نشریه علوم زمین
پیاپی 64 (تابستان 1386)

Nannofossils are good for biostratigraphy, since they are abundant, planktonic, rapidly evolving and largely cosmopolitan especially in the Late Cretaceous. With respect to this reality and due to the lack of any precise paleontological study, the nannofossils of Gourpi Formation were investigated in the north of Gachsaran. This formation consists of marl and clay limestone. In this study, for the first time, 23 genera and 43 species of nannofossils were identified. Based on the obtained nannofossils, the study section is Late Santonian to Late Maastrichtian in age corresponding to CC16-CC26.

A moderate earthquake (Ms=6. 3) struck the coastal region of north of Iran and the central Alborz on 28 May, 2004 was responsible of several damages and about 35 casualties. The mainshock was followed by a large number of aftershocks, the largest one reaching Ml=4. 8, based on the analysis of local waveforms. We study the mainshock, first major aftershock, and about 240 aftershocks recorded by Iranian National Seismic Network (INSN), Tehran Telemetry Seismic Network (Institute of Geophysics, Tehran University), and our temporary local seismological stations, which were installed on 30 May, around the epicentral area of this earthquake. Using waveforms of all permanent stations, the coordinates of the mainshock was determined as 36. 30 °N for latitude and 51. 60 °E for longitude. The analysis of aftershocks indicates that the seismic activity migrate from east where the mainshock occurred toward west close to the location of the largest aftershock (36. 36 ° N, 51. 45 ° E). 140 selected aftershocks recorded at a minimum of 6 stations, having rms less than 0. 15 sec and uncertainties less than 2 km, were used to infer a precise geometry of the fault region. The aftershocks distribution has 30 km long and trends NW-SE parallel to the North Alborz and Khazar faults. The focal depths comprised between 10 and 28 km, unusually deep for Iran. Distribution of aftershocks cluster on cross-section defines a fault plane which dips at 40-50 degree south-westward. Its upward continuation can be related to either North Alborz or Khazar faults. Most of the focal mechanisms are consistent with reverse faulting on NW-SE trending faults, parallel to the main active structures of the region. Well constraint focal mechanisms which dip gently at a rate of 25-40ْ indicate the activity of the second mentioned faults during the Firozabad-Kojour earthquake. Existence of focal depths up to 28 km indicates an unusual brittle lower crystalline crust in this part of central Alborz.

Development and evolution of Tethys basins during geological history from the Precambrian to Paleogene has been considered by many geoscientists. The first sign of various basins propagation of Tethys which resulted in separation of supercontinents, ancient lands (such as Eurasia, Gondwana) and blocks or microplates among them, are found in the Precambrian. One of these old basins in the north of Iran has been called Ortho-Tethys, its evidence can be found in the primary structures of Alborz and its Precambrian units. Some other researchers believe that the Ordovician and Silurain volcanic series of Iranian Plateau and Alborz were formed after genesis of Early Tethys basin or Paleotethys due to extension-shear system along the Paleo-Tethys between Turan plate and Alborz-Kopet-Dagh belts. By increasing the rate of subsidence in the Permain basin, the dominating marks of extension system between Iranian microcontinents and Arabian plate were recorded, however, the related alkaline volcanic rocks of the Neotethys rifting found in Triassic succesion. In addition, the most important tectonic changes of the Caspian and Black seas occurred in the Triassic time. Structural upheaval of Tethys basins among blocks and plates, such as Turan in the north Kopet-e-Dagh, Caucasus, Alborz, Sabzevar, Central Iranian micro-continents and Arabian plate in the south is traceable periodically. The succession of extension and compression tectonic events resulting from opening and closing of such basins during Assynitic-Pan african and Alpine orogenies which sometimes correspond to metamorphic facies from Caldonian and Hercynian orogenies, resulted in tectonic evolution of sedimentary basins in the areas impressed by various orogenies. Alborz as an extended block in active tectonic zone and impressed by most distinguished geological events in both supercontinents of Eurasia and Gondwana is evolved between two ancient continents. The total of thinning and thickening of Alborz basement during ancient events in comparison to other parts of Iran indicate approximately constant rate and little negative gradients due to thickness decrease of crystalline crust and lower crust towards Caspian basin. Detachment folding system with uplift of ancient facies in hanging wall of basic fault in north Alborz is one of the apparent features of central part of this structural block that occurred in flower structure. More folding and movement in western part of this fault comparing to eastern part, is another structural feature in the area. North Alborz, Mosha and Taleghan faults as principal faults appearing in internal part of Central Alborz, where some propagation faults like the North Tehran and Khazar can be derived from them to the south and north side of Central Alborz.

