Alborz Mountain Belt Research Articles

Tectonic geomorphology assessment of neotectonics in the north of Damghan Region, Alborz Mountain Belt, Northern Iran

In mountain ranges, recent and active tectonics can be viewed as the main factor contributing to rock uplift, their present-day topography being the result of the competition between tectonic and erosional processes. The region coincides with three faults which show signs of activity during the late Quaternary period: The Astaneh, North Damghan, and Damghan faults, which collectively make up the SW section of the left‐lateral Shahrud fault system. In this study we have analyzed morphotectonic and neotectonic indices to evaluate tectonic activity of the study area.Tectonic geomorphological indices that are used in this study are drainage basin based (drainage basin asymmetry factor (Af), Hypsometric integral (Hi)) and mountain front based (mountain-front sinuosity (Smf), valley floor width – valley height ratio (Vf)) which can be useful tools for studying active tectonics. The study area includes pop-up structures that are formed during inverse tectonic activities of Alborz mountain belt. Strike-slip faults and flower structures suggest transpressional tectonic regime at eastern part of Alborz.

Assessment of Landslide Susceptibility Using Statistical- and Artificial Intelligence-Based FR–RF Integrated Model and Multiresolution DEMs

Landslide is one of the most important geomorphological hazards that cause significant ecological and economic losses and results in billions of dollars in financial losses and thousands of casualties per year. The occurrence of landslide in northern Iran (Alborz Mountain Belt) is often due to the geological and climatic conditions and tectonic and human activities. To reduce or control the damage caused by landslides, landslide susceptibility mapping (LSM) and landslide risk assessment are necessary. In this study, the efficiency and integration of frequency ratio (FR) and random forest (RF) in statistical- and artificial intelligence-based models and different digital elevation models (DEMs) with various spatial resolutions were assessed in the field of LSM. The experiment was performed in Sangtarashan watershed, Mazandran Province, Iran. The study area, which extends to 1072.28 km2, is severely affected by landslides, which cause severe economic and ecological losses. An inventory of 129 landslides that occurred in the study area was prepared using various resources, such as historical landslide records, the interpretation of aerial photos and Google Earth images, and extensive field surveys. The inventory was split into training and test sets, which include 70 and 30% of the landslide locations, respectively. Subsequently, 15 topographic, hydrologic, geologic, and environmental landslide conditioning factors were selected as predictor variables of landslide occurrence on the basis of literature review, field works and multicollinearity analysis. Phased array type L-band synthetic aperture radar (PALSAR), ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), and SRTM (Shuttle Radar Topography Mission) DEMs were used to extract topographic and hydrologic attributes. The RF model showed that land use/land cover (16.95), normalised difference vegetation index (16.44), distance to road (15.32) and elevation (13.6) were the most important controlling variables. Assessment of model performance by calculating the area under the receiving operating characteristic curve parameter showed that FR–RF integrated model (0.917) achieved higher predictive accuracy than the individual FR (0.865) and RF (0.840) models. Comparison of PALSAR, ASTER, and SRTM DEMs with 12.5, 30 and 90 m spatial resolution, respectively, with the FR–RF integrated model showed that the prediction accuracy of FR–RF–PALSAR (0.917) was higher than FR–RF–ASTER (0.865) and FR–RF–SRTM (0.863). The results of this study could be used by local planners and decision makers for planning development projects and landslide hazard mitigation measures.

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.

Exhumation of the Deylaman fault trend and its effects on the deformation style of the western Alborz belt in Iran

