Slickenline orientations as a record of fault rock rheology

James D Kirkpatrick,Emily E Brodsky

doi:10.1016/j.epsl.2014.09.040

James D Kirkpatrick, Emily E Brodsky

Open Access

https://doi.org/10.1016/j.epsl.2014.09.040

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Abstract

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.

Highlights

Fault slickensides display an array of tool marks, corrugations, intersecting fractures, scoops and mullions
Our results show that at the small scales well resolved by the LiDAR datasets the slickenline rakes are consistent over the fault surfaces, and the measured orientations do not appear correspond to any fault geometry control (Fig. 3)
These stresses are thought to perturb the tectonic stress field and cause the fault surface shear tractions to vary spatially. Boundary conditions such as the fault finiteness can result in a rotation of the slip direction relative to the shear tractions, for an elastic, isotropic fault zone the superposition principle requires that perturbations to the stress field generate a local rotation of the displacement field (Pollard et al, 1993). It follows that if the slickenlines were locally parallel to the maximum shear traction directions, stresses induced by deformation of the rough fault surfaces would cause deflection of slickenline trajectories

Summary

Introduction

Fault slickensides display an array of tool marks, corrugations, intersecting fractures, scoops and mullions These features are used as kinematic indicators that record the relative motion of opposing sides of a fault (Petit, 1987; Doblas, 1998). Together, they define the shape of a fault surface, which for a large variety of faults has been shown to be non-planar at all scales of observation with increasingly large geometric asperities (bumps in the topography of the fault surface) with increasing length scale (Brown and Scholz, 1985; Power et al, 1987; Lee and Bruhn, 1996; Renard et al, 2006; Sagy et al, 2007; Sagy and Brodsky, 2009; Candela et al, 2009, 2012; Fondriest et al, 2013; Siman-Tov et al, 2013).

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