Kopeh Dagh Mountains Research Articles

3-D thermochemical structure of lithospheric mantle beneath the Iranian plateau and surrounding areas from geophysical–petrological modelling

SUMMARY While the crustal structure across the Iranian plateau is fairly well constrained from controlled source and passive seismic data, the lithospheric mantle structure remains relatively poorly known, in particular in terms of lithology. Geodynamics rely on a robust image of the present-day thermochemical structure interpretations of the area. In this study, the 3-D crustal and upper mantle structure of the Iranian plateau is investigated, for the first time, through integrated geophysical–petrological modelling combining elevation, gravity and gravity gradient fields, seismic and petrological data. Our modelling approach allows us to simultaneously match complementary data sets with key mantle physical parameters (density and seismic velocities) being determined within a self-consistent thermodynamic framework. We first elaborate a new 3-D isostatically balanced crustal model constrained by available controlled source and passive seismic data, as well as complementary by gravity data. Next, we follow a progressively complex modelling strategy, starting from a laterally quasi chemically homogeneous model and then including structural, petrological and seismic tomography constraints. Distinct mantle compositions are tested in each of the tectonothermal terranes in our study region based on available local xenolith suites and global petrological data sets. Our preferred model matches the input geophysical observables (gravity field and elevation), includes local xenolith data, and qualitatively matches velocity anomalies from state of the art seismic tomography models. Beneath the Caspian and Oman seas (offshore areas) our model is defined by an average Phanerozoic fertile composition. The Arabian Plate and the Turan platform are characterized by a Proterozoic composition based on xenolith samples from eastern Arabia. In agreement with previous studies, our results also suggest a moderately refractory Proterozoic type composition in Zagros-Makran belt, extending to Alborz, Turan and Kopeh-Dagh terranes. In contrast, the mantle in our preferred model in Central Iran is defined by a fertile composition derived from a xenolith suite in northeast Iran. Our results indicate that the deepest Moho boundary is located beneath the high Zagros Mountains (∼65 km). The thinnest crust is found in the Oman Sea, Central Iran (Lut Block) and Talesh Mountains. A relatively deep Moho boundary is modelled in the Kopeh-Dagh Mountains, where Moho depth reaches to ∼55 km. The lithosphere is ∼280 km thick beneath the Persian Gulf (Arabian–Eurasian Plate boundary) and the Caspian Sea, thinning towards the Turan platform and the high Zagros. Beneath the Oman Sea, the base of the lithosphere is at ∼150 km depth, rising to ∼120 km beneath Central Iran, with the thinnest lithosphere (<100 km) being located beneath the northwest part of the Iranian plateau. We propose that the present-day lithosphere–asthenosphere topography is the result of the superposition of different geodynamic processes: (i) Arabia–Eurasia convergence lasting from mid Jurassic to recent and closure of Neo-Tethys ocean, (ii) reunification of Gondwanian fragments to form the Central Iran block and Iranian microcontinent, (iii) impingement of a small-scale convection and slab break-off beneath Central Iran commencing in the mid Eocene and (iv) refertilization of the lithospheric mantle beneath the Iranian microcontinent.

The role of active strike-slip faults and opposite vertical axis rotations in accommodating eurasia-arabia shortening in central iran

In Central Iran, most of the Late Cenozoic to Present shortening has been accomplished in the Zagros, in the Alborz, and Kopeh-Dagh mountain ranges. Between these active mountain belts, a minor amount of shortening is observed and the deformation style is characterized by active strike-slip faults that bound relatively aseismic blocks. Two main strike-slip fault systems are presently active in different regions of Central Iran: South of latitude 34°N, active deformation is accommodated on ∼N–S oriented right-lateral strike-slip faults, and north of latitude 34°N, E–W left-lateral strike slip faults prevail.In this work, we investigated two different structures, representative of the two different strike-slip systems occurring in Central Iran: the Yazd fold system located in an area dominated by the occurrence of NNW-SSE oriented right-lateral strike slip faults, and the Ferdows fold system that developed at the western termination of the E–W left-lateral strike-slip Dasht-e-Bayaz fault. Paleomagnetic results show opposite vertical-axis rotations related to the different orientation and sense of movement of strike-slip fault systems, suggesting that in Central Iran the N–S oriented right-lateral and E–W oriented left-lateral strike-slip faults play significant roles in accommodating the Arabia-Eurasia convergence, by rotating counterclockwise and clockwise in the horizontal plane, respectively.

