The results of our study strongly suggest that the older, Cretaceous ages found in the Western Alps are not the accidental result of random processes such as the migration of excess argon, but instead a fundamental attribute of the Alpine orogenic belt that can be traced back in time from the Present to at least the early Miocene.

Ages around 107-120 Ma can be interpreted as the result of cooling following UHP metamorphism due to the onset of the Ligure-Piemontese oceanic subduction in the Western Alpine domain. We argue that these ages are consistent with a model that assumes an Early Cretaceous oceanic subduction and subsequent fast cooling/exhumation. The ages around 82-92 Ma could potentially be indicative of the diachronicity of this process, as it probably occurred earlier in the area at present represented by the Sesia Lanzo zone than in the area of the Internal Western Alpine Massifs. Support for this interpretation is that oldest Eo- Alpine ages are recorded at present in the Sesia Lanzo zone (e.g Ruffet et al., 1997; Cortiana et al., 1998; Agard et al., 2002 and references therein). Ages around 68-75 Ma could be indicative of cooling following HP to UHP metamorphism related to the onset of continental collision and subsequent exhumation. Furthermore, a possible mechanism, which would explain the preservation of the HP-UHP metamorphic signals today, could be exhumation of crustal rocks close to the downgoing slab through shear zones parallel to the subducting plate. The subducting plate provides the necessary cooling to counter the radioactive heat production overprinting the thermochronological signal during exhumation (Wijbrans et al., 1993 and references therein).

Recent models on the Western Alpine evolution all dismiss Early Cretaceous ages (e.g. Dal Piaz, 1999; Agard et al., 2002; Reddy et al., 2003) and consider the HP to UHP metamorphism related to the onset of subduction to be no earlier than 70 Ma. On the other hand, evidence of Late Jurassic-Early Cretaceous subduction related to the Tethys, exists in the Eastern Alps and Western Carpathians (Dal Piaz et al., 1995). Therefore, if our interpretation of the new data is correct then detrital ⁴⁰Ar/³⁹Ar Cretaceous ages would be evidence for the relicts of a diachronous (earlier in the east than in the west) Tethys subduction. We conclude that the metamorphic history of the Western Alps domain is substantially more complex than current models assume, and the Pre-Eocene tectonic history needs to be seriously re-considered.