M Baghernejhad; M.R Ghassemi; B Oveisi
Abstract
There is a close relationship between resistance to slip along decollement surfaces and presence of deep and shallow decollement levels in thin-skinned fold and thrust wedges. Decollement units in lower (Upper Red Formation) stratigraphic levels in Mianeh-Mahneshan fold belt have an effective role onthe ...
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There is a close relationship between resistance to slip along decollement surfaces and presence of deep and shallow decollement levels in thin-skinned fold and thrust wedges. Decollement units in lower (Upper Red Formation) stratigraphic levels in Mianeh-Mahneshan fold belt have an effective role onthe geometry and kinematics of deformation of the area. In order to understand the fold geometry and folding mechanisms, and to exploredepth-to-the-decollement surface, we carried out data collection and field study in an area between Mianeh and Mahneshan cities. Folded structures in the study area are different from other structures within the area, as well as from the structures in the neighboring Alborz Mountains.The rise of salt domes along with the plasticity of marls in the Upper Red Formation have resulted in extreme complexities in folding pattern. In order to analyze syn-sedimentary structural features and interpret the geological evolution of the area, we used detailed structural measurements, sedimentological and sedimentary environment features, sedimentary rock studies, and paleogeography.One of the results of this study was the interpretation of syn-sedimentary growth structures in the Mianeh-Mahneshan area, which helped us to construct six structural cross-sections (AA’, BB’, CC’, DD’, EE’ and FF’)across the folded structures. Measured shortening along two Sections AA’ and DD’is 46.65% and 38.05%, respectively, with an average of 42.3%. These values are different from those estimated forthe neighboring Alborz and Zagros Orogens, where shortening ranges between 16-30%. We attribute this difference to local intense shortening in the study area caused by several factors such as basin slope, deep faults and weak beds along decollement surfaces. This study indicates that dominant folding mechanisms in the study area are detachment folding, and fault-propagation or fault-bend folding. The presence of evaporitic material (gypsum and salt) within the succession has played a major role in the kinematics of folding.
M.A Shokri; M Foroutan; M Nemati; M.J Bolourchi; SH Javadipour; B Oveisi
Abstract
The Touchahi earthquake of Aug 27, 2010 (MN 5.9; IRSC- Mw 5.7; USGS) occurred at 19:23:49 UTC (23:53:49 local time on 5 Shahrivar 1389) in south of Damghan city. No foreshock were reported before this earthquake whereas 85 aftershocks (MN 1-5) were registered by IRSC until 1 month after the mainshock. ...
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The Touchahi earthquake of Aug 27, 2010 (MN 5.9; IRSC- Mw 5.7; USGS) occurred at 19:23:49 UTC (23:53:49 local time on 5 Shahrivar 1389) in south of Damghan city. No foreshock were reported before this earthquake whereas 85 aftershocks (MN 1-5) were registered by IRSC until 1 month after the mainshock. According to our field study after the event, surface rupture of causative fault was not observed but we measured some fractures related to this event with dominant strike of N120º-140º. According to our observations of 32 towns and villages that were damaged in this seismic event, maximum intensity (I0) of VIII+ in MMI scale occurred near the Touchahi village in ~85 km south of Damghan city. Unfortunately in this earthquake 4 people were killed. Focal mechanisms of the Touchahi seismic event and its greatest aftershock is solved using the first P motion method. The fault plane solution show near vertical plane for the causative fault of the earthquake and suggests a left- lateral mechnism. The mechanisms associated with the fault show mainly left-lateral strike–slip motion, on a NE –SW striking fault plane. Based on location of the earthquake epicenter, its aftershocks location, the fault plane solution (left-lateral strike-slip with N039º strike and dip direction toward NW) and field observations, the causative fault of Touchahi earthquake is one of the active fault branches that is situated in north of Darestan mountain and south of Touchahi, Koohzar and Kooshahi villages. This fault with left-lateral strike-slip mechanism by general strike of NE-SW and dip direction toward NW is indicated as Touchahi fault.
S Alimardan; Sh Solaymani Azad; M Ghorashi; M.R Ghorashi; B Oveisi; A Hatami
Abstract
Due to urban population growth in recent decades and the need for decentralization of cities with limited facilities and increasing marginalization in the larger cities, construction of new cities around large towns has been considered by decision makers to develop a plan to create new cities around ...
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Due to urban population growth in recent decades and the need for decentralization of cities with limited facilities and increasing marginalization in the larger cities, construction of new cities around large towns has been considered by decision makers to develop a plan to create new cities around these towns. One of the most important parameters for the construction of new population centers is seismotectonic studies. Hashtgerd New Town in southern slopes of the Alborz (located 65 km northwest of Tehran) is located on the uplifted deposits of the Plio-Quaternary. Vertical tectonic movements in the range caused the height difference of about 300 m between Hashtgerd and its southern plain elevations. Due to the emplacement of the city on the uplifted Plio-Quaternary deposits, and the lack of information about the causative active fault in this region, an attempt has been made to investigate the morphotectonic characteristics of the area on the basis of survey satellite image, aerial photo, digital elevation model, and field observations. In the present study, an active fault zones with NW-SE trends has been identified with oblique-slip movements (compression with left-lateral strike-slip component). The N-NE stress direction, in regards to the abovementioned trend is oblique, and hence, the result could be a transpressional regime for the area. Recognition of mentioned active faults is very important for earthquake hazard assessment studies for the new town.