Clockwise paleomagnetic rotations in northeastern Iran: Major implications on recent geodynamic evolution of outer sectors of the Arabia-Eurasia collision zone

In this study, an extensive paleomagnetic sampling (70 sites) was carried out in north-eastern Iran with the aim of reconstructing the rotation history of the outer margin of the Eurasia-Arabia collision area represented by the Ala-Dagh, Binalud and Kopeh-Dagh mountain belts. We sampled the red beds units from the Lower Cretaceous Shurijeh Fm. and from the Middle-Upper Miocene Upper Red Fm (URF). Paleomagnetic results from all the sampled areas show a homogeneous amount of CW rotations measured in the above-mentioned Formations. These paleomagnetic results suggest that the oroclinal bending process that caused the curvature of Alborz mountain belt in north Iran after the Middle-Late Miocene, also extended to the Ala-Dagh, Binalud and Kopeh-Dagh mountain belts, at the north-eastern border of the Arabia-Eurasia deforming zone.Based on our paleomagnetic results and on GPS, seismological, geomorphological and structural data available in the area, a hypothesis of tectonic evolution of the northern Iran-South Caspian Basin area, from Middle-Late Miocene to Present, is here proposed. In this model, the initiation of the oroclinal bending processes in northern Iran occurred about 6–4 myr ago, related to the impinging of North Iran between the South Caspian block and the southern margin of the Turan platform, driven by the northward subduction of the South Caspian basement under the Aspheron-Balkhan Sill. As paleomagnetic results from this study show a pattern of vertical axis rotations that is inconsistent with the present-day kinematics of the northern Iranian blocks as described by seismicity and GPS data, we suggest that the tectonic processes responsible for the bending of northern Iran mountain chains are no longer active and that the westward motion of the South Caspian basin, and therefore the initiation of opposite strike-slip motion along the Ashk-Abad and Shahrud faults, occurred very recently (∼2 My ago). We therefore propose that initiation of the northward subduction of the South Caspian basin below the Apsheron-Balkhan Sill and the westward extrusion of the South Caspian block did not occur at the same time, with the former occurring between the late Miocene and the Pliocene, and the latter during the Pleistocene.

Coseismic displacement of the 5 April 2017 Mashhad earthquake (Mw 6.1) in NE Iran through Sentinel-1A TOPS data: New implications for the strain partitioning in the southern Binalud Mountains

