Summary
The study area is located at the NW segment of the Zagros Fold and Thrust Belt in Kurdistan Region of Iraq, and it includes four major thrusted anticlines with adjoining synclines. These anticlines are Bradost, Pirs, Berat, and Maqloub BardaRash, which are located respectively in the Imbricate, High Folded and Low Folded zones. Based on the bedding plane attitudes, the studied anticlines have been analyzed geometrically. All the studied anticlines are NW-SE trending, double plunging and asymmetrical. The hinge regions, forelimbs and back limbs of each structure are affected by a number of major fault ramps NW – oriented, either foreland (SW) verging or hinterland (NE) verging and dip-slip displacement partitioned between them. The traces of these thrust faults have been laterally shifted by few strike-slip faults. The exposed layered rocks of the studied area range between Early Jurassic up to the Pliocene.
The studied area has been mapped, and six structural balanced cross-sections and their corresponding restored sections have been constructed. Four of these sections passed through Bradost, Pirs and Berat structures, whereas the remainder cut Maqloub-BardaRash structure. All balanced cross-sections were built to unravel changes in the studied fold and fault architectures, from the surface to subsurface.
The studied sections imply that both Berat and Pirs structures are represented thrust-related anticlines with pop-up geometry, whereas the Bradost structure comprises a combination of two different geometries and styles of deformation along strike at each of the segments. So, the Bradost structure appears as a positive flower structure in one of the segments and as a ramp-related anticline in another segment. This structural phenomenon suggests the development of two different segments instead of one embryonic anticline in the Bradost structure, which were later connected via transverse Hadhar-Bekhme basement fault, during a progressive lateral growth of the two segments.
Based on the top of Early Cretaceous Qamchuqa Formation, the shortening values that were accommodated by the thrusts are ~ 1000 m, ~ 430 m and ~172 m at the Bradost, Berat and Maqloub structures respectively. This variation in shortening is due to the intensity of the thrusting process which is higher in the Bradost structure than that in the Berat and Maqloub structures.
The current study suggests that the underlying duplex structures underneath Bradost, Pirs and Berat structures are primarily responsible for the accommodation of the potential uplifting of these structures, rather than the imbricate thrust system. Based on the obtained deformed geometry of the folds from the cross-sections, the Bradost, Pirs and Berat structures were decoupled within the Upper Paleozoic Ora Shale Formation. The overall deformed geometry of the cross-sections indicates a combination of thin- and thick-skinned deformation.
One of the main goals of this work is to build a plausible fold model, as well as to explain the kinematic evolution of the Maqloub–BardaRash structure. The middle part of the anticlinal hinge line of the Maqloub–BardaRash structure shows deviation and anticlockwise rotation. The Maqloub segment is inferred as a fault-propagation fold, but the BardaRash segment is interpreted as a faulted detachment fold based on the geometrical analysis of deformed models. Based on the resulting deformed geometries from the balanced cross-sections, the propagation of the Maqloub segment is associated with the activation of an underlying basement fault, whereas the BardaRash segment is decoupled along the Triassic ductile sequence. The structural characterization of the two different models of the fault-related deformation along the individual Maqloub–BardaRash folded structure will enhance the accurate well position of the crestal reservoir.