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.

Kinematics and geometry of structures in the southern limb of the Paraíba do Sul divergent structural fan, SE Brazil: a true transtensional shear

“…Such features have been described in the Caledonides (Marten and Dewey 1998, Fossen 2000, Stra-374 NOLAN M. DEHLER et al chan et al 2001, in the Himalaya (Herren 1987, Pêcher 1991, in the Andes (Dalmayrac and Molnar 1981), in the western Cordillera (Crittenden et al 1980, Wernicke 1981, Davis 1983, Spencer 1984, in the Alps (Selverstone 1988, Ratschbacher et al 1989, and elsewhere.…”

Mechanisms for folding of high-grade rocks in extensional tectonic settings

“…Tibetan plateau, Dewey, 1988) or oblique (Central Andes, Dewey, 1988) to the direction of initial convergence across the orogen. Selverstone (1988) and Sawyer and Barnes (1994) describe cases where normal displacements are interpreted synchronously on structures striking at a high angle to each other, suggesting local variations in the maximum extension direction. Because of such variations in strain patterns, axes of folds formed during orogenic collapse may therefore be sub-parallel or oblique to those formed during previous crustal shortening.…”

Geochronological evidence for continuous exhumation through the ductile‐brittle transition along a crustal‐scale low‐angle normal fault: Simplon Fault Zone, central Alps

“…[3] The Simplon Fault Zone (SFZ) [Bearth, 1956a;Steck, 1984Steck, , 1987Steck, , 1990Steck, , 2008Mancktelow, 1985Mancktelow, , 1990Mancktelow, , 1992Merle et al, 1986;Mancel and Merle, 1987] in the central Alps and the Brenner Fault Zone [Behrmann, 1988;Selverstone, 1988;Fügenschuh et al, 1997] in the eastern Alps are the most prominent low-angle detachment systems developed in the European Alps. In this paper, we specifically concentrate on the SFZ for several reasons.…”