The outward expansion of the northeastern Iranian Plateau is mainly accommodated by a series of widely distributed range-normal thrust and range-parallel strike-slip faults. However, the role of the strike-slip faults in this region's strain-partitioning kinematics remains unclear. The occurrence of the 5 April 2017 Mw 6.1 Mashhad earthquake provides us a rare opportunity to study this topic because this earthquake struck the southern Binalud Mountains, which are experiencing an oblique convergence between the northward-moving Lut Block and the NW-striking Binalud and Kopeh Dagh mountain ranges. We process two paired ascending and descending Sentinel-1A radar-image observations to determine the relative contributions from the dip and strike-dip subcomponents in such an oblique collisional domain. The interferograms along the line-of-sight direction and their converted horizontal and vertical displacements indicate that a thrust with a right-lateral strike-slip fault controlled the rupture process of the Mashhad earthquake, with the maximum dextral shearing and vertical displacements reaching 19.5 cm and 14 cm, respectively. Coseismic surface deformation measurements are also used to estimate the fault geometry and invert the slip distribution along the underlying seismogenic fault. Our best-fit faulting model suggests that the coseismic rupture occurred on a fault plane with a dip angle of 37.5° and strike angle of 324°. The strike-slip subcomponent was more significant than the dip-slip, approaching ∼95 cm, and the dip-slip varied from 10 cm to 47 cm. The seismic-moment release of our preferred fault model is 1.71 × 1018 Nm, equivalent to a Mw 6.16 earthquake event. We also use the preferred fault model to calculate the Coulomb failure stress (CFS) change at the nearby receiver faults. Combined with an analysis of historical earthquakes and the consistent dextral river deflections along the strike-slip fault systems of the southern Binalud and Kopeh Dagh Mountains, we speculate that the seismogenic structure that triggered the Mashhad earthquake should have been one strand fault of the NW–SE-striking Kashafrud fault system. The strike-slip faults in NE Iran play an important role in accommodating the lateral transport of crustal material from the convergence front of the Lut Block and Binalud fragments and providing commonly distributed anticlockwise rotation around a vertical axis.

Discrimination of sand dunes and loess deposits using grain-size analysis in northeastern Iran

Identification and characterization of aeolian deposits in arid environments provide information on mechanisms of loess and sand accumulation. The objectives of this study were to (i) identify the distribution of aeolian deposits, (ii) discriminate loess and sand deposits using granulometric data, and (iii) describe the aeolian deposition in Sarakhs area, northeastern Iran. Particle size distributions of 26 surface samples were determined using a laser grain-size analyzer. Fine sand, very fine sand, and very coarse silt were dominant fractions in studied sediments, and the sum of these fractions ranged from 46.9% in loess deposits to 93.8% in sand dunes. The mean grain size (M z ) of sand dunes ranged from 3.31 to 3.54 ϕ, which gradually changed to 4.09 to 5.50 ϕ in loess deposits. Sorting, skewness, and kurtosis ranged from 0.84 to 1.94 ϕ, 0.18 to 0.49, and 0.76 to 2.38, respectively. Aeolian deposits in the area resulted in the incorporation of Hariroud River system and Kopeh Dagh Mountains for aeolian particle production and accumulation. Alluvial comminution in Hariroud River is suggested the main mechanism of sand and silt production and flood plain environment the main reservoir of these particles. The mountains of Kopeh Dagh act as a barrier and play a key role for sand and loess accumulation.

Lithospheric structure of the Eastern Iranian plateau from integrated geophysical modeling: A transect from Makran to the Turan platform

We present a 2D profile of density and temperature distribution in the lithosphere across Iran along a more than 1600km long profile extending from the Oman Gulf in the South to the Kopeh-Dagh and the Turan platform in the North. Gravity, geoid, topography and surface heat flow data were used for modeling, assuming local isostatic equilibrium. As much as possible, crustal structure has been constrained by seismic data. Crustal thickening is found under the Taftan-Bazman volcanic-arc (up to 47km), under the Binalud foreland (∼54km) and beneath the Kopeh-Dagh mountains (up to 50km) whereas thin crust has been obtained under the Oman Gulf (20km). Moho depth under the Lut block and the Turan platform is about 40km. The lithospheric thickness is ∼90km under the Oman Gulf and increases slightly until the Jazmourian depression. Then the lithospheric-asthenospheric boundary (LAB) bends significantly and sinks to ∼260km under the Taftan-Bazman volcanic-arc. The LAB depth is about 190km beneath the Lut block. A slight increase of LAB depth under the Binalud foreland towards the North may indicate a suture zone. Under the Turan platform, the LAB depth reaches ∼210km. We also modeled two possible positions of the deep suture zone in NE Iran (along the main Kopeh-Dagh fault or along the Atrak River) and concluded that, when the suture zone is along the Atrak River, we obtained the better fit between calculated and measured data.

