Arabia-Eurasia Collision Zone Research Articles

Earthquake-induced paleo-landslides in the Tehran Region and its role in assessing the seismic hazard, Iran

In the central portion of the Arabia-Eurasia collision zone, the Tehran domain is positioned at a transitional boundary between seismotectonic zones of the Central Iranian lowland (to the south) and the Alborz highland (to the north). Consequently, numerous destructive seismic events have occurred in this active tectonic domain. This study delves into the tectonic geomorphology of the region within its northern highland domain, specifically focusing on the hanging wall of the E-striking north-dipping North Tehran fault (NTF) zone. Our findings in this northern domain emphasize several prominent topographic scars as significant co-seismic features. These include huge landslides, rockfalls, rock avalanches, and offset geomorphic surfaces and could be present as the main indirect co-seismic morphological features. Within this seismically active region, the extensive dimensions of these geomorphic pieces of evidence reveal the seismic potential of the Tehran Region to experience really strong earthquakes (i.e. M>7.5). These results contrast with the previous Maximum Credible Earthquake (MCE) magnitude estimated for the Tehran Region (i.e. M~7.2) through different approaches in Seismic Hazard Assessments (SHAs). Consequently, the previous SHAs of the Tehran Region might have underestimated the seismic risk, and therefore, it is necessary to conduct an updated and complementary deterministic SHA based on the more detailed seismogenic geological features in this crucial area.

Seismicity and state of stress in northeast Arabia

Abstract Eastern Arabia exhibits low seismic activity although damaging earthquakes are historically known. Despite the proximity to convergent plate boundaries in the Arabia-Eurasia collision zone, the state of stress in northern Oman with its persistent topography of the Oman Mountains (OM) remains enigmatic. We revised the earthquake catalogue of northern Oman and confirm detection and location accuracy of the permanent network by comparison with a temporarily densified network across the OM. For the first time, we infer focal mechanisms (FM) for earthquakes in Oman from P-wave polarities. Seismic activity is high in the Northern OM but diminishes rapidly south of 24.5°N. In the Central and Eastern OM, low-magnitude earthquakes occur along topography bounding faults with mostly transtensional FM. Offshore, seismicity follows NE-trending lines in extension of the Semail Gap and the Masirah Fault Zones, up to the Makran trench. Except for an isolated patch of repeated small-magnitude earthquakes, the western Makran trench is seismically quiet. Inversion of FM confirms a NE-SW direction of maximum horizontal stress that aligns with Arabia-Eurasia convergence. In the Central and Eastern OM, maximum horizontal and vertical stresses are balanced. The topography of the OM appears, therefore, not to be generally sustained by compressional forces.

Middle-Late Miocene to Pleistocene Post-Collisional Magmatism in the Arabia-Eurasia Collision Zone, an Example from Northwest Iran

Abstract Post-collisional volcanism contains important clues for understanding the processes that prevail in orogenic belts, including those in the mantle and the uplift and collapse of continents. Here we report new geochronological and geochemical data for a suite of post-collisional Miocene to Pleistocene volcanic rocks from northwest Iran. Four groups of volcanic rocks can be distinguished according to their geochemical and isotopic signatures, including: (1) Miocene depleted lavas with high Nd and Hf but low Pb and Sr isotopic ratios, (2) less depleted lavas with quite variable Pb isotopic composition, (3) lavas with non-radiogenic Nd and Hf isotopic values, but highly radiogenic Sr and Pb isotopic composition, and (4) Pleistocene adakitic rocks with depleted isotopic signatures. The isotopic data reveal that the Miocene rocks are derived from asthenospheric and highly heterogeneous sub-continental lithospheric mantle sources. Evidence suggests that the lithospheric mantle contains recycled upper continental material and is isotopically similar to the enriched mantle two (EMII) end-member. Analysis of Sr-Nd-Pb-Hf-O isotopes in both mineral and rock groundmass, in conjunction with energy-constrained assimilation and fractional crystallization (EC-AFC) numerical modeling, demonstrates that the incorporation of continental crust during magma fractionation via AFC had an insignificant impact on the isotopic composition of the Miocene lavas. Moreover, adakites are the youngest rocks and show a geochemical signature consistent with the partial melting of a young and mafic continental lower crust. Both seismological data and geochemical signatures on these Miocene to Pleistocene volcanic rocks indicate the initiation of asthenospheric upwelling and orogen uplift in the Arabia-Eurasia collision zone, which occurred after slab break-off, following the Neotethyan closure.

