Zagros Mountain Belt Research Articles

A revised model for Neogene Zagros foreland sedimentation in the Lurestan arc based on new geochemical data

AbstractThe Zagros foreland basin is an important sedimentary archive for the tectonic and paleoclimatic evolution of the Zagros Mountains and the entire Neotethyan Arabia–Iran collision zone. By combining new geochemical high‐resolution whole rock XRF data with clay mineralogy and soluble salt geochemistry we propose an evolution of the sedimentary environment in the Lurestan arc from the Serravallian to the early Pleistocene, closing a gap in understanding the complex exhumation history of the central Zagros mountain belt. An increase in ultramafic sedimentary input indicates a shift from provenance 1 to provenance 2 by ophiolite exhumation at ca.10 Ma in the Imbricated Zagros north of the Lurestan arc. Our data further indicates that the sedimentary environment of parts of the Lahbari Mb within the Lurestan arc represents a piedmont deposition of fine‐grained alluvial fans and siltstones with aeolian contribution deposited under hyper‐arid climate conditions. These represent provenance shift 3 and were likely sourced from evaporites of the underlying Gachsaran Fm and fluvial deposits of the Lower Aghajari Mb (provenance 1 and provenance 2), uplifted by the Mountain Front Flexure at around 5.6 Ma. Combining XRF whole rock data with clay mineral data refines formation conditions of the clay minerals in the foreland basin such as palygorskite, which is revealed to be authigenic in origin in the Lower Aghajari Mb. as a function of varying Mg‐content due to variations of erosion of the ultramafic and mafic rocks in the Imbricate Zagros belt. Palygorskite in the Lahbari Member is likely both inherited from the Neogene Gachsaran evaporites as well as of authigenic origin.

REWORKED MESOZOIC RADIOLARIANS IN MIOCENE-PLIOCENE FORELAND SEDIMENTS IN THE ZAGROS BELT, IRAN

Micropaleontology can give important insights into the provenance and paleoenvironmental conditions in terrestrial sedimentary archives. For the current study, 84 samples representing a 2.6 km thick sedimentary profile from the SimplyFolded Zagros Mountain Belt were investigated. They span ca. 10.2 my from the late Middle Miocene (Serravallian) to the earliest Pleistocene (Gelasian), and comprised floodplain sediments and saline mudstones with an aeolian contribution. The mudstones revealed a unique Cretaceous radiolarian assemblage comprising largely of cryptothoracic Nassellarians and spherical spumellarians. This record highlights the reworking of sediments derived from Cretaceous Qulqula- Kermanshah radiolarian claystones and radiolarites in the Imbricated Zagros belt into the distal Neogene Zagros foreland sediments in Lurestan (Lurestan Arc). The high abundance of Holocryptocanium barbui (Dumitrica) and other cryptothoracic taxa compared to the Qulqula- Kermanshah radiolarian claystones and radiolarites potentially indicates a preferred erosion of softer units such as the Red Radiolarian Claystone Unit (RRCU) compared to harder radiolarian cherts. The observation of a reworked largely cryptothoracic assemblage might also point to additional sorting effects during fluvial and aeolian transport as well as during redeposition, depending on the morphology and hydrodynamic properties of individual radiolarian taxa.

Evidence for enhanced aridification since 13 Ma in the Qom back-arc basin, Central Iran

The Iranian Plateau was once occupied by the Neotethys Sea, which connected with the Indian Ocean in the southeast and the proto-Mediterranean in the west during most of the Cenozoic era. However, the Neotethys Sea has undergone a stepwise retreat, and the Tethyan Seaway also closed in the Miocene driven by both Arabia-Eurasia collision and global eustatic sea-level drop. Changes in the land-sea distribution and uplift of the Zagros Mountain belt must have affected paleoclimatic changes in Iran. To date, a high-resolution paleoclimatic record is lacking for Iran. In this paper, we present new environmental magnetic and color parameter data for terrestrial sediments in the Qom back-arc basin, Central Iran from the middle to late Miocene (17–11.5 Ma). All of our parameters indicate an abrupt climatic change at ∼13 Ma. We suggest that the less reddish color and a shift in environmental magnetic parameters after ∼13 Ma implies weakened chemical weathering under a colder/drier climate. Enhanced aridification was mainly controlled by closure of the Tethyan Seaway and shrinkage of the Neotethys Sea after ∼13 Ma, which greatly reduced moisture transport to the downwind Central Iran by westerlies, whereas the rain shallow effect of the initially uplifted Zagros Mountain belt and reduced seawater evaporation related to lower sea-surface temperatures after the Mid-Miocene Climatic Transition only played minor roles. Therefore, our results provide a window to discuss relationships between plate tectonic movements, land/sea distribution, and climatic change in a tectonically active region during the late Cenozoic.

