Evidence for east‐west crustal extension in the Eastern Alps: Implications for the unroofing history of the Tauern window

Abstract

The contact between the western border of the Tauern Window and the overlying Austroalpine nappes (Austria and Italy) is marked by a structural, petrologic, and geochronologic discontinuity that developed in the early stages of unroofing of the window. Within the window, pressure‐temperature‐time paths (J. Selverstone, 1985) indicate that significant ductile thinning occurred shortly after the cessation of thrusting (Paleocene ‐ Eocene). Sense‐of‐shear indicators (rotated porphyroblasts, asymmetric augen and pressure shadows, S‐C fabrics) associated with a west plunging stretching lineation indicate that this thinning occurred in response to top‐to‐the‐west low‐angle normal shear during metamorphism. The same sense of normal shear is present in the Mesozoic members of the Austroalpine sequence west of the window. In these rocks, however, west directed shear occurred at low temperatures (<300°C) after the thermal peak of metamorphism. Structural data and the regional geometry imply that the western end of the Tauern Window is a low‐angle normal fault (Brenner Line) that juxtaposed brittlely deformed rocks of the Austroalpine nappes against more ductilely deformed rocks from the window. Biotite K/Ar and Rb/Sr data suggest a Miocene age for final movement on the Brenner Line. These data indicate a prolonged history of east‐west extension in the Eastern Alps that affected all crustal levels (early ductile thinning of lower crust followed by mid‐to‐upper‐crustal low‐angle normal faulting). A model is presented that relates the young east‐west extension to displacement transfer from dextral movement on the Periadriatic Lineament. In this model, the Brenner Line and, by analogy, the Simplon Line can be thought of as detachment faults that tectonically unroofed the Tauern Window and Lepontine thermal highs, respectively.