Thin-skinned fold-and-thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer. The resulting surface expression frequently consists of anticlines and synclines spaced in a fairly regular manner. It is thus tempting to use this spacing along with other geological constraints to obtain insights into the dynamics and rheology of the crust on geological time scales. Here we use the Zagros Mountains of Iran as a case study, as it is one of the most spectacular, well-studied thin-skinned fold-and-thrust belts in the world. Both analytical and numerical models are employed to study what controls fold spacing and under what conditions folding dominates over thrusting. The models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover, deformation is dominated by thrusting, which is inconsistent with the data of the Zagros fold belt. If we instead take into account additional decollement layers that have been documented in the field, a switch in deformation mode occurs and crustal-scale folding is obtained with the correct spacing and time scales. We show that fold spacing can be used to constrain the friction angle of the crust, which is similar to 5 degrees in the Zagros fold belt. This implies that on geological time scales, the upper crust is significantly weaker than previously thought, possibly due to the effect of fluids.