معرفی و بررسی ساختاری پهنه گسلی عرضی- برشی عزالدین- راهجرد؛ یکی از ساختارهای با توان فعالیت در گستره شهرستان تفرش

شناسایی دقیق چشمه و پتانسیل‌های لرزه‌ای از مهم‌ترین اقدامات لازم و قدم نخست برای مطالعات لرزه‌خیزی هر منطقه به شمار می‌رود. گستره مورد مطالعه که در بخش میانی کمان ماگمایی ارومیه دختر واقع است، یکی از مناطقی است که شناسایی و مطالعه دقیق چشمه‌های لرزه‌ای آن ضروری به نظر می‌رسید. در این پژوهش با تفسیر تصاویر ماهواره‌ای و بازدیدهای میدانی و بررسی‌های ریخت‌زمین‌ساختی منطقه، برای نخستین بار سامانه گسلی عزالدین-راهجرد با روند 150-n و طول بیش از 70 کیلومتر معرفی می‌شود. این پهنه گسلی، همروند با گسله های عرضی-برشی دیگر همچون گسله‌های بیدهند، قم زفره و دهشیربافت، سنگ‌های آتش‌فشانی کمان ماگمایی را بریده است. خش لغز گسلی، خمش راست‌بر روند محور چین‌های اصلی در راستای گسله و تشکیل ساختارهای فرعی مرتبط با آن بیانگر حرکت راست بر این گسله است. بررسی زمین‌ریخت‌شناسی و جابه‌جایی راست‌بر آبراهه‌ها با گسله، بیانگر فعالیت گسله در زمان کواترنری است. انطباق مرکز سطحی زمین‌لرزه‌ها بر روی این گسله، اهمیت شناسایی و معرفی این پهنه گسلی را در مطالعات لرزه‌خیزی منطقه نشان می‌دهد. لذا احتمال وقوع زمین‌لرزه ویرانگر در اثر فعالیت پهنه گسلی عرضی-برشی عزالدین-راهجرد به‌عنوان گسلی که فعالیت کواترنری داشته است، دور از انتظار نیست و ممکن است در خطر لرزه‌ای شهرستان تفرش و مناطق هم‌جوار نقش تعیین‌کننده داشته باشد.

Introduction and Structural Study of the Ezedin-Rahjerd Transverse Fault Zone, One of the Seismic Potential Structures in Tafresh Area

Accurate identification of seismic potential is the first step and one of the most important for seismic studies in each region. The study area, located in the middle part of the UrmiaDokhtar magmatic arc belt, is one of the areas where identify and study of its seismic sources is necessary. In this study, for the first time, the EzedinRahjerd fault system is introduced according to the interpretation of satellite images, field study and tectonic surveys of the area. The Cenozoic (Eocene) volcanic rocks of the Tafresh area, which are covered with Lower and Upper Red Formations have been cut by this N150 transverse fault of 70 km. This fault system, like other transverse faults such as the Bidhend, QomZefreh and Dehshirbaft faults, has cut volcanic rocks of the magmatic arc belt. Geometry pattern detection of the structures using satellite images and field observations in the area showed that the effect of this fault are observed as the change of the main structure trends such as the reverse faults and fold axes bending and the formation of minor structures with a different trend from the main structures trends of the UrmiaDokhtar belt. The slicken lines of the fault plane and the rightlateral bending of the main fold axes along the fault zone and the formation of minor structures indicates the right lateral movement of the EzedinRahjerd fault zone.One of the prominent features of the study area is the existence of several parallel dyke sets that are not very distant and sometimes outcropping at a distance of less than 200 m. These dykes with a thickness of about 25 m and a length of about 1520 m with dip of more than 80 degrees are divided into two sets with dominant trends N4045W and N2030E. The first set of dykes with N4045W trending are a semideep dolerite and porphyro gabbro diorite that exposed around the Gyan and Lalaein villages and cutting Eocene and older units. Whereas, the second N2030E trending shallow dikes with combining of hornblende, andesite and basaltic andesite also cuts Miocene volcanicsedimentary rocks, Qom Formation and Lower Red Formation. Alteration of the second dyke set is significant. The trend, including rocks and tectonic setting of the dykes, show their influence in several stages. These intrusive dikes also show tension fractures, so will assist in structural analysis of the area. In such a right lateral transverse fault zone, development of the tension fractures with N2030E trend is not unexpected. Therefore, the second shallow N2030E dyke set are formed in these tension fractures due to the EzedinRahjerd transverse fault activity. While the first set with N4045W trending that is older than the second one, has been created due to the main structures of the UrmiaDokhtar magmatic belt.The most important geomorphological feature observed along the EzedinRahjerd fault is the conformity of the Kamar (Tafresh) river as the main river of the area with the fault trend. The Kamar River flows northsouth along the EzedinRahjerd fault, passing through the Barezjan, Dadghan, and Ezedin villages, and then reaches the GharehChai River in Jalaier. Instrumental earthquakes of the area show 2