Abstract
The study area as a part of Arabian plate, is located in the northwestern part of the Zagros Fold-and Thrust Belt within the Kurdistan Region of Iraq. This area comprises four thrusted anticlines includes Asos, Kosrat, Dokan, and Taq Taq along with adjoining synclines, which are distributed across three different tectonic zones: the Imbricated, High Folded, and Low Folded zones.
A comprehensive geological map with two stratigraphic columns and five balanced cross-sections and their corresponding restored sections have been constructed for the studied region, depending on the surface field data and subsurface (seismic and well) data. The sections were constructed to understand the variation in the structural architecture, kinematic evolution, and plausible fold model of the structure. Moreover, a geometric analysis has been performed on the studied anticlines depending on the 649 bedding plane attitudes.
The exposed rocks of the studied region range in age from Late Triassic (Baluti Formation) up to the Pliocene-Pleistocene (Bai-Hassan Formation), represented by 24 geological formations besides the Dokan Conglomerate and Recent deposits.
The studied structures are double plunging and asymmetrical features, with broad to wide hinge zone, gentle to open interlimb angles, non-cylindrical curvi-planar fold, and sub-horizontal to horizontal in the NW-SE direction and plunged with a very shallow angle. A number of major fault ramps with either SW-foreland verging or NE-hinterland verging have affected the forelimbs, back limbs, and hinge regions of each structure. The dip-slip displacement along these NW-SE oriented faults on the surface varies between several meters to ~1000m that increase toward Imbricated Zone.
The thrusts in the Asos structure are regarded as being emergent imbricate fan faults with listric thrust geometry converging at the main detachment. There is a general gradual forelandward decrease in dip amount in sequence, and fault displacement on frontal younger thrusts resulted in rotation of older thrusts rearward.
The Asos, Kosrat, and Dokan structures are fault-related folds that have been kinematically propagated over northeast-directed main ramps. These ramps converge at the main detachment, and bifurcate up-section forming numerous thrust faults. These thrusts have a foreland progression (break-forward) thrusting system in which older thrusts are carried piggy-back and folded over ramps in lower more frontal thrusts. Furthermore, the attributed back thrust cutting up from the fore-thrust can create a pop-up geometry and lead to the lifting of the Kosrat, Dokan, and Taq Taq structures.
The average horizontal shortening results at the top Qamchuqa Formation (Early Cretaceous) of the Asos, Kosrat, Dokan, and Taq Taq Anticlines are 17.70%, 10.11%, 6.79%, and 6.68% respectively. Generally, thrust related shortening values in the Asos structure are greater than their corresponding values in the other structures. This is attributed to the greater intensity of deformation via thrusting processes in the Asos Anticline, which indicates that intensity of the deformation increases toward the hinterland.
The obtained deformed geometry of the cross-sections indicate that the Asos, Kosrat, and Dokan Anticlines were decoupled within the Ora Shale Formation (Upper Paleozoic). According to the study, the main factor responsible for the accommodation of the potential uplifting of these structures is the underlying duplex structures, not the imbricate thrust system, and the deformed geometry of the cross-sections suggests a combination of both thin- and thick-skinned deformation.
The balanced and interpreted seismic sections show that the Taq Taq anticline is faulted detachment fold bounded by a main blind thrusts that dip toward each other. The back thrust broke up at the related fore-thrust resulting in growth of pop-up structures. The listric shape of Taq Taq forethrust is interpreted to be floored and detached along a deep-lying décollement in Kurra-Chine Formation (Upper Triassic) that did not involve basement fault. Thus, the deformation style of the anticline is interpreted as thin-skinned. The Taq Taq symmetric geometry changed to a slightly reverse asymmetrical anticline, because deformation comparatively concentrated on the backlimb during further propagation of the structure. The along strike variation in geometry, asymmetry, and shortening rate may reflect displacement variation on individual blind thrusts and fold wavelength and amplitude, or related to the effect of NE-SW strike-slip faults corresponding to subsurface lineaments.
Ranya-Bingrd Depression is affected by various types of faults, two of the main dextral reverse faults exert a couple force on the area, resulting in the formation of a shear zone between them and developing Ranya depression. This shear zone plays a crucial role in shaping the various structures observed in the area, as depicted in the Strain Model.