ارزیابی تکتونیک فعال حوضه آبریز کرگانرود در دامنه شرقی تالش ( بغروداغ)، با استفاده از شاخص‌های ژئومورفیک

حوضه آبریز کرگانرود به عنوان یکی از زیر حوضه‌های مستقل دریای خزر در غرب استان گیلان واقع شده است. با توجه به فعّال بودن حرکات کوهزایی در برخی از نواحی ایران و برای آگاهی از میزان فعّالیّت نیروهای درونی و تکتونیکی در منطقه‌ی مطالعاتی، از هفت شاخص ژئومورفیک که عبارتند از: شاخص پیچ و خم پیشانی کوهستان (smf)، شاخص نسبت عرض کف دره به ارتفاع دره (vf)، شاخص گرادیان طولی رودخانه (sl)، شاخص تقارن توپوگرافی عرضی (t)، شاخص عدم تقارن حوضه زهکشی (af)، شاخص پیچ‌ و خم رودخانه اصلی (s)، شاخص انتگرال هیپسومتریک (hi) استفاده شده است. در این زمینه، مجموعه‌ای از روش‌های توصیفی، میدانی و تحلیلی به کار گرفته شد. نقشه‌‌های توپوگرافی، زمین‌شناسی و عکس‌های هوایی و مدل رقومی ارتفاعی منطقه (dem) با قدرت تفکیک 12/5 متر و نرم افزار arc gis 10.1 و تصاویر گوگل ارث به عنوان داده‌های اصلی تحقیق مورد استفاده قرار گرفتند. از ترکیب شاخص‌های ژئومورفیک با یکدیگر می‌توان رابطه‌ی شاخص فعالیت‌های تکتونیکی (lat) را به دست آورد. نتایج بررسی نشان داد که طبق مقادیر (lat) حوضه کرگانرود در کلاس فعّالیّت‌های نئوتکتونیکی زیاد قرار دارد. شواهد ژئومورفولوژیکی به دست آمده مانند پادگانه‌های رودخانه‌ای ارتفاع یافته، وجود تراس‌های قدیمی و آبراهه‌های عمیق شده و دره‌های v شکل نتایج تحقیق را تایید می‌کند.

Assessment of Active tectonics in the Karganroud Basin in the eastern slope of the Talesh (Baghrodagh), by Using Geomorphic Indices.

Extended AbstractIntroductionTectonic geomorphology is the study of the influence of active tectonics on the landscape. Geomorphic indices are known as a useful tool in active tectonic studies, and they have been tested in different areas around the world.The aim of this research is to examine tectonic activity in the eastern slope of the Talesh (Baghrodagh). Some geomorphic indices such as hypsometric integral(Hi), asymmetry factor (Af), valley floor width to height ratio (Vf), stream lengthgradient index (Sl), transverse topographic symmetry factor (T), mountainfront sinuosity (Smf), sinuosity (S) have been used to evaluate neotectonic activity for study area. Karganroud basin is located in the west of Gilan, between eastern longitudes 48°34ʹ to 48° 58ʹ and northern latitudes 37° 42ʹ to 57° to 37ʹ.MethodologyThe method used in this research, is a combinational of, descriptive and analytic method with field and library studies. The geomorphic indices are used to evaluate the neotectonic of the region.This research used topographic maps on a scale of 1:250000 , geologic map on a scale of 1:100000, digital elevation model (DEM) with a resolution of 30 meters and in combination with aerial photos. These data were then analysed in GIS10.1 to calculate the geomorphic indices and prepration river profiles and needed maps. The following geomorphic indices were determined: the asymmetry factor (Af) was used to detect possible tectonic tilting in the karganroud basin. In order to discriminate between Vshaped and Ushaped flatfloored valleys, the valley floor widthtoheight ratio (Vf) has been calculated for the selected mountain fronts of the region. The hypsometric integral (HI) is controlled by lithological and tectonic factors. A simple way to characterize the shape of the hypsometric curve for a given drainage basin is to calculate its hypsometric integral HI. The integral is defined as the area under the hypsometric curve. Mountainfront sinuosity (Smf) has been employed to evaluate tectonic activity along mountain fronts, apart from its dependence of climate and lithology . In active mountainfronts, uplift will prevail over erosional processes, producing straight fronts with low Smf values. The SL index shows the variation in stream power along the river reaches and it is very sensitive to changes in channel slope. For this reason it allows the evaluation of recent tectonic activity and/or rock resistance. The sixth geomorphic index is the transverse topographic symmetry factor (T). The transverse topographic symmetry factor (T) estimates the amount of asymmetry of a river basin and the variation of this asymmetry in different segments of the valley.  The seventh geomorphic index is sinuosity(S). Straight rivers will show low values of S and indicate active regions.Finally we used (lat) index for final evaluation of active tectonism of the rigion.Results and DiscussionGeomorphic analysis carried out by GIS techniques is considered to be a useful tool for evaluating the effects of active tectonics in an area.The asymmetry factor (Af) value of the karganroud basin is (Af=42/8).It indicates that the left of the basin has uplifted and the karganroud River has shifted to the right (to the west ) of the downstram of the drainage basin,which according to the classification of El Hamdouni et al. (2008)  is primarily of class 2 with moderately active tectonic. The mountainfront sinuosity (Smf) value of the karganroud basin equals 1/53 (smf =1/53) that belongs to class 2 with moderately active tectonic according to El Hamdouni et al. the valley floor widthtoheight ratio (Vf) has been calculated for the selected mountain fronts of the region. The mean value of vf for the basin equals 0/27 (vf = 0/27) that belongs to class 1 with the highest active tectonic according to El Hamdouni et al (2007) and Keller &Pinter (1996). The mean value of SL shows high value (sl = 627/29) that belongs to class 1 with the highest active tectonic according to El Hamdouni et al (2007). The mean value of  T  for the basin equals 0/20(t = 0/20) that belongs to class 2 with moderately  active tectonic. The hypsometric integral (Hi) value of the karganroud basin is (Hi =0/49) that belongs to class 1 with with highest active tectonic according to El Hamdouni et al (2008) and Strahler (1952) that shows mature topographic . Finally, the value of the seventh geomorphic index ( sinuosity) for the study area equals (s = 1/2)  that shows the karganroud river is straight and it still does not reach to its equilibrium, indicating the influence of tectonic activity in the study area.ConclusionThe calculated geomorphic indices in this research indicate that the Karkaganroud basin is tectonically active. The lat index was used for final evaluation of tectonic activity of the region. According to results of Lat index, the karganroud basin belongs to class 2 with high active tectonic. Also, Geomorphologic evidence rectifies the effect of active tectonics.