Mesozoic diapirism in the Pyrenean orogen:: Salt tectonics on a transform plate boundary

The Mesozoic history of the Pyrenean region was dominated by the opening of the North Atlantic Ocean and the left-lateral movement of Iberia relative to Europe. Most deformation was tensional or transtensional but this style was interrupted by several short compressional events, perhaps related to bends in the transform plate boundary or minor changes in plate movement directions. These compressional events were recorded by deformation adjacent to and above salt structures, which were the weakest parts of the system and so served as especially sensitive strain barometers. The alternation of extension and shortening controlled the location and style of salt structures. Evaporites were initially deformed during the latest Jurassic-Early Cretaceous Neocimmerian transpression, which formed salt-cored anticlines above west-northwest- east-southeast-trending basement faults. Transtension in the Aptian-Albian caused salt domes to pierce to the surface at the intersections of these anticlines with reactivated northwest-southeast- and northeast-southwest-trending basement structures. Transtension was at least locally interrupted by brief periods of transpression at the Aptian-Albian and Albian-Cenomanian boundaries. Transpression shortened the diapirs, causing rotation, uplift, and erosion of beds near the salt. Most diapirs were buried during the Late Cretaceous. In some cases, postburial hydrothermal circulation dissolved much of the halite, causing the diapirs to sag and the basins to form in roof strata. Finally, the entire region was shortened during the Late Cretaceous-Tertiary Pyrenean orogeny, which greatly distorted many of the preexisting geometries.