Crustal Thickness of Iran Inferred from Converted Waves

The Iranian plate is part of the Alpine-Himalayan orogenic belt, which has been formed by the continental collision between the Arabian and Eurasian plates. The present-day Iranian plate is characterized by diverse tectonic domains including mountain belts (e.g. Zagros and Alborz, Kopeh-Dagh) and oceanic plate subduction (e.g. Makran). Here we present the lateral variations of the Moho discontinuity beneath Iran using a detailed P receiver function study. Our results allow for more precise estimations of the crustal thickness and enable us to provide a detailed Moho depth map for all of Iran for the first time. We used the teleseismic events recorded from 1995 to 2011 at 77 national permanent stations (24 broadband and 53 short period stations). Our results show significant variations in the crustal thickness, which are related to the different geological features within Iran. In general, the average crustal thickness beneath Iran is about 40–45 km. A relatively thick crust of about 54 ± 2 km due to the shortening is observed beneath the Alborz mountain ranges. The crust beneath the Alborz zone shows a thickness changing from 47 ± 2 to 45 ± 2 km from west to east and reaches a thickness of about 50 ± 2 km beneath the Kopeh-Dagh mountain range. We find the thinnest crust of about 33 ± 2 km beneath the Makran subduction zone in southeast Iran showing a normal continental crust, which has not been influenced by collisional processes. The thickest crust (~66 ± 2 km) is locally observed beneath the Sanandaj-Sirjan Zone, which is considered the suture zone of the collision between the Arabian and Eurasian plates.

The deep structure of the Iranian Plateau

High resolution structures of the lithosphere–asthenosphere system beneath a seismic profile in Iran are obtained by the simultaneous inversion of data from receiver functions and fundamental mode Rayleigh wave group velocity and validated by modeling Bouguer gravity anomaly data. The seismic data are gathered over a profile extending across Zagros, Sanandaj–Sirjan Zone (SSZ), Urumieh–Dokhtar magmatic arc (UDMA), Central Iran, Alborz–Binalud Mountain ranges and Kopeh Dagh Mountain ranges. The results confirm the presence of crustal roots at the north and south of Iranian Plateau where it meets the Arabian Plate and Eurasia. The high velocity lithosphere of the Arabian Plate gently plunges NNE-ward beneath Central Iran supporting the subduction of the continental lithosphere responsible for the seismicity of the area. The crust and lithosphere are thinner beneath Central Iran, where two low velocity structures are very likely related to magma sources of the UDMA and in east of Iran, around Lut block, where the volcanism shows calcalkaline subduction-related geochemistry. The crustal-lithospheric root to the north of the Iranian Plateau may represent the relict of a previous “cimmeric” subduction zone. Therefore the Iranian lithosphere–asthenosphere system could be the result of the coalescence of two separate subduction zones.

A detailed receiver function image of the lithosphere beneath the Kopeh-Dagh (Northeast Iran)

The Kopeh-Dagh fold belts are among the most seismically active areas in Northeastern Iran, which build the northern part of the Alpine–Himalayan orogen in western Asia. They act as the abrupt northeastern limit to active deformation in Iran. We perform a combined P and S receiver function analysis to detect the major discontinuities within the lithosphere beneath Northeast Iran. Our results obtained from 12 short period and broadband seismological stations significantly map the lateral variations of the Moho boundary. Based on P receiver functions, we show that the Moho depth varies from ∼43 km beneath the southern Kopeh-Dagh foreland basin to ∼49 km below the northern part of the basin. S receiver functions reliably reveal an average Moho depth of ∼50–55 km beneath the Kopeh-Dagh mountain range showing the regional shortening in response to the collision of Arabia with Eurasia. Furthermore, we observe clear conversions with negative polarity at ∼8.5–9.5 s in S receiver functions, which could be related to the conversion at the lithosphere–asthenosphere boundary. This may show a relatively thin continental lithosphere of about 85–95 km beneath the Kopeh-Dagh implying that the lithosphere was influenced by geodynamical reworking processes in the past.