Anisotropic gradients in Iran: Quasi-Love waves illuminate the deep structure and deformation style of the Zagros, Alborz, and Kopet Dagh

We investigate the presence of the quasi-Love wave (qL) at 51 seismic stations of a temporary seismic network across the western Arabia-Eurasia collision zone. We quantify the intensity of the qL observations from the April 12, 2014 Solomon Islands earthquake by calculating the peak-to-peak amplitude ratios of the qL and Love waves, and compare them with predicted qL intensities from previous shear-wave splitting results. We determine the polarity, timing, and period-dependence of the qL observations within the period range of 50–100 s. Our analysis reveals that the qL observations at stations in the Zagros and Alborz mountain belts exhibit opposite characteristics. In contrast to the Alborz stations, the intensity of qL observations at the Zagros stations exhibits relatively negligible dependence on the period, while their receiver-scatterer distances are considerably period-dependent. We approximately locate the anisotropic gradients that generate the qL waves. Our results suggest that a lithospheric gap is responsible for the shallow and abrupt variation in the belt-parallel trend of fast-axis orientations in the westernmost part of the Zagros. Additionally, the period/depth dependence of the anisotropic gradients along the boundary between the central Zagros and central Iran provides insight into the variation in the downward dip of the Arabian lithosphere. The anisotropic gradient located to the north of the Doruneh fault in eastern Iran indicates its role as a major shear zone and lithospheric boundary. Finally, we observe that the spatial distribution of the anisotropic gradient in northeastern Iran matches the higher strain-rate areas in the Kopet Dagh Mountains, suggesting coupling between the lithospheric mantle and crust in that region.

Early-middle Miocene Inversion of the Abu Jir Fault in the Western Iraq: a Possible Consequence of the Arabian Plate Northward Movement

The Abu Jir fault zone in western Iraq that marks the western limit of the Zagros foreland basin merges with the Anah graben in the north. The northern segment of the Abu Jir fault zone and the western Anah graben are inverted structures. In this study, we investigated the timing and mechanism of the northern Abu Jir fault zone inversion in comparison with other inverted structures like Tel Hajar and Sinjar, which are located in the north and closer to Arabia–Eurasia collision zone. In the study area the Abu Jir fault zone is characterized by a local-scale northwestern-trending asymmetric fold, associated with inverted normal faults, positive flower structures, right–lateral strike-slip faults, and seismically induced early middle Miocene slump structures. Based on the structural synthesis of the surface geology and age constraints from syntectonic deposits at the northern Abu Jir fault area, the Abu Jir fault inversion and its western block northward movement were inferred to be resulted from forces that originated from the south to the north, unlike the northern younger inverted structures. This early middle Miocene age of the Abu Jir fault inversion is comparable to the arrival time of the Afar plume hot mantle material flow from the south to the north in western Arabia. Therefore, we argue that the Abu-Jir fault inversion is more likely to be due to dragging of the Arabian plate by mantle flow from the south to the north rather than shortening propagation from the north to the south.

Active seismogenic faulting in the Tehran Region, north of Iran; state-of-the-art and future seismic hazard assessment prospects

