پديد آورندگان :
احمدي، طيبه دانشگاه خوارزمي , صفاري، امير دانشگاه خوارزمي - دانشكده جغرافيا , يماني، محتبي دانشگاه تهران - دانشكدۀ جغرافيا , رضايي، خليل دانشگاه خوارزمي - دانشكده زمين شناسي
كليدواژه :
رود هراز , درياچه هاي سدي , پادگانه , آتشفشان دماوند
چكيده فارسي :
در طي كواترنري، نيروهاي تكتونيكي با دخالت تغييرات اقليمي منجربه ايجاد لندفرم‌ها و يا تغييرشكل در فرم‌هاي قديمي دره‌هراز شده‌اند. پادگانه‌ها، لندفرم‌هاي به‌جامانده از دوره كواترنري هستند كه مطالعه آنها تغييرات محيط را طي اين دوران مشخص مي‌كند. در پژوهش‌حاضر، فرآيندهاي دخيل در تشكيل پادگانه‌هاي دره‌هراز و نقش رسوبات آذرآواري دماوند در تغيير رفتارآبي رودخانه‌هراز و تشكيل درياچه سدي در محل آب‌اسك مورد بررسي قرارگرفته است. براي دستيابي به اين هدف روش تحليلي و بازسازي زماني تغييرات به‌كار گرفته شده‌است. تكنيك كار، مقايسه مورفومتري پادگانه‌هاي درياچه‌اي و سدهاي يخچالي- پيروكلاستي به چهار روش بازه‌بندي منطقه مورد مطالعه، مقايسه ارتفاع و ضخامت رسوبات‌آبرفتي، تعيين حدود گسترش رسوبات درياچه‌اي و بازيابي حدارتفاعي موانع محصوركننده دره‌هراز استوار است. نتايج به‌دست آمده نشان‌داد كه سطوح ارتفاعي پادگانه‌ها هم داراي اختلاف ارتفاع بوده و هم توالي نامنظم دارند. بالاترين سطح پادگانه در كوه‌نل 630 متر از بستر دره‌هراز ارتفاع داردكه نشان مي‌دهد علاوه بر تغيير سطح‌اساس، فرآيندهاي يخچالي از عوامل مرتفع شدن سطوح پادگانه درمنطقه بوده‌است. بررسي‌ها نشان‌داد طي دو دوره براثر بسته شدن دره توسط فعاليت يخچال و آتشفشان همگام باهم، در محل آب‌اسك درياچه‌اي ايجاد شده‌است. اين سد گدازه‌اي به‌تدريج طي هولوسن شكسته شده و در حال‌حاضر پادگانه‌هاي نهشته‌گذاري درياچه قديمي در دوسوي دره‌هراز قابل مشاهده هستند. با توجه به‌شواهد به‌جا مانده از اين رسوبات در دره‌هراز درنهايت مدل رقومي جريان يخچالي- لاهاري بازسازي شد.
چكيده لاتين :
Introduction
During the Quaternary, tectonic forces have been implicated in climate change as a result of landforms or deformations in the ancient forms of the Haraz Valley. The terraces are solid landforms from the Quaternary period whose study of environmental changes during this period. But in the studied basin, in addition to climate change and intrusive fault-tectonics, volcanic activity has had the greatest impact on the processes governing the region. Haraz basin Although it has an area of more than 3779 square kilometers and is located on the border between East and Central Alborz, its basin is different from other neighboring basins. So that their forms and processes are genetically incompatible with processes in the region and exhibit specific structural complexity. One of the available forms is the diversity of alluvial surfaces that can be observed along the Haraz valley. Given that the totality of these alluvial surfaces, due to the youngness of the Haraz Valley, does not coincide with the present river process, the most important issue that may arise here is that these erosional surfaces have their effects on the slope. The Haraz Mountains are visible, were they merely influenced by the flow dynamics of the Haraz River? Otherwise what factor or factors contributed to the formation of these levels. There are three hypotheses for achieving the morphogenesis of Haraz terraces in the study area:
A: Damavand lava intrudes through the valleys into the main valley of Haraz and forms a barrier lake in its upstream, forming morphogenesis of alluvial terraces of Haraz;
B: Morphogenesis of haraz terraces is caused by glacier activity in the area;
C: Damavand volcano deposited its pyroclastic material along the main arterial path of the Haraz and formed this vast surface of alluvial terraces in the area;
Materials and methods
To this end, analytical and temporal reconstruction methods of change have been employed. The technique is based on comparing the morphometry of lake terraces and glacier-pyroclastic dams with four methods of rehabilitating the study area, comparing the height and thickness of alluvial sediments, determining the extent of lake sediment distribution, and recovering the extent of barrier confinement.
Result and discussion
Investigating the findings of the research and their comparisons and analyzes, it can be concluded that the terraces of the Haraz valley in this area are significantly different in terms of the thickness and height of the sediment and in the processes affecting their formation and alteration. The height of the alluvial surfaces at the outlet of the Gazane valley is 630 m. However, the height of alluvial surfaces in the Abe Ask reaches 200 meters above the river bed. The highest elevation of the terrace is at the outlet of Fire Valley to Haraz 2065 m. Therefore, this difference in altitude of the alluvial surfaces is not only related to the change in the basement level, but also to the interplay of glacial and volcanic activity of Damavand. The sequence of alluvial terraces is not evident in all the studied periods. While in the water of the ski in the travertine (interval 1) four levels, in the six intervals, four levels and in the Nova range three levels are visible. Other areas have an alluvial terrace surface. The genesis of the sediments of the terraces under study indicates a significant difference. Although the glacier has the most significant activity in producing and consequently the formation of alluvial terraces in this region and is dominant in genus, volcanic pyroclastic sediments have been active in sync with the glacier and evidence of temperature volcanic activity is observed among all glacial sediments. So that they are sometimes deposited on ice in the form of a Lahar layer and occasionally as a layer of ash or other pyroclastic material.
Various researchers have already commented on the creation of lava barrier lakes in the Haraz Valley and the valleys around Damavand Peak (Alaei Taleghani, 2012; Yamani et al., 1977). Many are full of ice, not yet. Damavand's new volcano, which coincides with glacial and interglacial periods, has been able to throw pyroclastic and lava materials out of successive eruptions.
Conclusion
The results showed that the elevation levels of the terraces had both an elevation difference and an irregular sequence. The highest level of alluvial terrace in the Nell Mountains is 630 m above the Haraz valley bed, indicating that in addition to changing the basin level, glacial processes have been a factor in elevating alluvial levels in the area. Surveys showed that the lake was formed during two periods due to the closure of the valley by the activity of glacier and volcano together. This lava barrier was gradually broken during the Holocene and nowadays the depositions of the old lake are visible across the Haraz valley.
