Late Miocene Exhumation in the Ligurian Alps (NW Italy) as Evidenced by Detrital Apatite (U-Th)/He Thermochronometry

We present new detrital apatite (U-Th)/He ages reconstructing the exhumation history of the Ligurian Coast of NW Italy in the Western Mediterranean. The Ligurian coast is the seaward extension of the ENE-WSW trending Ligurian Alps, the southern segment of the Western Alps located between the Po Plain foredeep basin in the North and the Liguro-Proven‡al Basin to the South. The apatite (U-Th)/He ages are the first ages that constrain the low temperature cooling history of the Mediterranean domain. In total 35 samples, originating from a Cretaceous and Eocene flysch substratum were collected. Sampling took place along two vertical (elevation) and two horizontal profiles. He-ages range between 5.69 to 24.63Ma. Analysis of Induced Fission Tracks for 5 samples representative for apatite grains used for (U-Th)/He analysis shows that U-content is homogeneously distributed in the apatites. Although the data show some spread several clusters of He ages are identified between ~1600m (10.05Ma) and 300m of elevation (7.71Ma). The near vertical slope of the age-elevation profile is interpreted as reflecting fast exhumation. Applying a statistical fit through the data (ranging in elevation between ~1600m and sea level) indicates to exhumation rates of 0.5 - 1 km/Ma. The ages obtained suggest that the Ligurian Alps have been exhumed rapidly during Late Miocene times. These results are particularly interesting because they point to fast vertical movements taking place after the main contractional stages in the Western Alps and hypothetical shoulder uplift associated with rifting in the Liguro-Proven‡al Basin. Exhumation pre-dates the Messinian salinity crisis, which might have caused additional isostatic uplift. We propose that the fast exhumation in the Ligurian Alps coincides with a regional increase in exhumation rates as evidenced by the western Alpine External Massifs.