--> Halokinetic Deformations During the Development of Detachment Folds on Multilayered Syn-Orogenic Evaporitic Sequences: Results From Analogue Modeling and Comparison With the Sanaüja Anticline (Ebro Foreldand Basin, Iberian Peninsula)

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Halokinetic Deformations During the Development of Detachment Folds on Multilayered Syn-Orogenic Evaporitic Sequences: Results From Analogue Modeling and Comparison With the Sanaüja Anticline (Ebro Foreldand Basin, Iberian Peninsula)

Abstract

Detachment folds are common structures in fold-and-thrust systems containing evaporitic layers. The geometry of these structures, their kinematics, and their mechanical features have been widely analyzed from real case studies, analogue and numerical models as well as geometrical analysis. However, almost all the literature has considered pre-kinematic evaporites and practically not attended syn-kinematic evaporites. Consequently, the influence of a syn-tectonic evaporitic level in the detachment folds evolution is rather unknown. This knowledge gap is striking taking into account that syn-tectonic evaporites are rather usual in several deformed foreland basins (i.e., Tarim, Zagros or Ebro basins). Specifically, the Ebro basin (southern Pyrenean foreland basin) is formed by a thick syn-orogenic sequence that includes several evaporitic layers with salt that are involved in the contractional belt. The Sanaüja anticline is a 5 km width and 25 km long detachment fold probably developed at the southern pinch-out of the syn-orogenic Cardona salt unit that also involves younger syn-contractional evaporitc unit (Barbastro Fm.) and the Solsona fluvial succession. This anticline is characterized by several minor folds with opposite vergences detached at the bottom of the Barbastro Formation. Based on this natural analogue and using an experimental approach (sandbox models) we propose a new kinematic model for the development of detachment folds with multilayered evaporites in the Ebro basin. This model denotes that the overall fold geometry is constrained by the deep detachment level (Cardona salt), but also by the overlying evaporitic layer (Barbastro evaporites) that controls the formation of disharmonic folds in the interlayered non-evaporitic strata and the development of secondary tight folds and thrusts. Detachment anticlines can evolve to diapirs or salt walls without erosion. Indeed, the presence of a thin roof for the incipient detachment anticline together with the deposition of thick syn-kinematic sequence in the adjacent synclines result in salt pressure gradients. This gradient trigger salt migration and salt piercement in the anticline crest. Simultaneously, the adjacent synclines become strongly subsident if the contraction progresses. Analogue modelling of detachment folds suggests a new scenario for diapirs formation in compressional tectonic settings.