Discovery of sheath folds in the Adula nappe and implications forthe tectonic evolution (Central Alps)

Orogenic deformation patterns show intricate overprinting and structural relations, variations of style and orientation of folds and sense of shear, which are traditionally interpreted as due to polyphase deformation, i.e. distinct deformation phases separated by periods of tectonic quiescence. The Adula nappe in the Central Alps displays exceptional exposures of complex internal structures involving heterogeneous rocks (meta-pelitic and meta-granitic gneisses, micaschists, amphibolites, eclogites, minor quartzites and limestones). The Adula structures are distinguished through the style and the orientation of folds, schistosity and the observation of refolded folds. Structural features show a great variability within the unit, making the structures along the nappe difficult to correlate. However, the Adula deformation patterns are classically interpreted as generated by multiple, distinct deformation phases (five deformation phases; D1-5), despite only one schistosity and lineation may be clearly recognized in the field. Kinematic indicators indicate dominant top-to-N sense of shear, although local top-to-S shear is interpreted as developed during the D3 backfolding phase (e.g. Löw 1987; Nagel 2008). In this contribution, we show a recognition of sheath folds from the central part of the Adula nappe, the largest high-pressure nappe of the Central Alps. We performed detailed geological mapping (scale 1:10000) and structural characterization of the spectacular outcrops of the Piz de Cressim glacial cirque. Here a large antiform is described as the main structure classically associated with the D3 backfolding phase. We show that the meso/leucocratic heterogeneous rocks (orthogneisses, micaschists, migmatitic gneisses, amphibolitic lenses) form highly non-cylindrical folds. Sheath folds are highlighted by several cm to km scale omega and elliptical eye-structures in cross sections perpendicular to the shear direction (y-z plane). Local variations of style and orientation of folds and sense of shear are easily explained by the three-dimensional structure of the sheath folds. All lithological units show one penetrative foliation and a related stretching lineation with variations in orientation. We suggest that the Cressim antiform formed during a progressive, highly non-cylindrical folding under top-to-N deformation accomplished within rheological heterogeneous rocks.