نوع مقاله : پژوهشی

نویسندگان

• غلام حسن جعفری ¹
• نسرین حضرتی ²

¹ استادیارگروه ژئومورفولوژی دانشگاه زنجان

² دانشجوی کارشناسی ارشد هیدروژئومورفولوژی، گروه جغرافیا، دانشگاه زنجان، زنجان، ایران.

چکیده

چکیده
واحد شمال غربی که محل تلاقی رشته‌کوه‌های شمالی و جنوب غربی ایران است، یک منطقه­ ی کوهستانی به‌حساب می‌آید. شواهد فراوانی از فعالیت‌های یخچالی کواترنری در این واحد باقی‌مانده است. در راستای همین مسئله، این مقاله درصدد آن است که با توجه به آثار یخچالی موجود در منطقه به برآورد ارتفاع برف مرز کواترنری آن بپردازد. با استفاده از نقشه‌های توپوگرافی 1:50000 و انعکاس فرم اشکال یخچالی بر روی نقشه‌ها، بیش از 4000 سیرک یخچالی شناسایی شد که در بین حوضه‌های آبریز ارس، سفیدرود و دریاچه­ ی ارومیه پراکنده‌شده است. با توجه به قرارگیری مناطق کوهستانی این منطقه، سیرک‌های یخچالی در قسمت‌های غرب، شرق و مرکز واحد شناسایی شدند. ارتفاع برف مرز به روش‌های رایت، ارتفاع کف سیرک، نسبت پنجه به دیواره و نسبت‌های ارتفاعی برآورد شد. در این واحد ژئومورفیک سیرک‌های یخچالی از ارتفاع 1800 متر به بالا امکان شکل‌گیری داشته‌اند. تجزیه‌ و تحلیل ارتفاعات برآورد شده گواه این است که ارتفاع برف مرز دائمی در روش ارتفاع کف سیرک پورتر، به دلیل انعکاس اثرگذاری جهت بر ارتفاع برف مرز نسبت به سایر روش‌ها با واقعیت انطباق بیشتری دارد. ارتفاع برف مرز کواترنری بین 2453 متر (حوضه­ ی آبریز ارس) تا 2685 متر (حوضه­ ی آبریز سفیدرود) متغیر برآورد گردید (232 متر اختلاف ارتفاع). این اختلاف نشان‌دهنده­ی کاهش ارتفاع برف مرز از جنوب به شمال است. متوسط ارتفاع برف مرز این واحد 2586 متر است. تجزیه‌ و تحلیل یافته‌ها نشان می‌دهد که این واحد در دوره‌های سرد تحت حاکمیت سیستم شکل­زایی یخچالی بوده است.

تازه های تحقیق

-

کلیدواژه‌ها

• کلمات کلیدی: کواترنری
• سیرک
• برف مرز
• ارتفاع کف سیرک

20.1001.1.23833254.1398.6.18.5.4

عنوان مقاله [English]

The Boundary of the Quaternary Glacial Morphogenes System in the Water Basins of the Northwest of Iran

نویسندگان [English]

• Gholam hassan jafari ¹
• Nasrin Hazrati ²

¹ Assistant professor geomorphology, University of Zanjan, Zanjan, Iran, (Corresponding Author), E-mail:jafarihas@yahoo.com.

² - M.A of physical hydro geomorphology, University of Zanjan, Zanjan, Iran

چکیده [English]

Introduction
The climate change that has started since the Quaternary period continues to shape the current morphology of the Earth. During the Quaternary period, glacial and interglacial periods have been continuously occurring and have undeniable geomorphologic evidence. Using this evidence, it is possible to determine the ELA and the extent of glacial expansion and the study of climate change. This is because of the fact that exogenous Earth-changing processes have not been able to completely eliminate the remaining effects of quaternary glacial erosion (Yamani & Zamani, 2007: 100). The Northwestern unit, which is the intersection of the northern and southwestern mountain ranges of Iran, is a mountainous region. There is an extensive evidence of Quaternary glacial activity remained in this unit. In this regard, this article seeks to estimate the Quaternary ELA considering the glacial effects of the region. Talrghani (2012) has introduced the Northwestern unit as a mountainous region of Iran, as the north and southwest mountain ranges of Iran meet with different structures in the area; in addition, since the northwest is the intersection and the density of the three plateaus, it has led to its complexity and disorder of its terrains.
 
 




Methodology
Among the basic issues in glacial studies are the ELA and the water and ice equilibrium line .Using topographic maps with the scale of 1: 50000 and the reflection of the glacial evidence on maps, more than 4000 cirques, dispersed among the basins of Aras, Sefidrud and Lake Urmia, were identified. The overall direction of the identified cirques was determined according to the continuity of the terrains in a way that the main ridge of each basin determined the direction of the formation of the cirque. Using the geological map with the scale of 1: 100000, the lithological conditions of the cirques location were investigated. The data of those cirques located in sediments that had not undergone diagenesis such as conglomerate and were loose (such as marl, clay, and evaporite sediments) were extracted. Considering the mountainous nature of this region, glacial cirques were identified in the western, eastern and central parts of the unit. The ELA was estimated by Wright, Cirque-floor altitude, Terminus-to-Head Altitude Ratio (of Wright and Porter), and Altitude Ratios' methods.
Results
In this geomorphic unit, glacial cirques can be formed from a height of 1800 m above the surface. The analysis of the ELA estimated by cirque-floor altitude method indicated that it was more consistent than the other methods due to the reflection of the direct effects on ELA. The ELA between 2453 m (Aras basin) to 2685 m (Sefidrud basin) was estimated (with 232 m height difference). This difference indicated a decrease in the ELA from the south to the north. The average elevation of the ELA was 2586 m. The average elevation of the ELA in the Aras basin, Lake Urmia, and Sefidroud were respectively 2453, 2621, and 2685 m. The analysis of the findings showed that this unit was controlled by the glacial system during the cold periods.
Discussion and conclusion
Since the quaternary survey was about 12000 years ago, it was obviously impossible to accurately estimate the ELA, and there were

 
inevitable differences in the estimated ELA by different methods. The highlands of the Northwestern geomorphic unit were located in the western, eastern and centeral parts. The Aras River, Lake Urmia and Sefidrud basins were located in this unit. The terrains’ direction of this unit was in six directions of north-south, east-west, and northeast-southwest. There was a possibility of the formation of a cirque glacial. 4059 cirques identified in this unit with the dispersion of 1,215 cirques in the Aras Basin, 1442 cirques in the Sefidrud Basin,  and 1643 cirques in the Lake Urmia Basin. After the removal of pseudo-cirques (2720), remained the landform Cirques. The percentage of cirques was then estimated in different directions. The findings showed that while 71.25% of the identified cirques were in the Nesar slopes, 28.69% of them were in the Negar slopes. After identifying the cirque, the Wright method was used to estimate the ELA.  By applying this method, in the first stage, the lowest ELA was estimated in the Aras Basin (1826 m). In the second stage, with the elimination of cirques pseudo-data, it was allocated to the Lake Urmia Basin (2360 m). Estimated ELA in different methods indicated that the ELA was less 232 m from the southern basin (Sefidrud) to the northern basin (Aras).

کلیدواژه‌ها [English]

• Keywords: Quaternary
• Cirque
• ELA
• Cirque-floor altitude
• Northwestern