GOT SALT? RECOGNIZING THE FORMER PRESENCE OF DIAPIRIC SALT BODIES IN OROGENIC BELTS

Orogenic belts may invert, overprint, and deform former rift basins and overlying passive margin successions that often contain long-lived mobile salt systems. Orogenic shortening can effectively “squeeze” the salt completely out of the system leaving behind apparent faults and folds that may be mistakenly interpreted as reflecting only orogenic tectonic processes and not the precursor salt tectonics. In addition, diapiric salt may carry sedimentary, igneous, and/or metamorphic inclusions that can be recycled into higher stratigraphic levels leading to erroneous sediment source exhumation level interpretations in the associated sedimentary basin. Understanding the contribution, location, and timing of salt diapirism is critical to structural restoration, burial history, thermal history, and reconstruction of the tectonic evolution of orogenic systems.

Presented here are a series of sedimentologic, stratigraphic, and mineralogic tools that can be applied to recognize the former presence of diapiric salt bodies that have been removed or overprinted by orogenic processes. Salt welds and fault welds will display halokinetic sequences in their wall rocks. Wall rocks may contain recycled sedimentary facies from the diapir with unusual lithoclasts/grains (for example doubly-terminated quartz) or display zircon populations derived from non-evaporite diapiric inclusions (diapir-derived detritus). Orogenic igneous and metamorphic bodies that contain unusually high Na, Cl, K, Al, and/or B bearing mineral assemblages may reflect the incorporation of evaporite-concentrated elements from assimilation or metamorphism of salt bodies. Other features that are not exclusive to, but that hint at possible salt diapirism, are polygonal structural patterns and unusual thrust map traces. Scrutinizing structures for a potential precursor mobile salt history is especially important when working orogenic belts that deform former basins associated with the two major periods of continental break-up and salt deposition in Earth’s history (i.e. late Neoproterozoic breakup of Rodinia and early Mesozoic breakup of Pangea).