Striation and slickenline development on quartz fault surfaces at crustal conditions: Origin and effect on friction

Fragments of optically flat silica discs embedded in synthetic gouge were deformed to examine the relationship between the development of striations and slickenlines, and deformation mechanisms, conditions, and fault rheology. Experiments were performed under hydrothermal conditions in a rotary shear apparatus at 100°C or 450°C, to shear strains of 2.02 < γ < 8.25. Slip hardening and softening prevail at low and high temperatures, respectively. In recovered samples, disc fragment surfaces are decorated by fine gouge, sometimes arranged in trails, pits, and scratch marks. Prominent grooves—inferred slickenlines—with constant orientation, wavelength <10 μm, and amplitude <0.7 μm are only observed on disc fragments deformed at 450°C. Some parts of the grooves below the original disc fragment surface contain scattered rounded beads of silica ~200 nm diameter. Conversely, close examination of pits in 100°C experiments reveals they contain angular particles <2 μm diameter. The 200 nm diameter crystalline quartz can precipitate at 450°C in only 250 s, well within the time frame of the experiments but precipitation at 100°C would take at least 8 years. No systematic dislocation arrays were observed in the quartz disc fragments, but microfractures are sporadically present. This indicates that at both temperatures brittle failure generated microfractures and microcomminution occurred where gouge particles impacted disc fragment surfaces. These observations suggest formation of silica beads by precipitation from amorphous silica facilitates slip weakening, smoothing of the fault surface parallel to the slip vector, development of undulations perpendicular to the slip vector by a pressure solution creep mechanism, weakening, and maintenance of a constant slip direction.