شناسایی زمین‌ساخت فعال در شمال گسله شمال تبریز به یاری شاخص‌های زمین‌ریختی و تحلیل جنبشی

پس از زمین‌لرزه اهر- ورزقان (21 مرداد 1391)، مشخص شد که در نقشه گسله‌های فعال در شمال گسله شمال تبریز باید بازنگری صورت بگیرد. در راستای ارزیابی زمین‌ساخت فعال1 شمال باختر ایران، از ترکیب روش برگشتی برای تعیین تانسور تنش، مطالعات میدانی، زمین‌ریخت‌شناسی و شاخص‌های زمین‌ریختی بهره گرفته شد. شاخص‌های کیفی که در بررسی مورفومتری محدوده مورد مطالعه قرار گرفته‌اند عبارتند از: انتگرال فرازسنجی (Hi)، منحنی‌های فرازسنجی و شاخص طول- شیب رودخانه (SL). منحنی‌های فرازسنجی، پراکنش ارتفاعات را در یک بخش از زمین، در یک حوضه زهکشی توصیف می‌کنند. از میان تمام شاخص‌های زمین‌ریخت‌شناسی، شاخص طول- شیب رودخانه (SL)، ابزاری مهم برای نشان دادن تغییرات آنومالی در شیب یک رودخانه است. منطقه مورد مطالعه واقع در شمال گسله شمال تبریز است. بازدیدهای محلی، داده‌های جنبشی و زمین‌ریختی، یک گسله فعال با سازوکار راستالغز فشاری را نشان می‌دهد که به نام گسله نهند (با طول تقریبی 168 کیلومتر) از آن یاد شد. تحلیل جنبشی نشان‌دهنده‌ی چندین راستای تنش بوده که قدیمی‌ترین آنها در راستای شمال خاور- جنوب باختر، با گذر از یک مرحله تنش شمالی- جنوبی، به تنش عهد حاضر شمال باختری- جنوب خاوری تبدیل شده است.Active Tectonic in the North of North Tabriz Fault (NTF), Iran, Using Morphometry and Kinematic AnalysisShohreh Rafiee, Mohammad Reza Abbassi, Abdollah Saidi and Hadi TabassiThe study area is a part of the Alpine-Himalayan orogen. It is formed by the Greater Caucasus Mts., the Lesser Caucasus Mts., the Talesh Mts., the Kura Basin, and the South Caspian Basin. Present-day structures of the study area are controlled by the ongoing collision of the Arabian and African plates with Eurasia. The study area extends over 1342 km2 located in Northwest of Iran, which can be regarded generally as the continuation of structural grain of Lesser Caucasus. However, the existence of rigid block of South Caspian Basin and the different lithology of Azerbaijan including Quaternary volcanic masses such as Sahand and Sabalan has resulted in a complex distribution of deformation in the studied area.Northwest Iran is a region of vigorous inflection, deformation and seismicity situated between two thrust belts namely Lesser Caucasus to the north and the Zagros thrust belt to the south.What is known about Seismotectonics of the area located in north of Tabriz fault is limited to the recent works done by Masson et al. (2006). Using dense GPS, they determined that the deformation in NW Iran is characterized by ~ 8 mm/yr of right-lateral movement on the North Tabriz fault, and ~ 8 mm/yr of extension within Talesh Mnt. The duplicate Ahar-Varzeghan earthquake focal mechanisms contradicted the GPS results. Although both main shocks have probable fault planes that strike roughly east–west, it is likely that the mapped surface faulting should be associated with the first main shock because field observations record nearly pure strike-slip motion that would be inconsistent with the transpressional mechanism of the second main shock (Ghods et al., 2015). The fault study of Coopley et al. (2013)introduced three Segments with a length of 400 m. till 8 km. In 2012 August 11 (12:23 UTC), a moderate earthquake with MW=6.4 (USGS) occurred between Ahar and Varzeghan towns in Azerbaijan Province at northwest of Iran, in a region where there was no major mapped fault or any well-documented historical seismicity. In order to solve this problem, Ghods et al. (2015) have introduced a model to resolve this earthquake focal mechanisms and many active fault earthquake rupture in the range of Ahar - Varzeghan.A combined study of active tectonic parameters such as geomorphic indices and stress tensor measurement, allowed to recognize a new fault in the northern part of North Tabriz Fault. In addition to the seismicity of North Tabriz fault, seismicity map of Azerbaijan shows the same distribution ratio in the NW part of the North Tabriz fault. In the absence of active faults, the relation of this seismicity is not known. This study was an attempt to introduce one of the active faults at the North of the North Tabriz fault in order to improve the understanding of seismotectonic characteristic of this area.In order to inquire the relevance between rock rigidity and SL index based on a simplified geological map of the area and field observation, rocks were categorized by their resistance as below: Very low (young alluvial deposits), Low (old alluvial deposits, poorly consolidated conglomerates, marl), moderate (gypsum, gypseous marl), High(limestone, sandstone, dolomite, shale, conglomerate, tuff, schist, flysch sediments), Very high (andesites, trakiandesite, gabbro, dacite) (El Hamdouni et al., 2007). Based on the mentioned category, the distribution map of lithological resistance was obtained using GIS. To assess tectonic activities in the area, geomorphic indices such as: the stream length- gradient index (SL) and hypsometric integral (Hi) have been used to reveal vertical active movements along a particular fault, namely Nahand fault. This fault lying in north of Tabriz fault strikes parallel to it with a length of 168 Km. The combination of morphometric studies and kinematic study show clear vertical movement along Nahand fault. Determination of stress tensor and geomorphic indices has shown that the Nahand fault is a right-lateral strike-slip with a minor vertical component. Field observations and the inversions of stress tensor (geologic and seismologic fault kinematics) revealed that the Nahand fault is an active fault, acting in a transpressional tectonic regime, with the Sh-max oriented NW-SE.Keywords: Nahand Fault, North Tabriz Fault (NTF), Geomorphic Indices, Stress Tensor, Active Tectonic.