فصلنامه زمین شناسی ایران
پیاپی 28 (زمستان 1392)

Ziaran area is located in the southwestern part of the central Alborz range (northern Iran) where a complete series of rocks from Paleozoic to Tertiary are exposed. Thus, the area is suitable for temporal evolution of structures in this part of the range. The study area is bounded by the Taleghan Fault to the south, the Tekie Fault to the east, and the Ahurak Fault (western continuation of the North Qazvin Fault) to the North. In this research, kinematic analysis of these faults and other related structures are presented to elucidate their structural evolution during Tertiary, considering the relationship between present structure and the dominant Tertiary deformation phases. The data obtained from analysis of the Taeghan Fault implies the basement involvement nature of the fault and its evolution during the inversion of an initial normal fault since Late Cretaceous (at least). This inversion led to thrusting of the Paleozoic and Mesozoic rocks over the Eocene units. Based on E-W strikes of the Taleghan and Ahurak faults and their reverse components, it might be suggested that they have been formed as a result of a N–S compression phase before the Miocene. During the Miocene, the final collision between the Arabian Plate and Central Iran has occurred (with approximate oblique trend of SSW-NNE). This change of trend resulted in variation on the main faults kinematics across the area such as change in kinematics of the Taleghan Fault to left lateral strike-slip and that of the Ahurak Fault to right lateral strike-slip. Due to SSW motions of the Caspian basin toward the central Iran, left lateral transperssion tectonics have dominant since Pliocene. This movement led to reactivation of the basement faults and their effect have resulted in the formation of several left lateral strike slip faults, as its riddle shear faults inside the study area.

Mineral exploration is a complex task that often requires the use of satellite imagery, geochemical and geophysical data in order to achieve a better result. The following study is a result of a research about Artificial Neural Network (Back propagation) in GIS environment to determine areas of Cr anomalies in Baneh (Kurdistan). Images of Landsat TM (the date 11/2002) has been used to determine rock alterations in the region. For determination of rock alterations and pyrite oxidation, we used band ratios 1 +2 +3 / 4 +3 and 2-3 +1 / 8 +4. Also field observations and petrography of thin section were used to control the results. GIS data and the results of the geochemical analysis were used in neural network data processing. Four types of neurons were used in the first layer. Also, 50 neurons in the second layer and 1 neuron in output layer were used. By using of the network, Cr potential maps with high accuracy of 86% were estimated (compared to ground data). This method is based on the numerical data and compared to conventional techniques (knowledge base) which has more advantages. The results showed that the use of satellite imagery and neural cells may play a significant role in geochemical mapping of mineral potential.

Soltan Abad metamorphic complex is located in NE of the Sabzevar ophiolites. Migmatitic metabasites are exposed next to the trondhjemitic body, in the western part of this metamorphic complex. Thermobarometric calculations of these metabasites indicate that the metamorphic process has taken place at temperature higher than 650°C and pressure more than 15 Kb. Under these conditions, migmatitic structures have been created by partial melting process as Grt-hornblenditic melanosome and tonalitic-trondhjemitic leucosome with adakitic characteristics. Rare earth elements pattern of melanosome as positive slope and heavy rare earth elements enrichment and amphibole-bearing leucosome as negative slope and light rare earth elements depletion indicating partial melting event has occurred at high pressure garnet stability conditions as well. Upward convex of rare earth elements pattern in the amphibole-rich leucosome and middle rare earth elements enrichment are compatible with amphibole accumulation. Adakitic composition of leucosome in the migmatitic metabasites and the age of their generation are similar to the adjacent tonalitic-trondhjemitic bodies. Probable scenario for the migmatitic metabasites contain metamorphism and partial melting of subducted oceanic slab at depth higher than 50 Km in a hot subduction zone and the creation of garnet-hornblenditic melanosome and tonalitictrondhjemitic leucosome with adakitic characteristics.

The NE oriented Dasht-e- Arjan graben is located about 65 Km west of Shiraz and is formed as a result of segmentations of the active Kare-e-Bas Fault. This extensional graben bounded by two boundary east-Arjan and west-Arjan Faults perpendicular to the Shahneshin and Salamati anticlines. Analysis of data from slickenlines was used to reconstruct the orientations effective stress using graphic method (fault-slip inversion technique). The position and trend of the local stresses are, σ1 is NNE, N23°E, 72º and σ3 is SE, S58°E, 26° and σ2, is SW S34°W, 10°, respectively. Based on the study of fault-slip data and slickenside lineation, using the multiple inverse methods, two phases of stress are extracted. An early tension stress field caused faulting with the SW trend in the area, while, the later stress phase is resulted in occurrence of young joint system in the area.

