STRUCTURAL STYLE IN THE SOUTH DEZFUL EMBAYMENT, SW IRAN: COMBINED INFLUENCE OF THE ZAGROS FRONTAL FAULT SYSTEM AND THE DETACHMENT IN THE MIOCENE GACHSARAN FORMATION

Islam Tavakolian^*, Ali Yassaghi^*+ and Mahdi Najafi^*

^* School of Petroleum Geology, Faculty of Engineering, University of Zulia, Maracaibo 4002, Venezuela.

⁺ corresponding author: yassaghi@modares.ac.ir

Key words: South Dezful Embayment, Iran, Zagros Frontal Fault, fold style, detachment horizon, Gachsaran Formation, Miocene evaporites, decoupling.

Structural decoupling in the Zagros fold-and-thrust belt, SW Iran, is controlled by the presence of intermediate detachment horizons within the Phanerozoic stratigraphic column such as intervals rich in evaporites. The effectiveness of the detachment horizons varies across the Zagros, and controls the style of folding and therefore the formation of structural traps. In this paper, field mapping, seismic interpretation and well data is used to investigate the complex structural style at six large-scale anticlines located on either side of the surface trace of the NW-SE trending Zagros Frontal Fault system in the South Dezful Embayment (Central Zagros). At some of these anticlines, lower – middle Miocene evaporites in the Gachsaran Formation decouple the folded overlying succession from the competent, underlying interval which includes the Asmari Formation reservoir. Anticlines to the northeast of the surface exposure of the fault have undergone four times more shortening than anticlines to the southwest. This localized shortening compared to regional values has resulted in some cases in complex asymmetric folding, and in severe structural decoupling above and below the Gachsaran Formation. However, other anticlines to the NE of the fault which underwent a similar amount of shortening, but in which the thickness of the Gachsaran Formation is reduced and the salt content is lower, show a harmonic style of folding above and below the formation. Thus, the efficiency of the Gachsaran detachment varies throughout the study area and appears to be controlled mainly by the total thickness of the formation, the net thickness of salt-rich intervals within it, and the amount of structural shortening across the Zagros Frontal Fault system. These observations could be relevant to similar structural domains both in the Zagros and in other fold-and-thrust belts where intermediate detachment horizons and regional-scale faults have together controlled the configuration of folds and structural traps.

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