Late stages of exhumation constrained by structural, fluid inclusion and fission track analyses (Sesia-Lanzo unit, Western European Alps)

In metamorphic areas, paleogeothermal gradients are difficult to properly estimate, leading to great uncertainty in exhumation rate calculations based on fission track analysis. In this work, a new multidisciplinary approach based on fluid inclusion analysis, structural analysis and fission track dating has proven to be able to constrain the late stages of exhumation of metamorphic rocks. The study area is located in the Sesia–Lanzo unit of the Western European Alps, along the Hone shear zone. Kinematic analysis at different scales unraveled a polyphase postmetamorphic deformation history, constrained in the P–T space thanks to laboratory investigation of kinematically referenced fluid inclusions coupled with the analysis of the rheological behavior of quartz and feldspar inside syn-kinematic veins. The resulting P–T path points to an increasing paleogeothermal gradient from 18 to 30 °C km−1 after greenschist facies metamorphic conditions, and provides a reference framework for a quantitative interpretation of fission track data in terms of exhumation rates and fault throws. According to fission track data, throws accommodated by the Hone shear zone between 33 and 30 Ma are on the order of 4.5 km, and would be underestimated by ∼25% if a “normal” steady gradient was assumed for the whole post-metamorphic history of the area. The deepest portions of this structure were exhumed thanks to the activity of the E–W fault system that accommodated a throw of ∼4 km between 28 and 20 Ma, and a throw of ∼0.8 km during the last 20 Myr.