Effects of Morphotectonic in Longitudinal Profile of Vaz Stream, Northen Alborz, Mazandaran Province

بررسي اثرات مورفوتكتونيك در نيمرخ طولي رودخانه‎ي واز؛ البرز شمالي، استان مازندران

رودخانه‌ها نسبت به حركات تكتونيكي حساس هستند و رابطه‎ي نزديكي بين لندفرم‌هاي رودخانه‌اي و حركات تكتونيكي وجود دارد. شاخص‌هاي ژيومورفيك به‎عنوان ابزاري براي مشخص‎كردن ساختارهاي جديد و فعال به‎كار مي‌روند. منطقه‎ي مورد مطالعه، حوضه‎ي آبريز واز است كه در شهرستان نور در استان مازندران واقع شده است. در اين حوضه گسل‌هاي متعددي مانند، گسل شمال البرز و گسل كلرد سازندهاي زمين‎شناسي را جابه‎جا كرده‌اند. در اين مطالعه شاخص‌هاي ژيومورفيك، مانند منحني‌هاي هيپسومتري، شاخص تقارن حوضه، نيمرخ طولي نرمال‎شده‎ي رود و شاخص گراديان طولي رود با استفاده از مدل ارتفاعي رقومي (DEM) در نرم‎افزار ARC GIS تهيه و مورد تحليل قرار گرفتند. نيمرخ طولي نرمال‎شده‎ي رود (SLK) براي زيرحوضه‌ها ترسيم شد. به‎دليل بي‎بُعد بودن اين شاخص، حداكثر تقعر (Zmax ) دره‌هاي مختلف مورد مقايسه قرار گرفتند. سپس با محاسبه‎ي مقادير SL شاخه‌هاي مختلف رود و ميان‎يابي اين مقادير، نقشه‎ي SL رود تهيه شد. مقادير SL بين 75 تا 1375 متغير بوده است. تقريبار 75 درصد مقادير SL كمتر از 700 متر را نشان مي‌دهد. بيشترين مقدار SL در قسمت جنوبي حوضه و در مجاورت گسل بزرگ البرز شمالي قرار گرفته است. مقادير بالاي SL تنها در برخي موارد با وجود گسل‌ها در ارتباط است. با استفاده از آزمون كروسكال ـ واليس، اين مقادير مورد ارزيابي قرار گرفت كه نشان‎دهنده‎ي تفاوت معنا‌داري بين سنگ‌شناسي حوضه و مقادير SL است.

Extended Abstract Introduction The river is sensitive to tectonic movement. There are close relationship between the fluvial landform and tectonic movement. Geomorphic indices are indicators capable to detect landform responses to recent deformation processes and therefore have been broadly used as a recognition tool to characterize sectors deformed by active faults. Some geomorphic indices such as hypsometric integral (Hi), drainage basin asymmetry (Af) , ratio of valley floor width to valley height (Vf), stream length–gradient index (SL), transverse topographic symmetry factor (T) and basin drainage shape (Bs) have been used extensively to detect tectonic activity at different places around the world. The Vaz watershed is located in the northeastern Alborz, Mazandaran province and county of Nour. Vaz watershed geologically consists of marine and continental sequences, which deposited through the Msozoic to Cenozoic. Several faults system such as North Alborz fault (mangol) and Kelerd fault have been defined in Vaz watershed. Methodology Quantification of geomorphic response to tectonic activity may be done by using geomorphic indices. Digital elevation models (DEMs) were used to calculate geomorphic indices. The asymmetry factor (AF) of catchments was used to detect possible tectonic tilting at the Vaz catchment. Values of AF above or below 50 indicate that the basin is asymmetric. To avoid possible confusions between the catchments AF expressed as the absolute value minus 50, with an arrow indicating the asymmetry direction in the map. The hypsometric curve of a catchment represents the distribution of area and altitude within it. In this study, the curves have been depicted by plotting the relative area (0–1) above each relative height (0–1). The shape of this curve is related to the degree of dissection of the basin. Convex hypsometric curves characterize relatively “young” weakly eroded regions, S-shaped curves characterize moderately eroded regions, and concave curves characterize relatively “old” highly eroded regions. Normalized longitudinal river profiles were applied to describe the geomorphic response of streams in regions with active tectonic. The advantage of these profiles is the direct comparison of valleys with different lengths and absolute gradients because they are dimensionless. The abscissa is d/D, where D is the profile length and d is the distance of the individual data points from the valley summit or watershed at one end of the profile. The ordinate represents normalized elevation to the absolute gradient along the valley (e/E). Normalized profiles characterize the degree of grading of a river where zmax is the maximal concavity on graph, and ?d/D is the normalized distance between zmax and the source. The computation of SL is based on the interpolation of a discrete value. SL allows for direct comparisons between small rivers that typically have steep profiles and larger rivers that have more gentle profiles. The values needed to compute SL are measured from topographic profiles. The calculated SL of each section of the principal stream is assigned at the midpoint of the section. Then, the spatial distribution of SL is obtained by interpolating the values of the midpoints. Results and Discussion The hypsometric curves show differences between the curves of the Vaz catchment. We distinguished three groups with a particular curve shape. The basins located in the south western sector (3 and 4) characterized by a concave shapes. The second group corresponds to the basins in the south eastern sector (1 and 2) have S-shaped curves. The basins located in the western sector (5 and 6) characterized by a convex shapes. We have divided AF absolute values in four classes: AF < 5 (symmetric basins), AF=5–10 (gently asymmetric basins), AF=10–20 (moderately asymmetric basins). AF values in the south eastern part of the Vaz catchment present a pattern with asymmetries at north east side, that coinciding with the north Alborz fault (Mangol fault). In the Vaz catchment, there is no defined pattern in AF values. For Normalized longitudinal profiles an objective comparison of the stream profiles and in order to recognize the similarities between them, ?d/D was plotted against zmax values. The streams in the Vaz drainage basin can be divided into two groups based on the computed ?d/D and, zmax, parameters from the normalized longitudinal profiles. The first group consists of 1, 2 and 7 rivers, which are denoted by values of ?d/D ( < 0.288) and a maximal concavity (zmax < 0.14). The second group contains 3, 4, 5 and 6 rivers, and these are characterized by higher values of ?d/D ( > 0.33) and varying zmax between 0.177 and 0.237. On the interpolated SL map, the SL values range between 75 and 1357 m. Most of the study area (75%) shows SL values smaller than 700 m. The highest values are located in South of the Vaz watershed and near North Alborz fault. Conclusion The geomorphic indexes calculated in this work indicate that the Vaz catchment is tectonically active in near north Alborz fault. But, in the other sector of Vaz catchment, a Kruskal–Wallis test shows that the bedrock lithology has impact on the SL value. Therefore, the SL variations can be related mainly to lithology and less important to uplift due to tectonic forcing.