Not so simple “simply-folded Zagros”: The role of pre-collisional extensional faulting, salt tectonics and multi-stage thrusting in the Sarvestan transfer zone (Fars, Iran)

Abstract

The Sarvestan plain is bounded by highly elevated anticlines associated with thrusts or transpressional faults and hosts the NNW–SSE Sarvestan transfer zone. Surface and subsurface geological data, and 22 seismic lines allowed us to reconstruct the 3D geometry of the area. Mixed layer illite–smectite and 1D burial and thermal modelling were used to constrain the complex geological evolution of the Sarvestan plain where inherited structures strongly controlled the geometry of syn- to post-collisional contractional structures. Paleozoic–Mesozoic rifting related extension generated E–W and NNW–SSE normal fault systems. Such faults were associated with changes in the thickness of the sedimentary cover. Lateral facies changes were later induced by the Cretaceous obduction of ophiolites, cropping out some tens of km north of the study area. During the Miocene the footwall and the hanging wall of the Sarvestan Fault had different thermal evolution. This is tentatively explained by flow of Cambrian salt from the plain area towards the hanging wall of the Sarvestan Fault, associated with salt diapirism during Lower–Middle Miocene time. Salt tectonics is invoked also to explain, at least in part, the development of the overturned anticline in the hanging wall of the Sarvestan Fault. An early phase of contractional deformation occurred in the Middle Miocene (since 15My, i.e., after the deposition of the Agha Jari Fm) generating the E–W oriented folds buried below the plain, likely inverting inherited normal faults. The erosion of these structures was followed by the deposition of the Bakhtiari Fm conglomerates in Middle–Late Miocene times. A later phase of contractional tectonics generated the thrust faults and the anticlines bounding the Sarvestan plain some 6–5My ago. The Sarvestan dextral transpressional fault, that likely acted as a strongly oblique ramp of the Maharlu thrust, mainly structured in this period, although its activity may have continued until present.