The mylonites of the Austroalpine Dent Blanche nappe along the northwestern side of the Valpelline Valley (Italian Western Alps)

Alpine deformation in the Dent Blanche nsppe (Austroalpine, Western Alps) comprises three main folding phases (D1-D3). Regional schistosity is composite. Development of penetrative mylonitic foliation is associated with both D1 and D2 deformations (S1m, S2m). The northwestern side of the Valpelline Valley has been selected for this study of mylonites. Magmatic (“Arolla Serie” Auct.) and Pre-alpine high-T granulite-amphibolite facies metamorphic (“Valpelline Serie” Auct.) protoliths were largely converted into fine-grained S-L tectonites. The mylonitic foliation S1m is preserved in areas of D2 low-strain where D2 structures are open to isoclinal folds with axial-plane crenulation cleavage. D2 fold axes are orthogonal to L1 mineral stretching lineation. New mylonitic foliation S2m develops along D2 high-strain zones. Mylonitic lineation L2 is parallel to L1 and trends NW-SE. D3 gives rise to open folding, kinking with flat to steeply-dipping axial planes and subhorizontal axes around NE-SW. D3 does not produce a new penetrative foliation. Alpine evolution includes an early relatively high-P phase (aegirinaugite, garnet, blue amphiboles, phengite, kyanite, zoisite-clinozoisite) followed by white mica-chlorite-actinolite-albite-epidote assemblages typical of Meso-alpine greenschist facies metamorphism. Relatively high-P assemblages are stable on S1. D2 and D3 develop under greenschist facies conditions. Kinematic analysis on microscopic indicators in the mylonites records a top-to-NW sense of tectonic transport.The parallelism between L1 and L2 suggests roughly constant bulk kinematic regime through the Eo-alpine (late Eo-alpine?)/Meso-alpine evolution.