Slickensided shear surfaces are ubiquitous in many fault zones. However the internal structure, the micromechanics and the evolution of these structures are not fully understood and the contributions of crystal plasticity, grain-boundary sliding, microfracturing, solution-precipitation and mineral transformation under different conditions are subject of debate.

We studied well- preserved core samples from the Main Fault, an up to 3 m wide zone of approximately 10 m offset in the Mont Terri Underground Research Laboratory (CH), a site to evaluate long-term safety of radioactive waste disposal. The drill core breaks easily along many slickensided shear surfaces indicating reverse slip, which form an anastomosing network connected by branch lines.

Broad ion beam polishing and scanning electron microscopy shows that the slickensides are invariably revealed by fracture of the drill core along a few μm thick shear zone, which acts as a crack guide for fracturing the samples. In this zone, a complex set of processes is inferred, leading to extreme localization of strain, development of strong particle preferred orientation, the formation of nanoparticles, and local precipitation of calcite veins in releasing sections. In lenses between shear zones, homogeneous gouge is formed with a well-developed oblique foliation and removal of calcite grains by pressure solution. We infer that with progressive deformation, the number and density of slickensided shear surfaces increases, generating tectonically derived scaly clay and more homogeneous gouge. In all deformed elements of the Main Fault, porosity is much smaller than in the undeformed Opalinus Clay. An interesting observation is the almost complete absence of cataclastic microstructures. Transmission electron microscopy (TEM) of focused ion beam lamellae of this micron-wide shear zone shows a strong preferred orientation of clay minerals, including nano-sized illite particles. In TEM, the shear zones envelop hard particles and confirm an almost complete loss of porosity compared to the protolith.

We propose that inter- and transgranular microcracking, pressure solution, clay neoformation, crystal plasticity and grain boundary sliding are important micro-scale processes during the early stages of faulting in Opalinus Clay and thus need to be considered in extrapolating laboratory results to long-term mechanical behavior.