The Alborz range in northern Iran stretches along the southern coast of the Caspian Sea and finally runs northeast and merges into the Pamir mountains in Afghanistan. Alborz mountain belt is a doubly vergent orogen formed along the northern edge of the Iranian plateau in response to the closure of the Neo-Tethys ocean and continental collision between Arabia and Eurasia. The south Caspian depression—the Alborz basin of Mesozoic age (with W–E trend) in northern Iran—inverted in response to the Arabia–Eurasia collision. Pre-existing extensional faults of the south Caspian–Alborz system preferentially reactivated as contractional faults because of tectonic inversion. These contractional structures tend to run parallel to the trends of pre-existing extensional faults and acquire W and WNW–ESE orientations across the previous accommodation zones that were imposed by the reactivation of adjacent extensional faults with different directions. The NNE to N dipping faults show evidences of reactivation. The Deylaman fault is one of the important faults of western Alborz in Iran and is an example of inversion tectonic style of deformation in the western Alborz mountain range. The Deylaman fault, with an E–W trend, contains three discontinuous fault segments in the area under investigation. These fault segments have evidence of oblique right-lateral reverse motion and links eastward to the dextral Kandavan thrust. The importance of this fault is due to its effect on sedimentation of several rock units from the Jurassic to Neogene in western Alborz; the rock facies on each side of this fault are very different and illustrate different parts of tectonic history.

Geometrical and structural setting of landslide dams of the Central Alborz: a link between earthquakes and landslide damming

The combination of remote sensing observations, geological field investigations and geomorphological mapping and measurements along with faults and landslides inventory were employed to identify the landslide-dams of Alborz Mountain belt in northern Iran. The study comprises main landslide-dams of Central Alborz, including those with saturated and dried lakes. Valasht, Chort, Shoormast, Evan, Tar, Havir and Imamza-e-Ali were the lakes discovered as being formed by a landslide-damming process. Several other ancient lakes were identified as the remnants of breached and dried landslide-dams. The geometry and structural setting of each case is described; however, more investigations are required to evaluate the potential hazards for downstream populations. Most of these cases are believed to be co-seismic or earthquake-induced landslide dams formed due to large (M: ~7.0) known historical and derived pre-historic earthquakes. This paper emphasizes the relationship between active faulting, fault valleys and slope instability that leads to fault valley blockage by landslide. The recommendation from this study is that more consideration of active structures is needed in tectonically active regions, where the fault damage zones are more susceptible to slope instability due to progressive deformation and earthquakes.

Microseismicity of the Tehran region based on the data recorded in a local monitoring network: 2004-2010

Evaluation of the seismicity of a region depends on records of earthquakes. This article assesses the microseismicity of Tehran region, bounded by the coordinates 34.43°-36.87° N and 49.07°-53.13° E. This was investigated by the Tehran Disaster Mitigation and Management Organization (TDMMO) network from 2004 to 2010. The main purpose of this study was to substantiate the current tectonic activity of the region. We checked whether the observed faults in the region are active or not. Some active trends were found with the strongest activity in the eastern part of the region, especially associated with the North Tehran fault. In addition, records exist of strong historical earthquakes in this zone. These confirm that the area east of Tehran is more active than the west. The magnitude of completeness in the E, NE and SE is Mc=2. Major faults criss-crossing Tehran are located in the foot of the Alborz mountain belt. The released seismic energy map of the region demonstrates that the main active zones with high energy values for microearthquakes are in the east of the region.

Rapid Estimation of Peak Ground Velocity and Earthquake Location Using Small Magnitude Earthquakes in the Tehran Region, Iran

Online Material: Supplemental tables. The Tehran metropolis is located in an extensive area between the Alborz mountain belt to the north and the central Iranian plateau to the south. Tehran, as the political and economic capital of Iran, is considered one of the most populated cities in the world. From the point of view of vulnerability to natural disasters, Tehran is of great concern because of its basic infrastructures and population concentration. The Alborz mountain belt is a part of the Himalayan–Alps orogenic belt with numerous active faults, including the Mosha reverse fault, the North Tehran thrust fault, the Parchin, the Kahrizak, and the South and North Rey faults, with high seismic potential in the study region (Fig. 1). The Mosha fault is one of the fundamental structures of the central Alborz mountains and is situated north of Tehran. This fault is concave to the north and extends from the edge of the mountain range in the west to the eastern Alborz mountains and can be considered an upthrust in some places and an overthrust in others (Moinfar et al. , 1994; Berberian and Yeats, 1999). The North Tehran fault is the most prominent tectonic structure in the immediate vicinity of Tehran for which the trace is east–west to east‐northeast–west‐southwest and slightly concave to the south (Berberian and Yeats, 1999). The Tehran urban district is extended across thick alluvium layers over bedrock with complex geological structure. Based on historic seismic documents, Tehran is surrounded by several specific faults with return periods of about 150 years, but no damaging earthquake has occurred since 1830 (Moinfar et al. , 1994; Berberian and Yeats, 1999), when an estimated 45,000 …