Using clay mineralogy and micropalaeontological observations to unravel Neogene climate variations in Northern Arabia

The Zagros Mountain belt in Iran comprises an extensive record of Mesozoic and Cenozoic deposits uplifted by collision of the Arabian and Iranian continental plates. This area has been shown to provide important information to decipher the climatic history of Western Asia, especially regarding the evolution of deserts in Mesopotamia and Northern Arabia. In a high-resolution geological record of 84 samples, we combine clay mineralogy and micropalaeontological observations to refine the late Neogene climate record in Northern Arabia. We investigated a 2.6-km-thick sedimentary profile from the Simply Folded Zagros Mountain belt spanning ca. 10.2 myrs from the late Middle Miocene (Serravallian) to earliest Pleistocene (Gelasian) comprising floodplain sediments and saline mudstones with a strong aeolian contribution. The clay fraction < 2 µm reveals palygorskite, smectite, illite, chlorite and the zeolite mineral erionite. Using clay mineralogy, arid conditions during the Serravallian and early Tortonian in Northern Arabia are identified, followed by semi-arid conditions during the late Tortonian. Clay mineralogy suggests a transition to sustained arid conditions during the early Messinian and provides a more complex picture on a previously proposed period of hyperaridity (NADX) in Northern Arabia penecontemporaneous with the apex of the Messinian salinity crisis.

Coseismic and post-seismic characteristics of the 2021 Ganaveh earthquake along the Zagros foredeep fault based on InSAR data

SUMMARY The Ganaveh earthquake on 2021 April 18 (Mw 5.8) occurred in the southwest of the Dezful embayment of the Zagros Mountain belt, Iran, as a mainly compressive event. The InSAR coseismic displacement maps reveal a maximum of 17 cm of surface displacement in the satellite line of sight direction. InSAR inversion indicates a low-angle NE-dipping causative fault plane with a maximum slip of 95 cm at ∼6 km depth. It highlights the occurrence of the Ganaveh earthquake within the competent layers of the Zagros sedimentary cover, beneath the Gachsaran formation. A slight sinistral slip component (2.9 cm) is retrieved which is compatible with the USGS focal mechanism. Time-series analysis of SAR images after the main shock until the end of 2021 indicates a maximum of 7 cm of post-seismic surface displacement with a similar strike and pattern as the coseismic phase. This similarity and the distribution of aftershocks suggest an afterslip mechanism for the post-seismic phase. The inversion of post-seismic cumulative displacement evaluates a maximum of 30 cm slip at a depth of ∼5 km along the coseismic causative fault. A regional compressional stress regime (N041°E for the direction of the σ1 stress axis) is constrained by using the focal mechanisms of 39 earthquakes occurring between 1968 and 2021, including the Ganaveh main shock and its five larger aftershocks. Applying this direction of compression on the Ganaveh fault plane also results in a minor sinistral movement, consistent with the geodetic results. The relocated main shock and aftershocks as well as our InSAR coseismic displacement situations on the hanging wall of the Zagros Foredeep fault highlight it as the causative fault of the Ganaveh earthquake. To fit the geometry of the Ganaveh rupture plane with the Zagros Foredeep fault, we modelled a listric fault plane and its slip distribution using the available geological data. The retrieved slip variation on the listric plane is in close agreement with the slip pattern on the modelled planar geometry. The low dip angle of the rupture plane combined with a listric geometry highlights the thin-skin characteristics of the Zagros Foredeep fault as the causative fault of the Ganaveh earthquake. The occurrence of this moderate magnitude earthquake on the Zagros Foredeep fault underlines its role as the western structural boundary for the recurrence of Mb &amp;gt; 5 events in the Dezful embayment.

