Nappe stacking and metamorphic evolution in the Western Tauern Window

The western Tauern Window is a key area for the reconstruction of the tectonic evolution of the Eastern Alps. Here basement units derived from the European margin are stacked together with their parautochthonous post-Variscan cover in a south-vergent crustal scale duplex that also involves the Glockner nappe (calcschists and ophiolites derived from the Penninic ocean). The major lithostratigraphic units comprise: i) a polymetamorphic basement consisting of mainly amphibolites, migmatites, orthogneisses, and locally pre-Mesozoic ultramafic to mafic bodies; ii) the Tux and Zillertal orthogneiss (part of the “Zentralgneiss Auct.”) that intruded unit (i) during early Permian times; iii) the parautochthonous metasedimentary cover including meta-conglomerates, calcitic and dolomitic marbles, calcareous micaschists, quartzites, and subordinate greenschists. iv) the Glockner Nappe, it consists of km-thick sequences of calcschists with interbeddings of prasinites, amphibolites and ultramafic bodies. Along the tectonic contact between the northern and southern duplex antiforms a slice (about 1km-thick, wedging out westward) of graphite-rich garnet micaschists with minor quartzites and calcschists (“Greiner schists” Auct.) occurs. Two phases of roughly coaxial isoclinal folding, the first one of these responsible for the regional foliation, have been recognized in the post-Variscan cover of the “Zentralgneiss”. A pronounced stretching lineation, parallel to fold axes and ascribed to a strong flattening is well developed in quartz-rich rocks. An amphibolite facies metamorphic stage characterized by static recrystallization of micas, garnet and amphibole (“Garbenschiefer” Auct.) is well recognizable in the post-Variscan metasedimentary units whereas its occurrence remains controversial in the Glockner nappe. This point still need to be resolved as the formation of the regional scale duplex, involving all described unit, is generally interpreted to had predated the static crystallization (“Tauercristallisation” Auct.). Late stage doming of the duplex, associated to folding at the mesoscale, caused eventually the present day structure with the two antiformal Tux and Zillertal domes. In the frame of the of the South Tyrol Province CARG project, devoted to 1:10000 mapping of the Vipiteno sheet, the goal of this work is to provide structural and petrological constraints to identify tectonometamorphic units with coherent structural and metamorphic evolution. Preliminary results display significant differences among the reconstructed P-T paths of the Glockner schists and metasedimentary rocks of the post-Variscan cover of the Tux and Zillertal gneisses. The generally accepted model for the tectonic evolution of the western Tauern Window does not provide full account for these features. Further analyses are therefore needed to unravel earlier stages of metamorphic history of the several units and define the ambient conditions at which nappe stacking occurred.