<p>This contribution focuses on the timing of metamorphism within the Lepontine dome, located in the Penninic domain of the Central European Alps (Switzerland). The Lepontine dome is formed by crystalline basement nappes bent towards the south in the migmatites of the Southern Steep Belt. The Lepontine nappes are formed by metamorphic rocks, mainly ortho- and para-gneisses, whose foliation dip-direction together with the attitude of thrust sheets define a dome shape. The Lepontine dome is characterized by a widespread Barrovian metamorphism of Tertiary age whose expressions are: an asymmetric concentric zonation of mineral-zone boundaries, locally dissecting the tectonic nappe contacts, and a NW-SE directed mineral and stretching lineation developed during peak metamorphic conditions, which suggests non-coaxial deformation during thrusting.</p> <p>In a recent work, we dated the upper amphibolitic non-coaxial deformation. We performed U-Pb zircon dating on multiple samples which resulted in two groups of ages at ca. 31 Ma and 22 Ma. We attribute the development of the amphibolite facies syn-kinematic metamorphism to the widespread-recorded event at 31 Ma. This time constraint still lacks of specific information on the duration of the temperature peak, the subsequent cooling and the nature of the cooling process. To solve the temporal character of the formation and evolution of the high-grade metamorphic rocks, we applied a method to determine cooling rates calculated using post-peak-T estimates as initial temperature in the metapelites of the Lepontine dome. We selected garnet-paragneisses from the core of the Lepontine dome at different levels in the nappe pile, being the structural lowest one at the base of the Simano nappe and the uppermost in the Cima Lunga unit. Their mineral assemblage is marked by quartz, feldspar, garnet, biotite, white mica, kyanite, local staurolite, rutile and minor phases. Garnets are pre- to syn-kinematic with respect to the amphibolite facies metamorphic foliation. Furthermore, in the migmatitic paragneisses of the Southern Steep Belt we analysed one sillimanite-rich sample, where we found textural evidences of the presence of melt and k-feldspar.</p> <p>We exploited garnet compositional re-adjustment due to major-element diffusion at the borders of the crystal to extract cooling rates, whose estimates where constrained by temperatures obtained via geothermometry and phase equilibria modelling. The post-peak temperatures of re-equilibration were estimated at ca. 600 ± 50 °C at the border garnet-biotite, where a step in garnet major element composition was seen. The diffusion time necessary to fit garnet-rim profiles along short transects (less than 1 mm length) was calculated as a preliminary result, giving a value < 2 Ma for most of the samples.</p> <p>Note that a cooling time < 1 Ma is typical of transient thermal regimes, however the type of thermal regime can be properly evaluated only with the calculation of the cooling rate. High cooling rates are consistent with high temperatures in a localized area developed in a small time frame, such in the case of thrust-related shear heating during metamorphism. Slow cooling rates indicate instead a regional thermal history. Our preliminary results suggest high cooling rates for the high-grade metapelitic rocks of the Lepontine dome.</p>

