Apport de l'analyse microstructurale à la compréhension des mécanismes de plissement : Exemples de structures plissées aux USA (Wyoming) et en Iran (Zagros)

The link between the mechanisms of macroscopic deformation and the mechanisms of microscopic deformation during folding is not completely established, in spite ofvarious studies carried out on this topic. With the aim at better understanding the mechanism of folding, this work consists in studying how, where and when internal deformation occurs during a fold's ―life‖. This objective could be reached by a new combination of the results of micro- and mesostructural analyses. This combination includes results of calcite twin analyses (using Etchecopar's method and Groshong's method), results of magnetic and elastic properties analysis (Anisotropy of Magnetic susceptibility; AMS, anisotropy of P-waves Velocity; APV) and results of fracture and fault analyses. Calcite twin analysis (Etchecopar method) and fault analysis are good tools for the quantification of paleostresses associated with folding, while the analysis of the AMS and the APV fabrics and calcite twin data obtained by Groshong's method helps characterize the microdeformation recorded by the folded layers. During this study we faced some problems related to the operational limits of these methods, like the significance of the magnetic signal in the diamagnetic rocks or the rocks with low magnetic susceptibility, or like the critical value of calcite twinning for very small or very big crystals. The combination of these various methods of analysis and their data helps overcome these difficulties. These methods were applied to two fold structures: Sheep Mountain Anticline (Wyoming, USA) which is an analogue of mixed reservoir sandstone/carbonates, and Kuh-e Khaviz (Iran) which is an analogue of fractured carbonate reservoirs, and on the Zagros folded belt in the Fars area (Iran). The complete Laramide tectonic history of the Sheep Mountain Anticline has been reconstructed, including a mechanical scenario with the evolution of paleostress magnitudes and pore fluid pressure through time. At a regional scale in the Fars the comparison of the structural data obtained by these various methods demonstrates the consistency at different space and time scales of the record of orogenic stresses related to the convergence between the Arabia-Eurasia plates since ~20 My. The fact that the Arabia-Eurasia convergence is not very oblique in the Fars explains the absence of a partitioning in this area. This partitioning of the oblique convergence does occur in the area of Dezful, where the Khaviz anticline was investigated using calcite twin analysis. This analysis has made it possible to reconstruct the evolution of stress orientations before, during and after folding, which led to a better understanding and interpretation of local /regional fracture development. At the regional scale, the combination of the calcite twin data with those of the fracturing and the faults in the Dezful confirms the partitioning suggested by previous works and better constrains its time of initiation. In addition to the local and regional conclusions, the combination of the results of these various methods has made it possible to confirm the existing complementarity between the mechanisms of deformation on various scales, with a prevalence of the microdeformation before and after folding s.s. while macro- and meso-deformation prevail during folding s.s., and to establish a complete tectonic history of folding.