Sensitivity of ground-motion scenarios to earthquake source parameters in the Tehran metropolitan area, Iran

The northern Tehran fault (NTF) is a principal active fault of the Alborz mountain belt in the northern Iran. The fault is located north of the highly populated Metropolitan Area of Tehran. Historical records and paleoseismological studies have shown that the NTF poses a high seismic risk for the Tehran region and the surrounding cities (e.g. Karaj). A series of ground-motion simulations are carried out using a hybrid kinematic-stochastic model to calculate broadband (0.1–20Hz) ground-motion time histories for deterministic earthquake scenarios (M7.2) on the NTF. We will describe the source characteristics of the target event to develop a list of scenario earthquakes that are probably similar to a large earthquake on the NTF. The effect of varying different rupture parameters such as rupture velocity and rise time on the resulting broadband strong motions has been investigated to evaluate the range of uncertainty in seismic scenarios. The most significant parameters in terms of ground-shaking level are the rise time and the value of the rupture velocity. For the worst-case scenario, the maximum expected horizontal acceleration, and velocity at rock sites in Tehran range between 128 and 1315cm/s/s and 11–191cm/s, respectively. For the lowest scenario, the corresponding values range between 102 and 776cm/s/s and 12 to 81cm/s. Nonlinear soil effects may change these results but are not accounted for in this study. The largest variability of ground motion is observed in neighborhood of asperity and also in the direction of rupture propagation. The calculated standard deviation of all ground-motion scenarios is less than 30% of the mean. The capability of the simulation method to synthesize expected ground motions and the appropriateness of the key parameters used in the simulations are confirmed by comparing the synthetic peak ground motions (PGA, PGV and response spectra) with empirical ground-motion prediction equations.

Cenozoic exhumation history of the Alborz Mountains, Iran: New constraints from low‐temperature chronometry

The Alborz Mountains in the north of the Turkish‐Iranian Plateau mainly developed in the Cenozoic as a consequence of the closure of Neo‐Tethys and continental collision between Arabia and Eurasia. Cenozoic growth of the fold‐and‐thrust belt exploited an older Palaezoic‐Mesozoic crustal fabric but the extent to which this governed the overall form of the mountain belt is unclear. To determine when and how the Alborz mountain belt has grown, apatite fission track (AFT) and (U‐Th)/He (AHe) thermochronometry were performed on 46 bedrock samples collected along 8 transects across the range. AFT central ages range from 157 ± 24 Ma to 10 ± 1 Ma with most ages falling between 40 Ma and 10 Ma. AHe ages range from 17 ± 2 Ma to 6 ± 1 Ma. The data revealed enhanced exhumation ca. 35 ± 5 Ma, minor exhumation between ca. 30–20 Ma and an increase in exhumation thereafter. This pattern matches the tripartite Mid‐Cenozoic stratigraphy of Central Iran, where Upper Oligocene‐Lower Miocene carbonates are sandwiched between terrestrial clastic formations. The most intriguing thermochronometric signal found is a major acceleration of exhumation that initiated in the late Miocene to early Pliocene, recorded by the AHe data. There does not appear to be a direct tectonic cause for Pliocene intensified erosion, as convergence rates between Arabia and Eurasia have remained relatively constant. Enhanced exhumation at this time may reflect a climatically induced intensification of erosion during Caspian isolation and base level fall, or a regional tectonic re‐organization of the Arabia‐Eurasia collision, or both.

