Megaflaps flanking salt walls: strain and stress distribution from field observations to numerical modeling.

Abstract

The megaflaps are halokinetic objects of multi-kilometer extension corresponding to vertical or overturned strata flanking salt walls or resulting welds. Although their geometry and mechanical behavior can be considered similar to detachment folds, their development mechanism and the consequences regarding strain record remain to be specified. Megaflaps record strain before, during, and/or after tilting, and their development involves mechanisms specific to each megaflap (i.e. force folding with limb rotation or kink-band migration, passive folding), to their geological characteristics and the local and regional context. We highlight that the initial carapace, covering the evaporites and forming the future megaflap, constitutes a continuous mechanical unit allowing the transmission of an early homogeneous stress (either local or regional). Folding is then initiated by limb rotation, hinge migration or passive folding. During diapirism, the piercement of the salt structure generally accommodates the stress, that preserves the adjacent sediments from regional deformation. However, due to the evaporites loading, we observe a very localized compressive strain, which is perpendicular to the evaporite wall and results in the development of salt-related meso- and microstructures. We compared the stress states determined on field analogs are with 2D uncoupled geomechanical models. The stress-strain distribution of two types of megaflaps were studied: a single development step megaflap, presenting a main mechanical unit (e.g. Karayün megaflap, Turkey), and a sequential development megaflap, presenting several mechanical units (e.g. Cotiella megaflap, Spain). Our models are based on well-defined geometries of known field analogs. In each model, the layers constituting the future megaflap record compressive strains, varying in extent and localized near the salt wall. Horizontal compression parallel to the layers, induced by salt push, is consistently observed and matches with field observations. Our models also depict layer folding, which can be accommodated through various mechanisms (extrados and intrados deformation, compressive mechanical guidance). Finally, it seems that the late stage tightening deformation would occur through an intensification of stress magnitude induced by complete welding. Our models show that welding significantly increases the maximum stress magnitude. The ratio between compressive stresses and background stresses is thus four to five after welding, compared to a ratio of two without welding. This matches our observations regarding the formation of new mesostructures during the late stage tightening within the Cotiella megaflap, induced synchronously in all layers (regardless their attitudes, already vertical and overturned, or gently dipping). In contrast, the preservation of salt bodies is correlated with the absence of late micro- and mesostructures perpendicular to the vertical layers as observed in other known field analogs (e.g., Sivas Basin, Paradox Basin, Witchelina diapir). Consequently, some megaflaps may remain unaffected by late-stage tightening even within a regional compressive system undergoing shortening.