مجله زمین ساخت
پیاپی 8 (زمستان 1397)

SE trending (S62E) and NE dipping (80NE) mylonitic foliation is the main structural feature developed in ~557-561Ma Deh-Zaman mylonitic granite in the N70˚E trending Kuh-e-Sarhangi shear zone at the northwest of the Lut Block. 35˚ plunge of the stretching lineation on this mylonitic foliation in combined with microstructural evidence, especially on feldspar crystals confirmed dominant sinistral shear with the little compressional component. Sinistral shear proposed to initiate after emplacement of the granitic body and deformed it at ~300-500˚C during Early Cambrian. Our new and detailed microstructural analysis indicates considerable overprinting of dextral shear on the former older sinistral shear in the Deh-Zaman mylonitic granite. There is no radiometric age constraint on the timing of the shearing shift from sinistral to dextral kinematics; however, based on the structural and stratigraphic evidence, we propose post-Late Cretaceous- Paleocene age of the kinematic change. This shear sense change matches with Structural evolution of the Kuh-e-Sarhangi region in which sense of shear has been changed several times during its evolution.

The application of Experimental modeling is an important key to understanding the complex tectonic phenomena, specially the extremely complex salt structures. In this study, using an analogue sandbox modeler, the salt structures of Zagros and their effect on the general shortening of Zagros had been simulated, considering the low density and creep of the salt. The study area was divided into three subregions, according to the existence or the lack of salt structures and the thickness of Hormuz salt series. At the western part of the region the lack of salt series was assumed, and gradually the thickness of Hormuz salt series increases until it reaches the Qatar-Kazerun Fault System (KFS), then at the eastern part there is a thick layer of Hormuz salt series. For every subregion, a model was designed, considering the low density of salt and its creep. Finally, a schematic outline proposed to describe the existence and distribution of salt structures at Zagros. According to this outline the KFS is an important agent of general thickness changes at the base, because at its eastern part there is an abundance of salt structures, but conversely, at its western part except of a few buried small salt structures there is no sign of Hormuz salt structures.

The sub-vertical fault planes with sub-horizontal slickenlines and correlation of these date with anisotropy of magnetic susceptibility indicates a predominantly transpressive regime including right lateral strike slip faults with reverse components in the Bibi-Maryam area. An assemblage of en-echelon and double plunge folds with slip indicators such as duplexes in their limbs confirms progressive transpressive deformation there. Accompanying hydrothermal alteration with transpression has created ideal conditions for reactivating shear fractures during late Cenozoic times that they record the last phase of tectonic activity. The sets of the shear fractures indicate an N025° direction of compression (σ1) that is compatible with the Plio-Quaternary stress; also the results of faults analysis show a N085° direction of compression (σ1) that is compatible with the middle-late Miocene in the area. Also, the granitoid bodies are exposed along faults that these faults are branches of the Nehbandan Fault System (NFS) in this area. Dextral transpressive regime activity along the NFS may have largely affected on the late Cenozoic exhumation of the granitoid bodies in the Bibi-Maryam area and thus Sistan suture zone. Field evidence indicates that exhumation of the granitoid bodies cannot have preceded Oligo-Miocen in this area.

With regard to drying of Urmia Lake, and an extensive unloading from the earth’s crust, due to the loss of water from the lake surface, the evaluation of b-value, and the probable increase of earthquakes, has been investigated in this study. The evaluation of b-value variations due to the loss of water from the lake’s body, represents a decrease in b-value after water level dropping in the lake, subsequent unloading about 20 Milliard tones, and the increase of stress in the study area. Furthermore, the water level change-seismicity diagram, including 1990-2017 data, is clearly show an increase in seismicity and its relation with the lake water level. Research of the radius of the b-value variations have shown that Van M=7.5, 2011 earthquake and Ahar and Varzghan M=6.4, 2010, earthquakes, are the Induce earthquakes due to the loss of water of Urmia Lake.

The morphotectonic features and seismic assurances are some indicators for the study region as a neotectonic place. In this study, faults have been classified based on the seismic and tectonic evidences in which the role of each fault system and their importance in the seismic assessment that have been revealed. The analysis of morphotectonic in this region indicate the progressive deformation along the right lateral strike-slip zones with N-S striking and the pressure-shearing zones with NW-SE one. The Neotectonic and Morphotectonic studies reveal so active faults which there are no report about their activities. These seismic active faults have cut the Quaternary sediments and also cause some earthquakes. According to the fracture intensity map, the most fracturing (6.5-7) are observed in the south parts of Kuhbanan fault and its segments, the north Gowk segment, the south part of Nayband fault, and along the Jorjafk and Bardsir faults. These fracturing intensity are related to pressure-shear fault zones in this region where occurred many small to large earthquakes.

 Active faults as seismic faults, cause a great deal of damage each year through the creation of seismic impulses and surface ruptures in different parts of the world. These faults, due to the persistence of tectonic stresses, have a significant potential for re-motions, which subsequently cause more casualties. Therefore, understanding these faults the concepts associated with them is very useful in the strategic planning of a country. One of the most important of these concepts is fault territory. The most important causes of earthquake damage are construction within the fault territory. To understand this concept, other concepts such as fault scarp, fault migration, blind fault, fault system evolution, density maps, avoidance zones, geometrical parameters, and fault splay are to be considered. In this study, with an overview of these concepts as well as studies conducted in the east of Iran including the 470 m left-lateral displacement along the Dasht-e Bayaz fault, we attempt to provide a clearer concept of fault territory that includes these definitions.