Sheath-folds form when ductile shear is strong enough to amplify initial irregularities of the folded layers. Penetrative shearing of the layers progressively reorients the fold axes that become curvilinear, forming structures resembling eyes in section (or even more complex shapes). They are signs of intense shear within shear zones accounting for large amounts of tectonic displacement. Sheath-fold structures were first described at 10 cm to meter scale as the ones below, from the alpine Corsica, elegantly sketched by Jacques Malavieille (from the paper Faure and Malavieille 1980).
The following sketch illustrates the way sheath folds form by progressive shearing (also by J. Malavieille, from his PhD thesis, 1982)
Below is another drawing again by J. Malavieille showing the different structures that result from intense simple shear in metamorphic rocks (label 3 points to the sheath folds). This sketch is from a paper I co-signed (Malavieille, Lacassin and Mattauer 1984) about stretching lineations (label 5 on the drawing), which we interpreted as markers of tectonic transport direction in the western Alps.
Almost at the same time, we managed to explain a strange kilometre-scale "eye fold" from the Swiss Alps as a section of a giant sheath-fold. This story made the cover of Nature in June 1985 (Lacassin & Mattauer, Nature, 1985). The Nature's paper followed a preliminary description, in French, by Maurice Mattauer (see last figure below). It allowed us to assess the minimum amount of tectonic transport - several tens of km - on one of the major alpine thrusts.
- Faure M, Malavieille J. (1980). Les plis en fourreau du substratum de la Nappe des Schistes lustres de Corse; signification cinematique. Comptes Rendus Hebdomadaires des Seances de l’Academie des Sciences, Serie D: Sciences Naturelles, 290(21):1349-1352
- Malavieille, J., Lacassin, R., & Mattauer, M. (1984). Signification tectonique des linéations d’allongement dans les Alpes occidentales. Bull. Soc. Geol. Fr, 26, 895-906.
- Lacassin R, Mattauer M. (1985). Kilometre-scale sheath fold at Mattmark and implications for transport direction in the Alps. Nature (London). 315(6022):739-742.
