Tectonometamorphic evolution, tectonic coupling and exhumation history of the Ötztal, Schneeberg and Texel unit (Eastern Alps, NE Italy)

The Austroalpine nappe stack to the west of the Tauern Window is made of three main tectonometamorphic units that are, from top to bottom: the Ötztal-Stubai complex, the Schneeberg and the Texel units. The Ötztal-Stubai complex consists of a polymetamorphic basement and Permian-Mesozoic sedimentary rocks that underwent metamorphism and deformation during the Alpine orogeny. E and NE of Vipiteno (Schneeberg, Telfer Weissen, Tribulaun) the nonconformity surface between basement and the metasedimentary succession has been mapped in details in the frame of the CARG project of the Provincia Autonoma di Bolzano-Alto Adige. As the non-conformity is still roughly preserved, structural investigations across such discontinuity are useful to reconstruct the Alpine deformation history. Mafic to intermediate dikes that cut across the basement regional foliation but do not intrude the post-Variscan cover are additional structural markers. The Ötztal cover consists of a metaconglomerate (Basal Conglomerate Auct., Permian), containing quartz, gneiss and garnetite clasts, a thin level (1-20 meters thick) of quartz-carbonatic schists, calcitic-dolomitic marbles, graphitic calcschists and, at the top, dolomitic marbles. Even if the non-conformity between the basement and the overlying metaconglomerates is variably interested by shear zones, the occurrence in the metaconglomerate of clasts strictly deriving from the underlying basement suggests that displacement due to shearing is limited: garnetite clasts occur only in the Schneeberg area where garnetite levels, not found elsewhere, occur within Ötztal micaschists. Alpine deformation resulted in a penetrative foliation in the sedimentary cover coupled with a stretching lineation due to a top-to-W (NW) shearing. Mafic-intermediate dikes are substantially undeformed, displaying only a faint foliation close to the contacts with host gneisses. The Ötztal basement did not have suffered penetrative recrystallization and deformation during the Alpine cycle: dikes cut across not only the regional foliation but also the last fold system that fold it. P-T estimates constrain metamorphic condition in metaconglomerates and dikes at 450-500°C and 0.8-1.0 GPa. Ar-Ar dating of micas aligned along the main foliation in conglomerates and recrystallized biotite in dikes provide ages of about 85 Ma. These data are very close to the age of the eclogitic metamorphism in the Texel unit and the HP amphibolite facies metamorphism in the Schneeberg complex. All together these data suggest that metamorphic peak conditions were experienced roughly contemporaneously by the three units when they were at different crustal levels. During exhumation, the HP Texel unit was first coupled with the Schneeberg unit (the two units show a common evolution since the amphibolite facies re-equilibration of the Texel eclogites) and then the Schneeberg-Texel was brought in contact with the Ötztal basement and cover at shallow crustal level.