A series of scaled analogue models are used to study (de)coupling between basement and cover deformation. Rigid basal blocks were rotated about a vertical axis in a ‘bookshelf’ fashion, which caused strike-slip faulting along the blocks and in the overlying cover units of loose sand. Three different combinations of cover–basement deformations are modelled: (i) cover shortening before basement fault movement; (ii) basement fault movement before cover shortening; and (iii) simultaneous cover shortening with basement fault movement. Results show that the effect of the basement faults depends on the timing of their reactivation. Pre- and syn-orogenic basement fault movements have a significant impact on the structural pattern of the cover units, whereas post-orogenic basement fault movement has less influence on the thickened hinterland of the overlying belt. The interaction of basement faulting and cover shortening results in the formation of rhombic structures. In models with pre- and syn-orogenic basement strike-slip faults, rhombic blocks develop as a result of shortening of the overlying cover during basement faulting. These rhombic blocks are similar in appearance to flower structures, but are different in kinematics, genesis and structural extent. We compare these model results to both the Zagros fold-and-thrust belt in southwestern Iran and the Alborz Mountains in northern Iran. Based on the model results, we conclude that the traces of basement faults in cover units rotate and migrate towards the foreland during regional shortening. As such, these traces do not necessarily indicate the actual location or orientation of the basement faults which created them.

Microstructural, finite strain and vorticity analyses of quartz-rich mylonites were used in order to investigate kinematics of rock flow and deformation temperature in the Sirjan thrust sheet exposed in a structural window within the Sanandaj–Sirjan High Pressure – Low Temperature (HP–LT) metamorphic belt that forms part of the hinterland of the Zagros orogenic belt of Iran. A dominant top-to-the-SW sense of shear in the study area is indicated by several shear sense indicators such as asymmetric boudins, rotated porphyroclasts, mica fish and S/C fabrics. Quantitative analyses reveal approximately plane strain deformation conditions with Rxz values ranging from 2.5 to 4.3 and increasing towards the Sirjan thrust. Opening angles of quartz c-axis fabrics and recrystallization regimes suggest deformation temperatures vary from 430 to 625 ± 50°C in the hanging wall rocks. Oblique grain shape and quartz c-axis fabrics were used to estimate the degree of non-coaxiality during deformation. The obtained vorticity profile indicates a down-section increase in kinematic vorticity number (Wm) from 0.6 to 0.89. This range of vorticity numbers confirms contributions of both simple (41–68 %) and pure shear (32–59 %) deformation components. The structural characteristics of the study area ultimately were controlled by oblique motion of the Afro-Arabian plate relative to the Iranian plate.

Kinematic analysis of mélange fabrics provides critical information concerning tectonic processes and evaluation of the kinematics of ancient relative plate motion. Systematic kinematic analysis of deformed structures within a tectonic mélange exposed along the Zagros Suture Zone elucidates that this zone is an ancient transpressional boundary. The mélange is composed of a greywacke and mudstone matrix surrounding various lenses, blocks and ribbons of radiolarian chert, limestone, sandstone, pillow lava, tuff, serpentinite, shale and marl. The deformation fabrics of the mélange suggest that the mélange units were tectonically accreted at shallow levels within a subduction complex, resulting in layer-parallel extension and shearing along a NW–SE-trending suture that juxtaposes the Afro-Arabian continent to the south and the Central Iranian microcontinents to the north. The tectonic mélange is characterized by subhorizontal layer-parallel extension and subsequent heterogeneous non-coaxial shear resulting in alternating asymmetric and layer-parallel extensional fabrics such as P–Y fabrics and boudinaged layers. Kinematic data suggest that the mélange formed during oblique subduction of the Neo-Tethys oceanic lithosphere in Late Cretaceous time. Kinematic shear sense indicators reveal that the slip direction (N9°E to N14°E) during accretion-related deformations reflects the relative plate motion between the Afro-Arabian continent and Central Iranian microcontinents during Late Cretaceous to Miocene times.

