Kinematics and dynamics of tectonic nappes: 2-D numerical modelling and implications for high and ultra-high pressure tectonism in the Western Alps

• A new dynamic model for tectonic nappes in the Western Alps is presented.
• High and ultra-high pressures can occur in crustal depth due to tectonic overpressure.
• Thickness of shear zones due to shear heating is independent on numerical resolution.

We present two-dimensional numerical simulations of lithospheric shortening with a crust containing weak and strong inclusions. Thermo-mechanical coupling is included, and a crustal-scale shear zone develops self-consistently due to viscous heating and thermal softening of temperature dependent viscosities. Several tests for crustal conditions are performed showing that 1) the thickness of and strain rates within the shear zone are independent on the numerical resolution and applied numerical method (finite element and finite difference method), 2) the shear zone is stable and rotates during large strain deformation, 3) the numerical algorithm conserves total thermal and mechanical energies, and 4) the bulk horizontal force balance is fulfilled during large strain deformation. A fold nappe develops around the shear zone in the lithospheric shortening simulation. In this simulation the stresses in the crust are limited by a friction angle of 30°. Significant tectonic overpressure (PO) occurs in strong lower crustal rocks and in strong inclusions. Significant PO also occurs in a weak inclusion that is only partly surrounded by strong crustal rock suggesting that a continuous strong “vessel” is not required to generate significant PO in weak rocks. Maximal values of PO are ~ 2.2 GPa with corresponding deviatoric stresses ~ 1.5 GPa and occur in a depth of ~ 42 km. Maximal pressure of ~ 3.4 GPa and maximal temperatures > 700 °C occur during the formation of the fold nappe in crustal depth. Synthetic pressure–temperature paths exhibit entire cycles of pressure and temperature increase and decrease, and suggest that crustal rocks in depths < 50 km can reach the ultrahigh pressure metamorphic facies fields. Applications to tectonic nappes with high and ultra-high pressure rocks in the Western Alps are discussed, and a dynamic model for the evolution of fold nappes in the Western Alps is proposed.