Biostratigraphy and palaeogeographic implications of Ordovician and Silurian chitinozoa from the High Zagros Mountains, Northern Persian Gulf, Iran

A siliciclastic succession is exposed at the Faraghan mountain, northern Persian Gulf, southeastern Iran. A detailed, high-resolution palynological analysis was performed on the Zardkuh, Seyahou, Dargaz, and Sarchahan formations to verify the exact age and palaeogeographic position of the High Zagros Mountains. Two hundred surface samples from this succession were collected and analysed. Most samples yielded abundant and well-preserved chitinozoans, acritarchs, scolecodonts, and cryptospores. Fifty-three chitinozoan species (21 genera) were identified that permitted to establish the Eremochitina brevis, Desmochitina ornensis, Belonechitina henryi, Siphonochitina formosa, Belonechitina robusta, Tanuchitina fistulosa, Acanthochitina barbata, Armoricochitina nigerica, Ancyrochitina merga, Tanuchitina elongata, Spinachitina oulebsiri, and Spinachitina fragilis biozones in ascending stratigraphic order as was previously established for the North Gondwana Domain. These results indicate that the Zagros Mountain Belt of Iran was part of the North Gondwana palaeo-province during the Ordovician-Silurian. On the other hand, these chitinozoan biozones and other associated fauna (e.g. graptolites, trilobites, brachiopods, and conodonts) suggest a late Early Ordovician (Floian) to Early Silurian (Rhuddanian) age range, with unconformities, for this succession. Based on chitinozoan biozones, a distinctive hiatus is present between the Zardkuh and Seyahou formations at Faraghan mountain, which corresponds to the absence of jenkinsi-tanvillensis chitinozoan biozones, encompassing the latest Middle Ordovician (latest Darriwilian: stage slice Dw3) and the early Late Ordovician (Sandbian: the stage slices Sa1 to Sa2 and time slices 5a to 5b). Two chitinozoan species were erected: Belonechitina bifurcaspina sp. nov., Angochitina persianense sp. nov., and Eremochitina cf. brevis was left in open nomenclature.

Seismic attenuation tomography of the Sn phase beneath the Turkish-Iranian Plateau and the Zagros mountain belt

AbstractThe Turkish-Iranian Plateau and the Zagros highlands are among the most prominent physiographic features in the Middle East and were formed as a result of continental collision between the Arabian and Eurasian plates. To better understand the nature of the lithospheric mantle and the origin of the observed seismic anomalies in this region, we investigated seismic attenuation of the uppermost mantle by detailed measurements of the quality factor of the Sn seismic phase (Sn Q). To that end, we collected a large data set consisting of 30 years (1990–2020) of waveforms recorded by 1266 permanent and temporary seismic stations, applying both the two-station method (TSM) and reverse two-station method (RTM) to measure path-averaged Sn Q. Finally, we performed a tomographic inversion on the path-averaged Sn Q to map the lateral variations of the upper-mantle attenuation across the northern Middle East. Our Sn attenuation maps show moderately low Q (&amp;lt;250) values beneath the Turkish-Iranian Plateau and high Q values (&amp;gt;350) beneath the Zagros and northern edge of the Arabian plate. Furthermore, our Sn Q model is broadly consistent with seismic velocity models in the region suggesting that most of the seismic anomalies are the result of thermal rather than compositional effects.

Palaeogeographic reconstruction and sequence architecture of the middle‐upper Jurassic successions of Hawraman Basin (NW Iran): Implications for tectono‐depositional processes of the northeastern passive margin of the Arabian Plate