3D modelling of geological structures is a key method to improve the understanding of the geological history of an area, and to serve as a drive for exploration. Geomodelling has been performed on a large 60000 km2 area of the Zagros fold-and-thrust belt of Iran, to reconcile a vast but heterogeneous dataset. Topography, geological surface data and dips, outcrop surveys, and well and seismic data were integrated into the model. The method was to construct a key surface maximizing the hard data constraints. The Oligo-Miocene Top Asmari layer was chosen, as this formation was regionally deposited before the main Zagros collision phase and because the numerous outcrops allow proper control of the bed geometry in the fold cores. Interpreted seismic data have been integrated to interpolate the surfaces at depth within the synclines. Several conceptual models of fold geometry have been applied to estimate the best way to convert seismic time signal to depth. Several deeper horizons down to Palaeozoic strata were deduced from this key horizon by applying palaeo-thickness maps. During the construction, the 3D interpolated surfaces could be reconverted to time, using a velocity model, and compared with previous seismic interpretations. This exercise obliged us to revise some early interpretations of seismic lines that were badly tied to wells. The 3D modelling therefore clearly improves regional interpretation. In addition, the 3D model is the only tool that allows drawing consistent cross-sections in areas where there are no seismic lines. Emerging Hormuz salt diapirs were added to the model. Dimensions and shapes of the individual diapirs were modelled using a statistical survey on the cropping out Hormuz structures. Modelling reliably demonstrated that the diapirs, when piercing, show a constant mushroom shape whose diameter depends on the stratigraphic depth of observation. This observation allowed us to exemplify relations between the pre-existing diapirs and the anticlines of the area, and to highlight the morphological changes from the inner onshore areas to the coastal and offshore areas. In addition, one of the surprising results of this study was the observation of the increasing diameter of the diapirs at the time of the Zagros collision and folding event, with growth strata and overhangs on the flanks of the diapirs.

Accessory chrome spinels are scattered throughout the serpentinite masses in two allochthonous thrust sheets belonging to the Penjween–Walash sub-zone of the northwestern Zagros Suture Zone in Kurdistan. Based on field evidence, the serpentinites are divided into two groups: (1) highly sheared serpentinites (110–80 Ma), which occupy the lower contact of the ophiolitic massifs of the Upper Allochthonous sheet (Albian–Cenomanian age), and (2) ophiolitic mélange serpentinites of mixed ages (150 and 200 Ma) occurring along thrust faults on the base of the volcano-sedimentary segment (42–32 Ma) of the Lower Allochthonous sheet. The Cr-spinels of both groups show a wide range of YCr (Cr/(Cr + Al) atomic ratio) from 0.37 to 1.0, while the XMg (Mg/(Mg + Fe2+) atomic ratio) ranges from 0.0 to 0.75. Based on the Cr-spinel compositions of the entire dataset and in conjunction with back-scattered electron imaging, from core to rim, three spinel stages have been recognized: the residual mantle stage, a Cr-rich stage and a third stage showing a very narrow magnetite rim. These three stages are represented by primary Cr-spinel, pre-serpentinization metamorphosed spinel and syn- or post-serpentinization spinel, respectively. The chemical characteristics of primary (first-stage) Cr-spinels of both serpentinite groups indicate a tectonic affinity within a fore-arc setting of peridotite protoliths. The second stage indicates that Cr-spinels have undergone subsolidus re-equilibration as a result of solid–solid reaction during pre-serpentinization cooling of the host rock. Here the primary Cr-spinel compositions have been partly or completely obscured by metamorphism. During the third stage, the Cr-spinels have undergone solid–fluid re-equilibration during syn- or post-serpentinization processes. Both the second and third stages point to diachronous metamorphic paths resulting from continuous tectonic evolution influenced by either slow or fast uplift of mantle protoliths. In the fast metamorphic paths, the primary chrome spinels are flanked by a very narrow magnetite rim. The presence of two groups of distally separated serpentinites with different emplacement ages and fore-arc tectonic affinity could indicate that the closure of the Tethys Ocean culminated in two fortuitous subduction processes.

