Seismic imaging of the lithospheric structure of the Zagros mountain belt (Iran)

International audienceWe present a synthesis and a comparison of the results of two temporary passive seismic experiments installed for a few months across Central and Northern Zagros. The receiver function analysis of teleseismic earthquake records gives a high-resolution image of the Moho beneath the seismic transects. On both cross-sections, the crust has an average thickness of 43±2 km beneath the Zagros fold-and-thrust belt and the Central domain. The crust is thicker beneath the hanging wall of the Main Zagros Reverse Fault (MZRF), with a larger maximum Moho depth in Central (69±2 km) than in Northern Zagros (56±2 km). The thickening affects a narrower region (170 km) beneath the Sanandaj-Sirjan zone of Central Zagros and a wider region (320 km) in Northern Zagros. We propose that this thickening is related to overthrusting of the crust of the Arabian margin by the crust of Central Iran along the MZRF considered as a major thrust fault cross-cutting the whole crust. The fault is imaged as a lowvelocity layer in the receiver function data of the Northern Zagros profile. Moreover, the crustal-scale thrust model reconciles the imaged seismic Moho with the Bouguer anomaly data measured on the Central Zagros transect. At upper mantle depth, P-wave tomography confirms the previously observed strong contrast between the faster velocities of the Arabian margin and the lower velocities of the Iranian micro-blocks. Our higher-resolution tomography combined with surface-wave analysis locates the suture in the shallow mantle of the Sanandaj-Sirjan zone beneath Central Zagros. The Arabian upper-mantle has shield-like shear-wave velocities, while the lower velocities of the Iranian upper mantle are likely due to higher temperature. But these velocities are not low enough and the low-velocity layer not thick enough to conclude on a delamination of the lithospheric mantle lid beneath Iran. The lack of a high-velocity anomaly in the mantle beneath Central Iran suggests that the Neotethyan oceanic lithosphere is probably detached from the Arabian margin

Testing observables for teleseismic shear-wave splitting inversions: ambiguities of intensities, parameters, and waveforms

We assess the capabilities of different observables for the inversion of core-refracted shear waves (XKS phases) to uniquely resolve the anisotropic structure of the upper mantle. For this purpose, we perform full-waveform calculations for relatively simple, canonical models of upper-mantle anisotropy. The models are characterized by two and four domains of different anisotropic properties. Specifically, we assume hexagonal symmetry with arbitrarily chosen strength of the anisotropy and orientation of the horizontal fast axis. XKS waveforms, generated from plane-wave initial conditions, traverse through anisotropic models and are recorded at the surface by a single station (in case of vertical variations) and by a dense station profile across the laterally and vertically varying structure. In addition to waveforms, we consider the effects of anisotropic variations on apparent splitting parameters and splitting intensity. The results show that, generally, it is not possible to fully resolve the anisotropic parameters of a given model, even if complete waveforms (under noisefree conditions and for the complete azimuthal range) are considered. This is because waveforms for significantly different anisotropic models can be indistinguishable. However, inversions of both waveforms and apparent splitting parameters lead to similar models that exhibit systematic variations of anisotropic parameters. These characteristics may be exploited to better constrain the inversions. The results also show that splitting intensity holds some significant drawbacks: First, even from measurements over a wide range of back-azimuth, there is no characteristic signature that would indicate depth variations of anisotropy. Secondly, identical azimuthal variations of splitting intensity for different anisotropic structures do not imply that the corresponding split waveforms are also similar. Thus, fitting of observed and calculated splitting intensities could lead to anisotropic models that are incompatible with the observed waveforms. We conclude that (bandlimited) XKS-splitting inversions and related tomographic schemes, even if based on complete waveforms, are not sufficient to fully resolve the heterogeneous anisotropic structures of the upper mantle and that combinations with alternative methods, based on e.g., receiver-function splitting, P-wave travel-time deviations, or surface waves, are required

Crustal and uppermost mantle shear-wave velocity structure beneath the Middle East from surface-wave tomography

