ارزيابي تكتونيك فعال حوضه آبريز كرگانرود در دامنه شرقي تالش ( بغروداغ)، با استفاده از شاخص‌هاي ژئومورفيك

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

حوضه آبريز كرگانرود به عنوان يكي از زير حوضه‌هاي مستقل درياي خزر در غرب استان گيلان واقع شده است. با توجه به فعّال بودن حركات كوهزايي در برخي از نواحي ايران و براي آگاهي از ميزان فعّاليّت نيروهاي دروني و تكتونيكي در منطقه‌ي مطالعاتي، از هفت شاخص ژئومورفيك كه عبارتند از: شاخص پيچ و خم پيشاني كوهستان (Smf)، شاخص نسبت عرض كف دره به ارتفاع دره (Vf)، شاخص گراديان طولي رودخانه (Sl)، شاخص تقارن توپوگرافي عرضي (T)، شاخص عدم تقارن حوضه زهكشي (Af)، شاخص پيچ‌ و خم رودخانه اصلي (S)، شاخص انتگرال هيپسومتريك (Hi) استفاده شده است. در اين زمينه، مجموعه‌اي از روش‌هاي توصيفي، ميداني و تحليلي به كار گرفته شد. نقشه‌‌هاي توپوگرافي، زمين‌شناسي و عكس‌هاي هوايي و مدل رقومي ارتفاعي منطقه (Dem) با قدرت تفكيك 12/5 متر و نرم افزار ARC GIS 10.1 و تصاوير گوگل ارث به عنوان داده‌هاي اصلي تحقيق مورد استفاده قرار گرفتند. از تركيب شاخص‌هاي ژئومورفيك با يكديگر مي‌توان رابطه‌ي شاخص فعاليت‌هاي تكتونيكي (Lat) را به دست آورد. نتايج بررسي نشان داد كه طبق مقادير (Lat) حوضه كرگانرود در كلاس فعّاليّت‌هاي نئوتكتونيكي زياد قرار دارد. شواهد ژئومورفولوژيكي به دست آمده مانند پادگانه‌هاي رودخانه‌اي ارتفاع يافته، وجود تراس‌هاي قديمي و آبراهه‌هاي عميق شده و دره‌هاي V شكل نتايج تحقيق را تأييد مي‌كند.

Introduction Tectonic 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 length-gradient index (Sl), transverse topographic symmetry factor (T), mountain-front 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. Methodology The 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 V-shaped and U-shaped flatfloored valleys, the valley floor width-to-height 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. Mountain-front 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 Discussion Geomorphic 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 mountain-front 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 width-to-height 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. Conclusion The 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.