Kinematic, thermal and petrological model of the Central Alps: Lepontine metamorphism in the upper crust and eclogitisation of the lower crust

Abstract

Seismic and seismological studies as well as gravimetric models indicate that a slab of European lithospheric mantle and lower crust is currently underthrust below the Apulian crust. We assume a simple kinematic model in which the lower and upper subducted European crusts are decoupled along a decollement. The lower crust goes into subduction without deformation. The upper crust deforms by pure shear with a horizontal compressional axis. The total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry, different distributions of erosion being used. The computed temperature field is steady-state if the kinematic model applies during a minimum time of 40 Myr for a convergence rate of 8 mm/yr. Equilibrium mineral assemblages and densities are determined from the computed P, T conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust. Assuming local isostasy, the density model fits the average topographic profile across the Central Alps. The P-T-t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure Oligocene metamorphism in the Lepontine dome. The peak calculated temperature for the deepest non subducted crustal rocks is 600°C for a pressure of 0.8 GPa, near the lower limit of high-pressure amphibolites. We thus propose that the Lepontine metamorphism corresponds to the steady-state thermal regime. However, either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt. The computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km. We conclude that the Moho limiting the deepest part of the root may correspond to the eclogitisation phase change. Lower crust eclogites have a density comparable to or higher than that of the mantle, depending on their chemical composition (3.37 for andesitic eclogites, 3.56 for gabbroic eclogites). Thus, andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink.