Exhumation of the Central Alps: evidence from Ar-40/Ar-39 laserprobe dating of detrital white micas from the Swiss Molasse Basin

Ar-40/Ar-39 single-grain laserprobe dating of detrital white micas from ear ly Oligocene to middle Miocene (31-14 Ma) sedimentary rocks of the central Swiss Molasse basin reveals three distinct clusters of cooling ages for the hinterland. Two Palaeozoic age clusters reflect cooling after the Variscan orogeny with only limited reheating during the Alpine orogeny. The third T ertiary age cluster reflecting late Alpine cooling is restricted to sedimen ts younger than 20 Myr old. Micas with cooling ages < 30 Myr are interprete d to originate from the footwall of the Simplon detachment fault, thus repr esenting formerly exposed upper levels of the present-day Lepontine metamor phic dome. Erosion of these levels is reflected by an increase of low-grade metamorphic lithic grains in the sandstones. This interpretation puts cons traints on the timing of exhumation as well as on the evolution of the drai nage pattern of the Central Alps.