مطالعه‎ي مورفوتكتونيك فعّال حوضه‎ي آبخيز حصارك (شمال‎غرب تهران) با استفاده از شاخص‎هاي مورفومتريك

The Study of Active Morphotectonic of Hesarak Aquiferous Basin (Northwest of Tehran) with the Use of Morphotectonic Indexes

حوضه‎ي آبخيز حصارك در شمال‎غرب شهر تهران، در ناحيه‎ي زمين‎شناختي البرز مركزي واقع شده است. با توجّه به فعّال بودن حركات كوه‎زايي در برخي از نواحي ايران و براي آگاهي از ميزان فعّاليّت نيروهاي دروني و تكتونيكي در منطقه‎ي مطالعاتي، از هشت شاخص ژيومورفيك كه عبارت‎اند از: شاخص‎هاي منحني هيپسومتريك و انتگرال هيپسومتريك، شاخص پيچ‎وخم پيشاني كوهستان، شاخص‎هاي عدم تقارن حوضه‎ي زهكشي و تقارن توپوگرافي عرضي، شاخص نسبت پهناي كف درّه به ارتفاع درّه، شاخص گراديان طولي رودخانه و شاخص پيچ‎وخم رودخانه استفاده شده است. به‎طوركلّي، دانش تكتونيك ژيومورفولوژي، مطالعه‎ي ساختارهايي است كه بر اثر تكتونيزم و عكس‎العمل ميان فرآيندهاي تكتونيكي و ژيومورفولوژيكي حاصل شده‎اند. رودخانه‎ي فصلي حصارك، حوضه‎ي آبخيز حصارك را زهكشي كرده و از به‎هم‎پيوستن دو شاخه‎ي اصلي تشكيل شده است. بنابراين، هر شاخه به‎عنوان يك زيرحوضه در نظر گرفته شده و شاخص‎هاي ژيومورفيك براي هر زيرحوضه، جداگانه مورد بررسي قرار گرفته است. با توجّه به ويژگي‎هاي خاصّ هر يك از شاخص‎ها و نتايج حاصل از آنها در سه زيرحوضه، مشخّص شد كه حوضه‎ي حصارك از نظر تكتونيكي جز مناطق فعّال است.

Introduction Active tectonic or active geomorphology tectonic studies effective dynamic processes in shaping the earth and existent prospects. In Geology the term tectonic points out to transformed buildings and evaluation of the tolls and structures. To evaluate aquiferous basin of Hesarak in terms of kind of tectonic, we used 8 morphotectonic indexes: Hypsometric curve and Hypsometric integral, Mountain front sinuosity, Drainage basin asymmetry factor, Transverse topographic symmetry factor, Ratio of valley floor width to valley height, Stream length-gradient index, River sinuosity. Hesarak aquiferous basin is 14/40 km2 and located in northwest of Tehran and seasonal river of Hesarak drains the basin. Methodology To obtain needed data for this research 1:25000 topographic map of Kan and satellite picture of Tehran IRS used. First needed information layers made of Arc Gis which is an information system then geomorphic indexes were calculated for Hesarak basin. With respect to Hesarak river in which 2 main branches of Chapdareh and Dochenaran at 1720 altitude joint, needed indexes for3 sub-basin at eastern branch (Chapdareh), western branch (Dochenaran) and sub-basin of Hesarak are calculated separately, and based on gained amounts, the rate of tectonic activity of aquiferous basin of Hesarak measured. Results and Discussion 1. Hypsometric curve and Hypsometric integral Hypsometric curve describes distribution of heights vertically on an area, for instance, in a drained area. This curve draws by indicating ratio of total height on total acreage. By drawing hypsometric curve and doing related calculations we come to know that Hesarak basin is a mature stage. 2. Mountain front sinuosity This index indicates balance between erosion forces and tectonic at mountain front usually, active mountain fronts have less amount of this index. In Hesarak it is equal with 1.84. 3. Drainage basin asymmetry factor Length of canal and minor drainage on both sides of main chanal are other morphotectonic indexes which used to evaluate active tectonic. When numerical amount of this index is around 50 existent of minor drainage symmetry of main canal is approved and less or higher amount of 50 stating rise on right and left bank of main canal in row. The amount of this index in eastern sub-basin is 67, in western sub-basin is 40, and Hesarak sub-basin is 63. 4. Transverse topographic symmetry factor This index is used to evaluate drained basin in terms of tectonic transverse and asymmetry if they exist. In fact the index of T is a vector with specific orientation and amount of 0-1. In fully symmetry basin the amount of this index is equal with zero. With decrease of basin symmetry, T index increases and it will be near to1. Amount of transverse topographic symmetry at 3 sub-basins are between 0.18-0.46. 5. Ratio of valley floor width to valley height This index is the factor of separation of valley floor width with V shape valley. In fact little amount of this index, indicates deep valley and V shape indicates existent of active tectonic. Numerical amount of this index at sub-basin of chapdareh is around 0.14, Dochenaran 0.15 and Hesarak 0.83. With respect to this fact that numerical amount of less than 1 is the sign of active tectonic, therefore Hesarak aquiferous basin is an active area in terms of tectonic. 6. Stream length-gradient index To calculate gradient index of river first, profile length of main chanal of 3 sub-basins on topography map1:25.000 are gained and the amount of this index has gained at distances of 100-300 m (the amount of length are not equal at each sub basin and linear profile basin has been determined based on shift of incline). At last to calculate total SL of stream, you should first calculate the average of all SLs. The average of this index in this studied area is 76.18 at Hesarak sub-basin till 473.38 at western sub-basin. 7. River sinuosity This index is stating the point that rivers which are very curvy are near to balanced position where as rivers which are direct are sign of an area where is ripe of area and has neo- tectonic activity. With respect to gained amounts of such an index, we can say basin is not in balanced position and tectonic is active in area. Conclusion In this research we used 8 geomorphology indexes to classify tectonic activity at Hesarak aquiferous basin, the result concerning all 3 sub-basins has been summarized at table1. With respect to gained amounts and related explanations of each index, we can state that Hesarak aquiferous basin is active in terms of tectonic.