Fluid inclusion analysis as a calibration tool for exhumation rate studies: interpretation of fission-track data from the Western Alps.

In metamorphic belts, as it is difficult to constrain P-T paths at temperatures lower than 250°C, the evaluation of the paleo-geothermal gradient represents a major uncertainty in exhumation rate calculations based on fission-track data. Fluid inclusion analysis, combined with a detailed structural analysis at different scales, provides a reliable tool to calibrate the thermal setting of the shallow crust during exhumation. The study area is located in the lower Aosta Valley (Sesia-Lanzo unit, Western Alps). Here, exhumation is driven by two crosscutting fault systems: the sinistral contractional Hone shear zone and the dextral normal E-W fault system, which define four distinct blocks (A to D in fig. 1). Zircon and apatite fission-track ages are different in these blocks, and span from 40 to 28 Ma (zircon), and from 36 to 20 Ma (apatite). Fluid inclusion analysis on veins sampled in different structural domains led to the recognition of three generations of primary inclusions, related to different deformation phases. Microthermometric measurements reveal that they lie on two near-parallel isochores. Their trapping temperature along the isochore has been evaluated on the basis of the rheological behaviour of quartz and feldspar during the coeval deformation phases. The resulting concave upward P-T path corresponds to a geothermal gradient of 30°C/km during cooling below 300°C (fig. 2). Derived exhumation rates range from 1.5 km/Ma (Hone shear zone hanging wall, A-C in fig. 1, during Early Oligocene) to 0.1 km/Ma (E-W fault system hanging wall, C-D in fig. 1, from Late Oligocene onward).