with early faults (i.e., those formed within ca. 100 kyrs of extension) exhibiting scaling ratios consistent with those characterising individual earthquake ruptures on active faults. Our models also show that while finite lengths are rapidly established (< 100 kyrs), active deformation is highly transient, migrating both along- and across-strike. Competing stress interactions determine the overall distribution of active strain, which oscillates locally between localised and continuous slip, to distributed and segmented slip. These findings demonstrate that our understanding of fault growth and the related occurrence of earthquakes is more complex than that currently inferred from observing finite displacement patterns on now-inactive structures, which only provide a spatial- and time-averaged picture of fault kinematics and related geohazard.