A Salehpour; B Oveisi; M.R Ghassemi
Abstract
In thin-skinned fold-thrust belts, the style of deformation is critically depends not only on the resistance to sliding along basal detachment, but also on the presence of intermediate decollement levels or/and ductile units within the wedge. In ZFTB the incompetent units known as detachment levels like ...
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In thin-skinned fold-thrust belts, the style of deformation is critically depends not only on the resistance to sliding along basal detachment, but also on the presence of intermediate decollement levels or/and ductile units within the wedge. In ZFTB the incompetent units known as detachment levels like the Gachsaran formation (Shallow level) and the Hormuz salt series (deep) affect strongly the deformation style. However, in many cases shallow level detachment of the Gachsaran unit controls this shallow deformations, but owing to the different facies of this unit in the studied area it is reasonable to other incompetent shallow units (e.g. the middle Miocene Mishan Fm. and the Upper part of Aghajari Fm.) to partially decoupling deformation form the underlying sedimentary units, and have permitted the re-activation of pre-existing syncline structures, through upward extrusion of the syncline core. Owing to the concentration of significant amounts of young deformation across the structure (specially northern hinge) in the style of erosional surfaces, the East-West trending Anve syncline in north of the Bastak town, considered to be one of the most active structures in the region. In this study, we try to define the vertical deformation of a geomorphic marker as well as using tectonic morphology techniques to shed light on the deformation extend and pattern in the eastern and western parts of the structure . However, the microseismic data do not suggest any significant event related to this kind of deformation, but studying deformation style and relative uplift rate estimation could be beneficial dealing with earthquake hazards in the region.
N Saboor; M.R Ghassemi; M Eskandari; A Nazari F; B Oveisi
Abstract
Erosion agents diffuse the scarps created by the earthquakes, are and their primary sharp shapes are gentled. This process is accomplished with the definite rate, which is accounted with the diffusion equation. Rate of diffusion depends on primary shape of scarp and diffusion coefficient that is coefficient ...
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Erosion agents diffuse the scarps created by the earthquakes, are and their primary sharp shapes are gentled. This process is accomplished with the definite rate, which is accounted with the diffusion equation. Rate of diffusion depends on primary shape of scarp and diffusion coefficient that is coefficient of diffusion equation. The coefficient depends on various parameters such as the climate and the value of erosion of scarp material. The coefficient is accounted with the modeling of earthquake scarp with the definite age and initial shape. We afforded three profiles from the Salmas earthquake scarp created in 1930. We diffused the reconstructive primary shape of them by the code that we wrote in the Matlab software. We present diffusion coefficient of the Salmas area, and the mean of definitive coefficient. This coefficient is used for other earthquake scarps and traces in this and other areas with the same climate. The age of their formation was calculated by this coefficient and the prepared profiles.
K Hashemi; B Oveisi; A Saeedi
Abstract
The Zagros fold-thrust belt is a young orogenic belt which is characterized by extensive folding in its sedimentary cover, and abundance of earthquakes in which, intends to inner parts of Zagros like a band. The Lar anticline is one of the active folds in this belt which is situated in the coastal Fars ...
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The Zagros fold-thrust belt is a young orogenic belt which is characterized by extensive folding in its sedimentary cover, and abundance of earthquakes in which, intends to inner parts of Zagros like a band. The Lar anticline is one of the active folds in this belt which is situated in the coastal Fars arc of the Zagros simple folded belt and intends to inner parts. Earthquakes have several times destroyed Lar town in the northeast of this structure. The most important event was 1960 earthquake (mb~6.0) that caused lots of destruction and casualties. The nearest fault to Lar town, which probably caused this earthquake, was the Lar fault. In order to illustrate the geometry of the Lar fault-related fold a structural cross section with ~27 km length was prepared. Percentage of total shortening along the cross-section obtained 17.3 % which is equal to 4.6 km. The maximum amount of this shortening equals to 15 % (~4 km) has been absorbed by the Lar anticline. Geometrical comparison between the Lar anticline and fault propagation fold models showed that the Lar anticline kinematically corresponds to a fixed axial surface fault propagation fold that associated with thinning in its forelimb. Investigations showed that the Lar anticline is a mature fault propagation fold, which its core is converting to an immature fault bend fold. This kinematic reorganization has led to elimination of the low viscosity Hormuz salt from anticline core and elimination of the low viscosity units in turn has led to changing ductile behaviour to rigid behaviour. According to the structural cross section, such a mechanical behaviour in the Lar anticline can occur in contact boundary of the Paleozoic rocks in anticline core which can lead to seismic potential increase in the sedimentary cover. So, it is expected that the depth of 1960 earthquake was low (6-9 km) and occurred in the core of the Lar anticline, in relation to its rigid mechanical behaviour. By using geometric equations, we showed that long term relative shortening rate for the Lar anticline is 2 ± 1 mm/yr which is associated with 4±1 mm/yr average slip rate upon the Lar fault.
