Implications of New Geological Mapping and U-Pb Zircon Dating for the Barrovian Tectono-Metamorphic Evolution of the Lepontine Dome (Central European Alps)

The Barrovian metamorphism of the Lepontine dome is manifested by isograds that crosscut tectonic nappe contacts, which is commonly interpreted as metamorphism that occurred after nappe emplacement. However, the pervasive mineral and stretching lineation in amphibolite facies, associated with top-to-foreland shearing, suggests that peak Barrovian conditions are coeval with nappe-overthrusting. Here, we combine mapping and U-Pb zircon dating to better constrain the relation between metamorphism and overthrusting. Metamorphic zircon rims show two age populations at 31-33 and 22-24 Ma. The younger population is locally observed in post-foliation dikes (and associated metasomatism) likely sourced from deep-migmatites exhuming along the Alpine backstop. The older population occurs regionally and is found in syn-kinematic migmatites which occur along a crustal-scale shear zone. Below this shear zone, magmatic and detrital zircon cores suggest that the Cima Lunga unit, previously interpreted as a tectonic melange with Mesozoic fragments, was a pre-Variscan metasedimentary sequence intruded by Permian granitic sills, now orthogneisses. This unit was strongly sheared along the top of the Simano nappe during overthrusting of a rock pile here-termed Maggia-Adula nappe. This large-scale nappe emplacement imprinted the regional lineation and peak temperatures until 31-33 Ma. Peclet (1-10) and Brinkman (0.002-1.8) numbers, estimated for the overthrusting, suggest an advection-dominated heat transfer caused by rock exhumation, with some diffusion (conduction) during nappe emplacement. Diffusion contributed to Barrovian isograds discordant to the thrust. Shear heating was important if stress times shearing rate >similar to 5.10(-6) W.m(-3) within the nappe. The thermal evolution after overthrusting was spatially heterogeneous until ca. 22 Ma.