Tectonometamorphic Evolution of the Austroalpine Nappe Complex in the Central Eastern Alps—Consequences for the Eo-Alpine Evolution of the Eastern Alps

Abstract

In the east-central part of the Eastern Alps, three major deformation events can be distinguished within the Koralm Complex and adjacent units (Plankogel Complex, Gleinalm Complex, Seckau Crystalline Complex, Paleozoic of Graz). A first deformation event D1 is characterized by the formation of a penetrative foliation and an E-W stretching lineation. Remnants of deformational micro-structures indicate a top-to-the-west sense of shear during this deformation event. Most of the D1-related fabrics were overprinted by subsequent metamorphism. This metamorphic event did affect the presumed tectonic boundary between the Koralm Complex and the Gleinalm Complex below. Particularly, D2 is related to the Plattengneis shear zone, which formed in the uppermost structural sections of the Koralm Complex, characterized by a N-S-oriented stretching lineation. Eclogites in the footwall have been affected by this deformation event, too. This deformation event is associated with pure shear in the central parts of the Koralm Complex, probably with top-to-the-south displacement in the southern parts, and top-to-the-north displacement in the northern parts. Deformation within the Plattengneis and the eclogites below occurred along the decompressional path, indicated by decreasing minimum pressures within the eclogites, and by northward and southward decreasing pressures and temperatures. The Plattengneis shear zone continuously passes into a low-angle normal fault in the northeastern part of the Koralm, forming the contact between the Koralm Complex and the Paleozoic of Graz. Thus, the Plattengneis shear zone primarily formed as an extensional structure and triggered exhumation of the eclogites. D3-related structures are restricted to distinct low-angle normal shear zones along the northern and southern margins of the Koralm Complex, with top-to-the-N/NE and top-to-the-S/SE displacement, respectively. These are related to the juxtaposition of exhumed high-pressure rocks of the Koralm Complex, and medium- to low-grade metamorphic units above. According to this evolution, the Cretaceous collisional process (Eo-Alpine cycle), which formed the present Austroalpine Nappe Complex, may be subdivided into two distinct phases: The first phase is the (ES)E-ward subduction and closure of the Hallstatt-Meliata Basin, resulting in the assembly of the Upper Austroalpine Nappe Complex. After closing of the Hallstatt-Meliata oceanic basin during the Late Jurassic, the Cretaceous orogeny in the Eastern Alps encompasses the collision between (south)easternmost parts of the Austroalpine continental crust and a continental fragment to the east. The second phase involves the southward underplating of the southern Apulian continental margin and resulted in the imbrication of the Middle Austroalpine basement complex. These units were additionally affected by pronounced metamorphim, increasing from greenschist-facies conditions in the northern parts to amphibolite- and eclogite-facies conditions in its southern-most parts. Continuous underplating was accompanied by extension in the internal parts of the orogen, resulting in the formation of an extensional detachment in the lower crust (Plattengneis shear zone), and exhumation of high-pressure metamorphic rocks during the Late Cretaceous. Coeval extension in the upper plate resulted in the formation of the Gosau sedimentary basins. Toward the north, the Plattengneis shear zone continuously climbed toward shallower crustal levels, and passed into a foreland-directed thrust. This thrust is conjectured to have affected the Upper Austroalpine Nappe Complex as well as the formation of distinct out-of-sequence thrusts.