E Hajizadeh Naddaf; B Oveisi; M.R Ghasemi; M.J Bolurchi
Abstract
The analytical models in rock mechanics represent suitable analysis of deformation and failure conditions of the samples bearing simple geometry and mechanical properties. Nowadays different methods of numerical modeling, like finite difference method (FDM), are used for analysis of continuous, non- ...
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The analytical models in rock mechanics represent suitable analysis of deformation and failure conditions of the samples bearing simple geometry and mechanical properties. Nowadays different methods of numerical modeling, like finite difference method (FDM), are used for analysis of continuous, non- and quasi- continuous behavior. This paper represents logical estimate of geomechanical properties of the Asmari formation with usage of the results of triaxial tests on intact rock samples of the upper limy part of this formation and also the finite difference methods. For this purpose, we designed some models containing simple boundary condition and only one rheological unit. To simulate the failure conditions of the samples, a geomechanical simulative software (Flac 2D) was used. The failure of samples under the experienced stress conditions in laboratory were of basic data for simulation of failure conditions by finite difference method. The results show that the cohesion coefficient and uniaxial tensile strength are of sensitive parameters in controlling the failure conditions. This study suggests that the values of Young's and Bulk modulus are 260GPa, 23GPa, respectively, when the uniaxial tensile strength and cohesion coefficient are considered 29MPa and 26MPa, respectively.
M. Nemati; B. Oveisi; M. Foroutan; M. J. Bolourchi
Abstract
A MN=5.8 (Institute of Geophysics Seismological Center, University of Tehran) earthquake in 2010 July 20 shook southeastern Zagros, over the active ramp of the Mountain Front Fault (MFF). A body killed during the earthquake and it’s aftershocks have affected hundreds of square kilometers around ...
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A MN=5.8 (Institute of Geophysics Seismological Center, University of Tehran) earthquake in 2010 July 20 shook southeastern Zagros, over the active ramp of the Mountain Front Fault (MFF). A body killed during the earthquake and it’s aftershocks have affected hundreds of square kilometers around the epicenter. The shook area has experienced many moderate to large earthquakes at the historical time and the MFF recognized as their responsible. With the combination of the waveforms of seismological networks for the Institute of Geophysics, University of Tehran, International Institute of Earthquake Engineering and Seismology of Iran and a local network of the Geological Survey of Iran at Fars province, the source parameters of the earthquake were modified. For this event the ML magnitude computed 5.4 and a depth of 10 km were estimated. This event is related to the growing of anticlines neighboring the MFF with a fault propagated through their cores as of the other reverse earthquakes in Zagros. Tilting and uplifting of young alluvial deposits neighboring them are evidences for this growing. The iso-seismal curves elongated along MFF and the maximum intensity in MM and JMA scales was estimated VII. A small conjugate shear zone with maximum 1 km elongation, 150 m width and 10 cm of both vertical and right lateral dislocations with the strike of N45˚E, was mapped. Finally for this earthquake there was not any rupture except this T kind rupture which not merits the earthquake magnitude.
Kh. Hashemi; B. Oveisi; A. Saeedi
Abstract
Young deformations and folds are one of the impressive characteristics of Zagros simple folded belt. The studied structure is Hormud fault-related fold which is situated in the coastal Fars and the southward of Lar town. In order to illustrate the geometry of the studied structure and its kinematic relationship ...
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Young deformations and folds are one of the impressive characteristics of Zagros simple folded belt. The studied structure is Hormud fault-related fold which is situated in the coastal Fars and the southward of Lar town. In order to illustrate the geometry of the studied structure and its kinematic relationship with north and south structures, a structural cross section with ~27 km length from NE to SW was prepared. This study showed that Hormud anticline has been formed in the footwall of Lar fault, synchronous with kinematic reorganization in the core of Lar anticline and increasing horizontal stresses. Interpretation of crustal young deformations in the vicinity of the footwall of Lar fault indicates that Hormud anticline is growing in the form of a constant limb length detachment fold. Estimation of relative shortening rate (0.9± 0.2) for south limb of Hormud anticline is showing its fast growth in the form of a rocket fold which was associated by fast propagation of Hormud fault toward the surface. As a consequence of this process, kinematic model of Hormud anticline changes from a constant limb length detachment fold to a shear fault bend fold. Estimations predict 0.7 ± 0.1 mm/yr relative uplift rate for young Hormud anticline in its crest.
