Detecting provenance variations and cooling patterns within the western Alpine orogen through ⁴⁰Ar/³⁹Ar geochronology on detrital sediments: The Tertiary Piedmont Basin, northwest Italy

The Tertiary Piedmont Basin is a synorogenic basin located on the internal side of the Western Alps. Because of its key position, the Tertiary Piedmont Basin represents an important record of processes occurring in the Alpine retrowedge for over the last 30 m.y. ⁴⁰Ar/³⁹Ar geochronology has been applied to detrital white micas as a provenance tool and to derive information on cooling and exhumation patterns within the surrounding orogen. The age distribution in the detritus shows that in the Oligocene the clastic sediments were fed mainly from a southern source area (Ligurian Alps) that widely records high pressure (HP) Alpine metamorphism (40-50 Ma) and, in part, Variscan metamorphism (ca. 320 Ma). From the Miocene, the main source area gradually moved from the south to a western Alpine provenance characterized by strong Late Cretaceous (70 Ma) and Early Cretaceous (120 Ma) signals. This enlargement in the source is likely linked to an evolution of the main paleodrainage system into the basin. From the Serravallian, Variscan ages reappear; this is attributed to the exposure of the Argentera Massif as a new source for the Tertiary Piedmont Basin. The lack of thermal overprinting of the main detrital signals through time suggests that the western Alpine orogen has been regulated by episodic fast cooling and exhumation events followed by periods of slower erosion. Also, detrital ⁴⁰Ar/³⁹Ar Cretaceous signals in Miocene and Present sediments suggest the presence of real Eoalpine events in the Alps.