Protrusions and lenses of serpentinite–matrix mélanges occur at several places along the thrust faults of the Zagros Suture Zone. They separate the lower allochthonous thrust sheet, the ‘Lower Allochthon’ (i.e. Walash–Naopurdan nappe), of Paleocene–Eocene age from sediments of the Arabian platform and the upper thrust sheet of Mesozoic, ophiolite-bearing terranes termed the ‘Upper Allochthon’ (i.e. Gemo–Qandil nappe). The serpentinite–matrix mélanges occur mostly as stretched bodies (slices) on both sides of the Lower Allochthon (Hero, Halsho and Pushtashan (HHP) and Galalah, Qalander and Rayat (GQR)). Their overall chondrite-normalized rare earth element (REE) patterns form two main groups. Group One exhibits enrichment in the total REEs (> 1 × chondrite) whereas the Group Two pattern shows depletion (i.e. < 1 × chondrite). Bulk-rock MORB-normalized profiles of Group Two are almost flat in the MREE–HREE region with flattening profiles in the Gd–Lu range (> 3 times the MORB composition). In comparison with Group One, Group Two has extremely high REE content and displays variable depletions in the moderately incompatible high-field-strength elements (HFSEs) (Zr, Hf, Y) relative to their adjacent REEs. The REEs in the GQR serpentinite–matrix mélanges have a noticeably high LREE content, and a positive Eu anomaly, and their HREE content never reaches more than 1 × chondrite (i.e. < 0.01 to 1 × chondrite). The latter indicates that the hemipelagic sedimentary, melt-like components (i.e. high LREE, U/La, La/Sm and low Ba/Th) control the geochemical peculiarities of this type of serpentinite. The HHP serpentinite–matrix mélanges, however, are either equally divided between the two REE pattern groups (e.g. Hero, Halsho) or inclined towards Group One (e.g. Pushtashan). Contrary to GQR serpentinites, the variation in HHP serpentinite–matrix mélanges spans a compositional spectrum from U/La-rich to more Ba/Th-rich. Such ratio variations reflect the large variation in these two subducted sedimentary components (i.e. carbonate and hemipelagic sediment mix). The obvious differences in the trace element signatures of the GQR and HHP serpentinite–matrix mélanges might be related to plate tectonic parameters such as convergence rate, subduction age and thickness and type of subducted slab. It is more likely that the influx of subducted components to the mantle wedge relied heavily on the composition of the sedimentary inputs. These vary considerably with time from the relatively deepwater hemipelagic sediments (Qulqula Radiolarite Formation) to platform carbonate sediments (Balambo limestone). The trace element signatures of the GQR and HHP serpentinite–matrix mélanges might suggest multi-staging of the allochthonous sheet emplacement on the Arabian platform sediments.

This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia–Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ~ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj–Sirjan, Kermanshah and Urumieh–Dokhtar magmatic arcs), the P–T–t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ~ 150 to 0 Ma across two SW–NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115–85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene–Eocene (60–40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (~ 30–0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj–Sirjan Zone: 20–15 Ma, High Zagros: ~12–8 Ma; Simply Folded Belt: 5–0 Ma) and with partial slab tear at depths (~10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes.

The Zagros fold-and-thrust belt of SW Iran is a young continental convergence zone, extending NW–SE from eastern Turkey through northern Iraq and the length of Iran to the Strait of Hormuz and into northern Oman. This belt reflects the shortening and off-scraping of thick sediments from the northern margin of the Arabian platform, essentially behaving as the accretionary prism for the Iranian convergent margin. Distribution of Upper Cretaceous ophiolites in the Zagros orogenic belt defines the northern limit of the evolving suture between Arabia and Eurasia and comprises two parallel belts: (1) Outer Zagros Ophiolitic Belt (OB) and (2) Inner Zagros Ophiolitic Belt (IB). These belts contain complete (if disrupted) ophiolites with well-preserved mantle and crustal sequences. Mantle sequences include tectonized harzburgite and rare ultramafic–mafic cumulates as well as isotropic gabbro lenses and isolated dykes within the harzburgite. Crustal sequences include rare gabbros (mostly in IB ophiolites), sheeted dyke complexes, pillowed lavas and felsic rocks. All Zagros ophiolites are overlain by Upper Cretaceous pelagic limestone. Limited radiometric dating indicates that the OB and IB formed at the same time during Late Cretaceous time. IB and OB components show strong suprasubduction zone affinities, from mantle harzburgite to lavas. This is shown by low whole-rock Al2O3 and CaO contents and spinel and orthopyroxene compositions of mantle peridotites as well as by the abundance of felsic rocks and the trace element characteristics of the lavas. Similarly ages, suprasubduction zone affinities and fore-arc setting suggest that the IB and OB once defined a single tract of fore-arc lithosphere that was disrupted by exhumation of subducted Sanandaj–Sirjan Zone metamorphic rocks. Our data for the OB and IB along with better-studied ophiolites in Cyprus, Turkey and Oman compel the conclusion that a broad and continuous tract of fore-arc lithosphere was created during Late Cretaceous time as the magmatic expression of a newly formed subduction zone developed along the SW margin of Eurasia.

A sedimentological investigation of the Neogene deposits of the Zagros foreland basin in SW Iran reveals a continuous and largely gradational passage from supratidal and sabkha sediments at the base (represented by the Gachsaran Formation) to carbonates and marine marls (Mishan Formation with basal Guri carbonate member) followed by coastal plain and meandering river deposits (Agha Jari Formation) and finally to braided river gravel sheets (Bakhtyari Formation). This vertical succession is interpreted to represent the southward migration of foreland basin depozones (from distal foredeep and foredeep to distal wedge-top and proximal wedge-top, respectively) as the Zagros fold–thrust belt migrated progressively southward towards the Arabian foreland. This vertical succession bears a striking similarity to modern depositional environments and sedimentary deposits observed in the Zagros region today, where one passes from mainly braided rivers in the Zagros Mountains to meandering rivers close to the coast, to shallow marine clastic sediments along the northern part of the Persian Gulf and finally to carbonate ramp and sabkha deposits along the southeastern coast of the Persian Gulf. This link between the Neogene succession and the modern-day depositional environments strongly suggests that the major Neogene formations of the Zagros foreland basin are strongly diachronous (as shown recently by others) and have active modern-day equivalents.