We have constructed a 3-D shear-wave velocity (Vs) model for the crust and uppermost mantle beneath the Middle East using Rayleigh wave records obtained from ambient-noise cross-correlations and regional earthquakes. We combined one decade of data collected from 852 permanent and temporary broadband stations in the region to calculate group-velocity dispersion curves. A compilation of > 54000 ray paths provides reliable group-velocity measurements for periods between 2 and 150 s. Path-averaged group velocities calculated at different periods were inverted for 2-D group-velocity maps. To overcome the problem of heterogeneous ray coverage, we used an adaptive grid parametrization for the group-velocity tomographic inversion. We then sample the period-dependent group-velocity field at each cell of a predefined grid to generate 1-D group-velocity dispersion curves, which are subsequently inverted for 1-D Vs models beneath each cell and combined to approximate the 3-D Vs structure of the area. The Vs model shows low velocities at shallow depths (5–10 km) beneath the Mesopotamian foredeep, South Caspian Basin, eastern Mediterranean and the Black Sea, in coincidence with deep sedimentary basins. Shallow high-velocity anomalies are observed in regions such as the Arabian Shield, Anatolian Plateau and Central Iran, which are dominated by widespread magmatic exposures. In the 10–20 km depth range, we find evidence for a band of high velocities (> 4.0 km/s) along the southern Red Sea and Arabian Shield, indicating the presence of upper mantle rocks. Our 3-D velocity model exhibits high velocities in the depth range of 30–50 km beneath western Arabia, eastern Mediterranean, Central Iranian Block, South Caspian Basin and the Black Sea, possibly indicating a relatively thin crust. In contrast, the Zagros mountain range, the Sanandaj-Sirjan metamorphic zone in western central Iran, the easternmost Anatolian plateau and Lesser Caucasus are characterized by low velocities at these depths. Some of these anomalies may be related to thick crustal roots that support the high topography of these regions. In the upper mantle depth range, high-velocity anomalies are obtained beneath the Arabian Platform, southern Zagros, Persian Gulf and the eastern Mediterranean, in contrast to low velocities beneath the Red Sea, Arabian Shield, Afar depression, eastern Turkey and Lut Block in eastern Iran. Our Vs model may be used as a new reference crustal model for the Middle East in a broad range of future studies

Mantle-flow diversion beneath the Iranian plateau induced by Zagros' lithospheric keel.

Funder: German Research Foundation (DFG)Funder: Projekt DEALPrevious investigation of seismic anisotropy indicates the presence of a simple mantle flow regime beneath the Turkish-Anatolian Plateau and Arabian Plate. Numerical modeling suggests that this simple flow is a component of a large-scale global mantle flow associated with the African superplume, which plays a key role in the geodynamic framework of the Arabia-Eurasia continental collision zone. However, the extent and impact of the flow pattern farther east beneath the Iranian Plateau and Zagros remains unclear. While the relatively smoothly varying lithospheric thickness beneath the Anatolian Plateau and Arabian Plate allows progress of the simple mantle flow, the variable lithospheric thickness across the Iranian Plateau is expected to impose additional boundary conditions on the mantle flow field. In this study, for the first time, we use an unprecedented data set of seismic waveforms from a network of 245 seismic stations to examine the mantle flow pattern and lithospheric deformation over the entire region of the Iranian Plateau and Zagros by investigation of seismic anisotropy. We also examine the correlation between the pattern of seismic anisotropy, plate motion using GPS velocities and surface strain fields. Our study reveals a complex pattern of seismic anisotropy that implies a similarly complex mantle flow field. The pattern of seismic anisotropy suggests that the regional simple mantle flow beneath the Arabian Platform and eastern Turkey deflects as a circular flow around the thick Zagros lithosphere. This circular flow merges into a toroidal component beneath the NW Zagros that is likely an indicator of a lateral discontinuity in the lithosphere. Our examination also suggests that the main lithospheric deformation in the Zagros occurs as an axial shortening across the belt, whereas in the eastern Alborz and Kopeh-Dagh a belt-parallel horizontal lithospheric deformation plays a major role

