Shear zones are areas of intense deformation that localize the movement of one block of the crust with respect to another. In previous posts, we have seen that shear zones contain some very deformed rocks called mylonites, lineations that tell us the direction of movement, and useful kinematic indicators, such as S-C fabrics, that allow geologists to understand which way the rocks moved. However, we have just started to scratch the surface of the complexity of shear zones, which may also contain a special (and very complicated) kind of fold: sheath folds.
What are sheath folds?
Imagine holding a piece of fabric, like a curtain, tightly between your hands. If you start to slide your hands past each other, the curtain will slide too but also start to develop irregular folds, as the friction between the cloth and the irregular surface of your hands drags some parts of it more than other. The same happens in rocks. We are used to imagine shear zones as sharp zones of high deformation, but they are commonly irregular, with areas of different thickness and various rock types on their margin. If there is a small irregularity on the shear zone margin, deformation can ‘pinch’ it and drag it with the flow, progressively amplifying it to a strongly irregular fold.
Sheath folds are incredibly irregular. The exact term in geology is ‘non-cylindrical’: they have strongly elongated and curved hinges, like a long nose! In shear zones, they are stretched parallel to the direction of tectonic transport. Due to their complex shape, sheath folds are commonly difficult to visualize. In most outcrops, we ‘see’ only sections through sheath folds. In sections parallel to the direction of transport (i.e. parallel to the stretching lineation), they may look like regular folds, mainly tightly deformed, but with recognizable hinges and limbs.
Things become interesting when we look at sections perpendicular to the stretching lineation, because these folds are so strongly sheared and elongated that they form tightly closed surfaces, resembling eyes.
Do you see the outline of the nearly flat fold right behind the coin? Here is another example where I have highlighted the fold:
A strongly non-cylindrical fold is not enough to make a sheath fold!
We now have a list of ingredients for our sheath folds:
- They must be strongly non-cylindrical.
- They develop in shear zones.
- They show a constant orientation of the stretching lineation on their surface, parallel to the sense of tectonic transport.
Point #3 is very important, because there other ways to produce very irregular folds, like folding and refolding (interference folding), but in this case you will get also folded lineations. On the other hand, since sheath folds develop in a shear zone, where rocks are dragged past each other in a specific direction, they show a constantly oriented lineation on their surface, normally perpendicular to the sections where they appear as eyes or ‘flat’ pancakes if you prefer a food analogy. Fold hinges, indeed, tend to be parallel to this lineation.
As you can see, it is not easy to document sheath folds and I needed to look at the same outcrop on all the available sections to describe them. As you might imagine, documenting sheath folds in one shot is very rare, but the spectacular Cap de Creus Shear Zone (Catalunya, Spain) ‘delivers again’, quoting Emily Finch. Here is the most spectacular example of sheath folds you will see today:
Extras from the EGU community
References and further reading
Alsop, G. I., & Holdsworth, R. E. (2006). Sheath folds as discriminators of bulk strain type. Journal of Structural Geology, 28(9), 1588-1606.
Alsop, G. I., & Carreras, J. (2007). The structural evolution of sheath folds: A case study from Cap de Creus. Journal of Structural Geology, 29(12), 1915-1930.
Bell, T. H., & Hammond, R. L. (1984). On the internal geometry of mylonite zones. The Journal of Geology, 92(6), 667-686.
Cobbold, P. R., & Quinquis, H. (1980). Development of sheath folds in shear regimes. Journal of structural geology, 2(1-2), 119-126.
Fossen H. (2010). Structural Geology. Cambridge University Press.
Krabbendam, M., & Leslie, A. G. (1996). Folds with vergence opposite to the sense of shear. Journal of Structural Geology, 18(6), 777-781.
Platt, J. P. (1983). Progressive refolding in ductile shear zones. Journal of Structural Geology, 5(6), 619-622.
Tikoff, B., & Greene, D. (1997). Stretching lineations in transpressional shear zones: an example from the Sierra Nevada Batholith, California. Journal of Structural Geology, 19(1), 29-39.
Turner, F. J. & Weiss L. E. (1963). Structural analysis of metamorphic tectonites, Francis J. Turner, Lionel E. Weiss. McGraw-Hill. New York. US.