ECHELON FAULT DEVELOPMENT DEPENDS ON STRAIN RATE IN WET KAOLIN

We investigate strike-slip fault development using viscoelastic wet kaolin under different strain rates. The 15-minute Maxwell relaxation time for wet kaolin means that slower experiments, run >1 hour, will have greater stress relaxation. Because the Earth’s crust also has viscoelastic behavior over long time scales, the sensitivity of strike-slip fault growth in wet kaolin to strain rate may replicate strain rate sensitivity of crustal strike-slip systems.

In one suite of experiments, the clay is placed over juxtaposed basal plates, one of which is driven by a motor to produce dextral motion. In another suite of experiments, the plates are separated by an elastic sheet to apply distributed basal shear. For each suite of experiments, clay strength and thickness are constant and we vary strain rate. We use digital image correlation of overhead photos taken at regular intervals to map the incremental strain field throughout the experiment. The incremental strain fields provide insight into the partitioning of deformation as well as the patterns of active faulting.

Our experiments show that faster strain rate produces more closely spaced and shorter echelon faults than slower strain rate experiments. The increased degree of stress relaxation in the slower experiments reduces stress concentration at small flaws so that only the largest flaws develop faults, which accounts for the wider spacing of initial faults. Furthermore, the closely spaced echelon faults propagate into each other’s stress shadow, which limits their length. Tracking of individual echelon faults during their propagation reveals that they do not rotate; rather, the linkage of faults gives the appearance of rotation of the active fault surface. Numerical simulations of the onset of linkage show that shorter and more closely spaced faults don’t accommodate as much slip as faults that are longer and farther apart, which account for the higher kinematic efficiency in the initial stage of echelon fault development for the slower experiments. The shorter echelon faults of the faster strain experiment link to produce smoother and more efficient mature fault surfaces, while the longer echelon faults of the slower strain experiments produce larger, inefficient irregularities along the mature fault that persists throughout the experiment.