Folded and faulted Kopeh Dagh-Binalud belt in the northeast of Iran is a part of Eurasia and southern margin of Touran plate. What is now observed is the result of the latest Phase of Alpine folding. The morphology of the area is in early stage and its young folds express direct relevance between topography and geological structures. In order to evaluate uplift and distribution of compressional forces, stream- gradient (SL) and hypsometric (HI) analyses were carried out for 85 subbasins and 98 rivers of the major basins of the area (Atrakrud, Kashafrud, Jajarm, Gorganrud and Tourkamanestan plain rivers). Integration of the morphometric maps of SL and HI illustrates clearly the tectonic anomalies. Five highly uplifted morphotectonic zones were identified based on the morphotectonic indices as follows: 1) The middle part of Ashkabad Fault between geographical coordinates of: 56 47 E, 38 35 N to 57 34 E, 37 56N. 2) Gorganrud's sub-basins particularly located between Shavard fault (in the south) and eastern part of the Khazar fault (in the north). 3) Basins associated with Nokhandan, Qareh -Dagh, Sorkhdeh and Amrudak Faults. 4) Basins associated with Shogan and Gelli Faults are the Jajarm subbasins. 5) The northern foothill basins of Binalud Mountain, compared with the southern foothill basins, show remarkable higher uplifting and shortening which seem to be related to the Binaloud and North Neyshabur fault. In addition to high uplifted zones, some intermediate - low uplifted zones such as west - southwest part of Sarakhs, southern foothill subbasins of Atrakrud (associated with Takalkuh Fault and Ashkhaneh fault zone), Jajarm's subbasin in the eastern part of Aladagh Mountain were recognized.

The magnetotelluric (MT) method is a natural source electromagnetic geophysical technique, which is used mainly in petroleum, mineral and geothermal exploration. As in this method, the quantity of the measured data is bulky and have a complex structure, their modeling, compared with the modeling of the other electrical data, is a very complex task or even impossible in some instances. The main objective of this paper is to use the ability of the artificial neural networks (ANN) to find a solution for two-dimensional (2D) joint TE (transverse electric) and TM (transverse magnetic) modes inverse modeling of MT data. To achieve the goal, a multilayer perceptron (MLP) network with back propagation (BP) learning algorithm is used. In order to learn the designed network, many synthetic 2D models with the same category, have been created and their responses have been calculated for each polarization mode by forward modeling. Synthetic data include apparent resistivity and impedance phase in 9 stations and 11 frequencies in two polarization modes. After a comprehensive study, a perceptron with 3 layers and architecture of 396-9-9 has been designed and used to model the data. This study show that the designed network is capable enough to produce an acceptable 2D underground model so that the correspondence mean relative modeling error is 3.9% and 6.9 % respectively for noise free data and 5 percent randomly added noisy data. This indicates that if ANN is designed and trained properly, then it would be capable enough to perform 2D inverse modeling of MT data. It has also shown that once the designed network has been trained properly it is able to perform the inverse modeling precisely in a short time. At the end, the performance of the designed network has been evaluated by a set of field MT data and its results has been compared with those produced by a common smooth rapid relaxation inversion (RRI) method. The comparison indicates that the results of these two different procedures are in close agreement.

In experimental sciences we often need to solve inverse problems. That is, we want to obtain information about the internal structure of a physical system from indirect noisy observations. Information about the errors in the observations is essential to solve any inverse problem; otherwise it is impossible to say when a feature ‘fits the data'. In practice, however, one seldom has a direct estimate of the data errors. Here, we exploit the trade-off between data prediction and model or data structure to determine model based estimates of the noise characteristics from a single realization of the data. Noise estimates are then used to characterize the set of reasonable models that fit the data. By intersecting set of prior model parameter constraints with the set of data fitting models, we obtain a set of models that fit the data and are in agreement with prior constraints. This prior information can also be used to set bounds on the bias. We illustrate our methods with a synthetic example of vertical seismic profiling (VSP).

Generally the most important factor in the structural evolution of Alborz is thrust and reverse faulting that often has an east-west trend, parallel to the mountain chain. Mosha fault, one of the main faults, in this study covers the area between longitudes 51˚, 30´ to 51˚, 45´ including central-western part of Mosha fault. The stress evolution on the available faults in northeast of Tehran city was studied by means of inversion of 120 striated fault planes and related striations. Finally, two different movement systems were identified. One of them generated dextral reverse faulting and the other one which is younger created sinistral reverse faulting. The dextral system, known to resulted from the movement of the Arabian plate toward the north, is determined to be Miocene and probably older. The sinistral system originates from structural transition of Alborz Mountain (progressive deformation in Alborz). Obtained result shows that the movement vector on the thrust Mosha is the consequence of the combination of sinistral and reverse faulting. The ages of stress changes were determined by using the cross cutting relations of individual fault planes and differentiation of events.