This study summarizes the state-of-the-art investigation into active-seismogenic faulting in the Tehran Region, and presents new complementary data to provide an overall view on potential seismic sources to strike the capital of Iran. The study region lies in the central portion of the Arabia-Eurasia collision zone. The Tehran Region is located in a transitional zone between the seismically active regions of Central Iran to the south and Alborz to the north and exhibits a tectonic context characterized by both dip-slip and strike-slip E-striking fault systems such as the Mosha, Parchin, Garmsar and North Tehran faults. Seismic catalogues record numerous strong and moderate earthquakes in the Tehran Region. Although the mapping of active faults and paleoseismological records have defined the seismic sources of several historical and prehistorical earthquakes, still many of them need to be constrained. Based on successive fault displacements and related microseismicity, five previously unknown or poorly understood major active seismogenic fault zones in the Tehran Region were introduced. From the west to the east, and in both near- and far-field contexts, the Qezel Hessar, Mahdasht-South Karaj, Kuhak-Lavizan, Arakuh, and Kajan define the fault zones. Critically, in the last decades, the rapid urban development in Tehran, including the new settlements of Karaj, Hashtguerd, Pardis, and Parand, has covered some of the surface expressions of these fault zones. The Kuhak-Lavizan fault zone, however, is known to cut across inner city parts of Tehran. Within the Tehran Region, seismicity on these faults demonstrates ongoing active structural evolution. The active faults of Tehran City are generally discussed in three northern, central and southern ∼E-striking basement zones. The major zones, which contain deep-seated faults, are capable of producing moderate to strong seismic events near and within the metropolitan areas of Tehran and Karaj and nearby settlements.

Late Miocene–early Pliocene tectonic to erosional exhumation in the northwest of the Arabia–Eurasia collision zone

AbstractThe exhumation of actively deforming orogenic systems is commonly due to the incision of river systems in response to tectonically driven surface uplift. However, a regional base level and/or climatic variations can also cause erosional exhumation. The temporal and spatial distinction of the dominant mechanism during the topographic evolution of orogens is challenging. Using detailed morphological analyses and thermal modelling of an intrusive body through triple dating (zircon U–Pb, ZHe, and AHe), combined with the previously published structural, detrital, and bedrock low‐temperature thermochronometry (apatite fission track and apatite helium) analysis, the transition from early Oligocene (~27 Ma) and middle Miocene (~12 Ma) tectonic exhumation to late Miocene–early Pliocene (~7–5 Ma) river incision and erosional exhumation in the Talesh Mountains in the NW of the Arabia–Eurasia collision zone was documented. The late Miocene‐early Pliocene incision and the contemporaneous rapid exhumation documented in the Talesh Mountains are related to the base‐level fall of the Caspian Sea due to its isolation from the Paratethys domain. The effect of this base‐level fall then migrated to the margin of the Iranian Plateau and captured the overspilled basins (~4 Ma).

The coherence function and lithospheric elastic thickness of the Zagros fold and thrust belt

SUMMARY This study derives the spatial variation of the elastic thickness (Te) and its implications for understanding the structure, geodynamic and seismicity of the lithosphere for the Zagros fold and thrust belt region of the Arabia–Eurasia collision zone. Te is calculated using the coherence function in the fan wavelet domain based on recent terrestrial Bouguer gravity and topography data as input signals. Utilizing the load deconvolution method and Brent's method of 1-D minimization, the final Te for the survey region is estimated for each grid node of the studied area. To illustrate the mass distribution in the studied area, the subsurface loading fraction (F) is calculated simultaneously with Te in the inversion. The crust thickness and density from three different global crustal models are tested and the results obtained for these input models do not yield substantially different Te patterns. The final results are in accord with the global Te models as well as previous rheological, geodynamical and flexural studies, however, this study establishes much more detailed regional information. The calculations yield a mean value of Te of 61 km for the Zagros, with a mean estimated error of about 5 km. The high-Te values (>70 km) are observed in the southeast of the studied area (some parts of the Sanandaj–Sirjan zone, Urumieh–Dokhtar magmatic arc and most of the Central Iranian blocks); while over most of the northwest of the studied area, the value of Te is about 58 km. The Te results are consistent with the lithospheric structure of the study area and also support the idea of the crust–mantle decoupling. Further, there is a positive and negative correlation between the surface wave velocity and surface heat flow, respectively. The mean value estimated for the internal loading friction (F) of 0.4 means in most of the studied areas we may consider that the surface loading is dominant, or at least the ratio of the surface and subsurface loading can be assumed equal. Based on earthquake distribution in the period 1900–2020, seismicity is more likely to occur in areas with a relatively low value of Te.

