Slickenlines are lineations thought to record slip motion and mechanical
wear within shear fractures. Their formation mechanisms and effect on
friction and fault rheology are poorly understood. We investigate
natural slickenlines from strike-slip, normal, and low-angle detachment
faults formed in volcanic, quarzitic, and mylonitized sedimentary
lithologies, respectively. Slickenline surfaces exhibit non-Gaussian
height distributions and anisotropic self-affine roughnesses with
corresponding mean Hurst exponents in directions parallel– 0.53±0.07–
and perpendicular –0.6±0.1– to slip. However, there is a significant
variability in the fractal roughness descriptors obtained from multiple
hand samples per fault surface.
Microstructural analyses reveal that the principal slip surface is
formed by a thin (≤100 µm) nanoparticulate- and phyllosilicate-rich
layer, followed by a ~10 μm thick layer of increased
cohesion, wherein several smaller grains coalesce into bigger
aggregates. These microstructures are present in most analyzed samples
suggesting that they commonly form during fault slip regardless of
lithology or tectonic setting.
Our results 1) suggest that deformation immediately adjacent to fault
surfaces is energetic enough to comminute the rocks into nanometric
grains and 2) highlight the intricacies of fault systems not fully
captured by current models, which are likely to impact stress
distributions and frictional responses along faults.