Structure varies along strike in the Zagros fold-and-thrust belt of Iran, which is a principal element in the Arabia–Eurasia continental collision. Pre-collision, Late Cretaceous ophiolite nappes (Kermanshah, Neyriz) and related nappes of deep marine sediments (Radiolarite Series) were emplaced next to two regions (Pusht-e Kuh arc, Fars) which later developed a consistent structural style across the range from the High Zagros Fault to the foreland limit of deformation. The intervening area has a zone of highly imbricated Arabian plate strata (the Bakhtyari Culmination) thrust southwest towards and over a low relief, low elevation region (the Dezful Embayment). There are no ophiolite nappes northeast of the Bakhtyari Culmination. Isopachs reflect these different structural patterns from Late Cretaceous time but not earlier. In Late Cretaceous time the Dezful Embayment recorded less deposition than adjacent areas to the northwest and southeast. In the Palaeogene there was little net difference between the Dezful Embayment and its margins. The Dezful Embayment has been a depocentre since roughly 35 Ma, which is the likely time of initial collision between Arabia and Eurasia. We propose that the syn-collision structure and stratigraphy of the Zagros is therefore strongly influenced by the variation in Late Cretaceous ophiolite emplacement, but the original cause of this variation is not clear.

We present and use seismic images to constrain possible scenarios for the evolution of the Middle Miocene Gachsaran Formation in the Central Iranian Zagros during the Neogene folding. This evaporitic sequence plays an important role in sealing the Oligocene–Miocene Asmari reservoirs in the Dezful Embayment and offshore regions. It was deposited in the flexural basin south of the Mountain Front Fault, during the progressive southwestward propagation of the Zagros folding. Its thickness changes very rapidly from several hundred to more than 4000 m. This thickness variation is related to faulting, folding and flow. Seismic and subsurface data show that the Gachsaran evaporite sequence is a major disharmonic level in the Dezful Embayment and shallow, molassic synclines often overlie anticlinal axes at depth. The lithological composition of the Gachsaran Formation is mainly a combination of salt, shale and anhydrite, which have a high interval velocity contrast. Anomalous velocity behaviours of these sedimentary units after deformation affect the seismic quality of the surrounding area, especially the southern flank of the main anticline structures.

The motion of Arabia was stable with respect to Eurasia over the past 22 Ma. Deformation and exhumation in the Zagros is seen to initiate at the same time as argued by new detrital thermochronologic constraints and increasing accumulation rates in synorogenic sediments. A recent magnetostratigraphic dating of the Bakhtyari conglomerates in the northern Fars region of the Zagros further suggests that shortening and uplift in the Zagros Folded Belt accelerated after 12.4 Ma. Available temporal constraints from surrounding collision belts indicate that shortening and uplift focused in regions bordering the Iranian plateau to the south between 15 and 5 Ma. As boundary velocity was kept constant this requires concomitant decreasing strain rates in the Iranian plateau. Slab detachment has been proposed to explain the observed changes as well as mantle delamination, but the insignificant change in the Arabian slab motion and lack of unambiguous constraints make both hypotheses difficult to account for. It is proposed based on a review of shortening estimates provided throughout the Arabia–Eurasia collision that the total 440 km of convergence predicted by geodesy and plate reconstruction over the past 22 Ma can be accounted for by distributed shortening. I suggest that the topography and expansion of the Iranian plateau over Late Miocene–Pliocene time can be reproduced by the progressive thickening of the originally thin Iranian continental lithosphere presumably thermally weakened during the Eocene extensional and magmatic event.