Various diagenetic processes have affected reservoir quality of the Asmari Formation in Ahvaz Oil Field in well No. 19 with a thickness of 357 meters. It is composed of limestone dolomite, dolomitic limestone, sandy dolomitic limestone, sandstone, siltstone and shale. Study of 1100 thin sections from available cores at this well as well as the well’s numeral data, porosity and permeability of cores, indicates that the most important diagenetic processes is anhydritic cement with different types of textures. It appears as poikilotopic, porefilling and pervasive, nodular, fracture filling, sparce crystals, and evaporite veins. During different steps of diagenesis, these textures show replacement and pore filling porosity that have affected different facies to some degrees. Results of this study show that anhydrite cement occurs mainly in dolomitic and sandstone facies and to some extent, affected the reservoir quality. This is due to the effect of sulphate rich brines during dolomitization. Where as, the presence of anhydrite cements in limestone facies is sparse and rare, it has little effect on reservoir quality. However, pore filling and pervasive anhydrite texture filling all pore-space in dolograinstone and dolopackstone grain-supported facies during shallow burial intensively reduced reservoir quality. Dissolution of cements at later stage of diagenetic processes (creating secondary porosity) improved reservoir quality. In addition, dissolution of poikilotopic anhydrite texture in sandstone facies and reprecipitated as patchy anhydrite with poikilotopic texture, results in reduction of porosity but it doesn’t make any changes in throat pores. Hence, patchy anhydrite with poikilotopic texture that reduces porosity converts the sample from Lucias class 2 to class 1.

Three species of Spatangoid Echinoid belonging to genus Douvillaster: D. douvillei Gauthier, 1902; D. longesulcatus Cotteau and Gauthier, 1895; D. valamtarensis Gauthier, 1902, and a species of Neitheidae Bivalve belonging to subgenus Neithea (Neithea): Neithea (Neithea) coquandi Peron, 1877, were described from the Aptian deposits (Dariyan Formation) at the Mish anticline in the Zagros basin, southwest of Iran. Douvillaster was recorded from the Aptian to the Turonian of the Tethyan margins (Middle East, Africa, Madagascar and Europe). The first appearance of Neithea in the North Pacific is in the Berriasian, and was continuously present until the late Albian and reached their maximum diversity in the late Aptian (this coincides with the Late Aptian global warming phase). The Neithea appeared in the Berriasian in the Tethyan realm (like to North Pacific region), thrived in the Mid-Cretaceous (Cenomanian), then decreased in diversity and finally became extinct at the end of Cretaceous.

Jovien aquifer is located in north-east of Iran and is recharged by the mountains with ultramafic rocks. The purpose of this study was to identify the effective factors on the quality of Jovein groundwater using principal component analysis (PCA) multivariate statistical technique. Groundwater sampling was collected from 22 wells (May 2009) in the study area. Physical parameters including T, DO, pH and EC were measured in situ. Major and minor ions and heavy metal compositions of groundwater have compared with standards. Then multivariate statistical techniques, such as cluster analysis and factor analysis/PCA, were employed to examine the chemical compositions of groundwater and to identify effective factors on the groundwater quality. According to the results, four principle factors were identified including hydro chemical evolution, chromites ores and chromium mining, weathering and oxidizing environment.

Eshtehard plain has been subjected to over exploitation of ground water in recent years and disregarding to the quality of ground water was led to pollution and salinity of aquifer. In this research the spatial distribution of some of the qualitative properties of ground water such as Electrical Conductivity, the concentration of sulfate and chloride and finally the drawdown of water table are studied. For this purpose, initially on the base of existing data in the field of water table during 1376-90 years, the graph of long-term changes in water levels in the aquifer is plotted and the average annual drawdown is calculated 0.69 meter. Also on the base of continuous measurement of groundwater level of the 16 wells in the study area, the map of iso-level groundwater is produced for the year of 1389-90. Then, 14 resources including 9 wells and 5 aqueducts are selected and two sampling is done in the 1389-90. After chemical analysis of samples, the concentrations of cations and anions suchas sulfate and chloride are determined and the maps of EC, SO4 and Cl are drawn. Also annual comograph of aquifer is produced using arithmetic average for 8 years period between 1382-90. Results showed that owing to limited recharges of the study area, unbalanced exploitation is led to drawdown of water table and changing the flow path in the aquifer. On the base of annual hydrograph, the average annual recharge is 29.7 m cubic meter and average annual drawdown of water table is 0.69 meter which shows the over exploitation of aquifer. The maps of EC, SO4 and Cl indicated that brackish waters has been spread from the north of plain to the southern parts of plain so the EC is increased from 4258.1 micromoss in the 1382-83 to the 6733.9 between 1389-90. Given that in the northern part of plain, the concentration of chloride is high, the infiltration of brackish waters with high chloride has had greater increase than sulfate. So the gradual salitinization of groundwater and the spreading of saltwater front into the fresh water front can be seen in this area as one of the consequences of continuous drawdown of water table or over exploitation. Therefore continuation of the current trend does not draw a clear vision for the plain.