2D-thermo-mechanical modelling of spatial P-T variations in heterogeneous shear zones

Leonardo Casini (1) & Matteo Maino (2)
(1) Dipartimento di Scienze della Natura e del Territorio, Università di Sassari, via Piandanna 4, 07100 Sassari, Italy. (2) Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, via Ferrata 1, 27100 Pavia, Italy. Corresponding author e-mail: matteo.maino@unipv.it.

We present a suite of two-dimensional thermo-mechanical simulations of a compositionally heterogeneous shear zone characterized by viscous matrix and inclusions. The experiments have been performed using a visco-elasto-plastic 2D finite difference code that reproduces the stress, strain, temperature and pressure variations occurring within the shear zone at lithospheric scale. From here, we extrapolated and discuss the results of a narrow (<10 km) domain where strong inclusions with respect to a weaker matrix are inserted. The experimental setting simulates the geological conditions common to several (ultra-) high-pressure (U-HP) rocks worldwide. Particularly, we shaped the model in order to roughly reproduce the distribution of the (U)HP ultramafic rocks of the Adula-Cima Lunga nappe system. Results indicate that significant overpressure (up to 0.5 GPa) within the strong inclusions and viscous shear heating (up to 100°C) can be generated already with a short time of deformation (ca. 1.2 Myr) in small rock units. These data suggest that, in specific structural domains, pressure and temperature deviations may play a not trivial role in the pressure-temperature-time paths recorded by the rocks through the metamorphic reactions.

shear zones, tectonic overpressure, viscous shear heating, numerical modelling, Central Alps, High-Pressure metamorphism.