تاثیر تکتونیک در تغییرات نیمرخ طولی رودخانه‌ها (مطالعه موردی: رودخانه علامرودشت در جنوب استان فارس)

رودخانه ها به دلیل حساسیت نسبت به تغییرات تکتونیکی با تغییرات محسوسی در ویژگی های هندسی خود به فعالیت های زمین ساختی در یک منطقه واکنش نشان می دهند. شاخص های ژئومورفولوژیک استخراج شده از نیمرخ طولی رودخانه ها، ویژگی های نیمرخ طولی و پاسخ های آن ها به فعالیت های تکتونیکی و تغییرات سطح اساس را بیان می کنند. هدف این مقاله بررسی چگونگی تاثیر عوامل تکتونیکی در تغییرات نیمرخ طولی رودخانه علامرودشت در جنوب استان فارس و شناسایی شکل و محل این تغییرات است، بدین منظور با تقسیم محدوده به سه زیرحوضه اصلی فداغ، بیرم و علامرودشت، با استفاده از داده های استخراج شده از نقشه های توپوگرافی و زمین شناسی و مدل رقومی ارتفاع (dem)، شاخص های تقارن حوضه (af)، شاخص هیپسومتری (hi)، شاخص نرمال شده نیمرخ طولی رودخانه، شاخص گرادیان طولی رودخانه (sl) و شاخص تقعر (sci) در نرم افزارهایarcgis ، excel و matlab به صورت کمّی مورد تجزیه وتحلیل قرارگرفته و سپس نتایج این شاخص ها با یکدیگر و با داده های زمین شناسی و بازدیدهای میدانی مقایسه شدند. نتایج به دست آمده از شاخص های نیمرخ طولی، گرادیان طولی و شاخص تقعر رودخانه نشان دهنده تغییرات نیمرخ طولی رودخانه علامرودشت بر اثر عوامل تکتونیکی است. به دلیل فعالیت بیشتر گسل ها در زیرحوضه علامرودشت در شمال غرب محدوده، تغییرات نیمرخ رودخانه نسبت به سایر زیرحوضه ها بیشتر است. همچنین عدم وجود تغییرات سنگ شناسی در بخش اعظم طول بستر رودخانه، مقادیر و ناهنجاری های مشاهده شده در شاخص های ژئومورفیک نشان دهنده نقش مهم تر عوامل تکتونیکی در تغییرات نیمرخ طولی رودخانه علامرودشت است.

The Effects of Morphotectonic on the changes of rivers profile (Case study: Alamarvdasht River, South of Fars Province, Iran)

IntroductionTectonic geomorphology is defined as the study of landforms produced by tectonic processes, or the application of geomorphic principles to the solution of tectonic problems. The quantitative measurement of landscape is based on the calculation of geomorphic indices using topographic maps, aerial photos, satellite images and field work. Geomorphic indices are extremely useful for the study of drainage basins. The quantitative measurement of rivers allows geomorphologists to calculate parameters, or geomorphic indices, which are useful in establishing the characteristics of a river. The analysis of geomorphic indices gathered in the study of drainage basins, and their integrated comparisons, can contribute to the understanding of the morphotectonic structure. The study area of this paper is Alamarvdasht river basin that is located in south of Fars province, south of Iran. Active tectonics in the uplifting anticlines of Zagros structural zone, especially in Simply Folded Belt, has produced various fluvial landforms. Quantitative measurement of rivers helps to identify and to analyze tectonic deformation in an area. Alamarvdasht river basin is located in the Simple Folded Belt of the Zagros Mountains in Fars region. The Alamarvdasht basin area is 2925 Km2 and located between latitudes 27˚17'N 27˚55'N and longitudes 52˚37'E53˚46'E. The maximum elevation of study area is 1860 meters in the south of basin and the minimum elevation is 360 meters in northwest of the basin. The study area is located along a simply folded belt of southeastern Zagros and shaped by the anticlines and synclines with SW ndash;NE trending. The aim of this paper is assessment the effects of morphotectonic on the changes of Alamarvdashy river profile by using some geomorphic indices.MethodologyActive tectonic geomorphic indices were used as known to be useful in active tectonic studies. and has been previously tested as a valuable tool in different tectonically active areas. Spatial tools including ArcGIS, Excel and MATLAB software. In the following, geomorphic indices were analyzed by 15 meters Digital Elevation Model (DEM)) and gemological maps (1:100000). The Alamarvdasht basin was divided to 3 main subbasins (including Fedagh, Beyram and Alamarvdasht) and the following geomorphic indices were calculated: streamgradient index (SL), drainage basin asymmetry (AF), hypsometric integral (HI), normalized long profile and stream concavity index (SCI). To study the indices there is a formula which describes in results discussion part. The results have also combined field observations of the region and geodynamics data of the area.Results and DiscussionsThe asymmetry factor (Af) is an important geomorphic index that is useful in the geomorphological analysis of drainage basins and it has been used to measure the drainage basin asymmetry. The index is defined as follows:Af= (Ar/At) 100Where Ar is the right side area of the basin of the master stream (looking downstream) and At is total area of the basin that can be measured by ArcGIS software. Value of this index in Fedagh subbasin is higher than other subbasins due to long and thrust faults in this subbasin.The Hypsometric index (HI) is defined as the relative area below the hypsometric curve and it is an important indicator for topographic maturity and can shed light on the local effects of denudation and tectonic uplift. A simple equation to calculate this is:Hi= (average elevation ndash; min. elev.) ndash; (max. elev. minus; min. elev.)Computing the HI index for each subbasin shows that Alamarvdasht subbasin has higher values than other subbasins and higher tectonic activity.Normalized long profile is a good way to analyze and to compare long profiles of several rivers. Convexities in the profile commonly occur where a channel crosses a fault, resistant band of rock, a collection of instream boulders or a paleosurface. This index shows differences of long profile of Alamarvdasht river and convexity in northwest of river is an indicator of active faults that make a distinct knickpoint in the profile of river.The streamgradient index (SL) is used to discuss influences of environmental variables on longitudinal stream profiles, and can be used to evaluate relative tectonic activity. The calculation formula is in this manner:SL= (∆H/∆L) LWhere (∆H/∆L) is local slope of the channel segment located between two contours and L is the channel length from the divide to the midpoint of the channel reaches for which the index is calculated. Amount of Delta;H, Delta;L and L are gained from DEM. This index is calculated along the master river for each subbasin. The results of this index shows high values and anomalies in northwest of basin due to active faults.Stream concavity index (SCI) shows the change in its longitudinal slope helps find anomalies, the erosive state of bedrock, rock uplift, increases in drainage area, and various downstream relationships. To measure the concavity of a channel, we sused the areanormalized concavity in Excel and MATLAB that allows to compare a channel rsquo;s concavity with other channels made up of different lengths or gradients. This method is based on calculating the integral area between the channel profile curve and the line between channel endpoints. SCI shows a concave profile in Fedagh and Beyram subbasins (SCI>0) and a convex profile (SCI<0) in Alamarvdasht subbasin. ConclusionAccording the results, the high value of SL, HI and SCI indices were found in northwest of basin where the river crosses some active faults. The highest value of AF index was found in Fedagh subbasin which shows the effects of thrust faults along the main folds in this subbasin. Applying the method for three subbasins shows that the geomorphic indices considered to be useful for evaluating the effects of active tectonics on the Alamarvdasht river profile. Because there isn rsquo;t any distinct lithological changes along the river, high values and anomalies of indices are related to active tectonic factors which their evidences could be found by satellite images and field observations.