^PSControl in the Localization and Geometry of Thrust by Pre-Existing Salt Structures*

By

Jean-Paul Callot ¹, Sharam Sherkati ², Daniel Pillot ¹, Jean-Marie Mengus ¹, Jean Letouzey ¹, and Christophe Rigollet ³

Search and Discovery Article #30037 (2005)

Posted September 15, 2005

*Poster presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005

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¹Institut Français du Pétrole, Rueil Malmaison, France ([email protected])

²N.I.O.C, Tehran, Iran

³Gaz de France, Saint Denis la Plaine, France

Abstract 

Surface and subsurface structural data in the Dezful Embayment, the Fars, and the High Zagros provinces suggest that the deformation style is strongly controlled by the presence of the Eocambrian Hormuz and the Miocene Gasharan salt layers. Both units act as level of major disharmony and decollement during the Neogene Zagros folding. The Hormuz complex is known from emergent halite and anhydrite plugs in the Fars and High Zagros. There is some evidence of Hormuz salt movements triggered by tectonic events prior to the main Zagros folding phase; i.e., post deposition halokinesis, Permo-Triassic Tethyian rifting along High Zagros NW-SE trends, and Cretaceous-Paleogene obduction and compressive events with basement reactivation of N-S Arabian trends. The salt piercing of Hormuz evaporite plugs is closely associated with major thrusts parallel to the fold trend clearly associated with the Zagros folding event. To study the role of possible pre-existing salt structures (i.e., pillows and diapirs) in the mode of localization and geometry of newly formed thrust, we investigate analogue experiments imaged using X-ray tomography. Finger, pyramidal, and pillow shapes are tested with variable brittle ductile ratios. The shape and size of the salt structure, with respect to the sedimentary pile thickness, are the main controlling factor. Depending on its initial shape, the diapir may either be passively transported within the thrust sheet or be squeezed toward the surface, or else act as a ramp. The thrust sheet geometry is variable along strike, with short pinched fault-rooted folds above diapirs separated by large scale low angle thrust sheet. Modeling results are compared to the striking features of selected natural examples.

          Faults and emergent Hormuz salt plugs, Zagros fold and thrust belt.

          Representative cross-sections, Dezful Embayment, Zagros, and thrust belt.

Conclusions

We argue that the driving mechanism of Hormuz halokinesis extrusion during the Zagros folding and erosion is the squeezing of the pre-existing salt diapirs. These diapirs also influence the location of thrust and strike slip faults (weak zones). Emergent diapirs occurred above pre-existing domes along these faults, or at the plunging axes of the folds. Diapirs exposed in the core of the anticlines could be detached from their root, over the thrust ramp. Understanding the formation and development of the Hormuz diapirs is very important for reservoir prediction and hydrocarbon trapping in the Zagros province.

Models without pre-existing diapirs, but with several decollement levels within the sedimentary pile, show that lateral variation of geometrical characteristics of fold and faults correspond to different arrangements of ductile and brittle rocks.