Control in the Localization and Geometry of Thrust by Pre-Existing Salt Structures

Callot, Jean-Paul¹, Sharam Sherkati², Daniel Pillot¹, Jean-Marie Mengus¹, Jean Letouzey¹, Christophe Rigollet³ (1) Institut Français du Pétrole, Rueil Malmaison, France (2) N.I.O.C, Teheran, Iran (3) Gaz de France, Saint Denis La Plaine, France

Surface and subsurface structural data in the Zagros provinces suggest that the folding style is strongly controlled by the presence of the Eocambrian Hormuz. Two 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. Paleozoic post deposition halokinesis, Permo-Triassic Tethysian rifting along High Zagros NW-SE trends, and Cretaceous-Paleogene obduction and compressive events with basement reactivation of N-S Arabian trends. Much of the salt piercement of Hormuz evaporite are correlated with the Zagros folding event. To study the possible role of pre-existing salt structures (i.e. pillows and diapirs) in the mode of localiza­tion and geometry of newly formed fold and thrust, we investigate analogue experiments imaged using X-ray tomography. Finger, pyramidal and pillow shapes are modeled with vari­able brittle ductile ratios and shortening velocity. The shape and size of the salt structure, with respect to the sedimentary pile thickness, are the main controlling factors. Depending on its initial shape, the diapir may either be passively transported within the fold and thrust sheet or be squeezed toward the surface, or else acts 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 sheets. Modeling results are compared to the striking features of selected natural examples.