Mantle compositional structure and dynamics of the Arabia-Eurasia collision zone: Insights from integrated thermochemical modelling

The tectonic evolution of the Arabia-Eurasia collision zone is enigmatic due to our poor knowledge of the compositional structure of the Zagros keel and the sub-lithospheric mantle beneath the Iranian plateau. Here, we present a thermochemical model for the upper mantle, starting from the Arabian plate toward the Zagros keel (in SW Iran) and the Iranian plateau to the Kopet Dagh Mountains in NE Iran. The crustal and (sub) lithospheric mantle structure is constrained using receiver functions and high-resolution seismic tomography. The crustal density is obtained by converting local Vp velocity profiles to density. The presumed mantle composition/density of different tectonic units has been inferred from the composition of mantle-derived xenoliths in Pleistocene alkaline basalts of NE Iran, mantle xenoliths from Eastern Arabia, and integrated geophysical-petrological modeling of seismic tomography as well as Bouguer and geoid anomalies. Our results indicate that the lower part of the lithosphere in the Arabian plate is ∼ +25 kg/m3 denser than the sub-lithospheric mantle, which allows underthrusting of the Arabian lithosphere beneath the Iranian plateau. The Zagros core–an ultra-positive seismic velocity anomaly in the tomographic models– is modeled assuming a refractory “craton-like” composition, leading to a ∼-60 kg/m3 density contrast with its underlying fertile lithospheric mantle. We hypothesize that at least the upper ∼100 km of the hydrous Arabian lithosphere was compositionally cratonic before the collision-related shortening processes. In contrast, the anhydrous Iranian lithosphere has been refertilized during asthenosphere upwelling and mantle flows. The Iranian plateau is underlain by a cold sub-lithospheric body characterized by ∼ +25 kg/m3 density contrast with respect to the ambient asthenosphere.

Characteristics of the gold-bearing and barren quartz veins at the Zaylik-Sarilar epithermal deposit, Ahar-Arasbaran Zone, NW Iran: Evidence from mineralogy, alteration, texture and fluid inclusion

The Zaylik-Sarilar epithermal deposit is located 35 km southeast of Ahar in the Ahar-Arasbaran Zone in the Alborz-Azerbaijan Magmatic Belt, NW Iran. Exposed rocks in the Zaylik-Sarilar deposit are mainly composed of Eocene volcanic rocks and Miocene dacite-rhyodacite that are intruded by younger intrusions. The deposit occurs within a sequence of Eocene porphyritic andesite and andesitic lithic tuff. Mineralization occurs as open space filling, taking place as irregular veins, veinlets, and hydrothermal breccias. The mineralization zones can be categorized into two main groups according to sulfides and sulfosalts mineralogy, alteration assemblages, ore textures, and fluid inclusion data. The first group consists of gold-bearing quartz-sulfide ± breccia veins, silicified-phyllic ± argillic zones, and a few quartz-calcite veins. The second group is composed of barren milky (or white) quartz veins, silicified lithocaps, and argillic-kaolinite ± silicic zones. Pyrite, chalcopyrite, bornite, galena, sphalerite, and Au-Ag sulfosalts were identified in ore paragenesis of the gold-bearing veins. Pyrite is the main sulfide mineral in the barren quartz veins and silicified lithocaps. Gold and silver were enriched during oxidation and supergene alteration of the gold-bearing veins. Textural evidences from the gold-bearing veins, such as mosaic breccia monomicts, floating clast breccias, hydrothermal breccias, colloform-crustiform textures, ginguro bands, dendrite textures, and rarely lattice- and parallel-bladed calcite replacement textures, suggest that boiling occurred in the veins. Comb, cockade, reed molds, zonal, and feathery textures are common in the barren veins and silicified lithocaps. Fluid inclusion data show a decreasing trend in salinity and homogenization temperature (Th) from the gold-bearing quartz veins to the barren ones. The average salinity and homogenization temperature (Th) of fluid inclusions from the gold-bearing veins are 1.05 wt% NaCl equiv. and 297 °C, respectively; higher than the average salinity (0.68 wt% NaCl equiv.) and homogenization temperature (192 °C) of fluid inclusions from the barren quartz veins. Th versus salinity diagram suggests that boiling was the main mechanism of ore deposition in the gold-bearing quartz veins and silicified zones. These data propose that the barren quartz veins and silicified lithocaps were formed by cooling. A combination of the geologic, mineralogic, alteration, and fluid inclusion data from the Zaylik-Sarilar deposit represents an epithermal system of low-sulfidation type. Structurally, mineralization has been controlled through the NW-trending strike-slip principal displacement zone and it’s subsidiary Riedel fractures in response to a N-trending compression in the Arabia-Eurasia collision zone.