High resolution image of uppermost mantle beneath NE Iran continental collision zone

We invert 3775 relative P wave arrival times using the ACH damped least square method of Aki et al. (1977) to study upper mantle structure beneath the NE Iran continental collision zone. The data for this study were recorded by 17 three component broad-band stations operated from August 2006 to February 2008 along a profile from the center of Iranian Plateau, near Yazd, to the northeastern part of Iran on the Turan Platform just north of the Kopeh Dagh Mountains. The results confirm the previously known low velocity upper mantle beneath Central Iran. Our tomographic model reveals a deep high velocity anomaly. The surficial expressions of this anomaly are between the Ashkabad and Doruneh Faults, where the resolution and ray coverage are good. A transition zone in uppermost mantle is recognized under the Binalud foreland that we interpreted as suture zone between Iran and Turan platform. Our results indicate that Atrak Valley which is the boundary between the Binalud and Kopeh Dagh Mountains can be considered as the northeastern suture of the Iranian Plateau where Eurasia and Turan Platform under-thrust beneath the Binalud range and Central Iran.

Plio–Quaternary tectonic regime changes in the transition zone between Alborz and Kopeh Dagh mountain ranges (NE Iran)

This paper is concerned with the changes in the Plio–Quaternary stress states in NE Iran and provides evidence for regional active deformation across a wide transition zone between the Alborz and Kopeh Dagh mountains. The Arabia–Eurasia convergence is involved in the strike–slip faulting along NE-trending left-lateral faults in the eastern Alborz as well as NW-trending right-lateral faults in the western Kopeh Dagh. The inversion result of fault kinematic data (slip-vector measurement of fault plane and focal mechanisms) strongly indicates an active transpressional tectonic regime in the study area. We provide evidence for drastic temporal changes in the stress state during the Plio–Quaternary based on the inversions of fault kinematic data. A regional transpressional tectonic regime with a mean N036 ± 20°E trending horizontal σ 1 is representative for the modern stress state. The older state reveals compressional to transpressional tectonic regimes comprising a N135 ± 20°E trending horizontal σ 1. The modern transpressional tectonic regime characterized by a regional NE-trending σ 1 is consistent with the inversion of earthquake focal mechanisms that yield a N47°E trending σ 1 for the present day stress state. The kinematic results are significantly homogeneous and mechanically compatible with the active geological structures, in the way that reverse and strike–slip faulting are coherent with a single σ 1 direction.

Oroclinal bending, distributed thrust and strike-slip faulting, and the accommodation of Arabia-Eurasia convergence in NE Iran since the Oligocene

Regional shortening is accommodated across NE Iran in response to the collision of Arabia with Eurasia. We examine how N–S shortening is achieved on major thrust systems bounding the eastern branch of the Alborz (east of 57°E), Sabzevar and Kuh-e-Sorkh mountain ranges, which lie south of the Kopeh Dagh mountains in NE Iran. Although these ranges have experienced relatively few large earthquakes over the last 50 yr, they have been subject to a number of devastating historical events at Neyshabur, Esfarayen and Sabzevar. A significant change in the tectonics of the eastern Alborz occurs directly south of the Central Kopeh Dagh, near 57°E. To the east, shortening occurs on major thrust faults which bound the southern margin of the range, resulting in significant crustal thickening, and forming peaks up to 3000 m high. Active shortening dies out eastward into Afghanistan, which is thought to belong to stable Eurasia. The rate of shortening across thrust faults bounding the south side of the eastern Alborz north of Neyshabur is determined using optically stimulated luminescence dating of displaced river deposits, and is likely to be 0.4–1.7 mm yr^(−1). Shortening across the Sabzevar range 150 km west of Neyshabur has previously been determined at 0.4–0.6 mm yr^(−1), although reassessment of the rate here suggests it may be as high as 1 mm yr^(−1). Migration of thrust faulting into foreland basins is common across NE Iran, especially in the Esfarayen region near 57°E, where the northward deflection of the East Alborz range reaches a maximum of 200 ± 20 km (from its presumed linear E–W strike at the beginning of the Oligocene). West of 57°E, the tectonics of the Alborz are affected by the westward motion of the South Caspian region, which results in the partitioning of shortening onto separate thrust and left-lateral strike-slip faults north and south of the range. At the longitude of 59°E, published GPS velocities indicate that 50 per cent of the overall shortening across NE Iran is accommodated in the Kopeh Dagh. The remaining 50 per cent regional shortening must therefore be accommodated south of the Kopeh Dagh, in the eastern Alborz and Kuh-e-Sorkh ranges. Assuming present day rates of slip and the fault kinematics are representative of the Late Cenozoic deformation in NE Iran, the total 200 ± 20 km N–S shortening across the eastern Alborz and Kopeh Dagh mountains since the beginning of uplift of the Kopeh Dagh basin would be accommodated in 30 ± 8 Ma. Although this extrapolation may be inappropriate over such a long timescale, the age is nevertheless consistent with geological estimates of post Early-to-Middle Oligocene (<30 Ma) for the onset of Kopeh Dagh uplift.

Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma: COMMENT

[Hollingsworth et al. (2008)][1] proposed a “new” kinematic model for the complicated tectonic pattern in the eastern South Caspian Sea region. Our Comment is focused on the Ashkhabad fault and the Kopeh Dagh Mountains, examining the data and hypothesis used by the authors to support their model

New tectonic configuration in NE Iran: Active strike‐slip faulting between the Kopeh Dagh and Binalud mountains

In this paper, a combined approach of geomorphic analyses, detailed geological mapping on SPOT 5 satellite images and field surveys, complemented by radiometric dating (40Ar/39Ar) allows proposing a new and consistent tectonic configuration for the northeastern Arabia‐Eurasia collision zone. It favors a simple strike‐slip faulting mechanism in NE Iran accommodating the northward motion of central Iran with respect to Eurasia between the Binalud and Kopeh Dagh mountains. Our results demonstrate that ∼2 mm/a (∼25%) of this northward motion has been transferred from the Binalud to the Kopeh Dagh through the Meshkan Transfer Zone since ∼5 Ma. The modern state of stress deduced from fault kinematics analyses shows N30°E trending maximum stress axis and confirms a dominant strike‐slip faulting mechanism. Combining our geological data with other available data, we conclude that kinematics of deformation can be appropriately described by rigid block faulting in NE Iran; instead, the continuous deformation as it has been thought during the last 3 decades.

Quaternary slip rates along the northeastern boundary of the Arabia-Eurasia collision zone (Kopeh Dagh Mountains, Northeast Iran)