Post-collisional polycyclic plutonism from the Zagros hinterland: the Shaivar Dagh plutonic complex, Alborz belt, Iran

AbstractThe petrological and geochronological study of the Cenozoic Shaivar Dagh composite intrusion in the Alborz Mountain belt (NW Iran) reveals important clues to decipher complex relations between magmatic and tectonic processes in the central sectors of the Tethyan (Alpine–Himalayan) orogenic belt. This pluton is formed by intrusion at different times of two main magmatic cycles. The older (Cycle 1) is formed by calc-alkaline silicic rocks, which range in composition from diorites to granodiorites and biotite granites, with abundant mafic microgranular enclaves. The younger cycle (Cycle 2) is formed by K-rich monzodiorite and monzonite of marked shoshonitic affinity. The latter form the larger volumes of the exposed plutonic rocks in the studied complex. Zircon geochronology (laser ablation ICP-MS analyses) gives a concordia age of 30.8 ± 2.1 Ma for the calc-alkaline rocks (Cycle 1) and a range from 23.3 ± 0.5 to 25.1 ± 0.9 Ma for the shoshonitic association (Cycle 2). Major and trace element relations strongly support distinct origins for each magmatic cycle. Rocks of Cycle 1 have all the characteristic features of active continental margins. Shoshonitic rocks (Cycle 2) define two continuous fractionation trends: one departing from a K-rich basaltic composition and the other from an intermediate, K-rich composition. A metasomatized-mantle origin for the two shoshonitic series of Cycle 2 is proposed on the basis of comparisons with experimental data. The origin of the calc-alkaline series is more controversial but it can be attributed to processes in the suprasubduction mantle wedge related to the incorporation of subducted mélanges in the form of silicic cold plumes. A time sequence can be established for the processes responsible of the generation of the two magmatic cycles: first a calc-alkaline cycle typical of active continental margins, and second a K-rich cycle formed by monzonites and monzodiorites. This sequence precludes the younger potassic magmas as precursors of the older calc-alkaline series. By contrast, the older calc-alkaline magmas may represent the metasomatic agents that modified the mantle wedge during the last stages of subduction and cooked a fertile mantle region for late potassic magmatism after continental collision.

PFR model and GiT for landslide susceptibility mapping: a case study from Central Alborz, Iran

In northern parts of Iran such as the Alborz Mountain belt, frequent landslides occur due to a combination of climate and geologic conditions with high tectonic activities. This results in millions of dollars of financial damages annually excluding casualties and unrecoverable resources. This paper evaluates the landslide susceptible areas in Central Alborz using the probabilistic frequency ratio (PFR) model and Geo-information Technology (GiT). The landslide location map in this study has been generated based on image elements interpreted from IRS satellite data and field observations. The display, manipulation and analysis have been carried out to evaluate layers such as geology, geomorphology, soil, slope, aspect, land use, distance from faults, lineaments, roads and drainages. The validation group of actual landslides and relative operation curve method has been used to increase the accuracy of the final landslide susceptibility map. The area under the curve evaluates how well the method predicts landslides. The results showed a satisfactory agreement of 91% between prepared susceptibility map and existing data on landslide locations.

