بررسي تكتونيك فعال در قلمرو تراست زاگرس منطقه فارسان

شناسايي مناطق داراي پتانسيل خطر زمين‌لرزه همواره از دغدغه‌هاي اصلي رشته‌هاي مرتبط با علوم زمين بوده است و محققان مختلف با نگرش‌هاي گوناگون آن را مورد مطالعه قرار داده‌اند. دانش تكتونيك ژيومورفولوژي نيز با روش‌شناسي خاص خود به مطالعه اين پديده محيطي پرداخته است. از اين رهگذر تعبير و تفسير شواهد بر جاي مانده از لرزه‌ها و حركات تكتونيكي جديد بر چشم‌اندازها اصول موضوعه اين دانش نوپاست. شاخص‌هاي تكتونيك فعال از ابزارهاي كارآمد براي شناسايي وضعيت تكتونيكي و ارزيابي احتمالي مناطق لرزه‌خيز به‌شمار مي‌آيد. شاخص‌هايي چون سينوس جبهه كوهستان، شاخص‌هاي دره‌اي چون شاخص نسبت V شكل دره‌ها، شاخص پهنا به عمق دره‌ها، گراديان طولي رودخانه، شاخص‌هاي حوضه‌اي چون شاخص تقارن حوضه زهكش، شاخص نسبت V شكل و شاخص انتگرال هيپسومتري، از اين دست‌اند. از سويي هم ابرگسل زاگرس در زمره گسل‌هاي فعال و اصلي كشورمان است كه همواره منشا لرزه‌هاي بسياري در حاشيه خود بوده است و مرز دو زون ساختاري سنندج ـ سيرجان و زاگرس مرتفع است. در اين پژوهش حوضه فارسان، كه شامل دو زون سنندج ـ سيرجان و زاگرس مرتفع است، به‌وسيله شش شاخص متفاوت در محيط سيستم اطلاعات جغرافيايي (GIS) و از مدل رقومي ارتفاعي (DEM) با دقت 10 متر مورد آزمايش قرار گرفته و نرخ تكتونيك فعال آن سنجيده شده است. سپس نتايج به دست آمده، با بررسي ميداني مقايسه گرديده‌اند. نتايج شاخص‌هاي متفاوت تبديل به شاخص IAT شده‌اند و نقشه پهنه‌بندي تكتونيكي حوضه فارسان در چهار كلاس بسيار فعال، فعال، نيمه‌فعال و غيرفعال ترسيم گرديده است. نتيجه به‌دست آمده اين است كه در اين حوضه گسلي فرعي منشعب از گسل اصلي زاگرس در طول حدود 8350 متر داراي فعاليت تكتونيكي در حد بسيار فعال است. همچنين بخش‌هاي زيادي از حاشيه اين گسل داراي تكتونيك متوسط است. دو شهر باباحيدر و فارسان در حاشيه گسل‌هاي فعال واقع شده‌اند.

Introduction Concerns of earth sciences, tectonic geomorphology with its own methodology is too. The main subject in tectonic geomorphology science is the study of measurable forms and landforms produced by tectonic signals. Since judgment about paleoseismological characteristics of an area cannot be only based on instrumental paleosizemology records and/or historical earthquake, so the study of surfacial landscapes and landforms is very important. Active tectonics indexes are advantageous tools for assessment and investigation of tectonic status of mobile zones. Active tectonics indexes are such as stream length gradient, drainage basin asymmetry, drainage basin shape, hypsometry integral and ratio of valley- floor width to valley height. The, main Zagros fault is one of the major as well as active faults of our country which has been prudes many earthquakes in around area. In this study Farsan basin, which consists of two zone structural of Sanandaj-sirjan and high-Zagros, was tested by seven different indexes and active tectonic rate was measured. Farsan and Hafshijan basin with 32.5-32.9 latitude in north hemisphere, and 50.33-50.78 longitude. By is located in Chahar Mahal and Bakhtiari province Paying attention to paleosizemology situation on the zone and comparing it with field observation, the results have been evaluated. Materials and Methods All the indexes have been measured with high resolution DEM (10 meters) within GIS software. Finally, the results of the active tectonic indexes were converted to IAT index and the active tectonic rate map of Farsan basin has prepared. The aims of this study are as follows: First, to study the tectonic status of the Farsan Basin along the Zagros fault margin based on active tectonic indexes. Second, to show the status of this zone by comparing the results from the active tectonic indexes at both sides of this fault and at the structural zone, i.e. Sanandaj-Sirjan and high-Zagros. Third, to show that which index gives better and more reliable results, considering the geomorphologic and field observations. It should be noted here that measurements have been done by GIS software using digital elevation models (DEM) with 10-meter-pixel resolution. For studying this basin, it was divided to 30 sub-basins. Sub-basins 1-16 and 30 were located inside high Zagros zone, and 17-29 inside Sanandaj-Sirjan zone. These sub-basins had also been categorized into 3 classes (1, 2, and 3) based on their tectonic activity. Class 1 has the highest activity and class 3 has the lowest. Results and Discussion Shape-ratio Index (Bl): This index is calculated by dividing length of the basin (B) by greatest width of the basin (L), Bl=B/L. Based on this index, the results showed that 27.88% of basins are in class 1, out of which 15.38% are in Sanandaj-Sirjan zone and 12.5% in high Zagros zone. Drainage Basin ASymmetry Index (Af): This index shows the tilting caused by tectonics and is calculated by Af =100(Ar/At), Ar: the area at the right side of the channel, At: the total area if the basin. The results showed that 71.93% of the basins have a tilted shape and are in class 1 out of which 30.76% are in Sanandaj-Sirjan zone and 41.17% in high Zagros zone. Hypsometric Integral Index (Hi): It is calculated by , in which , Hmax, and Hmin are the average, maximum, and minimum height of the basin, respectively. According to the results, totally 27.14% of the basins have active tectonics and are in class 1, out of which 15.38% are in Sanandaj-Sirjan zone and 11.76% in high Zagros zone. Mountain-front sinuosity Index (Smf): This index is calculated by the direct distance (Lmf) divided by indirect distance (Ls), Smf=Lmf/Ls. The results based on this index showed that 77.7% of the mountain fronts are in high Zagros zone and 63.6% of them in Sanandaj-Sirjan zone, both of them in class 1. Ratio of Valley-floor Width to Valley Height Index (Vf): This index is calculated by Vf =2VFW/[(ELD-ESC)+(ERD-ESC)], VFW: Valley-floor width, ESC: Valley-floor height, ERD,LD: Elevations of right and left valley divides. Based on this index, narrow and deep valleys have active tectonics. The results showed that only 37% of the measured valleys in high Zagros are in class 1 and no valley from Sanandaj-Sirjan zone is in class 1. Stream length-Gradiant Index (SL): it is calculated by SL= (?H/ ?L)L, in which ?H is change in elevation of the reach; ?L is length of the reach, L is the total channel length. According to the results, 23.52% of the streams are in high Zagros zone (class 1) and 15.3% are in Sanandaj-Sirjan zone (class 1). To produce the IAT index, the authors have added all the results from these 6 indexes, calculated S/n, and finally drawn the zoning map of the Farsan basin in 4 classes. The results showed that 3.18% of total basin areas (25.85 km2) have active tectonics. Conclusion The average basin area categorized in class 1 is around 13 km2, while the average area of inactive basins is around 37.27 km2. The average height of the basins in class 1 is about 2650 m., while that of in class 3 is around 2470 m. So, in this basin the main Zagros fault has very active tectonics along 8350 meters and tectonically high-Zagros zone is more active than the Sanandaj-sirjan zone.