Strain Localization and Evolving Kinematic Efficiency of Initiating Strike-Slip Faults within Wet Kaolin Experiments: Results and Applications

Alex Hatem, University of Southern California
Michele Cooke, UMass Amherst
Kevin Toeneboehn,
Claire Johnson, University of Southern California

Using wet kaolin experiments, we document the evolution of strain localization during strike-slip fault maturation under variable boundary conditions (pre-existing fault, depth of and distribution of basal shear). While the nature of the basal shear influences strain localization observed at the clay surface, similarities between experiments reveal a general conceptual model of strain accommodation. First, shear strain is accommodated as distributed shear (Stage 0), then by development of echelon faults (Stage I), then by interaction, lengthening and propagation of those echelon faults (Stage II) and, finally, by slip along through-going fault (Stage III). Stage II serves as a transitory period when the system reorganizes after sufficient strain localization. Here, active fault system complexity is maximized as faults link, producing apparent rotation of active fault surfaces without material rotation. While the major shear zone across the entire fault zone narrows throughout the experiments, minor shear zones surrounding individual active and nascent echelon faults slightly widen while these faults interact during Stage II. As strain continues to localize, off-fault deformation decreases while fault slip and kinematic efficiency increases. We quantify kinematic efficiency as the ratio of fault slip to applied displacement. All fault systems reach a steady-state efficiency in excess of 80% in Stage III. Despite reducing off-fault deformation, the through-going fault maintains <1.5 cm structural irregularities (i.e., stepovers), which suggests that small (<3 km) stepovers may persist along mature, efficient faults in the crust. Scaled analog experiments such as these are a window into crustal processes, such as strain localization, that necessitate cumbersome time and length scales, that would otherwise be difficult to observe in a research lab, and also in a teaching setting. We explore a potential application for similar strain localization experiments to be carried out in teaching labs by deforming dry sand in a low-cost shear box. Deformation in teaching experiments can be monitored optically, using mounted cell phones as cameras and lights. These data can then be analyzed using particle image/tracking velocimetry methods utilized during the research grade, wet kaolin experiments. Dry sand and wet kaolin experimental results can be compared in the classroom for differences due to scaling and material properties, while still gleaning similar, general observations of strain localization during strike-slip fault growth.


Shear Zones