A Closer look into Slickenlines: The link between surface roughness and microstructure.

Abstract:

Slickenlines are lineations commonly found within shear fractures and fault surfaces that are thought to record slip motion and mechanical wear occurring during faulting. However, the mechanisms that give rise to slicken-surfaces and their effect on friction and fault rheology are not well understood. We investigate slickenline samples from 1) a strike-slip fault near Autlán de Navarro, Mexico, 2) a low angle normal fault in the Piute Mountains in the eastern Mojave Desert, CA, and 3) the Waterman Hills low angle detachment north of Barstow, CA. These faults formed in volcanic, quartzite, and mylonitized sedimentary lithologies, respectively. We performed roughness and microstructural analyses on the collected samples using a laser profilometer, petrographic microscope, and FE-SEM.

Preliminary results indicate samples exhibit an anisotropic self-affine roughness with corresponding Hurst exponents 0.56 ± 0.06 parallel to slip and 0.62 ± 0.06 perpendicular to slip and do not show any dependence on lithology or tectonic setting. Microstructural analyses reveal an abrupt decrease in grain size leading to the slicken-surface marked by a few microns thick nanoparticulate/partly-amorphous/ phyllosilicate-rich layer, which possesses fluidization-like structures. A ~10 μm thick layer of increased cohesion near the slip surface is also observed showing “healing microstructures” such as serrated, interlocked grain boundaries and coalescence of several smaller grains into bigger aggregates. These microstructures are present in most analyzed samples suggesting that they commonly form during fault slip with little to no influence from the lithology or tectonic setting.

Our results suggest that deformation immediately adjacent to the fault surface is energetic enough to fracture and comminute the rocks into nanometric grains, challenging the concept of grinding limit. As the consumed energy increases exponentially with decreasing grain size, the ubiquitousness of a thin nanoparticulate/partly-amorphous layer across lithologies might point towards a universal mechanism for slickenline formation and suggests that a reevaluation of the energy budget during fault slip is needed.