Slickenline orientations as a record of fault rock rheology

- We used ground based LiDAR to measure slickenline orientations on exposed faults.
- Slickenline directions vary little over distances of a few m across fault surfaces.
- Fault slip insensitive to stress perturbations from small-scale asperity contact.
- Scale-dependent inelastic asperity failure explains consistent slip directions.
- Inelastic/elastic crossover is recorded by fault core thickness.

Contact of geometric asperities across rough faults causes perturbations to the shear traction resolved on a fault surface that could potentially deflect the local slip direction. Slickenlines, which record the relative displacement of opposing sides of a fault, may therefore be sensitive to fault surface geometry. To investigate the relationship between fault geometry, shear traction and slip, we use ground-based LiDAR to measure the orientations of slickenlines defined by centimeter-amplitude corrugations on three fault surfaces. Slickenline rakes measured in the mean plane of each fault rarely vary by more than a few degrees. Deviations from the mean rake do not correlate with the fault surface topography at scales up to a few meters, but a weak correlation may be present at larger scales. Slip directions are therefore insensitive to any shear traction perturbations from the contact of geometric asperities at small scales. This observation is consistent with scale-dependent deformation. We show that the roughness of fault surfaces implies that short wavelength asperities fail inelastically because of flattening during fault slip. A crossover to elastic deformation occurs at lengths of 10−2 to 100 m, which defines the minimum dimension of a strength asperity that influences the displacement field. The roughness at the crossover length scale corresponds to the typical thickness of fault rocks in the fault core suggesting that fault rock thickness is related to the crossover length scale. The data requires that multiple processes combine to produce the fault surface geometry. We reconcile the variety of processes with the consistent surface roughness scaling by noting that all of the processes are governed in different ways by the elastic limit of rock.