تحلیل فضایی تقارن یا عدم تقارن شبکه ی زهکشی در باتولیت الوند

عقیده ی عمومی بر این باور است که در مطالعه و بررسی لندفرم‌های توپوگرافیکی و الگوی سیستم شبکه‌ی زهکشی، با استفاده از شاخص‌های ژئومورفیک، می‌توان وضعیت عملکرد تکتونیک فعال را ارزیابی نمود. این در صورتی است که ساختار اولیه ی زمین، نوع سنگ و عوامل محلی تعیین‌کننده ی میکروکلیما، مدنظر قرار نگیرند. در نظر گرفتن این عوامل، نتایج شاخص‌های ژئومورفیک را مورد تردید قرار می‌دهد. در این مقاله نقش عوامل مختلف در شاخص عدم تقارن زهکشی 117 زیرحوضه ی توده ی کوهستانی الوند همدان بررسی می‌گردد. برای این منظور پارامترهای جهت شیب غالب، متوسط شیب، متوسط ارتفاع، مساحت سمت راست و چپ، طول آبراهه ی اصلی، فاصله ی هوایی آبراهه ی اصلی، مجموع طول آبراهه‌ها، فاصله ی سرچشمه تا خط تقسیم آب و نوع سنگ بخش‌های مختلف زیرحوضه ها مشخص گردید. با استفاده از این پارامترها، ضریب گراولیوس، نسبت کشیدگی، تراکم زهکشی و شاخص عدم تقارن محاسبه و تجزیه‌وتحلیل شد. در زیرحوضه‌های وسیع شیب زیاد و متفاوت بودن جهت شیب دامنه، از جمله بردارهایی است که می‌تواند در عدم تقارن شبکه ی زهکشی اثر مهمی داشته باشد. کشیدگی بیشتر حوضه با تراکم زهکشی کم، در سنگ گرانیت، بر عدم تقارن شبکه ی زهکشی حوضه اثر دارد. بررسی شاخص مقدار شیب در کلاس‌های مختلف شاخص عدم تقارن در زیرحوضه‌هایی با سنگ غالب غیر از گرانیت، دال بر این است که تفاوت مقدار شیب چپ و راست در عدم تقارن حوضه نقش چندانی ندارد. بررسی ضریب گراولیوس دال بر این است که حوضه‌های پایدارتر ازنظر شاخص عدم تقارن، به شکل دایره‌ای نزدیک‌تر هستند.

Spatial Analysis of Symmetry or Asymmetry Drainage Index in the Alvand Batholith

1 Introduction The public opinion is that in the landformtopographic study, the drainage system, and the drainage pattern, by using geomorphic indicators one could assess the performance of active tectonics (Maghsoudi et al., 2011). This is true if we do not consider the primary structure of the earth, rock type, and local factors determining the microclimate (Abedini et al., 2015; Jafari and Barati, 2018). Considering these factors, the results question the geomorphic indicators. In this paper, the researchers study the role of various factors in the drainage asymmetric index of 117 subbasins in the Alvand mountainous of Hamadan. A subject not considered in the drainage asymmetry index of areas such as the Alvand mass of Hamadan is the shape of the magma lies between the sedimentary layers at the time of cooling. In relation to the drainage symmetry index, this paper tries to study the physical properties and the effect of local factors in the watersheds of the Alvand Hamedan Batholith. Although the drainage networks asymmetry of is analyzed with morphotectonic indices, the basic land structure, lithological and erosion properties can also affect the asymmetry of the basin.  2 Methodology For this purpose, parameters of the dominant slope, average slope, average height, the area of the right and left of the river, main river   length, river airy length total length of the rivers, distance of the source of the dividing line, and rock types in different parts of subbasins were determined. These parameters were used in calculation and analyses of Gravelius coefficients (compactness coefficient), elongation ratio, drainage density and asymmetry index. For the phenomenological investigation of the drainage network index status, with the help of topographical maps 1: 50000, the outlet point of the Alvand Batholith subbasin was identified in the mountainous area. Accordingly, 13 subbasins were classified in the Alvand Batholith. Class B subclasses are located inside the subclasses of class A and subclasses of class C are located inside class B subclasses. To separate subbasins in each of the 13 subbasins, the longest river was considered as the main river. All the streams connected to it are separated as subclasses of Class B. In the next step, the main river basin in the subbasins of class B was also determined and the rivers entering it were designated as the subclass C. Accordingly, a total of 117 subbasins was identified. 3Result and discussion  In order to investigate the factors affecting the asymmetry index (tectonics, lithology, microclimate and physiography’s characteristics of subbasins); at first the status of subbasin rock was studied. The largest area lies in the cordieriteGabbro Stone (15.4%), and the smallest area lies in the Granite Schist Gabbro and cordieriteGabbro (0.69%) groups. Investigating the elongation coefficient the subbasins in which the dominant stone is granite shows a very low elongation ratio of these subbasins. They lie in a group that is less elongated than the rest of the subbasins in other rocks. In terms of the Gravelius coefficient, it can be said that the more symmetrical the basins tend to be circular in shape, as the mean Gravelius coefficient of the stable subbasins for the asymmetric index is 3.35 vs. 2.64 in the relatively stable subbasins, which fully confirms this point. The ratio of bifurcation of unstable subbasins is very high, and according to the rocky area, the drainage density in the unstable subbasins is inversely related to the slope of the basin.   4Conclusion In general, subbasins of the northeast slopes are more asymmetrical than subbasins in the southwest slopes, which it can be due to the zhizman form of the Alvand Batholith, the differences in the physiographic properties, and finally the dominant subbasin rocks. In the wider subbasins of the slope and the varying direction of slope, are vectors that can have a significant effect on the drainage network asymmetry. The further elongation ratio of drainage of the basin with low drainage density, in granite rock, affects the drainage network asymmetry of the basin. Investigating the conditions of slope values in different classes of the asymmetric index in subbasins other than granite rocks indicates that the difference between the value of the left and right slopes of rivers is not a significant effect in the basin asymmetry. By decreasing the subbasins area, the effect of direction and slope value, elongation ratio and bifurcation coefficient on asymmetric basins decreases. The study of the Gravelius coefficient indicates that the shape of the more stable subbasins is closer to the circular shape. The bifurcation ratio of most unstable subbasins is very high. The drainage densities of unstable subbasins have an inverse relationship to the basin slope. The difference between the rock types on the left and right and the river’s position on the boundary between the two rocks have an important role in the basins asymmetry.