A simple thermo-mechanical shear model applied to the Morcles fold nappe (Western Alps)

We present a one-dimensional shear zone model with a power-law flow law and a temperature dependent viscosity. The analytical solution for the velocity profile across the shear zone depends only on the single dimensionless parameter β. β depends on the activation energy of the applied flow law, the temperature at the base of the shear zone and the temperature difference across it. The solution can describe three types of shear deformation: (1) homogeneous simple shear for β < < 1, (2) thrust-sheet emplacement on top of a thin ductile shear zone for β > > 1 and (3) heterogeneous simple shear potentially generating fold nappes for β ~ 5-10. We perform a systematic parameter search and determine the parameters that provide the best fit between a sheared model line and a reference line representing the first-order geometry of the frontal part of the Morcles nappe. We also apply a more elaborated pressure-driven model to test whether published flow laws for calcite and realistic pressure gradients provide realistic velocities and strain rates for the Morcles nappe. The value of β applicable to the Morcles nappe is estimated with three independent methods: (1) applying reported values for temperatures and activation energy to the analytical formula for β, (2) fitting the first-order geometry of the Morcles fold nappe and (3) comparing results of a velocity-driven (i.e. kinematic boundary condition) and pressure-driven (i.e. pressure source term and free surface) shear zone model. All three methods yield consistent estimates for β between 3 and 15. Our results show that previously published kinematic models, which explain the tectonic evolution of the Morcles fold nappe within a crustal shear zone, are physically feasible. The results further suggest that viscous heterogeneous simple shear played a dominant role during the formation of the Morcles fold nappe.

Highlights► A simple thermo-mechanical shear model is proposed for the formation of fold nappes. ► One dimensionless parameter controls transition from fold nappe to thrust sheet emplacement. ► Formation of Morcles fold nappe was dominated by heterogeneous ductile simple shear. ► Proposed model for Morcles fold nappe agrees with first-order geometry, kinematics and dynamics.