The middle‐upper Jurassic successions (Sargelu, Najmah, and Gotnia formations) of the NW sector of the Zagros Mountain belt are studied to generate a sequence stratigraphic model that helps to understand the Jurassic tectono‐depositional processes and palaeogeographic reconstructions of the northeastern passive margin of the Arabian Plate. The major geological structures observed in northwestern Iran indicate the predominance of an extensional deformation during the Toarcian to Aalenian owing to the eastern Mediterranean rifting. This extensional regime reached its maximum from the Early to Middle Jurassic, causing the fragmentation/drowning of the long‐lived Triassic to Early Jurassic carbonate platform and the development of the Hawraman intrashelf basin (HIB). Associated changes in marine current dynamics, trophic conditions, and degree of substrate oxygenation led to the bloom of oligotypical biota (Bositra limestones) creating unfavourable conditions for the development of other benthic communities (Sargelu Formation). Later, from the Callovian to Oxfordian, changes in carbonate production types (switch from the Bositra filament facies to the stromatolitic‐thrombolitic boundstones) remarkably decreased the dip angle of the depositional profile (Najmah Formation). A second‐order tectono‐eustatic cycle (a major long‐term transgressive/regressive [T/R] cycle) including six minor short‐term T/R facies cycles (TR1–TR6) is identified in the study area. The development of the T/R facies cycles during the Middle Jurassic (TR1–TR3; give‐up sequences; Sargelu Formation) as a result of combined global eustatic transgression and linear subsidence resulted in drowning and backstepping of the platform. In contrast, the short‐term cycles that developed in the upper Jurassic (TR4–TR6; keep‐up sequences; Najmah and Gotnia formations), following a sea‐level fall, resulted in progressive shallowing of the carbonate platform. This study suggests that extensional deformation of the area reached its peak in the transition of the Early to Middle Jurassic, leading to the creation of HIB, but either decreased or played a subordinate role to eustatic sea‐level variations during the deposition of the TR1–TR3 facies cycles (Bajocian‐Callovian). Changes in tectonic regime from an extensional deformation (Early Jurassic) to cooling subsidence (Middle Jurassic), accompanied by a long‐term transgressive sea‐level trend during the Middle Jurassic, were important factors in the development of anoxic trough environments with the deposition of organic‐rich intervals, represented by the TR1–TR3 facies cycles. Tectonic instability (tectonic uplift plus considerable differential subsidence due to the reactivation of faults) in the eastern margin of the Arabian platform during the Late Jurassic led to the development of the TR4–TR6 facies cycles.

Review of emUvarovistia/em (Orthoptera: Tettigoniidae; Tettigoniinae): genetic data indicate a new cryptic species, emUvarovistia/em emmunzurensis/em sp.n.

Genus Uvarovistia ranged along the Zagros Mountain belt. We presented first genetic data from three species of the genus and a taxonomic rectification indicated by these data. Sequences of three mitochondrial and two nuclear gene segments were obtained from different populations. Phylogenetic and automatic species delimitation analyses consistently suggested three distinct phylogroups as U. zebra, U. satunini and the third a new species, U. munzurensis Uluar Yahyaoğlu sp. n. Time estimation and population genetic analyses supported consistent results. The following conclusions were reached: (i) five species in the genus constitute two distinct species groups named as Zebra Group and Satunini Group, (ii) data suggest intra generic relationships as U. zebra + ((U. satunini + U. munzurensis) + (U. bakhtiara +U. iraka)), (iii) molecular clock estimations indicated a deep divergence time and no gene flow between U. satunini and U. munzurensis, (iv) although these two species cannot be clearly distinguished by morphology, range of U. munzurensis seems to be isolated from other by lowlands of Euphrates Valley, and (v) age of generic ancestor is around five million years a time corresponding to connection of Anatolian and Zagros plates, and genus radiated along Zagros belt after dispersal of ancestral stock here.

Probabilistic Modeling in Investigating the Factors Affecting knickpoints, Case Study: Zagros Mountain Belt, Ghaleh Shahrokh Basin

knickpoints are steep landforms that are important in the evolution of river systems. This study aimed to identify the effective factors in creating a knickpoint and to determine the susceptibility of the knickpoint in the study area by a binary logistic regression method. For this purpose, effective factors in the formation of knickpoint were selected, then its relationship with the distribution of knickpoint was investigated. The effective variables and their effect on the knickpoint were determined, then the prediction model was performed by logistic regression on these variables. The results of the likelihood test show that the geology and the distance from the boundaries of the geological formations are significant. The analysis of the results shows that with a decreasing amount of incision and isobase indexes, distance from the boundaries of the geological formations, and with increasing the vertical dissection, the probability of the existence of a knickpoint increases. There is no correlation between the other factors used. ROC curve indicates significant accuracy for the detection of knickpoints for testing data set with 87%. The result of the Yuden index is 0.66 that indicates accurate information about the probability status knickpoints. The result of the kappa agreement is 0.60, which indicates the agreement of the method with the observed values of the knickpoints. According to the results, geological and topographic formations play an important role in the occurrence of knickpoints in the study area and it can be stated logistic regression is suitable for the prediction of the knickpoint incidence.