The Kopeh Dagh is accommodating a large portion of the northward motion of central Iran with respect to Eurasia, involving a major right-lateral strike-slip fault system (Bakharden-Quchan). This fault system corresponds to the northeastern boundary of the Arabia-Eurasia collision, and can be considered as a lithospheric-scale tectonic feature. We present a well-constrained estimation of late Quaternary slip rates along two major strike-slip faults (the Baghan and Quchan faults) in this fault system, using in situ-produced 36Cl nuclide to date two offset alluvial fan surfaces. Combining detailed satellite image and digital topographic data analyses complemented with geomorphic fieldwork allows quantifying the cumulative offset values of 940 ± 100 and 360 ± 50 m of the fan surfaces along the Baghan and Quchan faults, respectively. A total of 12 carbonate boulders from the fan surfaces were collected and dated. This yields minimum age of two episodes of fan abandonment at 280 ± 16 (Baghan fault) and 83 ± 4 ka (Quchan fault). Age estimates and measured offsets of the fans are consistent with respective maximum long-term fault slip rates of 2.8 ± 1.0 and 4.3 ± 0.6 mm yr−1 for the Baghan and Quchan faults over the Middle-Late Pleistocene. Applying the slip rates to cumulative post-folding offsets along the Baghan and Quchan faults indicates that strike-slip motion within the Kopeh Dagh may have started ∼4 Ma. This constrains the timing of a major tectonic reorganization in the Kopeh Dagh, previously recorded through Arabia-Eurasia collision between 3 and 7 Ma. At the regional scale, the sum of total cumulative strike-slip offsets is about 35–40 km, which implies a total maximum slip rate of about 9 ± 2 mm yr−1 in the Central-Eastern Kopeh Dagh. This is resolved to average northward and westward slip rates of ∼8 and ∼4 mm yr−1, respectively, for the Western Kopeh Dagh with respect to Eurasia. Our results also suggests that the localized strike-slip faulting in the Central Kopeh Dagh can be considered as an intercontinental movement between northeast Iran and Eurasia, accommodating about 80 per cent of northward motion of central Iran with respect to Eurasia.

Strike-slip faulting, rotation, and along-strike elongation in the Kopeh Dagh mountains, NE Iran

SUMMARY The Kopeh Dagh is a linear mountain range separating the shortening in Iran from the stable, flat Turkmenistan platform. In its central part is an array of active right-lateral strike-slip faults that obliquely cut the range and produce offsets of several kilometres in the geomorphology and geological structure. They are responsible for major destructive earthquakes in the 19th and 20th centuries and represent an important seismic hazard for this now-populous region of NE Iran. These strike-slip faults all end in thrusts, revealed by the uplift and incision of Late Quaternary river terraces, and do not continue beyond the Atrak river valley, which forms the southern margin of the Kopeh Dagh. The cumulative offset on these strike-slip faults, and their associated rotation about vertical axes, can account for ∼60 km of N‐S shortening. This value is similar to estimates of the Late Quaternary N‐S right-lateral shear between central Iran and Afghanistan, which must be accommodated in NE Iran. The strike-slip faults also require ∼30 km of along-strike extension of the Kopeh Dagh, which is taken up by the westward component of motion between the South Caspian Basin and both Eurasia and Central Iran. It is probable that these motions occurred over the last ∼10 Ma.

The southern Caspian: A compressional depression floored by a trapped, modified oceanic crust"

The south Caspian intracontinental depression, floored by oceanic basement, is a relatively stable block, with minor deformation, surrounded by active fold-thrust belts of arcuate form (Talesh, Alborz, and Kopeh Dagh Mountains), which have undergone intense late Cainozoic crustal shortening. The basin is interpreted as a Neogene–Quaternary "compressional depression," bounded by multi-role mountain-bordering reverse faults, and apparently floored by a late Paleozoic – Triassic or late Mesozoic – early Tertiary "modified oceanic crust" trapped along an old geosuture. It may be a relic of an old (Paleozoic–Triassic) ocean, or else a marginal sea developed behind a Mesozoic–Paleogene ocean, and analysis of geological and geophysical data enables a scheme to be suggested. The general arcuate shape of the Alborz and the Talesh bordering mountain belts follows the pattern of the supposed rigid and thickened ocean crust of south Caspian depression.A tectono-sedimentary study of the south Caspian region, coupled with the body-wave modelling of a recent earthquake along one of the bordering faults, may suggest a possible flattening of the fault with depth (listric thrust), and that the estimate of the focal depth of the regional earthquakes based on teleseismic arrival time data is not accurate. The difference in elevation between the depression and the bordering active fold-thrust belts is caused by a difference in crustal structure and reverse faulting during a dominant compressional tectonic regime.The study may add support to the idea that old continental deep-seated multi-role faults, which have controlled the sedimentary facies and basins during different geological times and were responsible for the formation of the present physiographic feature, are the site of the present seismic activity in the orogenic belts.