Structural evolution of salt-influenced fold-and-thrust belts: A synthesis and new insights from basins containing isolated salt diapirs

Lateral shortening is expressed in unique ways in salt basins, especially if pre-shortening diapirs are present. We present an overview of previously-published and new physical models and present new 3-D conceptual models that capture the evolution of shortening structures in salt provinces dominated by precursor isolated diapirs (termed isolated-diapir provinces). In such provinces, isolated diapirs form only a minor volumetric component of a sedimentary basin, however, due to the relative weakness of rock salt and their ability to localize strain, during shortening they have a disproportionately large influence on structural development. We find three key mechanical principles govern the processes and structural styles developed during shortening of isolated-diapir provinces. First, salt diapirs shorten before surrounding sedimentary rocks due to their relative weakness, and so form salients in the thrust front during early shortening. Second, diapirs tend to nucleate folds and faults, which radiate out from the diapirs. Third, as diapir walls converge, the roof must shorten. Extrusive salt sheets are expelled through thin roofs, but thicker roofs resist piercement and so tend to undergo complex folding and faulting. As a result of these principles, the first-order controls on the structural styles expressed across a shortened isolated-diapir province are the configuration the diapir array prior to shortening, the connectivity of these diapirs prior to shortening, total strain magnitude, and diapir roof thickness. Second-order controls include the initial cross-sectional and map-view geometry of diapirs, diapir size, and diapir orientation with respect to the shortening direction.