On the Development of Shear Surface Roughness

From deformed quartzites in the Singhbhum Shear Zone, eastern India, we report shear fractures of varying surface roughness: very smooth, containing no lineation to strongly rough with prominent slickenlines. We reproduced them in analogue laboratory experiments, which suggest that the modes of shear failure (brittle versus ductile) and the fracture orientation are potential factors to control the fracture roughness. The experiments were conducted on cohesive sand‐talc models with varying sand:talc volume ratio. Pure sand models underwent Coulomb failure in the brittle regime; this failure mode switched to plastic yielding in the ductile regime with increasing talc content. Such a transition in failure behavior resulted in a remarkable variation in the fracture roughness characteristics. Shear fractures produced by Coulomb failure are smooth, and devoid of any slickenlines, whereas those produced by plastic yielding display strongly linear roughness, defined by cylindrical ridges along the slip direction. Such linear irregularities become more prominent with increasing fracture orientation ( θ ) to the compression direction ( θ  = 30 to 60°). We develop a new computational technique, based on controlled optical images to map the shear surface geometry from field casts and laboratory samples. Binarization of the irregular surface images (cantor set) provides 1‐D fractal dimension ( D ), which is used to quantify the roughness variability, and the degree of their anisotropy in terms of Δ D (difference in D across and along the slip direction). From numerical models, we finally show onset of wave instability in the mechanically distinct rupture zone as an alternative mechanism for slickenline formation.