Total Shortening Estimates Across the Western Greater Caucasus Mountains from Balanced Cross Sections and Area Balancing

The Greater Caucasus orogen forms the northern edge of the Arabia-Eurasia collision zone. Although the orogen has long been assumed to exhibit dominantly thick-skinned style deformation via reactivation of high-angle extensional faults, recent work suggests the range may have accommodated several hundred kilometers or more of shortening since its ~30 Ma initiation, and this shortening may be accommodated via thin-skinned, imbricate fan-style deformation associated with underthrusting and/or subduction. However, robust shortening estimates based upon surface geologic observations are lacking. Here we present line-length and area balanced cross sections along two transects across the western Greater Caucasus that provide minimum shortening estimates of 130-200 km. These cross sections demonstrate that a thin-skinned structural style provides a viable explanation for the structure of the Greater Caucasus, and highlight major structures that may accommodate additional, but unconstrained, shortening.

Quaternary deformation patterns in East–Central Iran, constrained by coseismic–postseismic displacements of the 2017 Hojedk triplet earthquake in the Kerman Province

The Central Iranian Microcontinent (CIM) in east-central Iran is located north of the active Arabia–Eurasia collision zone. Here, we report on the structure, deformation patterns, and earthquake occurrences along the dextral Lakar–Kuh and Godar fault systems in the CIM. The geometry of these fault systems marks a major restraining bend responsible for surface and rock uplift in the Plio–Pleistocene that produced the Mian Kuh mountain range. The 2017 Hojedk triplet earthquake (Mw = 5.8–6.0) occurred in the Mian Kuh Range. Sentinel–1 A Interferometer Synthetic Aperture Radar (InSAR) images (descending and ascending) were used to extract the coseismic displacements associated with the earthquake and its aftershocks. The results indicate a maximum displacement of ∼20 cm, corresponding to hanging wall uplift along the radar Line-of-Sight (LOS) direction. The Geodetic Bayesian Inversion (GBIS) of the coseismic deformation indicates that the causative faults of the Hojedk earthquakes were two reverse faults with NW–SE–strikes and SW–dips, with minor dextral slip components. Given the focal mechanism solutions and the epicenter locations of the triple earthquake sequence, we infer that these faults at the southern termination of the Lakar–Kuh Fault represent two segments (with different dip angles) of a previously unrecognized, blind reverse fault (a splay of the Godar Fault at depth). The Hojedk Earthquake and the geometry and kinematics of its causative faults highlight the strong potential of seismic hazard zones along the strike-slip fault systems in the CIM.

A Major Medieval Earthquake on the Main Köpetdag (Kopeh Dagh) Fault, Turkmenistan