Mantle-flow diversion beneath the Iranian plateau induced by Zagros’ lithospheric keel

Previous investigation of seismic anisotropy indicates the presence of a simple mantle flow regime beneath the Turkish-Anatolian Plateau and Arabian Plate. Numerical modeling suggests that this simple flow is a component of a large-scale global mantle flow associated with the African superplume, which plays a key role in the geodynamic framework of the Arabia-Eurasia continental collision zone. However, the extent and impact of the flow pattern farther east beneath the Iranian Plateau and Zagros remains unclear. While the relatively smoothly varying lithospheric thickness beneath the Anatolian Plateau and Arabian Plate allows progress of the simple mantle flow, the variable lithospheric thickness across the Iranian Plateau is expected to impose additional boundary conditions on the mantle flow field. In this study, for the first time, we use an unprecedented data set of seismic waveforms from a network of 245 seismic stations to examine the mantle flow pattern and lithospheric deformation over the entire region of the Iranian Plateau and Zagros by investigation of seismic anisotropy. We also examine the correlation between the pattern of seismic anisotropy, plate motion using GPS velocities and surface strain fields. Our study reveals a complex pattern of seismic anisotropy that implies a similarly complex mantle flow field. The pattern of seismic anisotropy suggests that the regional simple mantle flow beneath the Arabian Platform and eastern Turkey deflects as a circular flow around the thick Zagros lithosphere. This circular flow merges into a toroidal component beneath the NW Zagros that is likely an indicator of a lateral discontinuity in the lithosphere. Our examination also suggests that the main lithospheric deformation in the Zagros occurs as an axial shortening across the belt, whereas in the eastern Alborz and Kopeh-Dagh a belt-parallel horizontal lithospheric deformation plays a major role

Seismological evidence for crustal-scale thrusting in the Zagros mountain belt (Iran)

International audienceCrustal receiver functions computed from the records of 45 temporary seismological stations installed on a 620-km long profile across central Zagros provide the first direct evidence for crustal thickening in this mountain belt. Due to a rather short 14-km average station spacing, the migrated section computed from radial receiver functions displays the Moho depth variations across the belt with good spatial resolution. From the coast of the Persian Gulf to 25 km southwest of the Main Zagros Thrust (MZT), the Moho is almost horizontal with slight depth variations around 45 km. Crustal thickness then increases abruptly to a maximum of ~70 km beneath the Sanandaj-Sirjan metamorphic zone, between 50 and 90 km northeast of the surface exposure of the MZT. Further northeast, the Moho depth decreases to ~42 km beneath the Urumieh-Dokhtar magmatic assemblage and the southern part of the Central Iranian micro-continent. The region of thickest crust is located ~75 km to the northeast of the Bouguer anomaly low at –220 mgals. Gravity modelling shows that the measured Moho depth variations can be reconciled with gravity observations by assuming that the crust of Zagros underthrusts the crust of central Iran along the MZT considered as a crustal-scale structure. This hypothesis is compatible with shortening estimates by balanced cross-sections of the Zagros folded belt, as well as with structural and petrological studies of the metamorphic Sanandaj-Sirjan zone

La chaîn de collision continentale du Zagros (Iran): Structure lithosphérique par analyse de données sismologique