To revise the Abderaz Formation in Koppeh-Dagh basin, its type section was studied considering stratigraphical properties. The main lithology of Abderaz Formation includes gray to blue shale with a thickness of 286.86 meters. This Formation has an attitude of N 45˚ W and 80˚ SW. Its lower boundary with Aitamir Formation is unconformable and upper boundary with Abtalkh Formation conformable. In this study, 58 planktonic species of 26 genuses were identified and presented. The age of Abderaz Formation, based on the identified planktonic foraminifera, was determined from Early Turonian to Early Campanian. Planktonic foraminifera fossils found in Abderaz Formation are more abundant than benthic foraminifera ones that show deep sea and continental slope environment. Regarding the foraminifera content of the Abderaz Formation, 7 biozones were determined in the Mozduran section (Kopeh-Dagh) which shows Early Turonian - Early Campanian age for Abderaz Formation.

Using teleseismic body waves, this paper presents the crustal and lithospheric structure in the central part of the Alborz Mountains for the first time. The region has been known as a very active and seismogenic locations in the past, characterized by strong earthquakes and complex structures. Tehran, the capital of Iran, is located on the southern part of the region. The aim of this study is to determine Moho depth and its variation beneath the Central Alborz with high resolution and accuracy. The crustal structure of the Central Alborz beneath 26 broadband stations from a temporary dense seismological network was determined by using recorded data deployed for a period of 4-6 months. We first summarize an analysis of the teleseismic P-wave receiver function beneath each station. Next, we use joint inversion of receiver functions with surface wave dispersion data calculated for the center of network. Results of this research are shown as cross-sections through N-S azimuth of the profiles which passes across the Alborz Mountains. Our study shows a thickening in the central part of the Alborz Mountains crust down to 55 km relative to 48-50 km crustal thickness beneath northern part of the Central Iran. The crustal thickness is close to 44 km beneath the southern coast of the South Caspian Sea Block. These results are completely unlike with most of the other previous suggested crustal thickness. Based on our results, the Alborz Mountains with a high average elevation can be considered with a crustal root.

Hydrocarbon source rocks, the subject of various analyses and investigations, are well appreciated because of their capacity for oil generation. In the present study, attempts were made to evaluate source rock using a leap cost analytical method. For this purpose, some samples from these formations were analyzed by Rock- Eval. CO2 generated from these samples at 350ºС are calculated as mgHy/gr rock or Kg Hy/Ton rock. As this method requires more time and is relatively expensive, we offered a new method in which by calibration of data, obtained from analyzed samples, the result could be attributed to the whole interval of a formation. By calculation of S2, Tmax and TOC of analyzed samples collected from wells A and B for both Pabdeh and Gurpi formations in accordance with ∆LogR method, rescaling of Resistivity-sonic logs, Resistivity-Density logs and Resistivity-Neutron logs, TOC content of these formations were estimated. Comparison between these data and data obtained from direct sample measurements in lab and extrapolation of an equation that relates these data to S2 and TOC of sample analysis, TOC and S2 content of whole intervals of these formations were calculated through ∆LogR method. Then hydrocarbon generation potential of the Pabdeh and Gurpi formations were finally evaluated. These data were processed in a neural network method with forward back propagation capability designed as try and error structure within Matlab software. Final results are in good agreement with those data obtained from direct measurements.

Investigation on plant macrofossils of the Jurassic sediments of Shemshak Formation in Eshkevarat-e-Olia area has been resulted in introducing two species, consisting of Ptilophyllum sahnii and Ptilophyllum sakrigaliensis. These species belong to Phylum Cycadophyta and order Bennettitales. The above mentioned macrofossils are reported for the first time from Iran. Based on the stratigraphical distribution of these two plant macrofossils and accompanied species such as Todites williamsoni, Cladophlebis aktashensis, Nilssonia tenuinervis, Ptilophyllum cutchense, Klukia exilis, Klukia westii, Coniopteris hymenophylloides, Ptilophyllum acutifolium, Nilssonia harrisi, Nilssonia macrophylla, Pseudoctenis herriesi and Elatides thomasi the age of the Shemshak Formation for these sedimentary bed in Eshkevarat-e-Olia is assigned to the Upper Liassic (Toarcian) - Bajocian.

Shadyakh, commonly deemed as the headquarters of Neishabour City, has so far endured such various events as war, earthquake, and other natural disasters. What has been revealed through recent archeological excavations convincingly depicts a brilliant scientific, cultural, and political manifestation of this city in a glorious epoch between 9th to 12th centuries, which must have continued till 1205. Notwithstanding, owing to an unanticipated occurrence, the city was thoroughly devastated. The history of devastating events along with archeological evidence discovered recently indicates that the annihilation of Shadyakh has been an upshot of Genghis Khan's holocaust.