Mechanical constraints on the development of the Zagros Folded Belt (Fars)

Abstract

We synthesize available structural, seismotectonics and microtectonics studies, mechanical modelling of the topography as well as stratigraphic constraints on the timing of Plio-Pleistocene folding and Zagros basin evolution in order to examine which mechanical behaviour would explain the development of the Zagros Folded Belt at both local and regional scale. At the local scale we focus on the mechanism of cover folding and internal deformation of cover rocks. At the regional scale we focus on crustal rheology that led to the observed regional topography. Recent mechanical constraints derived from a critical wedge modelling of the regional topography together with available structural studies and seismotectonic studies confirm that the basement is necessarily involved in the deformation. Additionally, crustal rheology should involve a sufficiently strong lower crust to maintain the topography. Stratigraphic data on the basin scale suggests that the deformation in the Zagros Folded Belt initiated by inversion of the inherited N-S and NW-SE-trending marginal structures in the early Miocene. At 5-3 Ma, the intraplate stresses have increased sufficiently in response to ongoing convergence to exceed the brittle strength of the pre-fractured basement and then to produce the initiation of the Zagros uplift. This event occurred simultaneously with the rapid development of cover folding until the Bakhtyari conglomerates were deposited unconformably on these structures as the fold growth decreased. The Hormuz salts at the base of the pile allowed the upper sedimentary cover to be decoupled from the basement but there is no evidence of independent development through time. This is confirmed by the kinematical consistency of the Mio-Pliocene small-scale faulting in the cover and seismogenic faulting reflecting the internal deformation of basement and cover, despite the occurrence of the thick Hormuz salt layer. Buckling of the cover rocks, rather than thinskinned propagation of the Zagros Folded Belt, is proposed to be a more reliable mechanism to account for stratigraphic data, field observations, structural studies, microtectonic data and mechanical modelling. We finally conclude that the overall thick-skinned deformation that followed the initial margin inversion was probably coeval with cover folding (buckling). The way basement and cover deform is thus remarkably different; the basement is pre-fractured so it shortens preferentially by faulting. In contrast, the folding (buckling) of the sedimentary cover developed with the assistance of plastic-viscous processes.