The Zagros mountain belt situated on the northern margin of the Arabian plate, is one of the youngest belts of continental collision. This belt was built by the collision of the Arabian plate with the central Iranian micro-continent. A seismological experiment, called “ Zagros 2000-2001 ”, was realized by collaboration between LGIT and IIEES to study the lithospheric structure beneath this collision belt and some part of the central Iranian block. We used the data obtained during this experiment to characterise the structure of the crust and the lithospheric mantle under the stations network. The analysis of receiver functions was used to investigate the variations of the crustal thickness under the network. By this analysis, we have found a thickening by 20 km over a region of about 100 km width just on the northeast of the MZT (“ Main Zagros Thrust “). The average crustal thickness of 45 and 40 km were found respectively for the Zagros and central Iran. We've then proposed a crustal model in which the Moho was constrained using the results obtained by the receiver functions analysis. This model was tried to be compatible with gravimetric data. The thickening of the crust on NE of the MZT was interpreted to be related to overthrusting of the central Iranian crust onto the Zagros crust. We have also characterised the upper mantle structure to depth of 350 km by inversion of more than 5000 arrival times of teleseismic P waves. The results of this inversion show a fast upper mantle beneath the Zagros and a slow one beneath central Iran. We may relate the presence of a slow upper mantle beneath central Iran to delamination of the lithospheric mantle. In excess, the presence of a slow and light mantle beneath central Iran may explain the relatively high elevation of the Iranian plateau. The analysis of splitting of teleseismic S-waves show that the lithosphere of the Zagros and central Iran are different in terms of seismic anisotropy. This analysis underlined the absence of splitting beneath the Zagros as opposed to some regions in central Iran where we've observed an average splitting of more than 1 sec. On the other hand, we didn't observe any clear relation between the direction of the fast axes of the observed splitting and the direction of relative or absolute plate motions. The observed splitting should be produced by some “ fossil anisotropy ” into the lithosphere of central Iran. This anisotropy may be related to a tectonic episode before the continental collision between the two plates.La chaîne du Zagros située sur la marge septentrionale de la plaque Arabie, est l'une des plus jeunes chaînes de collision continentale. Elle a été structurée par la collision de la plaque Arabie avec le microcontinent d'Iran central. Une expérience sismologique, appelée « Zagros 2000-2001 », a été réalisée dans le cadre d'une collaboration entre le LGIT et l'IIEES pour étudier la structure lithosphérique sous cette chaîne de collision et une partie du bloc d'Iran Central. Le jeu de données de cette expérience nous a permis de caractériser la structure de la croûte et du manteau lithosphérique sous le réseau de stations. Les variations de l'épaisseur de croûte ont été mises en évidence par analyse en fonctions récepteur. Elles sont caractérisées par un sur-épaississement maximum de 20 km sur une largeur d'environ 100 km immédiatement au nord-est du MZT (« Main Zagros Thrust »). Une épaisseur moyenne de croûte de 45 km a été trouvée sous le Zagros et de 40 km sous l'Iran Central. Nous avons ensuite proposé un modèle de croûte, contraint par la géométrie du Moho tirée de l'analyse en fonctions récepteur, qui est aussi compatible avec les données gravimétriques. Le sur-épaississement est interprété comme lié à un redoublement crustal avec chevauchement de la croûte d'Iran central sur celle du Zagros le long du MZT. L'inversion de plus de 5000 temps d'arrivée P télésismiques nous a permis de caractériser la structure du manteau supérieur jusqu'à 350 km de profondeur. Les résultats de cette inversion montrent un manteau supérieur rapide sous le Zagros et lent sous l'Iran central. Ceci peut être lié à une délamination du manteau lithosphérique sous l'Iran central. La présence d'un manteau lent et léger sous l'Iran central peut expliquer la haute altitude moyenne du plateau iranien. L'analyse de la biréfringence des ondes S télésismiques montre une différence majeure entre la lithosphère du Zagros et celle de l'Iran Central en terme d'anisotropie sismique. Cette analyse met en évidence l'absence de biréfringence des ondes S télésismiques sous le Zagros par opposition à certaines régions d'Iran Central. D'autre part, aucun lien n'est observé entre la direction de l'axe rapide de la biréfringence observée en Iran central et le déplacement actuel relatif ou absolu des plaques. La biréfringence observée doit donc avoir son origine dans une anisotropie gelée dans la lithosphère du bloc d'Iran central liée à un épisode tectonique plus ancien que la collision continentale entre les deux plaques

Mantle Anisotropy in NW Namibia From XKS Splitting: Effects of Asthenospheric Flow, Lithospheric Structures, and Magmatic Underplating

