In the foreland thrust belts of the Pyrenees, the Triassic Keuper evaporites have long been recognized as the principal detachment level for thrusting. The deformed Late Cretaceous to Eocene foreland basins of the Pyrenees show evidence of diapirism that has been often overlooked due to the more obvious imprint of thrusts and fault-related folds. We explore the variation of the salt-tectonics structural style along two classic transects of the Southern Pyrenees (Noguera Pallaresa and Noguera Ribagorzana rivers) addressing the role of halokinesis in the structural and sedimentary development of the basin. Appreciating the diapiric role of the Triassic source layer provides new explanations with strong implications for the kinematics of compressional deformation and the amount of orogenic shortening.
The areas described include precursor diapirs, that started developing during the Mesozoic pre-orogenic extensional episode of the Pyrenees, and areas where halokinetic movements were triggered during the Pyrenean compression. We report the case of the Sierras Marginales foothills, a polygonal system of salt ridges and intervening synclines, initially outside of the area affected by rifting and then filled with early synorogenic sediments that now appears intensely imbricated. The second case study is the Montsec thrust and Ager basin, a linear salt wall and adjacent syncline with a long history dating back to the extension, subsequently squeezed during the orogeny. Also discussed is the case of the Pobla de Segur and Senterada intramontane conglomerate basins on the Boixols and Nogueres thrust sheets, which shows a complex history of sediment and thrust sheet loading onto a wide diapir which through salt expulsion causes the tilting of the entire basin.
Our field observations lead to fundamental questions regarding the relative roles of buckling and drape folding by salt evacuation, the mechanisms that drove the early, salt-cored folding to late imbrication, and the causes favoring polygonal salt ridges and minibasins vs. linear salt walls (i.e. basement faulting vs relative evaporite/overburden thickness).