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 (~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 ~37 to 25 Ma from high to deep tectonic levels. A second age group of ~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 overprint due to ductile extension of the overthickened orogenic wedge. ¿ 2001 American Geophysical Union