TIMING OF PENNINIC–AUSTRO-ALPINE KINEMATICS FROM 39Ar/40Ar DATING FROM THE NE TAUERN WINDOW, EASTERN ALPS

The Penninic–Austro-Alpine boundary represents the main Alpine suture in the Eastern Alps. In the region of the north-eastern Tauern Window, the Austro-Alpine nappes are characterised by an overall inverted Alpine metamorphic gradient from medium grade in the Middle Austro-Alpine (MAA) to low grade conditions in the Lower Austro-Alpine (LAA) nappes. Thrusting in the AA units is to the W, predating metamorphic peak conditions in the higher Bundschuh nappe, but is essentially syn-metamorphic in the deeper MAA Aineck and the LAA nappes. The Penninic nappes show an increase in metamorphism from top to bottom from low to medium grade conditions.

³⁹Ar/⁴⁰Ar-dating of mainly white mica from the Bundschuh nappe gave ages from ca. 130 to 105 Ma with some partially reset ages. From the deeper Aineck nappe, mica growing in the main foliation gave ages from 90-85 Ma, discordant mica ages around 80 Ma. In Mesozoic metasediments of the LAA unit, there is a general decrease in ages from ca. 75 Ma in higher to 50 Ma in lower nappes. Basement rocks preserve Permian white mica ages. Mica from the uppermost Penninic sequence display ages from 45 to 22 Ma, dependent primarily on lithology. Most ages cluster around 30 Ma.

As a result, thrusting in the AA units occurred over a long time-span. Alpine thrusting in the highest unit and cooling to about 400 °C predate 100 Ma. In the next lower unit thrusting could have persisted until ca. 85 Ma. The attainment of higher peak temperatures in higher nappes of the AA unit and subsequent thrusting onto progressively cooler units of the same mega-unit point to continuous accretion of parts of the footwall to the hanging wall. This progressive accretion explains the observed inverted metamorphic gradient without the need to invoke inverted temperature gradients. As there is no age break going down the nappe stack, also accretion of the LAA units and initiation of subduction of the Penninic oceanic lithosphere was a continuous process. The main deformation in the higher Penninic parts, related to their subduction and intra-Penninic stacking is pre- to syn-metamorphic. Hence the oldest age from that unit give a minimum age for N-directed shearing, the oldest ductile deformation. Ages of about 22 Ma place a lower age limit on ensuing WNW-directed shearing, occurring at about peak metamorphic conditions.