بررسی مورفوتکتونیکی غرب البرز مرکزی با استفاده از شاخص های ژئومورفیک

البرز، قسمتی از کمربند کوهزایی آلپهیمالیا، نتیجه همگرایی صفحه ایران مرکزی و اوراسیا از اواخر تریاس بوده است و به واحدهای البرز خاوری، البرز مرکزی و البرز باختری تقسیم می شود. گستره مورد مطالعه در غرب البرز مرکزی قرار گرفته است و شامل حوضه های آبریز پلرود، صفا رود و چالکرود می‌باشد. از آن‌جا که رودخانه ها به سرعت تحت تاثیر فعالیت های زمین‌ساختی واقع می شوند، می توان با بررسی الگوی حوضه‌های آبریز و ژئومورفولوژی آن‌ها به تاثیر فعالیت زمین‌ساختی بر روی سیستم های رودخانه‌ای و زیر‌حوضه های آن ها پی برد. ریخت سنجی حوضه آبریز شامل بررسی آبراهه ها با استفاده از شاخص های ژئومورفیک حوضه آبریز صورت می گیرد. در این پژوهش از شاخص ها ی ژئومورفیک  حوضه آبریز برای ارزیابی میزان فعالیت زمین‌ساختی فعال حوضه ها و رودخانه ها استفاده شده ‌است. بدین منظور هفت شاخص ژئومورفیک، گرادیان طولی رودsl) )، ناهنجاری سلسله مراتبی (δa)، شاخص انشعابات (r)، انتگرال و منحنی فرازسنجی (hi)، برجستگی نسبی (bh)، تراکم زهکشی (dd) و ضریب شکل (ff) به کار گرفته شده و در 38 زیرحوضه زهکشی گستره مورد پژوهش اندازه گیری شده ‌است. در نهایت با استفاده از شاخص زمین ساخت فعال نسبی (iat) گستره مورد بررسی به چهار رده فعالیت زمین ساختی بسیار بالا، بالا، متوسط و پایین تقسیم بندی شده ‌است. نتایج شاخص‌های مزبور به خوبی با نتایج بدست آمده از مطالعات صحرایی هم‌خوانی دارد. نتایج به دست آمده نشان می‌دهد که بخش مرکزی و جنوبی منطقه مرتبط با فعالیت گسل‌های خزر، لاهیجان، کشاچال، سماموس، زرین رجه، درفک، دیلمان و سهیل، دارای بیشترین فعالیت زمین‌ساختی می‌باشد. در حالی که در بخش های شمالی که در بخش فروافتاده ساحلی دریای خزر واقع شده اند، مقدار این شاخص خیلی کم بوده و بیانگر فعالیت زمین‌ساختی پایین می‌باشد.

Morphotectonic study of West Central Alborz by using geomorphic indices

Alluvial rivers react quickly and consistently to active tectonic deformation and by studying the relation of river with tectonics; the recent activity can be shown. Geomorphological analyses allow the study of modifications that affect river basins, particularly modifications due to active tectonics, and investigate the morphotectonic evidence of the area such as, bisected alluvial fans deflected streams basin and cut Quaternary deposits. Morphometric indices are useful to analyze the influence of active tectonics in large areas. Considering that the study area is extensive and has numerous rivers and main faults, the study of Morphometric indices is useful. The aim of this paper is to extract information on tectonic activity and landscape evolution of the study area. This study focuses on the morphotectonic indices of the west Alborz and includes three principle streams the polrud, safarud and chalkrud. The study area is located in the west Alborz. Alborz, a part of HimalayaAlp mountain range, has formed by convergence between Iranian microplate and Eurasia since Triassic Period and it is divided into West, Central and East Alborz. The study area comprised of a series of active faults and the mostly faults are reverse and thrust type with few normal faults. Main faults of study area include Khazar fault, samamus fault, kashachal fault, soheil fault, deylaman fault, zarinraje fault, lahijan fault. The DEM data of 30 m have been used to generate the drainage basins of all the catchments. The seven indices: Hierarchical anomalies (Δa), Bifurcation (R), Drainage density (Dd), Stream length–gradient index (SL) Form factor (Ff), Relative relief (Bh), and hypermetric integral (Hi) for each subbasin were measured and a unit index obtained as index of active tectonics (Iat). Afterwards the relationship between indices with morphotectonic characteristics and active faults of the area was investigated. In this study we use of survey satellite image1:100,000 geological maps of Geological Survey of Iran, digital elevation model. The field observations show that this study area has been active tectonic and the Basin morphometric indices to evaluate the active tectonic activity of river basins. In order to evaluate the active tectonic seven morphometric indices were used as follow: Hierarchical anomalies (Δa), Bifurcation (R), Drainage density (Dd), Stream length–gradient index (SL) Form factor (Ff), Relative relief (Bh), and hypermetric integral (Hi) and finally tectonic activity Index (Iat) was measured in 38 sub basins in the study area. Anomalies of the stream length gradient are associated with the faults Including Khazar fault, samamus fault, noosha fault, soheil fault. The drastic increase in SL occurs at the subbasin 35 related to kashachal fault. The High hypsometric integral and convex hypsometric curves indicate younger stage of landscapes and relate to neotectonic rejuvenation. The hypsometric Integral (Hi) index is calculated in range between 0 and 0.6, high values of the hypsometric integral are convex, and these values are generally greater than 0.35. The results of our study indicate that the highest mean value of Hi in subbasin 35 has undergone the highest intensity of neotectonic activity of kashachal fault. By using relative tectonic activity Index (Iat) the area was divided into 4 classes of tectonic activites as very high, high, medium and low. In the study area, about two thirds of the total area, Iat is classified as 1, meaning high to very high active tectonics In this research Various geomorphic features and landforms were observed in the study area where fault scarps ، unpaired terraces،Vshape valleys. Triangular facets, hanging valleys, deformed alluvial fan deposits, and deep gorges incised near mountain fronts exist. Results obtained show that central and south portion has the highest rate of tectonic activity and moderate relative tectonic activities level, has been found in the Nw and Sw part of the study area. While in the northern parts of the coast the value of this indicator is very low and tectonic activity is low.The results indicate the recent tectonic activity by movements of several faults such as Khazar fault, samamus fault, noosha fault, soheil fault, deylaman fault, zarinraje fault and lahijan fault of the study area.