This paper presents quantitative data on the finite strain, quartz crystal fabric, geometry of flow and deformation temperatures in deformed quartzite samples to characterize the ductile deformation along the thrust sheets constituting the Sanandaj–Sirjan Metamorphic Belt within the Zagros Mountains of Iran. The results of this study emphasize the heterogeneous nature of deformation in this belt, showing a spatial variation in strain magnitude and in degree of non-coaxiality. A dominant top-to-the-SE sense of shear is indicated by the asymmetry of microstructures and quartz c-axis fabrics. Quartz c-axis opening angles suggest deformation temperatures range between 435° ± 50°C and 510° ± 50°C, which yield greenschist to amphibolite facies conditions during the ductile deformation. Mean kinematic vorticity number (Wm) measured in the quartzite samples ranges between 0.6 and 0.9 with an average of 0.76, which indicates that extrusion of the metamorphic rocks of the region was facilitated by a significant component of pure shear strain. Traced towards the basal thrust of the Zagros Thrust System from northeast to southwest, the quartz grain fabrics change from asymmetric cross-girdle fabrics in the internal part of the deformation zone to an asymmetric single-girdle fabric at distances close to the basal thrust. This variation may depend on the structural depth and on the geometry of the ductile deformation zone. The observed increase in strain and vorticity within the study area is comparable with patterns recorded within metamorphic rock extrusions within other orogens in the world.

Pre-/early folding fracture patterns were recognized in several anticlines from three structural domains in the Simply Folded Belt of the Fars arc. These fracture sets were characterized in terms of opening mode and relative chronology and used to reconstruct the main compressional trends related to the early Zagros collisional history. The palaeostress reconstructions based on these fracture sets were further refined by combination with newly collected or already available fault-slip and calcite twin data. As an alternative to previous models of rigid block rotations or regional stress rotation, we propose that the complex pattern of pre-folding fractures and the contrasting palaeostress orientations through time in the different domains investigated are related to the presence of basement faults with N–S and WNW trends, above which basement and cover were variably coupled during stress build-up and early deformation of the Arabian margin. Beyond regional implications, this study draws attention to the need to carefully consider pre-existing fractures, possibly unrelated to folding, to build more realistic conceptual fold–fracture models.

Thrust-related anticlines in the Zagros Simply Folded Belt provide excellent exposed analogue structures for fractured reservoirs located in the more external sectors of the belt. In these structures it is possible to study the fracture network attributes and understand their relationships to the folding process, thus gathering fundamental information for fracture modelling in reservoirs. In this work we analyse the mesoscopic deformation pattern of the NW–SE-trending Bangestan anticline (SW Zagros, Iran) and discuss its relationship to the kinematic evolution of the hosting structure. The deformation pattern mostly includes extensional structures and pressure solution cleavages striking parallel to the fold axial trend (i.e. longitudinal), transversal extensional structures, and N–S- and E–W-striking extensional structures (oriented oblique to the fold axis). With the aid of deep wells and a transversal reflection seismic profile, we constructed a balanced cross-section of the anticline and propose a kinematic evolution pathway constrained by the mesoscopic deformation pattern. Longitudinal and transversal deformation structures developed before and/or in the very early stages of fold growth. During this stage, the Bangestan anticline grew as a set of unconnected décollement anticlines involving the Cambrian to Pliocenic sedimentary cover. In a later stage, inherited basement faults were reactivated with a right-lateral strike-slip component and the previously developed anticlines propagated laterally up to their complete linkage and thrust breakthrough. This produced the right-lateral strike-slip reactivation of longitudinal joints and the development of N–S- and E–W-striking extensional structures, which were also frequently reworked as strike-slip faults.

We describe the geomorphology of a large (~10000 km2) internally draining region within the Zagros fold-and-thrust belt of Fars province, Iran. A series of wind gaps through fold crests and a continuous line of low-slope pixels in digital elevation models indicate the presence of an older, and now abandoned, through-going river system. We suggest, from the presence of the wind gaps, that the original through-going river system was abandoned as a direct result of fold growth. At present, through-going drainage in Fars is restricted to only two major rivers, the Kul and the Mand, which bound the margins of the internally drained region. The low gradients of the Kul and the Mand rivers are similar to those in topographic profiles drawn along the course of the abandoned drainage. The Mand and Kul rivers may be defeated in the future, causing an expansion of the internally drained region, and resulting in a profound change in the distribution of sediment and surface elevations within the Zagros. The internally draining part of the Zagros resembles the Central Iranian Plateau both in its geomorphology and in the apparently slow rates of deformation within it. We speculate that the development of internally drained basins and distribution of shortening within the range may be causally linked. The geomorphology that we describe might, therefore, record a stage in the southward expansion of the non-deforming and topographically high Central Iranian Plateau.

The Yafteh cave in Iran has an intact Aurignacian sequence over 2m deep. First explored by Frank Hole and Kent Flannery in the 1960s, its strata and assemblage are here re-evaluated at first hand by a new international team. The authors show that the assemblage is genuine Aurignacian and dates back to about 35.5K uncal BP. They propose it as emerging locally and even as providing a culture of origin for modern humans in West Asia and Europe.