Effect of rift inheritance and salt layers on mountain building – a numerical modelling study motivated by the southern Pyrenean foreland fold-and-thrust belt

The existence of weak and shallow intra-crustal (salt) layers, syn-tectonic sedimentation as well as extensional inheritance have been shown to play a major role on deformational style and structural geometry of mountain belts on Earth. The Pyrenean orogen provides a well-constrained natural example of an inversion orogen strongly influenced by salt-detached foreland-fold-thrust belts. This study investigates the influence of pre-contractional salt and of syn-contractional salt deposition, together with inherited extensional structures, on crustal-scale mountain-building and associated foreland fold and thrust belt formation. To this end we use high-resolution thermo-mechanical numerical simulations based on the finite-element code FANTOM 2D, and explore the influence of salt viscosity, crustal flexural rigidity, extensional inheritance, crustal strength, and syntectonic sedimentation. By systematically varying those parameters, we can assess the impact of syn-tectonic salt deposition on the localization of deformation, thrust system evolution (both thick- and thin-skinned), and overall orogenic geometry. Comparison with the Pyrenees and other similar orogenic settings emphasizes the importance of extensional inheritance, syn-tectonic sedimentation and salt-tectonics during mountain building. Our models provide new insights into the mechanical role of evaporites in fold-and-thrust belt development.