ABSTRACTThe Main Köpetdag fault (MKDF) of Turkmenistan is one of the longest (≈500 km) and the most rapidly straining (9.1 ± 1.3 mm/yr) faults of the Arabia–Eurasia collision zone, and yet, in contrast to adjacent parts of Turkmenistan and Iran, it has little in the way of observed seismicity. Field observations indicate a fresh paleorupture along an ≈100 km long section of the MKDF with abundant streams offset across it. We use WorldView-2 optical satellite imagery to measure geomorphic offsets and compile them into a cumulative offset probability density (COPD) function. The COPD yields two peaks in offset density at 7.5 ± 1.5 and 12.5 ± 1.5 m, when considering the highest and the lowest quality measurements, whereas the medium-quality measurements present a single peak at 9 ± 3 m. We are unable to separate peaks into distinct events when exploring the contributions of offset measurements along strike that have significant variability. The paleorupture displaces archaeological remains, including extensive field boundaries of likely medieval age, and a paleoseismic trench brackets the age of the most recent displacement at 600–800 yr B.P. From our measurements of displacement, we estimate that the most recent paleoearthquake had a moment magnitude (Mw) 7.6 ± 0.4 and yet is missing from earthquake records in nearby Iran, indicating that the paucity of historical earthquakes in Turkmenistan may be misleading. The Baharly, Döwgala, and Gökdepe segments of the MKDF, which extend eastward toward Aşgabat, do not show fresh surface expression. Infrared-stimulated luminescence ages of 3.4 ± 0.5 and 2.5 ± 0.2 ka from a river-cutting exposure along the Baharly segment predate the most recent paleorupture. We conclude that individual segments of the MKDF are capable of rupturing in major (Mw>7.0) earthquakes, with ends of individual ruptures possibly controlled by segment boundaries. The occurrence of major earthquakes and rapid slip rate suggests that the MKDF constitutes a hazard to a populated region of Turkmenistan and northeast Iran.

Quaternary Collision-Zone Magmatism of the Greater Caucasus

Abstract The Greater Caucasus mountains (Cavcasioni) mark the northern margin of the Arabia–Eurasia collision zone. Magmatism in the central part of the Greater Caucasus began in the Pleistocene, up to ~25 Myr after initial collision. This paper presents bulk-rock and Sr–Nd–Pb isotope geochemistry from 39 Quaternary volcanic rock samples (<450 Ka) recovered from the Mt. Kazbek (Kasbegui) region of the Greater Caucasus, Georgia, to assess the sources and magmatic evolution of these lavas and the possible triggers for melting in the context of their regional tectonics. Compositions are dominantly calc-alkaline basaltic andesite to dacite (57–67 wt % SiO2). Although the lavas were erupted through thick continental crust, there is little evidence for extensive modification by crustal contamination. Trace element and isotopic systematics indicate that the lavas have supra-subduction zone signatures, most likely reflecting derivation from a lithospheric source that had been modified by melts and/or fluids from material subducted before and during the collisional event. Mass-balance modelling of the Sr–Nd isotope data indicates that the lavas require significant input from a subducted slab, with deep-sourced fluids fluxing the slab into the source region. In contrast with published data from Lesser Caucasus magmatism, data from the Mt. Kazbek region suggest that a compositionally distinct sediment source resides beneath the Greater Caucasus, producing characteristic trace element and Pb isotopic signatures. Two distinct compositional groups and therefore primary liquids can be discerned from the various volcanic centres, both derived from light rare-earth element enriched sources, but with distinct differences in Th/Yb and Dy/Yb ratios and Pb isotopes. Rare-earth element modelling of the lava sources is consistent with 3–4% melting starting in the garnet peridotite and continuing into the spinel facies or, potentially, sited in the garnet-spinel transition zone. Small-scale convection related to mantle upwelling provides a plausible mechanism for Greater Caucasus magmatism and explains the random aspect to the distribution of magmatism across the Arabia–Eurasia collision zone.

Late cenozoic exhumation of the Nodoushan plutonic complex constrained by low-temperature thermochronology: evidence for Arabia–Eurasia collision in the southeastern Iranian Plateau

