Kinematics and geochronological evolution of the Vinschgau Shear Zone (N Italy): Large-scale thrusting within the Austroalpine domain of the central-eastern Alps

The Vinschgau Shear Zone (VSZ) is an E-W striking ductile shear zone developed within the central-eastern Austroalpine domain. Along the VSZ the Ötztal-Stubai nappe with an amphibolite facies Alpine metamorphism overthrusted the Campo nappe (and respectively the Engadine Dolomites) characterized by low-to-medium metamorphic conditions (Schmid and Haas, 1989). In the eastern part the shear zone involves also the Texel Unit at the base of the Ötztal nappe. Foliation gently dips (20°-30°) northward and lineation constantly strikes E-W, with kinematic indicators that point to a top-to-NW sense of shear. The VSZ, deepening to the E, developed mostly under amphibolite to greenschist-facies conditions. A remarkable variation in finite strain along its length can be observed: lenses of protomylonites are bounded by deeply sheared ultramylonites, mylonites and phyllonites, these occurring in the western part of the VSZ. In order to fully assess the evolution of the VSZ a multidisciplinary approach based on structural and petrochronological analyses has been carried out on three representative transects of the shear zone from W to E: Eyrs, Schlanders and Juval transects. Our fieldwork based analyses suggest that the dip of mylonitic foliation increases from W to E and the lineation dips towards the W in Schlanders and Juval, whereas towards the E in Eyrs, probably due to a folding event related to Cenozoic shortening (Pomella et al., 2016). Chemical and microstructural analyses suggest that deformation temperatures of 350-400 °C have been reached during shearing, as testified by Ti content in biotite and by bulging and subgrain rotation as dominant recrystallization mechanisms in quartz. Timing of deformation along the VSZ has been constrained through 40Ar/39Ar dating of syn-shearing micas confirming previous geochronological data of Thöni, 1981, which reveal a Late Cretaceous age of the VSZ mylonites. A systematic younging trend of deformation towards the central part of the shear zone has been observed in the studied area. Vorticity analyses, both through shear bands and rigid porphyroclasts methods, show a clear decrease in simple shear component correlated to the younging direction of mica ages (i.e. towards the core of the shear zone). This evolution is clearly consistent with Type 2 evolution of Fossen and Cavalcante (2017), where strain localizes into the core of the shear zone during deformation. Our data confirm the age of the VSZ only supposed on the base of indirect observations and demonstrate that the Austroalpine domain was severely affected by W-NW verging thrust stacking much time before the Adria-Europe continental collision. References Fossen, H. & Cavalcante, G.C.G., (2017) - Shear zones–A review. Earth-Science Reviews, 171, 434-455. Pomella, H., Flöss, D., Speckbacher, R., Tropper, P., & Fügenschuh, B. (2016) - The western end of the Eoalpine High‐Pressure Belt (Texel unit, South Tyrol/Italy). Terra Nova, 28(1), 60-69. Schmid, S.M., & Haas, R. (1989) - Transition from near‐surface thrusting to Intrabasement Decollement, Schlinig Thrust, eastern Alps. Tectonics, 8(4), 697-718. Thöni, M. (1981): Degree and evolution of the Alpine metamorphism in the Austroalpine unit W of the Hohe Tauern in the light of K/Ar and Rb/Sr age determinations on micas. - Jb. Geol. B.A. 124, 1: 111-174.