Oligocene to Miocene clastic sediments of the Tertiary Piedmont Basin (northwest Italy) were derived from erosion of Western Alps source rocks. Detrital white micas from different stratigraphic units and from sands of three present-day rivers draining the internal Western Alps have been analyzed by 40Ar/39Ar geochronology. Our data suggest a widespread, fast cooling and exhumation event prior to ca. 38 Ma followed by a >30 m.y. period of slower cooling and exhumation combined with erosion of crustal rocks with uniform 40Ar/39Ar signatures. These processes have resulted in a pattern of regularly increasing lag time up section.