ABSTRACT The Nodoushan plutonic complex in the central part of the Urumieh–Dokhtar Magmatic Belt developed mainly in the Cenozoic in response to the final stages of subduction-related magmatism that preceded the post-Oligocene collision between the Arabian and Eurasian Plates. Despite numerous recent studies examining the deformation and related exhumation history of the Arabia–Eurasia collision zone, the exhumation history of the Urumieh–Dokhtar Magmatic Belt (making up the majority of the Iranian Plateau) is largely unknown. In this work, apatite and zircon (U-Th)/He thermochronometry and thermal modelling analysis are applied to the (mainly) granitoid intrusive bodies in the Nodushan Plutonic complex in order to unravel the exhumation history of this region in the context of collisional tectonics. The results document two main phases of exhumation in the early Miocene (~ 22–20 Ma) and middle Miocene (~ 11 Ma). The distribution of (U-Th)/He ages suggests that these deformation episodes are related to the collision between the Arabian and Eurasian Plates. The ‘soft’ initial stages of Arabia–Eurasia collision in the Nodoushan plutonic complex occurred when the stretched Arabian passive margin reached the subduction zone in the early Miocene. Subsequently, the final ‘hard’ collision with the harder continental margin occurred in the middle Miocene. The results of this work mark the first thermochronometric data related to SE Iranian Plateau evolution and identify exhumation that initiated in the early Miocene and developed regionally during middle Miocene times.

Middle-late Miocene normal faulting in the intermontane Tarom basin during the collisional deformation of the Arabia-Eurasia collision zone, NW Iran: A regional process or a local feature?

• Magnetic fabric in the central Tarom Basin records NE-SW layer parallel shortening. • Magnetic fabric in the SW margin of the basin is a primary sedimentary fabric. • NE-SW orogen-perpendicular syndepositional extension occurs in the SW margin. • Normal faults in the SW margin were induced by gravity instabilities. • Middle-late Miocene extension is not regionally pervasive. The upper plate of the Arabia-Eurasia collision zone experienced orogen-perpendicular to orogen-parallel extension from 25–22 to 10–9 Ma. Although such an extension occurred during widespread collisional deformation, it is not clear if it is a local feature or if represents a major phase of upper plate extension. In this study we combine anisotropy of magnetic susceptibility (AMS) with fault kinematic analysis and sedimentologic data from 16.2- to 7.6-My-old deposits of the Upper Red Formation of the intermontane Tarom Basin (NW Iran). These strata present syndepositional, normal faults and offer the possibility to gain new insights into the spatial extent of such a Miocene extension. AMS data from the central and northern sectors of the basin document a tectonic fabric with a magnetic lineation parallel to the strike of the orogen, suggesting a compressional tectonic overprint. Conversely, the southern margin of the basin presents a purely sedimentary magnetic fabric despite a ~NE–SW orogen-perpendicular extension. This suggests that basin formation was not driven by extensional tectonics. Rather, the normal faults are gravity instabilities induced as also documented by coeval landslide deposits. This allows concluding that the orogen-perpendicular extension observed in few sectors of the collision zone is not regionally pervasive and hence it is not controlled by large-scale processes. Combined, our results indicate that if orogen-parallel extension associated with tectonic denudation and metamorphic core complex development occurred in certain sectors of the collision zone (Takab complex), it must have ended before 19–16 Ma, when widespread upper plate contractional deformation started.

Fractal modeling and relationship between thrust faults and carbonate-hosted Pb-Zn mineralization in Alborz Mountains, Northern Iran

Central Alborz Metallogenic Belt is a major mineral province of northern Iran placed at the northern margin of the Arabia-Eurasia collision zone. This study aimed to at recognition the affiliation between reverse/thrust faults and Pb-Zn carbonate-hosted mineral deposition using fractal modeling. Thrusting is the major mechanism of faulting in this region in terms of length, frequency and density. The carbonate-hosted Pb and Zn occurrence/deposit were classified using the concentration-number fractal method. All faults then are categorized based on fault density, and concentration-area fractal methods are utilized for the thrust faults. According to our analysis, main fault density zones are situated at the central and western parts of the Alborz Mountains. Relationship among the major fault density zones and the Pb-Zn carbonate-hosted occurrences/deposits using logratio matrix reveals that there is a proper connection between reverse/thrust faults and the Zn-Pb carbonate-hosted occurrences/deposits. Moreover, the results were controlled by four ore deposits/prospects in this region which shows high-grade ores directly relate to faults especially reverse/thrust faults based on their distribution. This research indicates that this method can be used for other various ore deposit types that have been distributed by faulting.

