‌بازسازي تغييرات شبكه هيدروگرافي پيرامون دماوند براساس داده‌هاي مورفومتري و رسوب‌شناسي

Reconstruction of hydrographic network changes around Damavand based on morphometric and sedimentological data

يكي از عوامل زمين ساختي كه نقش قابل توجهي در شكل زايي دارد، آتشفشان ها و مواد مذاب ناشي از آن ها است. فوران هاي آتشفشاني باعث دگرشكلي مورفولوژي و ايجاد ساختمان لندفرمي جديد مي شود. آتشفشان دماوند با فوران هاي متوالي خود، طي كواترنري تغييرات قابل توجهي در پيرامون خود ايجاد نموده است. در اين پژوهش، نقش گدازه هاي دماوند در تغيير مسير رودخانه ها مورد بررسي قرار گرفته است. براي دستيابي به اين هدف، روش تحليلي و بازسازي زماني تغييرات به كار گرفته شده اند. تكنيك كار بر مقايسه مورفومتري رودخانه ها و دامنه هاي مشرف به دره ها به سه روش مقايسه ارتفاع و ضخامت رسوبات آبرفتي، تعيين حدود گسترش رسوبات رودخانه ايي و درياچه ايي و بازيابي آخرين حد ارتفاعي گدازه هاي مشرف به دره ها استوار است. رودخانه هراز به طور متوسط 4 كيلومتر، رودخانه دليچاي 3 كيلومتر و رودخانه لار 2 كيلومتر بر اثر گدازه هاي دماوند به مرور و طي فوران هاي متوالي تغيير مسير داده اند. مسير رودخانه ها عمدتا پس از شكل گيري درياچه ها بر اثر انسداد دره ها و شكسته شدن سد ايجاد شده اند. قابل ذكر است كه سطح اساس رودخانه هاي مذكور حدود 100 متر بالا آمده است.

Introduction One of the tectonic factors that play a significant role in the formation is the activity of volcanoes and the flow of volcanic materials. The volcanic eruptions cause morphology and create new landforms. Damavand volcano with its successive eruptions during the Quaternary has made significant changes to its surroundings. In this study, the role of Damavand lava flows in changing the bed rivers has been investigated. The main purpose of this study was to investigate the effects of the latest Damavand volcanic eruptions on the behavior and direction of these bed rivers around it.Materials and MethodsThe technique has been based on three main methods: 1) Comparison of the height and thickness of sedimentary deposits obtained through the morphometry of transverse profiles on the fieldworks. 2) Determination of the extent of lake sediment distribution in the Haraz, Lar and Delichai areas, which is transmitted on the map based on field data and GPS coordinates. 3) The extent and altitude of the lava flows overlooking the Haraz, Lar and Delichai valleys have been recorded and transferred to the maps. In addition, much of the data was obtained through indirect observations through satellite imagery and then combined with field findings. To achieve this, the length of the rivers is divided into two distinct intervals. Then morphometric data of valleys, sedimentary evidences and dynamics of rivers in the form of the mentioned intervals are studied and compared temporally and spatially. At the analysis stage, the above data have been adjusted with the radial pattern of the drainages around Damavand to the main grid as well as the volcanic cone structure and then the longitudinal routes of the rivers have been reconstructed.Results and Discussion Evidence indicates that there was no significant change in the waterway during the Abask interval toward Amol and the outlet of the basin (Interval 1). However, in the Abask-to-Plour (Interval 1) range, the presence of lava gravels carried by the Haraz River and deposited in the northwest of the Abask landslide indicates that the river changes over time. Also, the lava flows overlooking the Haraz valley represent major changes that have occurred in the past. In this regard, the Haraz River has dug up to a depth of 100 meters in the sediments of lakes and rivers. The amount of river diversion at the intersection of the Lasem River to the Haraz River (Interval 1) and the displacement of the river bed is approximately 4400 meters from its present bed. The modified route in the second interval is about 4200 meters. The bed river of the Haraz River redirection from the old Plour Bridge to the intersection of the Haraz and Lasem (interval 1) is about 1600 meters. Reconstruction of the old Haraz River bed route indicates that the stream has stretched past the eastern part of the Tizkuh sedimentary slope. The average redirection of the Haraz River in this interval is estimated at about 1000 meters. In the second interval, the most deliberate distance of the Delichai River is the intersection of this river with the Lar River. This displacement is estimated to be about 4100 meters. Damavand lava flows in this region have created a lava barrier along the Delichai River Route, changing the east-west direction. The mean flow direction of the Delichai River is estimated to be 2500 meters. In addition, studies indicate that as a result of the formation of lava dams, the base level of the above-mentioned rivers has also risen by an average of 100 m above the level of its original base level, prior to the Damavand eruption.Conclusion The results indicate that there have been major changes except for one in two intervals, and that these changes have occurred in two major forms. One is the formation of lava dams caused by Damavand eruption, which has caused major changes in the longitudinal profile of the rivers and the deepening, formation of gullies and then filling of the canals are three forms of this process. There are four levels of terrace that illustrate this. Secondly, the diversion of rivers occurred in the second and third intervals, where the presence of river deposits beneath the lava flows as well as the general topography of the valleys can clearly identify the longitudinal paths of the waterways in these two intervals.