SHEAR STRAIN LOCALIZATION AND EVOLVING KINEMATIC EFFICIENCY OF STRIKE-SLIP FAULT DEVELOPMENT WITHIN WET KAOLIN EXPERIMENTS

We document the evolution of strain localization and kinematic efficiency during strike-slip fault maturation within wet kaolin experiments with variable boundary conditions. Kinematic efficiency measures the percentage of applied displacement accommodated as fault slip rather than as off-fault deformation. To investigate different conditions of strike-slip fault development, we 1) test reactivation of a pre-cut fault, 2) vary the depth of the basal dislocation and 3) vary the distribution of basal shear. Fault systems above deep dislocations have delayed strain localization, and delayed increase in efficiency consistent with fault propagation from the experiment base. While the nature and depth of the basal shear influences observed strain localization and damage development, similarities between experiments reveal a general conceptual model for distinctive stages of fault evolution. Stage 0 represents distributed shear across a wide zone with no faulting. At the onset of faulting in stage 1 the shear zone begins to narrow and localize towards the incipient principal fault. Stage 2 sees the continued growth, interaction and abandonment of echelon faults. During stage 3, the shear zone narrows to a through-going, continuous fault by growing new linkages and abandoning other fault surfaces. Throughout these four stages, kinematic efficiency increases until strain is localized and supported by commiserate reduction of fault complexity (number of faults), shear zone width and distributed off-fault deformation. While strain localizes and organizes, linkage of echelon faults provides significant improvements to kinematic efficiency. However, the lengthening of echelon faults in stage 2, affords systems no changes in kinematic efficiency and some shear zones temporarily widen. Because the uncut experiments localize strain onto echelon faults, rather than reactivate a precut fault surface, the uncut experiments develop irregular final fault geometry. Whereas some stepovers comprising the irregular fault geometries of the uncut experiments are abandoned in favor of more planar segments, others persist throughout our experiments. The persistent geometric irregularities reveal the energetic cost associated with growing a new more efficient and planar fault.