Active Deformation and Relief Evolution in the Western Lurestan Region of the Zagros Mountain Belt: New Insights From Tectonic Geomorphology Analysis and Finite Element Modeling

AbstractTo unravel how and where coseismic and interseismic deformation impacts the spatial and temporal patterns of rock uplift of the Lurestan sector of the Zagros Mountains, we performed an investigation of the large‐scale features of topography and river network coupled with 2‐D finite element modeling. Geomorphological analysis and constraints from parameters such as elevation, local relief, normalized channel steepness index (ksn), river longitudinal profiles, and transformed river profiles (chi plots) were used to unravel the time‐space distribution of vertical motions. Whereas the much longer timescale over which topography grows and/or rivers respond to tectonic or climatic perturbations with respect to even multiple seismic cycles, the outputs of the finite element model yield fundamental information on the source of the late part of the spatiotemporal evolution of surface uplift recorded by the geomorphology. Model outputs shed new light into the processes controlling relief evolution in an actively growing mountain belt underlain by a major blind thrust. The outputs illustrate how coseismic slip controls localized uplift of a prominent topographic feature—the Mountain Front Flexure—located above the main upper crustal ramp of the principal basement thrust fault of the region, while continuous displacement along the deeper, aseismic portion of the same basement fault controls generalized uplift of the whole crustal block located farther to the NE, in the interior of the orogen.

Integrated interpretation of geophysical data from Zagros mountain belt (Iran)

Fluid composition and distribution, the key factors determining geoelectric structure in a seismically active region, are controlled by local and regional stresses and rheological contrasts. Two closely located magnetotelluric and seismic velocity profiles are jointly interpreted to recover more accurately the structural boundaries and fluid distribution within the crust in the central Zagros collision zone, one of the world’s most seismically active mountain belt. A multi-site and multi-frequency approach was used for the strike analysis of regional structure and decomposition of distortion effects on magnetotelluric data. Distortion corrected magnetotelluric data were then used for two-dimensional inversion modeling. The results indicate distinct conductive structures within the crust: (i) a thick conductive overburden in the southwest of the profile, (ii) high conductivities attributed to the fault zone conductors (FZCs) and (iii) an almost concave conductor extending from middle to lower crust in the central-eastern portion of the mountain belt, beneath the High Zagros (HZ). Comparison with the already available S-wave velocity structure, obtained by joint inversion of P-wave receiver functions and surface wave dispersion data shows that these main conductive features are spatially correlated with a low-velocity layer representative of the sedimentary cover overlying the Arabian platform and a velocity contrast bounded by the main Zagros thrust (MZT) fault, indicating the presence of fault zone fluids. The joint interpretation of magnetotelluric inverse modeling and seismicity data also shed light on fluid generation influencing rock deformation and seismicity in this region. It suggests that beneath the HZ, deep crustal fluids generated through metamorphism may promote aseismic deformations before high stresses are builtup and cause the north-eastern part of the Zagros Fold and Thrust Belt (ZFTB) to be seismically inactive compared to its southwestern part.

Geomorphic Analysis of the Southern Zagros Mountain Belt: Insight into a Remotely Sensed Fractal Approach

Geological processes that have acted on the Earth’s surface through cumulative influence have a consequence of forming topographic features. The Zagros is the most active collisional orogen during Cenozoic and is known as one of the largest mountain belts. Result of fractal analysis appears to bring useful information about the earth surface which is not contained in traditional morphometric measures. Fractal analysis of topographic data of Sentinel 1 DEM is used in this study as a means of characterizing landscapes. Northwestern and Southwestern parts of the study area show higher fractal dimension. Geological processes have impacted much to these relatively older and uplifted formations which are associated with enhanced erosion as compared to those having sooth topographies. Fractal dimension decreases with the direction of younging of the mountain belts trending northeast to the southwest.

