Mylonitic deformation of evaporites in décollements: examples from the Southern Alps, France

Abstract

The ductile deformation of Triassic evaporites is described in two décollements from the Southern Alps of France (the Arc de Nice and Arc de Digne nappes). We show that deformation is not chaotic as is commonly considered: on the contrary it is commonly well organized and is consistent with progressive shearing. Typical shear structures include mylonitic foliation, stretching lineations, folds of various shapes and orientations (“a” type folds and sheath folds), asymmetrical pressure shadows composed of gypsum fibres, shear bands and boudinage. At two sites, lineation directions and analysis of shear criteria allow the kinematics of nappe emplacement to be determined. Computer modelling of pressure shadows allows deformation mechanisms and strain measurements to be determined. The results of the analysis can be placed in the context of the regional framework of the two décollements. (1) Deformation at the Nice site corresponds to dextral wrenching on a lateral ramp of the décollement along the western limit of the Arc de Nice nappe. Deformation in the ductile fault zone is close to simple shear. (2) Deformation in the second site is associated with a ramp which affects the décollement in front of the Arc de Digne nappe. The deformation corresponds to shearing towards the southwest, including a pure shear component. This result can be explained by taking into account the effects of the ramp geometry upon the deformation in the décollement. This shows that deformation analysis of evaporites in décollements can be used to understand the mechanisms and kinematics of nappe emplacement.