40Ar/39Ar muscovite ages from the Penninic-Austroalpine plate boundary, Eastern Alps

The Radstadt Mountains, Eastern Alps, expose the tectonic boundary between the base of the Austroalpine continental plate (hanging wall) and the Penninic oceanic units (foot wall). Parts of the Austroalpine basement were penetratively deformed because of ongoing rifting of the continental crust during the Permian (290-250 Ma). Austroalpine Permo-Mesozoic cover rocks were deformed during the Cretaceous (similar to 80 Ma) and the Paleogene (55-50 Ma). These ages are interpreted as representing two distinct events of nappe stacking. The younger ages represent the collision of Austroalpine and Penninic tectonic units. The Penninic nappe complex displays a successive decrease of ages from similar to 37 to 25 Ma from high to deep tectonic levels. A second age group of similar to 22 Ma was found both in low-temperature release steps and as plateau ages close to the Penninic-Austroalpine boundary. It is attributed to a thermal over-print due to ductile extension of the overthickened orogenic wedge.