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.