Austro-Alpine Basement Complex to the South of the Tauern Window

Abstract

The pre-Upper Ordovician units of the Altkristallin and the Ordovician to Upper Devonian sedimentary and volcanic rocks of the phyllitic/fossiliferous Palaeozoic sequences show similar and parallel post-Upper Ordovician deformational structures but different P-T evolutions. In the Altkristallin, early prograde high pressure — low temperature metamorphism followed by a high pressure eclogitic stage and later decompression, evolved synchronous to a foliation-forming shearing deformation. Temperature-dominated amphibolite-facies overprinting with local formation of first melts post-dates this deformation. Greenschist-facies low pressure metamorphism, locally reaching the amphibolite facies during early shearing, is observed in the phyllitic sequences. A pre-Alpine age of medium-grade metamorphism of fossiliferous Palaeozoic sequences is not definitely proved. Folding of the foliation and later formation of shear bands are related to a common late Variscan uplift/cooling history and overprinted the foliation-parallel lithological contacts of the units. A Variscan process of progressive crustal stacking which first reworked the Altkristallin to the NW and then affected the phyllitic/fossiliferous Palaeozoic sequences to the SE can be inferred. An increasing degree of Alpine greenschist-to amphibolite-facies ductile overprinting from south to north is observed in the northern parts of the basement. In the southern parts, pre-Alpine structures and mineral assemblages remained well preserved during late Alpine brittle deformations.