What are Slickenlines ?
Slickenlines are scratches on fault surfaces resulting from shear motion (Figs 1, 2). 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 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.
Figure 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.
Figure 2. Slickenlines in Cretaceous shale core, West of Shetland, UK.
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. 3 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
Figure 3. Slickenlines and polished surface in Jurassic faulted sandstones from the North Sea.
Slickenlines can also help us understand the structural evolution of our study area. Ideally, they should be studied directly on fault planes in rock outcrops. It is usually more straightforward to take measurements in outcrop than on core as the core has to be oriented.
The fault in Fig 4. self-juxtposes Permian dune sandstones - given the accessibility of the fault plane it is worth checking it out for slickenlines and other structures. Most of the faults in this area are normal faults as is this one (although its not clear from the photo) and we’d expect any slickenlines to run up and down the fault plane. But the slickenlines run (or plunge) diagonally from top left to bottom right (Fig. 5) indicating that the fault has had some oblique movement. In other words, there has been some movement along strike as well as in the direction of dip. This is consistent with regional work on seismic data in the Moray Firth that the basin has also experienced strike-slip tectonics [1].
Figure 4. Clashach Fault, Hopeman Sandstone, Moray, UK.
Figure 5. Slickenlines on the fault plane in Fig. 4. Left: looking in towards the Fault plane. Right. Facing the fault plane where slickenlines have a ribbed appearance running top left to bottom right. Only one of these is highlighted in white so as not to obscure the others.
In Conclusion..
It is valuable to search for slickenlines in cored samples (e.g., amongst rubble) as they tell us if we are dealing with natural fractures. It is usually easier to use slickenlines in faulted outcrops for determination of sense/direction of movement as the core needs to be oriented. This is important in interpreting the structural history of an area.
References
[1]. Roberts, A.M., Badley, M.E., Price, J.D. and Huck, I.W. 1990. The Structural History of a transtensional basin: Inner Moray Firth, NE Scotland. J. Geol. Soc. London, 147,7- 103.