Velocity influence on restraining bend evolution
Hanna Elston, University of Massachusetts, Amherst
Michele Cooke, University of Massachusetts, Amherst
Abstract
How strike-slip restraining bends evolve depends on 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 rock materials within the crust, strength can depend on strain rate. For such materials, the initiation and propagation of new faults near restraining bends within strike-slip systems may depend on the applied loading rate. Because crustal restraining bends can evolve under a range of loading rates, discerning the impact of early strain rate on fault growth is difficult due to overprinting of early and late deformation. 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 long-lived and sharp faults that can reactivate. For the range of strain rates that we test, the viscoealstic wet kaolin shows strain-rate hardening as do many rock materials. We directly observe and record the horizontal surface deformation for six restraining bend experiments: three experiments simulate a gentle bend with a 15-degree bend angle and three experiments simulate a sharper bend with a 30-degree bend angle. 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 horizontal displacement and strain field data from overhead photos. For all six experiments, fault growth and abandonment impact slip rates along nearby faults and the system kinematic efficiency, which is the ratio of fault slip to applied displacement. Restraining bends deformed under different strain rates produce different faulting histories; experiments with slower applied loading produce more faults. Faster loading experiments produce fewer faults and greater off-fault deformation. The differences in fault evolution owe to variations of the wet kaolin strength with loading rate. The experimental results suggest that loading rate can impact fault geometry and partitioning of strain as off-fault deformation in crustal restraining bends.
Session
Session 1: Fault Zones from Top to Bottom