<p>The heat transfer through the nappes of the Lepontine Dome (Central Alps, Ticino, Switzerland) produced metamorphic amphibolite-facies isogrades that locally dissect the tectonic contacts. This large-scale observation, suggesting a thermal amphibolite-facies event after thrusting and nappe formation, is however at odd with the extremely pervasive mineral and stretching lineation (NW-SE directed) that attests non-coaxial deformation during shearing at similar metamorphic conditions.</p> <p>To solve this apparent paradox we performed 2D thermo-kinematic simulations in which we investigated the relationships between nappe geometry and the geometries of isogrades. The numerical simulations are based on the finite difference method. We evaluate the relative importance of velocity, thermal diffusion and advection, and geometry of the thrust sheets, on the geometrical relation between tectonic contacts and isogrades. We calculate the thermal evolution and peak temperatures in order to compare the numerical results with field and petrological data collected along the Simano and Cima Lunga nappes.</p> <p>In the field, the alternation of lithotypes is parallel to the nappe boundaries and constant over their whole length (order of kms). Passing from the Simano to the Cima-Lunga nappe, the transition between the nappes is marked by a progressive change in the texture of gneisses, in which the porphyroblasts become more stretched from the bottom to the top, and by the change in the constituent lithotypes. In the studied area, the Simano nappe is formed mainly by metagranitoids and by minor paragneisses. The Cima Lunga nappe is made of metasediments, mainly quartz-rich gneisses intercalated with amphibolite-gneisses, peridotitic lenses and local calcschists and/or marbles. Finally, the widespread paragneisses forming both the nappes frequently contain garnets of different sizes and internal microstructure. Published and own petrological data of these garnet-bearing rocks will be used to restrict the physical parameters of the numerical results.</p> <p>We intend to test multiple geological scenarios related to different sources of heat production, such as: internal heat sources (radiogenic heating); additional heat flux at the bottom of the nappes, such in the case of a magmatic underplating, slab break-off, lower crust delamination; and in situ-produced heat due to shear heating mechanisms at the tectonic boundary between the nappes (thrust surface).</p>

<p>Heat transfer during and after the emplacement of tectonic nappes within an orogeny is controlled by three fundamental processes: advection, diffusion and production of heat. Production is mainly caused by radioactive decay and shear heating. The relative importance and timing of these processes is often contentious. For example, in the Lepontine Dome of the Central European Alps the timing of the thermal evolution and the relative importance of advection, diffusion and shear heating is disputed. To better constrain and understand heat transfer in the Lepontine Dome, we apply a combined approach of petrological and structural analysis, zircon dating of migmatites and theoretical modelling.</p><p>We use data from an almost vertical transect (in the Ticino’s valleys) cutting from bottom-to-top the Simano, Cima Lunga and Maggia gneissic nappes. These nappes show an extremely pervasive mineral and stretching lineation (NW-SE directed) indicating non-coaxial deformation during shearing at amphibolite facies metamorphic conditions. The transition from the Simano to the Cima-Lunga nappe is marked by a progressive change in the texture of gneisses, in which the porphyroblasts become more stretched from the bottom to the top. Locally, at the tectonic contacts, syn-tectonic migmatites have been found. Their leucosomes contain metamorphic zircons with ages spreading from 40 to 31 Ma (U-Pb dating). <br>The widespread paragneisses frequently contain garnets of different sizes and internal microstructure. Published and own petrological data of these garnet-bearing rocks attest an inverted metamorphic gradient from ca. 700°C to 650-600 °C at intermediate pressures below the Cima Lunga unit during the peak-T amphibolite facies condition.</p><p>Overall, the field data depict a major km-scale shear zone that generated an inverted metamorphic gradient during the peak-T amphibolite facies condition between 40 and 31 Ma. These results hint that fast advection of heat or shear heating (or both component contempraneously) contributed to imprint the regional amphibolite facies metamorphism during nappe emplacement.</p><p>To take another step towards unravelling the controlling heat transfer processes in the Lepontine Dome and to test the relative importance of production, diffusion and advection, we employ three theoretical approaches with increasing complexity. First, we perform a dimensional analysis estimating dimensionless numbers, such as Peclet and Brinkman, for a range of reasonable parameters for the Lepontine Dome. Second, we apply numerical 2D thermo-kinematic simulations of trishear thrust-ramp evolution to test, for example, the impact of temperature-dependent viscosity and the geometrical relationship between temperature isogrades and nappe boundaries. Third, we apply state-of-the-art numerical 2D thermo-mechanical simulations of subduction and collision to investigate heat transfer and the resulting metamorphic facies distribution during the formation of an orogenic wedge.</p><p>Finally, we combine our modelling results with the available structural, age and metamorphic results to discuss potential scenarios for the heat transfer through the Lepontine dome.</p>

Barrovian metamorphism is characterized by a sequence of mineral assemblages associated with increasing metamorphic conditions, from chlorite through biotite, garnet, staurolite, kyanite to sillimanite-bearing