Velocity influence on deformation partitioning along evolving restraining bends

The evolution of strike-slip restraining bends depends on early fault geometry (e.g., bend angle & stepover distance) and material properties, yet the influence of loading rate on fault system evolution is unknown. Within viscoelastic materials, such as the crust, the relaxation of stresses depends on loading rate. Under faster strain, faults will have shorter recurrence intervals, which reduces the time for stress relaxation during the interseismic period. Because slow strain rates yield greater stress relaxation, the growth of new faults near restraining bends may depend on loading rate. While crustal restraining bends evolve under a range of strain rates, the field expressions of faulting have overprints of early and late deformation, which makes discerning the impact of early strain rate on fault growth difficult. Here, we use scaled physical experiments to directly investigate the impact of strain rate on the evolution of restraining bends. We use wet kaolin as an analog for the crust because it creates sharp faults that remain active even when the loading orientation deviates slightly from the ideal for fault slip. In addition, off-fault stresses within the wet kaolin dissipate over time just as stresses within the crust do via inelastic processes and are tracked with tests on Anton Paar MCR102 rheometer. We directly observe and record the horizontal surface deformation for three experiments with the same initial restraining bend geometry. Computer-controlled stepper motors drive a basal plate at a prescribed velocity to induce faulting within the overlying layer of wet kaolin. The three experimental loading rates of 0.25, 0.5, and 1.0 mm/min scale to crustal loading rates of 2-4, 4-8, and 11-22 mm/yr respectively. We use digital image correlation to calculate incremental displacement and strain field data from overhead photos. Restraining bend experiments with different loading rates produce different deformation histories; slower applied loading produces greater  off-fault deformation and more secondary faults. Furthermore, new oblique-slip faults that grow within the slower loading rate experiments accommodate greater slip than the new faults that grow within the faster loading rate experiments. This suggests that strike-slip fault systems in slow strain rate regions may have slip distributed among several faults whereas slip may localize along a few faults within high strain rate regions. Additionally, the restraining bend geometry becomes more open in the slower loading rate experiments due to greater off-fault deformation. The differences in fault evolution owe to the sensitivity of both the wet kaolin strength and the degree of stress relaxation to strain rate, supported by rheometer tests. The experimental data suggest that loading rate can impact strain partitioning and fault geometry in crustal faults.