Evolution of the Karebas Fault System and adjacent folds, central Zagros fold-and-thrust belt, Iran: Role of pre-existing halokinesis (salt structures and minibasins) and detachment levels

摘要

In the central part of the Zagros fold-and-thrust belt of Iran, several diapirs of the Latest Precambrian–Early Cambrian Hormuz Salt locally decorate the surface exposures of a series of N–S to NNW–SSE trending right-lateral tear fault systems. The Karebas Fault System (KBFS), as one of these fault systems, is associated with five salt diapirs. There are also several NW–SE trending anticlines, which are either cross-cut by the fault system or terminated against it and the adjacent salt diapirs. The Neogene Zagros folds are clearly affected by the surface expression of the Karebas Fault System, suggesting that the fault system is coeval with or post-dates the Neogene deformation. On the contrary, halokinesis as old as at least Late Jurassic is documented by near-diapir stratigraphic exposures, indicating pre-orogenic existence of the diapirs. Although the lack of older rock outcrops precludes tracking the pre-Jurassic records of diapirism, the geometry of strata, which flank these 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 (Neogene) tear fault system suggests an important role played by the presence of early-formed salt structures in inducing later strike-slip deformation along zones of preferential weakness, which was prepared by the pre-folding salt structures and thinner sedimentary pile overlying them. Salt diapirs and related changes in stratigraphic thicknesses (i.e. minibasins) also effectively grappled with the process of folding. Detachment folding is typically the main structural style across the area, and was mainly developed above the Hormuz Salt as the basal décollement and partly by involvement of the intermediate detachment levels. However, preferential weakness and squeezing of the diapirs on one hand and greater strength of the minibasins on the other hand gave rise to further complexities of deformation during shortening. Such first-order controls could led to perturbations, at the scale of diapirs and minibasins, in the stress pattern as the southwestward-advancing front of the Zagros deformation migrated across the stratigraphic section and the salt structures. It occurred because strain is localized at the diapirs and squeezes them whereas tends to avoid folding the thick, and hence stronger, minibasins into anticlines. In such a context, the localization and the longitudinal propagation of the folds in the cover sequence could be remarkably influenced and irregular. The consequence was variable fold geometries (location and dimension) and fold-axes rotations with respect to the diapirs and minibasins.