The Zagros folded belt (Fars, Iran): constraints from topography and critical wedge modelling

The Late Miocene tectonics of the Zagros folded belt (Fars province) has for long been related solely to folding of the cover controlled by a ductile decollement between basement and the sedimentary cover. However, geological constraints, topography analysis and seismotectonic studies reveal that basement thrusting may produce locally significant deformation in the cover. To determine how the deep-seated deformation in the basement may contribute to the overall topography we first examine the filtered large and short wavelengths of the topography. We find that the short-wavelength component of the topography (20-25 km), including the Zagros folds, is superimposed on the differential uplift at the regional scale. In other words, the regional base level of folded marker horizons remains parallel to the regional topography of interest. Modelling reveals that the salt-based wedge model, alone, is not able to reproduce the large-wavelength component of the topography of the Zagros Folded Belt. This reveals that when a thick (relatively to its overburden) layer of salt forms the basal decollement it is generally too weak and cannot support the growth of significant topography. We then test an alternative thick-skinned crustal wedge model involving the crust of the Arabian margin, which is decoupled above a viscous lower crust. This model satisfactorily reproduces the observed topography and is consistent with present-day basement thrusting, topography analyses and geological constraints. We conclude that basement-involved thickening and shortening is mechanically required to produce the shape of the Zagros Folded Belt since at least 10 Ma. Finally, the involvement of the basement provides mechanical and kinematic constraints that should be accounted for cross-sections balancing and further assessing the evolution of Zagros at crustal or lithospheric scales.