فصلنامه زمین شناسی ایران
پیاپی 58 (تابستان 1400)

Meyghan playa near Arak city is located at the boundary of the metamorphic zone of Sanandaj- Sirjan and Urmia-Bazman volcanic zone. Two main faults of Talkhab and Tozlugol (Tabarte) play a key role in the formation of this basin by right lateral movement with northwest - southeast trend. In this study, morphotectonic indices were investigated to study the tectonic activity of these two faults. The measured indices include altimeter integral index, asymmetry index, stream length gradient index and basin shape factor. Investigating the relative active tectonics using the above mentioned indicators shows that Arak region has a relatively moderate to high activity. Particularly, the region between the two Talkhab and Tozlugol faults is more active than the other regions. The graben of the Meyghan Desert is formed due to the activity of the Takhab and Thouzlugol faults during the Pleistocene by right lateral transpresion zone. The southwestern side of the lake is in accordance with the Tozlugol fault. It seems that according to the morphometric studies of this area, the activity of the Talkhab fault is more than the Tozlugol fault and plays an important role in the formation of the Meyghan playa.

Modeling of geological structures plays an important role in understanding the geometry of the structures and their relationships. In recent years, digital modeling using computers has attracted the attention of engineers as well as researchers in basic sciences. "MATLAB" is one of the softwares that is used with its extensive facilities for data analysis and modeling in various sciences, including structural geology. In this paper, possible models of interference patterns of the three generations of folds in metamorphic rocks of the Golpayegan area were reconstructed using the script written in MATLAB software. The required data include the attitudes of the mean axis and the mean axial planes of three consecutive generations of folds which were obtained during field measurements. Field studies show that the first and second generation folds are almost coaxial, however the third generation folds have a different axial trend. The results of modeling indicate that four classical fold interference patterns may be formed in horizontal (map view) and vertical sections in Golpayegan metamorphic rocks. Modeled fold interference patterns are closely consistent with the natural fold interference patterns observed in the outcrop and regional scales. Using the modeling, it can be determined that the different interference patterns of the folds in the northern Golpayegan metamorphic rocks are related to superposition of the related fold generations.

Lakhshak deposit is located 28 km northwest of Zahedan in the Sistan suture zone. The rocks in the Lakhshak area predominantly consist of Eocene volcanosedimentary and metamorphosed (greenshist facies) rocks. The mineralogy of the ore is simple and consists of stibnite, pyrite, arsenopyrite, chalcopyrite, sphalerite, and electrum. The Lakhshak deposit is hosted in the shear and altered calc-shist unit which is associated with quartz, sericite-muscovite and sulfide alteration minerals. Based on geophysical studies, using induction polarity and special resistance (IP/RS) in the Lakhshak sheared area, combined with the results of geological, metamorphic and mineralization information, calligraphic calcification units, fault zones and metamorphosed areas have a high potential for Au-Sb mineralization. Therefore, using the amount of changes in specific resistance and chargeability, as well as the intensity of chargeability in the profiles, can appropriately identify the promising area for gold and stibnite mineralizations. This study indicates that the main characteristics of the geology and mineralization of the Lakhshak, such as the nature of the host rock, the form of mineralization, metamorphism and associated alterations, is similar to orogenic gold deposits.

In this study, the upper part of the Garau Formation with a thickness of 24 m in the section of Sheikh Saleh anticline located in the northwest of Kermanshah was investigated from the calcareous nannofossils. In this section, the Garau Formation is mainly composed of medium bedded argillaceous limestone. In order to introduce the index species and biozones, 17 slides from the Garau Formation were studied, which resulted in the identification of three biozones. As a result of this study, 20 species and 15 genera of calcareous nannofossils were identified. Based on the obtained biozones, the age of the upper part of the Garau Formation in the studied section is suggested the late early Cenomanian to the late late Cenomanian. This age is corresponding to the upper part of Eiffellithus turriseiffelii Zone (CC9), Microrhabdulus decorates Zone (CC10), lower part of Quadrum gartneri Zone (CC11) from the Sissingh (1977) zonation and UC1, UC2, UC3, and lower part of UC7 from the Burnett (1998) zonation.

Sahand volcanic complex is located in the south of Tabriz and east of Urmia Lake and includes an alternation of lava, igneous materials and clastic sediments that were affected by many folds and fractures. Sahand volcano is relatively young in age (Miocene to Pliocene) and its structure is somewhat intact. Consequently, where there is a good lateral continuity, facies and volcanological studies can be carried out in the upper part of the complex. Based on these studies, it was determined that the Sahand volcanic structure can be divided into four central, near, medium and distant facies. Facies studies show that the structure of Sahand Volcano was created by several explosive phases, some of which were associated with water intrusion and there was not much time between eruptions. Characteristics, such as severe welding and discoloration of pyroclastic deposits show that these units had a high temperature at the time of formation and were formed on land. Overall, Sahand volcanic eruptions were more explosive, and thus, the number of pyroclastic deposits is much higher than lava flows. In the central, near and middle facies of Sahand, no traces of younger hydrothermal activities can be found, and only in the distant facies, there is a hot spring (Bostan Abad), the relationship of which needs further study to prove the connection of these springs with the Sahand Volcano.

Determining the direction of deformation migration in the fold- thrust belts, is one of the basic aspects for understanding the structural evolution of the fold and thrust belts. By studying the timing and mechanism of deformation and compare it with different regions of a fold thrust belt, it is possible to gain a better understanding the history of the structural evolution. The timing and development of deformation in the Zagros folded belt is one of the widely studied topics and the general belief is that collision of  two plates of central Iran and Arabian began in Miocene, and gradually developed from the northeast to the southwest. According to the present study, the timing of  folding of the Jaril anticline in the northern of Dezful embayment  is estimated to be end of Middle miocene (Serravalian). The results of the present study indicated that the onset folding of the Jarik anticline in this area coincides with the deposition of the lower Aghajari Formation.

The late-Eocene Kond Formation in Saran section, Central Alborz, is a carbonate, marl, volcano-clastic and evaporite succession and the lower and upper boundaries with Karaj and Lower Red Formation is unconformable. The main thickness of the studied section consists of evaporite deposits. Based on the field studies, petrography and XRD data, these evaporite deposits are mostly consist of gypsum, anhydrite, dolomite, calcite and quartz. Field and petrographic study, imply that the gypsum and anhydrite are present in three types: primary (type one), secondary (type two) and Tertiary (type three) and dolomites are related to very early diagenesis. The primary evaporites which are laminated and layered gypsum, were deposited in a subaqueous shallow environment (salina) due to solar evaporation. Secondary evaporites with nodular texture and enterolithic and chicken-wire structures, were formed during epigenetic and shallow burial diagenesis (coastal sabkha). Moreover, tertiary evaporites with alabasterian, porphyroblastic and satin spar textures are related to burial and uplifting of primary and secondary evaporites in telogenesis and phreatic meteoric conditions. The abundance of evaporite minerals as well as other evidences of this sediments in the Saran section indicates a warm and dry climatic conditions during deposition of the Kond Formation. Due to the layered nature of evaporites of the Kond Formation in the Saran section and their association with shallow marine facies and the presence of significant thickness of volcano-clastic deposits of the Karaj Formation below these deposits, Eocene seawater and also migration of ions from older tuffs is suggested as the source of ions required for the deposition of these evaporite units.