Dynamic constraints on crustal-scale rheology from the Zagros Mountains
Résumé
The Zagros Mountains are a spectacular example of a mountain belt that consists of crustal-scale folds with a regular spacing of ~15 km. Despite excellent geological constraints, the formation of these structures remains enigmatic. Here, we therefore use visco-elasto-plastic numerical models to understand the dynamics of the Zagros on geological time scales. Models with a brittle crust and a single basal salt layer produce fault-related deformation structures that are inconsistent with the data. If, on the other hand, we take into account the observation that there might be up to 3 intermediate weak layers within the brittle crust, our models produce crustal-scale folds with the correct wavelength. Physically, this is caused by a folding instability whose wavelength mainly depends on the friction angle of the crust and the viscosity of the detachment layers. By combining a new semi-analytical technique with independent constraints on the effective viscosity of salt, we show that the friction angle of the crust in Zagros should be smaller than ~10° to reproduce observations. Our results have implications for the deformation of the crust as they highlight the importance of thin detachment layers. Moreover, our results show how geological observations put constraints on the long-term rheology and dynamics of the crust.