Relocation and assessment of seismicity in the Iran region

SUMMARY More than 2000 instrumentally recorded earthquakes occurring in the Iran region during the period 1918‐2004 have been relocated and reassessed, with special attention to focal depth, using an advanced technique for 1-D earthquake location. A careful review of starting depths, association of teleseismic depth phases, and the effects of reading errors on these phases are made and, when necessary, waveforms have been examined to better constrain EHB focal depths. Uncertainties in EHB epicentres are on the order of 10‐15 km in the Iran region, owing to the Earth’s lateral heterogeneity and uneven station distribution. Uncertainties of reviewed EHB focal depth estimates are on the order of 10 km, as compared to about 4 km for long-period P and SH body-waveform inversions. Nevertheless, these EHB depth estimates are sufficiently accurate to resolve robust differences in focal depth distribution throughout the Iran region and, within their errors, show patterns that are in agreement with the smaller number of earthquakes whose depths have been confirmed by body-wave modelling or local seismic networks. The importance of this result is that future earthquakes with apparently anomalous depths can easily be identified, and checked, if necessary. Most earthquakes in the Iranian continental lithosphere occur in the upper crust, with the crustal shortening produced by continental collision accommodated entirely by thickening and distributed deformation. In the Zagros Mountains nearly all earthquakes are confined to the upper crust (depths <20 km), and there is no evidence for a seismically active subducted slab dipping NE beneath central Iran. By contrast, in southeastern Iran, where the Arabian seafloor is being subducted beneath the Makran coast, low-level earthquake activity occurs in the upper crust as well as to depths of at least 150 km within a northward-dipping subducting slab. Near the Oman Line, a region transitional between the Zagros and the Makran, seismicity extends to depths of up to 30‐45 km in the crust, consistent with low-angle thrusting of Arabian basement beneath central Iran. In north-central Iran, along the Alborz mountain belt, seismic activity occurs primarily in the upper crust but with some infrequent events in the lower crust, particularly in the western part of the belt (the Talesh), where the South Caspian basin underthrusts NW Iran. Earthquakes that occur in a band across the central Caspian, following the Apscheron‐Balkhan sill between Azerbaijan and Turkmenistan, have depths in the range 30‐100 km, deepening northwards. These are thought to be connected with either incipient or remnant northeast subduction of the South Caspian basin basement beneath the east-west trending Apscheron‐Balkhan sill. Curiously, in this region of genuine mantle seismicity, there is no evidence for earthquakes shallower than 30 km.

The Rudbar-Tarom earthquake of 20 June 1990 in NW Persia: Preliminary field and seismological observations, and its tectonic significance

Abstract The Rudbar-Tarom earthquake of 20 June 1990 in NW Persia ( M S 7.7, m b 6.4, M W 7.3) killed approximately 40,000 people, left more than 500,000 homeless, destroyed three cities (Rudbar, Manjil, and Lowshan) and 700 villages, and slightly damaged another 300 villages, in the densely populated area of the western Alborz mountains, southwest of the Caspian Sea. Both faulting and folding associated with the earthquake were observed in the epicentral area. Co-seismic surface faulting was associated with at least three main discontinuous, complex fault segments with a total length of more than 80 km. These three main fault segments are arranged in a right-stepping en-echelon pattern and are separated by gaps in the observed surface ruptures. Each segment has a strike of 95° to 120°, with oblique left-lateral and reverse motion on faults that are subvertical or have steep dips to the S or SSW. Maximum surface displacements were 60 cm horizontal (left lateral) and 95 cm vertical (south side up). Bedding-plane slip in a reverse or thrust sense was also observed at the surface following the earthquake and may have occurred during co-seismic folding. Seismic body waves of the mainshock confirm the left-lateral nature of the faulting and show that slip occurred in three main subevents that may correspond to the fault segments seen at the surface. The focal mechanism obtained from broadband body waves is almost pure strike slip, with little evidence of the vertical component of motion seen at the surface. There were numerous aftershocks, one of which had a thrust faulting mechanism, different from that of the mainshock. The complex system of surface faults on which the Rudbar-Tarom earthquake occurred had not previously been identified as active. The lack of obvious Quaternary scarps on the faults that moved in 1990 suggests that return periods for earthquakes of this type are relatively long; but this is difficult to quantify, as erosion rates are high. Although the Persian seismic network has insufficient coverage to locate small earthquakes with enough precision to identify active faults, there is much modern and historical seismicity in the area, and abundant geomorphological evidence of rapid active tectonic processes. Previous studies of focal mechanisms and active faulting had emphasized the reverse faulting and folding that are undoubtedly the main processes that build the Alborz mountain belt. The 1990 earthquake provided the first evidence that the belt also undergoes left-lateral shear. On the southern edge of the Alborz mountains the slip direction in earthquakes is approximately NE and occurs on oblique faults. We suspect that in the high Alborz, where the amount of shortening and the topography are greater, this oblique convergence between Persia and the southern Caspian Sea is partitioned into essentially pure reverse motion and left-lateral strike-slip.