Sigmoidal structures and their relationship with detachment layers including pre-exist Hormuz salt in Fars and Hormuzgan Zones of the Zagros Fold and Thrust Belt: Insights from experimental modeling and seismic data

Document Type : Original Article

Authors

1 Department of Geology, North Tehran Branch, Islamic Azad University, Tehran, Iran

2 Department of Tectonic Geology, National Iranian Oil Company, Tehran, Iran

Abstract

Introduction: The Fars and Hormuzgan Zones characterized by the outcrops and sub-crops of Precambrian-Cambrian Hormuz Salt. This suggests that part of halokinesis could have been localized along the Precambrian-Cambrian extensional faults. Differential loading is the most probable principal mechanism for initiating and driving halokinesis in the Hormuz Salt Basin in Early Paleozoic. Therefore, it is expected that dissimilar types of the earlier salt related structures have their own influences on the geometry of present-day structures. Therefore, objective of this study is understanding role of the Hormuz Salt as basal décollement on long term sedimentation and relatively short term shortening happened by the Zagros Orogeny. The presence of anomalies in buried salt domes plays an important role in this inequality, and for example, sigmoid-shaped anticlines can be mentioned.
Materials and methods: We arranged a sand box model to review structural evolution of the study area. In this experiment, we tried to simulate behavior of a viscous layer representing the Hormuz Salt at different stages.  This modeling was done with the objective of revealing role of the pre-exist Hormuz Salt bodies in geometry of the study area structures. The apparatus for this experiment is consists of four plates at four side of the sand box with the size of 20 × 25 cm. At the first stage of experiment we attempted to construct two column of Corn starch cream whit 3 cm diameter in sand layer to simulate expected pre- exist Hormuz salt. Three experiments were performed to indicate the effect of anomalies from the moving wall. Finally pattern of circles overlaid on the flat surface of the last dry sand layer. Total thickness of dry sand layers is 3 cm. In the last stage, shortening applied by the moving wall with a rate of 1 mm/min. This stage simulates superimposition of the Zagros Orogeny on the paleo-structures which were formed earlier than onset of the Zagros movements.
Results and discussion: Role of the basal and intermediate detachments of the Zagros Belt in deformation development in form of folds and thrusts have widely been discussed and the Hormuz Salt efficiently controlled fold wavelength in the Fars and Hormuzgan area. For instance, divergence reflectors (growth strata) beneath the syncline between Naura & Firouzabad anticlines denote early Hormuz Salt movement due to halokinesis. Results of the sandbox experiment confirm this matter as deformation in corn starch cream column and sand. Folding and thrusting happened at both sides of this column zone. In contrast, sigmoidal shape happened.
Conclusion: When the Zagros Orogeny shortened the belt, the localized Hormuz Salt bodies mainly affected geometry of the shallow structures. Therefore, structural evolution of the study area is simply summarized in two pre-Zagros halokinesis and the Zagros shortening stages. In fact, the Zagros compressional forces facilitated salt movement in form of salt diapirs as seen in the experiment. The results of this study also emphasis on importance of pre-exist salt body to unconventional shape folding whit one or both narrow and linear plunges.

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References

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