لرزه زمین ساخت پهنه های البرز خاوری و کپه داغ با استفاده از داده های شبکه لرزه نگاری محلی

The Kopeh Dagh and Binalud-Alla Dagh mountains are important structural elements located in the northeastern boundary of Arabia-Eurasia collision zone. Due to existence of large cities with a long history of civilization, there is a relatively rich body of data on historical seismicity in this area. Nonetheless, little adequate instrumental seismic data were available prior to this study. In this paper, we utilized the temporary China-Iran local seismic network data deployed in the area for 13 months. Based on this data, we determined 37 focal solutions through first motion polarity approach suggesting a combination of strike-slip, reverse and normal mechanisms. There is a significant concentration of epicenters and focal mechanisms around the eastern Alborz - Kopeh Dagh boundary along the Atrak River. The southern and western boundaries of the Binalud Mountains also show relatively high seismic activities. The Local magnitude (ML) of the events ranges from 3.5 to 4.9 with depths of up to 20 km, mostly concentrated at ~10 km. The principal stress axes and slip vectors obtained from the focal mechanisms agree well with kinematic state of main faults and tectonic regime of the area, confirming the seismogenic nature of these faults. In addition, they are consistent with right-lateral component of slip along thrust faults in eastern Kopeh Dagh and Binalud and both laft- and right-lateral motions in the central Kopeh Dagh.

Seismic anisotropy and mantle deformation in NW Iran inferred from splitting measurements of SK(K)S and direct S phases

SUMMARY We present the results of a shear wave splitting analysis performed on the teleseismic SK(K)S and direct S wave recordings of 68 temporary broad-band stations to investigate the mantle deformation on the northern side of the Arabia–Eurasia collision zone in NW Iran. We used the Reference Station Technique to overcome potential contamination from the source-side anisotropy on the direct S wave signals. This method enabled us to expand our splitting measurement database beyond the usual SK(K)S phases. The average splitting delay time over the entire region was found to be 1.14 ± 0.42 s for the SK(K)S wave and 1.36 ± 0.26 s for the direct S wave. In most parts of the study area, the fast polarization directions for both shear phases are consistent and show a uniform NE–SW direction with an average of 36° and 37° for SK(K)S and S wave-derived results, respectively. This direction is in close agreement with the direction of the absolute plate motion vector in NW Iran (N39°E). The fast directions are associated with neither the surface geological trends, nor the geodetic strain fields. We propose that the observed anisotropy is mainly controlled by the LPO fabric developed due to the shearing of the asthenospheric layer in response to the motion of the lithosphere relative to the deeper mantle. Only in a narrow region near the tectonic boundaries of central Iran with NW Iran and the Alborz, NW–SE oriented SK(K)S fast directions tend to align with the major geological structures. Fast directions obtained from direct S wave indicate significantly smoother variations in the same regions and mostly continue to be aligned in the NE–SW direction. We attribute these differences to the change in the structure of the lithosphere in the tectonic boundary zone. The western margins of central Iran possess a strong deformational fabric as evidenced by the major active strike-slip zones there. Considering that the depth extent of this fabric expands over a relatively narrow zone in the mantle, it can locally influence the SK(K)S phases. The direct S waves, on the other hand, have a larger footprint and therefore average over a larger region, and relative to the SK(K)S phases, are influenced more strongly by the asthenospheric fabric due to their larger angles of incidence, which results in a larger zone of influence for station average anisotropy parameters.

Multi-stage magmatic activity in the Neotethyan ophiolite of Penjween: Reply on comments on “Generation and exhumation of granitoid intrusions in the Penjween ophiolite complex, NW Zagros of the Kurdistan region of Iraq: Implications for the geodynamic evolution of the Arabia-Eurasia collision zone” by Mohammad et al.

Multi-stage magmatic activity in the Neotethyan ophiolite of Penjween: Reply on comments on “Generation and exhumation of granitoid intrusions in the Penjween ophiolite complex, NW Zagros of the Kurdistan region of Iraq: Implications for the geodynamic evolution of the Arabia-Eurasia collision zone” by Mohammad et al.