Midcrustal Thrusting and Vertical Deformation Partitioning Constraint by 2017 Mw 7.3 Sarpol Zahab Earthquake in Zagros Mountain Belt, Iran

Midcrustal Thrusting and Vertical Deformation Partitioning Constraint by 2017 Mw 7.3 Sarpol Zahab Earthquake in Zagros Mountain Belt, Iran

Early Jurassic Rifting of the Arabian Passive Continental Margin of the Neo‐Tethys. Field Evidence From the Lurestan Region of the Zagros Fold‐and‐Thrust Belt, Iran

AbstractThe Arabian passive margin formed at the southern margin of the Neo‐Tethys ocean during the breakup of Pangea. In the Lurestan region of the Zagros mountain belt, the deformed Arabian continental paleo‐margin can be reconstructed as originally consisting of distinct crustal domains, including a proximal sector and a distal continental ribbon, separated by a deep‐water trough, known as the Radiolarite Basin. Such an architecture was shaped by the continental rifting process, thus reflecting timing and style of continent separation, which is generally assumed to have occurred during the Permo‐Triassic interval. This study reports evidence of syn‐sedimentary extensional faults, unconformities, and facies changes in the Mesozoic stratigraphic succession of the Lurestan region, which point to a major Jurassic extensional pulse. In detail, extension reached its climax at the end of the Early Jurassic, when tectonically driven drowning of the long‐lived Triassic to Early Jurassic carbonate platform led to the transition from shallow‐ to deep‐water environments in large areas of the inner margin, coevally with the development of the Radiolarite Basin. Our findings suggest a two‐step continental rifting in this area, with the first Permo‐Triassic phase predating an Early Jurassic one.

The 2008 and 2012 Moosiyan Earthquake Sequences: Rare Insights into the Role of Strike Slip and Thrust Faulting within the Simply Folded Belt (Iran)

Abstract The Zagros mountain belt has an unusually large discrepancy between seismic and geodetic strain rates, implying very large aseismic release of strain. However, the spatial and depth relationship between seismic and aseismic deformation is poorly understood and controversial, with important implications both for understanding the role of aseismic deformation in regions of continental convergence and for characterizing seismic hazard for large urban populations in this region. Two recent earthquake sequences in 2008 and 2012 provide us with an ideal opportunity to use geodetic and seismological data to address this topic, not only for thrust faulting but also for rarer strike‐slip faulting in the Zagros. These aftershock sequences occurred on the southeast border between Iran and Iraq. We use Interferometric Synthetic Aperture Radar (InSAR) to obtain a mechanism for the 2008 mainshock and observe significant aseismic slip accompanying the earthquake. This aseismic slip occurred along strike from the seismic asperity and approximately doubled the seismic moment release, demonstrating that aseismic slip plays an important role for strike slip as well as thrust faulting in the Zagros. Depths are calculated by inverting surface‐wave amplitude spectra and depth phase observations, demonstrating that all events occur at depths less than 12 km, supporting the view that seismicity predominately occurs in the sedimentary cover in the Zagros. Using both the calculated depths and InSAR location, we relocate the aftershocks to reveal two distinct clusters, with the 2012 cluster occurring in the vicinity of the Zagros foredeep fault, challenging the traditional view that this region is aseismic. Electronic Supplement: Figures of Interferometric Synthetic Aperture Radar (InSAR) data, model, and resolution.

Fault Slip Rate of the Kazerun Fault System (KFS), Iran, Investigated Using Finite Element Modeling

A 3D non-homogenous finite element model (FEM) is developed to investigate the spatial variations of interseismic deformation for the Kazerun Fault System (KFS) in the Zagros Mountains of Iran. The model includes 19 fault segments that were extracted from geological maps and previous studies, and the average slips in the dip and strike directions on these segments were computed. The contemporary surface deformation is simulated using a free horizontal detachment surface. The dip angles of the faults in the model are varied at 90°, 70°, 50° and 30° to simulate different 3D representations of the fault systems. Tectonic loading at the boundaries of the region is applied using predicted GPS velocity vectors to the north (southern part of the Central Iran Block) and south (southern region of the Zagros mountain belt), which were obtained by solving inverse and forward problems. Where possible, the fault slip rates that are obtained using our non-homogeneous finite element model are validated using the long-term geologic and instantaneous GPS slip rates. The model is then used to estimate the dip- and strike-slip rates of the fault segments of the KFS for which no a priori information was available. We derive an upper bound of 1 mm/year for the average dip-slip rate in the region, which is consistent with estimates from geomorphologic observations. The modeling results show that in addition to the 4 main faults (Dena, Kazerun, Kareh Bas and Main Recent), other faults, such as the Zagros Front, Main Front, High Zagros and Mishan faults, accommodate up to 2.5 mm/year of the differential movement between the North and Central Zagros. We also investigated the contrast in rigidity between the southern and northern areas of the Zagros mountain belt and found that a rigidity contrast of 2 best explains the GPS data of contemporary surface deformation. Neglecting to account for the rigidity contrast in the model can lead to biased estimates of the fault slip rate of up to 50 %.

