The Isthmian salt basin in the southern Gulf of Mexico can be divided into the Yucatan and Campeche subbasins, separated by a base-salt high near the nose of the Yucatan platform. Despite their proximity, these two subbasins experienced radically different histories in the period immediately following salt deposition. Portions of the Yucatan subbasin are characterized by larges-cale (locally as much as 60 km [37 mi]) downdip translation of salt and suprasalt sediments during the Late Jurassic. This translation produced a major detached extensional province at the updip end of the basin, which is not compensated by observed shortening downdip. We interpret this history to be a result of unconfined seaward flow of salt and its cover during basin opening, a process mirrored on the conjugate Florida margin. The Campeche subbasin, in contrast, shows no evidence of significant Late Jurassic translation detached on salt. No larges-cale extensional or contractional provinces of Mesozoic age are evident, although some minor translation did occur. We suggest that salt in the Campeche subbasin was confined at its seaward end, which prevented the seaward salt flow experienced in the Yucatan subbasin. Furthermore, salt at the seaward end of the Campeche subbasin lies 2-3 km (1-2 mi) above oceanic crust, in contrast to salt lying on crust whose top sits at or below the level of oceanic crust at the seaward ends of the Tamaulipas, Yucatan, and Florida margins. The Campeche subbasin thus appears to have been perched relative to other parts of the Gulf of Mexico.