Balanced cross-sections across the Zagros fold-thrust belt in Iran are used to analyze the geometry of deformation within the sedimentary cover rocks, and to test the hypothesis of basement involved thrusting throughout the fold-thrust belt. Although the Zagros deformation front is a relatively rectilinear feature, the sinuous map-view morphology of the mountain front is a result of a 6 km structural step in the regional elevation of the Asmari Limestone that produces a pronounced step in topography termed the 'mountain front flexure'. Although the height of the mountain front flexure is sufficient to permit basement fault-bend folds at the front of the Lorestan and Fars regions, the taper of the orogen, low percentage of shortening, and consistent structural elevation from the mountain front flexure to the hinterland suggest that Lorestan and Fars segments of the fold and thrust belt are completely detached on lower Cambrian salt and that basement-involved thrusting occurs only in the hinterland of the orogen. Mass balance constraints necessitate that detachment folds throughout the fold-thrust belt are cored by faults that branch from the basal detachment. The steep dips of these faults and their depth within the lower Paleozoic sedimentary rocks can account for recorded earthquakes. This suggests that the ∼11±4-km-deep earthquakes throughout the fold-thrust belt may be nucleating within sedimentary rocks rather than in the basement as previously proposed. Total shortening in the Zagros fold-thrust belt is 70±20 km, which corresponds to ∼20% shortening of the Arabian block.