دگرریختی و جنبایی پهنه های گسلی قم، البرز و کوشک نصرت در محدوده شمال قم

حوزه درون‌قاره‌ای شمال قم در ایران مرکزی جای گرفته است. راستای عمومی گسل‌های مهم این پهنه شامل گسل قم، البرز و کوشک نصرت، شمال باختر جنوب خاور بوده و همگی با درازای بیش از 50 کیلومتر تحت تاثیر همگرایی صفحه عربی اوراسیا، در لرزه‌خیزی و دگرشکلی گستره قم نقش مهمی داشته‌اند. یکی از روش‌های ارزیابی زمین‌ساختی، بهره‌گیری از شاخص‌های زمین‌ریخت‌سنجی است. در این میان، با توجه به ویژگی‌های سنگ‌شناختی و نحوه قرارگیری و پراکندگی حوضه‌ها، شاخص‌های نسبت عرض به ارتفاع دره، هیپسومتری و گرادیان طولی رودخانه برای ارزیابی انتخاب شده و نتایج تحلیل شده‌اند. این داده‌ها، در کنار داده‌های هندسه گسل‌ها و جهت‌یابی تنش توانسته برخی ویژگی‌های زمین‌ساختی گستره را روشن کند. مطالعات زمین‌ریخت‌سنجی، گستره‌های اطراف گسل ساوه و گسل کوشک نصرت در بخش شمال ساوه و همچنین مناطقی از جنوب شهر قم را دارای توان بالاآمدگی نشان می‌دهد. سایر پهنه‌ها بالاآمدگی قابل توجهی ندارند.پایین بودن جنبایی قائم در برآوردهای زمین‌ریخت‌سنجی دلالت بر نبود جنبایی نیست بلکه می‌تواند کارکرد هم‌زمان مولفه‌های فشاری و برشی یا همان دگرشکلی خاص ایران مرکزی را نشان دهد. این نوع حرکت با دانستن جهت‌یابی تنش به‌سادگی قابل تفسیر خواهد بود. دگرشکلی برشی فشاری گستره قم، ناشی از حرکت رو به شمال خاوری ورقه عربی و محدودشدگی ناشی از پوسته سخت خزر است. حرکت‌های راستالغز هم‌زمان با جابه‌جایی معکوس در طول گسل‌ها باعث شکل‌گیری چین‌خوردگی‌هایی نیز شده است. بدون شک، گستره‌ی قم محدوده‌ای است که از نظر مطالعات زمین‌شناسی لرزه‌ای مورد توجه محققین با دیدگاه‌های مطالعاتی متفاوت بوده که مقایسه نتایج این پژوهش‌ها به کشف مجهولات جنبایی گستره کمک کرده است.

Deformation and Movement of Qom, Alborz and Kushk-e Nosrat Fault Zones in the Northern Part of Qom

The placement of the Iranian crust at the junction of the Arabian, Indian and Eurasian plates has caused tectonic disruption. The creation of numerous faults and many folds along with seismicity in the Iranian crust are signs of this disruption. The continental basin of northern Qom is located in central Iran. The general trend of the important faults in this area including Qom, Alborz and Kushke Nosrat faults, is northwestsoutheast. These faults with a length of more than 50 km affected by ArabEurasian plate convergence, and have played an important role in seismicity and deformation of Qom area.     Morphological evidence and the existence of numerous fault fragments that have cut Quaternary sediments indicate the activity of these structures around the city of Qom. Despite the valuable studies that have been done on the faults of Qom, Alborz and Kushke Nosrat, there are still ambiguous opinions and ambiguities about how these faults move and their seismic potential. Qom fault with a length of more than 50 km, has a northwestsoutheast trend that continues from the mountains southeast of Qom to the south of Zefreh in the northeast of Isfahan. Along this fault, OligoMiocene limestone and marl are driven on Quaternary sediments. Alborz fault has also a northwestsoutheast trend with a dip to the southwest that has cut the northern limb of the Alborz anticline. Alborz fault has also driven OligoMiocene units on Quaternary units. Kushke Nosrat fault is part of the fault system. It starts from the southeast of Hoze Soltan Lake and continues to the Avaj fault. The general direction of Kushke Nosrat fault is 280 to 290 degrees, and in most cases, it has a dip more than 80 degrees to the south. One of the objectives of this study is to identify how these faults displacement and activity are occurred.     One of the methods of tectonic assessment is the use of geometric indices. Among these, according to the lithological characteristics and the location and distribution of the basins, the indices of Vf, hypsometry and SL have been selected and the results have been analyzed. Results of morphometric indices with fault geometry and stress orientation data, have been able to elucidate some tectonic features of the area. Morphometric studies show the areas around Saveh fault and Kushke Nosrat fault in the northern part of Saveh as well as areas in the south of Qom have the potential to rise. Other zones do not have any significant uplift.     The geometry of the faults and orientation of the maximum compressive stress show that the maximum compressive stress direction is N30 to N330, and according to the direction of the fault, the fault mechanism is estimated to be Dextral. The stress inferred from the seismic event of 2007/06/18 also confirms the rightlateral shear motion along with the reverse motion in the faults.     The low vertical uplifts does not indicate the absence of activities, but can indicate the compressive and shear components or the specific deformation of Central Iran. This type of movement can be easily interpreted by knowing the stress orientation. The shearcompressive deformation of the Qom area is due to the northeastern movement of the Arabian plate and the limitation due to the Caspian hard crust.     Undoubtedly, Qom is an area that is considered and studied by different seismic researchers with different views and study styles. Comparing the results of these studies has helped to discover the Unknown activity zone.  In general, the northern part of Qom is an area with high seismicity in which the mechanism of earthquakes is mostly strikeslip with thrust. Therefore, moderate to large earthquakes are expected to occur in the future.