Constraint on foreland basin migration in the Zagros mountain belt using Sr isotope stratigraphy

We have constrained the time-space migration of the Zagros foredeep basin by performing Sr isotope stratigraphy on 31 samples of marine macrofossils from Neogene sediments now exposed in the Zagros mountain belt in southwest Iran. Our results show that these deposits (represented mainly by the Mishan Formation) are strongly diachronous, with ages ranging between 17.2
0.2 and 1.1
0.1 Ma. These deposits are older in the west (Dezful region) and become progressively younger towards the south and the south-east (Fars region). Our results show that the marine foredeep was replaced by a fluvial sedimentary environment between ca. 14 and 12 Ma in the western sector, while this occurred between ca. 8 and 1 Ma in the eastern sector, becoming younger towards the south. These results enable us to show that the foreland basin migrated perpendicular to the orogen at rates of between 17.5 and 50 mm year 1 throughout the Neogene, exceeding migration rates in the Alps, Pyrenees, Apennines and Himalayan foreland basins. The sporadically elevated rates in the Zagros appear to be related to times when major widely spaced pre-existing basement faults became reactivated. Finally, our results, when combined with published data, have enabled us to establish a new chronostratigraphic diagram for the Neogene portion of the Zagros foreland basin. Our study highlights that foreland basins are extremely dynamic settings where depocentres and palaeoenvironments may change rapidly in both time and space in relation to migrating deformation.

Neotectonics of Boroujerd Area, SW Iran by Index of Active Tectonics

Boroujerd area has located in the border zone of Zagros mountain and Sanandaj-Sirjan belt in the southwest Iran. Six geomorphic indices were calculated in the study area. Through averaging these indices we obtain index of active tectonics (Iat). The values of the index were divided into classes to define the degree of active tectonics. Therefore, relative tectonic activity was calculated and their values were classified and analyzed in two groups. Regions were identified as low and moderate levels. In analyzing data and combining them with tectonic setting the results were often associated and justified with regional geology. Our results show that the highest value has located along faulted area, which shows 3 classes of relative tectonic activity (moderate level). Also, other values have located along folded area (low level). Therefore, middle part of study area (sub-basin No. 4) is showing the more active uplifting related to surroundings region (sub-basin No. 1, 2 and 3). In other words, sub-basin No. 4 has got the more active uplifting by quaternary movements of several faults such as Doroud fault.

The role of the Zagros orogeny in slowing down Arabia‐Eurasia convergence since ~5 Ma

Large topographic belts along convergent margins have been recognized with the ability to slowdown the kinematics of subduction over geologically short time periods (i.e., few Myr), because their associated gravitational spreading provides significant resistive force within the framework of plate tectonics. The record of past and present‐day plate kinematics provides important constraints on the dynamics of the lithosphere, because plate‐motion changes must reflect temporal changes in the balance of driving and resisting forces. Here we focus on the convergence between the Arabian and Eurasian plates, across the Zagros mountain belt. Relative motion across this plate boundary is reconstructed since 13 Ma from published paleomagnetic and geodetic data, and features a slowdown of ~30% from ~5 Ma to the present day. We employ global dynamic models of the mantle/lithosphere system to test whether the most recent uplift across the Arabia‐Eurasia collision zone, including the Zagros orogeny, may induce the observed slowdown since 5 Ma. Specifically, we use constraints from the geologic record to infer past topography and quantify its influence on the convergence rate between Arabia and Eurasia. We test the sensitivity of our models to assumptions made in estimating the paleoelevation by perturbing the orogeny parameterization within reasonable ranges. Finally, we speculate on the potential effects of Tethys slab break‐off, changes of the deformation style within the collision zone, and the Afar plume on the dynamics of convergence. Our results indicate that orogenic uplift across the Arabia‐Eurasia collision zone played a key role in slowing down convergence since ~5 Ma.