Basement-controlled early halokinesis associated with later folding and strike-slip faulting in central Zagros, Iran: insights from evolution of the Karebas Fault System and adjacent structures

In the central part of the Zagros fold-and-thrust belt of Iran, several diapirs of the Latest Precambrian‒Early Cambrian Hormuz Salt decorate locally the surface exposures of nearly N–S trending dextral fault systems. The Karebas Fault System as one of these fault systems is associated with five salt diapirs. There are several NW–SE trending anticlines that either are cross-cut by the fault system or terminate against it and the adjacent salt diapirs. Field observations and construction of structural crosssections were integrated to (1) document timing of halokinesis and (2) constrain the relationship between the halokinesis and the structures that developed during the Zagros shortening in the Late Cenozoic. The Neogene Zagros folds are clearly affected by the surface manifestation of the Karebas Fault System, suggesting that the fault system is a young structure coeval with or post-dating the Neogene deformation. On the contrary, exposed evidences of halokinesis indicate that the diapirs were rising at least since the Late Jurassic, long before the Zagros was shortened during the Late Cenozoic, although the earliest halokinesis most likely started in the Earlier Paleozoic shortly after deposition of the evaporites. The geometry of strata flanking the diapirs indicates long-term pre-folding salt rise driven by downbuilding of lateral minibasins and progressive draping of sedimentary layers over the flanks of the rising diapirs. Therefore, presence of old salt structures along the young strike-slip fault system suggests an important role played by early halokinesis in inducing later strikeslip deformation along zones already weakened by pre-folding salt structures and thinner sedimentary pile above them. Salt diapirs and related changes in stratigraphic thicknesses (i.e. minibasins) also effectively grappled with the process of folding. Basically, detachment folding is the main structural style across the area, formed mainly above the Hormuz Salt as the basal décollement and partly by involvement of the intermediate detachment levels. The intermediate detachment levels resulted in tighter folds overlying gentler anticlines, flanking rabbit-ear structures, shallow structural decoupling and out-of-syncline thrusts. A further complication of deformation occurred because of involvement of the pre-existing salt diapirs and minibasins. As the southward-advancing front of the Zagros deformation migrated across the stratigraphic section and salt structures, weakness of the diapirs and relative rigidity of the minibasins led to perturbations in the stress field. Consequently, strain was localized and delocalized at the diapirs and the minibasins, respectively. This process is reflected by the variable fold geometries (location and dimension) and fold-axes rotations. Architecture of lateral minibasins, salt-related thickness variations, local rigidity of the minibasins and finally the dominant type of diapir-flanking halokinetic sequences controlled (1) longitudinal fold propagations, geometries and orientations, and (2) fold locations with respect to pre-existing diapirs.