What are Slickenlines ?
Slickenlines are scratches on fault surfaces resulting from shear motion (Fig. 1). Remember that shear fractures are smaller versions of faults that you’ll find in core - so slickenlines record the shear motion of fractures. You won’t find slickenlines on extension fracture or joint surfaces as these are opening mode fractures.
Can we tell the direction of travel ?
Sometimes it is possible to tell the direction of shear by running your fingers over the slickenlined surface - this would be the smooth direction (top right to bottom left for missing block in Fig 1).
What about Slickenfibres ?
Slickenfibres on the fault plane are mineral fibres that grow during fault movement. An example of calcite slickenfibres are shown on the fault plane in Fig. 1.
Fig. 1. Slickenlines in playa lake siltstones, North Sea. Direction of travel of missing block is (relatively) from top right to bottom left. The white patches are calcite slickenfibres.
Why are they important ?
Its very likely you’ll come across rubble zones when looking at rock core. Its well worth checking the pieces of rubble (as well as the intact core of course !) for evidence of slickenlines. The reason for this is that you’ll know you’re looking at natural (not induced) fractures. Secondly these will be shear fractures (small faults) not joints.
Fig. 2 shows bagged rubble from a North Sea appraisal well that was drilled through a seismically resolvable fault in sandstones. The piece of sandstone that the pencil is pointing at has a particularly well developed polished surface and slickenlines - indicating natural shear fractures and consistent with the seismic evidence in this case - of being close to a seismic scale fault.
So, finding slickenlines therefore tells you that you are dealing with natural fractures. There are other criteria that you can use as captured in this blog.. http://www.ogilviegeoscience.co.uk/blog/2021/3/16/natural-vs-induced-fractures-in-core
Fig. 2. Slickenlines and polished surface in Jurassic faulted sandstones from the North Sea.
Where do Mohr Circles come in ?
Mohr circles can be used to predict the critical state of stress required to create a shear fracture (Fig 4). The arrow points to where a roughly 60 deg dipping shear fracture with a 30 degree angle to Sigma 1 (as on core plug in middle) would plot.
