Unravelling the thermo-tectonic evolution of the Alps: a contribution from fission track analysis and mica dating

Summary Fission track analysis of apatite and zircon, and K-Ar and Rb-Sr dating of white mica and biotite provide time and temperature constraints for direct dating of deformation phases and in establishing cooling and uplift rates and patterns. In the western Alps, timing of the Sesia-Lanzo zone Eoalpine metamorphic temperature peak is confirmed between 110 and 85 Ma, possibly preceded by the pressure maximum. The slowly cooling Sesia zone reached 200–250°C by ∼33 Ma and ∼100°C by ∼25 Ma, with fast uplift during the late Oligocene to Miocene Insubric phase; cooling in the adjacent Gran Paradiso Massif occurred slightly later, with fission track zircon and apatite ages of 30±2 and ∼22 Ma respectively. No evidence to support an inverted metamorphism of the Gran Paradiso was found. In the Silvretta nappe (E Alps) three different uplift styles and periods are distinguished: uparching between 110 and 35 Ma, homogeneous uplift during 35 and 2 Ma and recent eastward tilting. The data imply that updoming of the Engadine Window occurred before ∼31 Ma. In the central Lepontine Alps, a uniform late Neogene to recent mean uplift rate of ∼0.5 mm/a succeeded early Miocene uplift at ∼2.2 mm/a in the southern steep belt (root zone), the latter resulting from a major phase of backthrusting of the central Alps along the Insubric Line. Clear-cut time-temperature paths are not found for the Suretta Nappe (central Alps): fission track data delimit the Miocene cooling, but speculative interpretation of phengite-mica and K-Ar amphibole ages suggests that the main deformation (possibly Ferrera phase) occurred between 55 and 36 Ma. Schematic models are advanced to describe the asynchronous thermal evolution of different parts of the central Lepontine Alps.