The presence of the Etendeka flood basalts in northwestern Namibia is taken as evidence for the activity of the Tristan da Cunha mantle plume during the continental breakup between Africa and South America. We investigate seismic anisotropy beneath NW Namibia by splitting analysis of core‐refracted teleseismic shear waves (XKS phases) to probe mantle flow and lithospheric deformation related to the tectonic history of the region. We present the results of the joint splitting analysis of XKS data collected from 34 onshore stations and 12 ocean‐bottom seismometers. The fast polarization directions (FPDs) are consistent with a model that combines the effects of lithospheric deformation and large‐scale mantle flow due to the NE motion of the African plate. The dominantly NNW‐SSE‐oriented FPDs in the northern part are likely caused by shallow lithospheric structures. Our observations do not show any strong evidence of a pervasive effect of the Tristan da Cunha mantle plume.Plain Language Summary: The geology of Northwest Namibia is characterized by the presence of flood basalts, originating from magma sourced in the Earth's mantle. The source magma of these flood basalts was produced during the passage of the African plate over a mantle plume, more than 80 million years ago, contemporaneous with the onset of the breakup of the South American plate from the African plate. The role of the mantle plume in the continental breakup can be examined by a seismological technique named shear wave splitting analysis. The mantle flow induces direction‐dependent physical properties, that is, seismic anisotropy, which causes a shear wave to split into two different components traveling at different speeds. The leading component is polarized in a direction representing the direction of the flow in the earth's mantle. Except for the northern part, the polarization direction of the fast shear wave is consistent with the model of mantle flow caused by the NE motion of the African plate and deformations in the lithosphere. The results of our study do not show any direct evidence for the direct impact of the mantle plume on the mantle beneath our region of study.Key Points: Upper mantle anisotropy beneath NW Namibia is a combined effect of the present‐day motion of the African Plate and lithospheric structures. No significant direct effect of the Tristan da Cunha mantle plume is observed in shear wave splitting measurements. Localized shearing in the lithosphere and crustal underplating are likely the main causes of the lateral variations in seismic anisotropy.https://doi.org/10.14470/KP6443475642https://doi.org/10.14470/1N134371https://doi.org/10.7914/SN/IUhttps://doi.org/10.18715/SKS_SPLITTING_DATABASEhttps://www.geophysik.uni-frankfurt.de/64002762/Softwar

La chaîne de collision continentale du Zagros (Iran) (structure lithosphérique par analyse de données sismologiques)

La chaîne du Zagros située sur la marge septentrionale de la plaque Arabie, est l'une des plus jeunes chaînes de collision continentale. Elle a été structurée par la collision de la plaque Arabie avec le microcontinent d'Iran central. Une expérience sismologique, appelée "Zagros 2000-2001", a été réalisée dans le cadre d'une collaboration entre le LGIT et l'IIEES pour étudier la structure lithosphérique sous structure de la croûte et du manteau lithosphérique sous le réseau de stations. Les variations de l'épaisseur de croûte ont été mises en évidence par analyse en fonctions récepteur. Elles sont caractériśées par un sur épaississement maximum de 20 km sur une largeur d'environ 100 km immédiatement au nord-est du MZT ("Main Zagros Thrust"). Une épaisseur moyenne de croûte de 45 km a été trouvée sous le zagros et de 40 km sous l'Iran central. Nous avons ensuite proposé un modèle de croûte contraint par la géométrie du Moho tirée de l'analyse en fonctions récepteur, qui est aussi compatible avec les données gravimétriques. Le sur-épaississement est interprété comme lié à un redoublement crustal avec chevauchement de la croûte d'Iran central sur celle du Zagros le long du MZT. L'inversion de plus de 5000 temps d'arrivée P télésismsiques nous a permis de caractériser la structure du manteau supérieur jusqu'à 350 km de profondeur. Les résultats de cette inversion montrent un manteau supérieur rapide sous le Zagros et lent sous l'Iran central. Ceci peut être lié à une délamination du manteau lithosphérique sous l'Iran central.GRENOBLE1-BU Sciences (384212103) / SudocSTRASBOURG-EOST (674822249) / SudocSudocFranceF