The role of mantle upwelling on the thermal history of the Tertiary‐Piedmont Basin at the Alps‐Apennines tectonic boundary

The Tertiary Piedmont Basin (NW Italy) is an episutural basin that developed from the late Eocene on the Alps–Apennines tectonic junction. Several coeval geodynamic processes, including the loading and exhumation of the Western Alps, the outward migration of the Apennine accretionary wedge and the opening of the Liguro-Provençal rift basin, controlled the basin evolution. We integrate fluid-inclusion microthermometry, low-temperature thermochronology and burial history with numerical modelling to constrain the paleo-geothermal gradients required and evaluate the mechanisms that governed the basin thermal history. Apatite fission-track and (U-Th-Sm)/He analyses of the basal late Eocene turbidites show reset ages of ~25 Ma and ~20 Ma, respectively which require temperatures to have been > 120 °C. Homogenization temperatures up to ~130 °C from fluid inclusion analyses from authigenic minerals confirm the thermochronometric data, supporting a significant post-depositional heating in the lower sequence of the basin. Stratigraphic reconstructions and decompaction of the basin fill indicate that the maximum burial experienced by the basal strata at 25 Ma is 2.3 ± 0.1 km, which is not sufficient to reset the AFT thermochronometric system when applying a typical geothermal gradient (~20-30 °C/km). An elevated geothermal gradient of 45 ± 5 °C/km is thus necessary to explain the thermochronometric dates and the elevated thermal signature at shallow depths. 2D numerical simulations indicate that such an elevated paleogeothermal gradient can be best explained by mantle upwelling, consistent with crustal thinning caused by the inception of the Liguro-Provençal rift basin and related outward migration of the Alpine and Apennine fronts during the Oligocene.