Feedbacks between brittle deformation and surface processes: Insights from extensional settings
Major fault systems are the primary manifestation of localized strain at tectonic plate boundaries. Slip on faults creates topography that is constantly reworked by erosion and sediment deposition. This in turn affects the stress state of the brittle upper crust. Numerical models commonly predict that surface processes can modulate the degree of strain localization, i.e., the partitioning of strain onto a given number of master faults and/or the lifespan of individual faults. The detailed mechanisms, potential magnitude, and geological evidence for such feedbacks however remain debated. We address this problem from the perspective of continental rifts, and at the scale of individual fault-bounded structures. Half-grabens in particular constitute ideal natural laboratories to investigate brittle deformation mechanisms (e.g., fault localization, elasto-plastic flexure...) in relation to continued erosion of the master fault footwall and sediment deposition on the hanging wall. Through an energy balance approach, we show that suppressing relief development in a half-graben can significantly enhance the lifespan of its master fault if the upper crust is moderately strong. Simple geodynamic simulations where tectonic topography is either entirely leveled or perfectly preserved confirm our analytical predictions.
Natural systems, however, lie somewhere in between these two endmembers. To better represent the true efficiency of surface processes at redistributing surficial masses, we couple a 2-D long-term tectonic code with a landscape evolution model that incorporates stream power erosion, hillslope diffusion, and sediment deposition. We identify a plausible range of landscape evolution parameters through morphological analyses of real normal fault-bounded massifs from the East African Rift and Western United States. This allows us to assess the sensitivity of half-graben evolution to a documented range of rheological, climatic, and lithological conditions. We find that half-grabens that reach topographic steady-state after a short amount of extension (~1 km) are more likely to accumulate master fault offsets on par with the thickness of the upper crust. Conversely, a longer phase of topographic growth ––for example due to low rock erodibility–– will favor the initiation of a new master fault and the abandonment of the initial one. A less erodible crust could thus be more prone to extension on a series of horsts and grabens, while more erodible units would deform as long-lived half-grabens. Lithological controls on erodibility could therefore constitute a form of structural inheritance in all geodynamic contexts.