Snidero, Marco (2021) ROLE OF INHERITANCES IN SHAPING THE ZAGROS THRUST AND FOLD BELT. [Tesi di dottorato]

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The claim that a place has a unique geology is made often, and never incorrectly. The Zagros fold and thrust belt represents an exceptional example where different structural and stratigraphic inheritances co-exist along the same mountain range, playing a key role in determining the lateral variability of the thrust and fold belt. To the south-east, the Zagros mountains are limited by the Makran subduction zone. The subduction is pinned laterally by the continental collision of Arabia and Asia in the Straits of Hormuz area, where the Zagros and Oman chains meet. This zone of transition forms a major structural reentrant where the Zagros deformation front and the main Zagros thrusts converge. This boundary represents the eastern limit of the Hormuz salt basin, characterized by a minimum of two kilometers thick salt unit deposited during the Neoproterozoic-Cambrian times. The Hormuz salt province aerially extends for over 500 km toward the north-west along the Fars Province, where spectacular diapirism developed previously to the Zagros contractive event. Besides, the presence of an effective basal decollement resulted in a wide detached thrust and fold belt shaping the structural salient of the Fars arc. The northwestern-ward thinning and finally pinchout of the Hormuz salt is progressively taken over by the presence of the Mountain Front Flexure, a major structure of the Zagros orogenic system underlain by the deeply rooted and seismically active Mountain Front Fault system. These coupled structural features divide the belt from its foreland and their trace is sinuous, forming a sequence of salients and recesses, formally named, Dezful embayment, Lurestan arc and Kirkuk embayment. In this work we combine the interpretation of on-shore and off-shore seismic reflection profiles, field data, earthquake data, geomorphic analysis, and, remote sensing interpretations, to build a series of geological maps, 3D geological reconstructions, geological and balanced cross-sections, and, sequential restorations in the eastern Fars province and the Lurestan region. We provide new evidence from different structures of the Zagros fold-and-thrust belt, to stress the role of inheritances related with the previous rift architecture and the presence of the lateral facies change to the Hormuz salt sequence, as an important lateral variations of the mechanical properties of the multilayer, and a dramatic change in the structural style related with the pre-contractional Hormuz salt diapirism. In the Fars province we propose new interpretations for several on-shore and off-shore pre-contractional salt structures. Our evolutional models show how the deformation of inherited salt structures predates thrust wedging and leads to squeezing, roof arching, crestal extension and finally extrusion. Further shortening result on secondary welding as evidenced by the collapse of the extrusion summit dome and reverse faulting nucleated at the secondary welds. Regional cross sections across the eastern Hormuz salt pinch-out aim to understand the switch in structural style from a salt-detached thin- to thick-skinned thrusting. In the Lurestan region we introduce a new interpretation of an hyperextended margin architecture segmented by inherited N-S and NE-SW striking faults, in an alternance of more proximal or distal rift domains. The integration of our results with previous knowledge indicates that the Mountain Front Fault system developed in the necking domain of the Jurassic rift system, ahead of an array of inverted Jurassic extensional faults, in a structural fashion which resembles that of a crustal-scale footwall shortcut. Within this structural context, the sinusoidal shape of the Mountain Front Flexure in the Lurestan area arises from the re-use of the original segmentation of the inverted Jurassic rift system.

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