Slip on normal faults induced by surface processes after the cessation of regional extension—Insights from three-dimensional numerical modelling

Abstract

In regions of active extension, normal faulting generates topography that is continuously modified by erosion, sediment transport and deposition. As shown by previous numerical models with full coupling between tectonics and surface processes, the redistribution of mass at the Earth's surface accelerates the rate of faulting by affecting the stress state of the crust. It remains unknown, however, how fault slip evolves as a result of ongoing surface processes if regional extension as the main driver of faulting ceases. Here we use three-dimensional finite-element modelling to show that surface processes acting on normal-fault bounded mountain ranges may sustain fault slip for millions of years even after regional extension has stopped. The models consist of two successive phases. During the first phase, the normal fault accumulates displacement owing to an extensional boundary condition, while erosion and sediment deposition are active on the model surface. At the beginning of the second phase, extension of the model is stopped while the surface processes remain active. The results show that in most models normal faulting continues during the second phase at rates of ~20 to ~70m/Ma for more than 1Ma. In some experiments, normal slip is maintained for ~3Ma, whereas in other models, a short phase of normal faulting is followed by slow reverse slip. The maximum amount of normal slip in different experiments reaches up to 90m during the second model phase. If erosion is intensified by increasing the diffusion constant by a factor of 5, the fault accumulates the additional normal slip at a faster rate, i.e. during a shorter time period. In contrast, a five-fold variation of the fluvial erosion constant does not significantly affect the fault slip evolution. Variations of the fault dip and length have a similar effect on the duration of the phase with additional normal slip as variations of the diffusion constant. The fault slip evolution is correlated with the temporal evolution of the erosion and sedimentation rates, which decrease more or less gradually after the end of extension. Ultimately, the fault slip behaviour is controlled by the evolution of the differential stress, which varies through time due to the redistribution of mass on the model surface that is induced by erosion, sediment transport and deposition. Our results imply that individual normal faults may remain active even if regional extension ceases because surface processes continue to modify the fault-generated topography.