بازسازی برف مرز کواترنری واحد ژئومورفیک زاگرس ایران

اگرچه دوره کواترنری در میان دوره‌های زمین‌شناسی ازنظر زمانی، بسیار کوتاه است، ولی نزد ژئومورفولوژیست ها بسیار مهم است. اهداف اصلی این مقاله، ارائه بهترین روش برآورد ارتفاع برف‌مرز دائمی، تجزیه و تحلیل اثر تغییر عرض جغرافیایی و جهت ناهمواری بر ارتفاع برف مرز دائمی کواترنری و درنهایت، ترسیم نقشه فرایند یخچالی برای واحد ژئومورفیک زاگرس در طی کواترنری است. به این منظور، ابتدا با توجّه به فرم خطوط منحنی میزان در نقشه‌های توپوگرافی 1/50000، بیش از 2000 نقطه سیرکی در واحد زاگرس شناسایی شد. به دلیل وسعت منطقه مورد مطالعه، ابتدا ارتفاع برف مرز دائمی در حوضه‌های آبریز به تفکیک برآورد شد. ارتفاع برف مرز به روش‌های رایت، ارتفاع کف سیرک، نسبت پنجه به دیواره و نسبت‌های ارتفاعی برآورد شد. امتداد ناهمواری‌های زاگرس سبب شکل‌گیری سیرک‌های یخچالی در جهات شمال شرقی –جنوب غربی شده است. پراکندگی سیرک‌های شناسایی‌شده دال بر این است که دامنه‌های شمال شرقی این واحد، شرایط مساعدتری برای شکل‌گیری سیرک داشته است. تجزیه و تحلیل ارتفاعات برآوردی برای برف مرز بر اساس روش‌های مختلف، دال بر این است که ارتفاع برف مرز دائمی در روش نسبت‌های ارتفاعی ازنظر عدد ارتفاعی و انعکاس اثرگذاری جهت سطوح ارضی بر اختلاف برف مرز دائمی، ارتفاع مناسب‌تری است. گستردگی زاگرس در عرض جغرافیایی، سبب تفاوت ارتفاع برف‌مرز قسمت‌های مختلف آن شده است، به‌طوری که ارتفاع برف مرز دائمی جنوبی‌ترین حوضه (بختگان مهارلو) 661 متر مرتفع‌تر از شمالی‌ترین حوضه (مرزی غرب) برآورد شده است. متوسّط ارتفاع برف مرز واحد زاگرس 2719 متر است. درمجموع، ارتفاع برف مرز دامنه‌های نسار این واحد 2561 متر و دامنه‌های نگار (دامنه‌های جنوب غربی) 2822 متر برآورد شد (261 متر اختلاف ارتفاع). تجزیه و تحلیل اثر مقدار شیب و ارتفاع قلل بر ارتفاع برف مرز بیانگر آن است که اثرگذاری این عوامل بر ارتفاع برف مرز دامنه‌های نگار بیشتر از دامنه‌های نسار است.

Reconstruction of Quaternary ELA of the Zagros Geomorphic Unit of Iran

Although the quaternary period is a very short period among the periods of geology, it is important for geomorphologists. The main objectives of this article are to provide the best method to estimate the ELA, analyze the effect of the change of latitude and the direction of terrains on ELA and finally, draw a map showing Quaternary glacial regions of the Zagros geomorphic unit. First, more than 2000 cirques points in Zagros units were identified according to the form of the equilibrium lines on topographic maps of 1:50000. The terrains protraction of the Zagros causes the formation of glacier cirques in the northeasternsouthwestern directions. Due to the extent of the study area, at first, ELA was estimated in the basins separately. The Zagros geomorphic unit is a very mountainous and upland unit, in which northeastern parts are higher than southeastern Zagros. According to the main protraction of the Zagros, from the northwest to the southeast, Cirque forms are formed in the northeastern and southwestern slopes. ELA was estimated by Wright, Cirquefloor altitude, TerminustoHead, Altitude Ratios, and Altitude ratios methods. The analysis of the estimated ELA indicates that ELA is more realistic by the Altitude Ratios method, the point of view of the number and the effect on the surface on the ELA difference. The extent of the Zagros in the latitude has led to the ELA difference of its various parts so that ELA of the southernmost basin 661 meters is higher than the northernmost basin. The average of ELA of the Zagros is 2719m. In total, ELA of the Nesar slopes was 2561 m and the Negar slopes were 2822m. The analysis of the effect of the slope and height of the peak on ELA indicate that the effect of these factors on Negar slopes is greater than the Nesar slopes.Extended Abstract1IntroductionAlthough the quaternary period is a very short period among the periods of geology, it is important for geomorphologists who examine surface forms of the earth in relation to internal and external processes. During this period, the climate has changed several times. Such changes have led to forms at the surface of the earth that are not consistent with current processes (Jafari, 2016). In the mutation and evolution of terrains, glaciers and per glacial and glacial period rule have played an important role in the processing and evolution of evolution, as during the glacial periods, the accumulation of large amounts of snow and ice on the slopes and during the per glacial periods, the melting of these ice masses has a significant role for domains (Yamani et al, 2013). The main objectives of this article are to provide the best method to estimate the ELA, analyze the effect of the change of latitude and the direction of terrains on ELA and finally, draw a map showing quaternary glacial regions of the Zagros geomorphic unit.2Materials and MethodsFirst, more than 2000 cirques points in Zagros units were identified according to the form of the equilibrium lines on topographic maps of 1:50000. The terrains protraction of the Zagros (northwesternsoutheastern) causes the formation of glacier cirques in the northeasternsouthwestern directions. Because of the extent of the study area, firstly, ELA was estimated in the basins separately. The West boundary, Karkheh, Karun and BakhteganMaharlou basins are located in this unit fully, but the western part of the AbarugoSirjan basins are located in the Zagros and its eastern part is located in Central Iran, Therefore, the cirques in the eastern of AbarugoSirjan was not used to estimate ELA. ELA was estimated by Wright, Cirquefloor altitude, TerminustoHead, Altitude Ratios and Altitude ratios methods.3Results and DiscussionThe Zagros geomorphic unit is a very mountainous and upland unit, in which northeastern parts (high Zagros) are higher than southeastern Zagros. According to the main protraction of the Zagros, from the northwest to the southeast, Cirque forms are formed in the northeastern and southwestern slopes. Due to the extent of the Zagros, estimating ELA in a single stage for the whole geomorphic unit is not appropriate; therefore, the first, ELA was estimated in the basins. Considering such conditions, 2366 cirques were identified in this unit (Fig. 2). An adaptation of the identified cirques with the lithology layer and according to their drawn longitudinal profile analysis of the Porter method was considered 753 cirques as pseudocirques. According to the data of the 1613 remaining cirques, ELA was estimated by different methods. The analysis estimated ELA indicates that ELA by the Altitude Ratios method, the point of view of the number and the effect on the surface on the ELA difference is more realistic. The extent of the Zagros in the latitude has led to the ELA difference of its various parts, so that ELA of the southernmost basin (BakhteganMaharlou) 661 meters is higher than the northernmost basin (West boundary). The average of ELA of the Zagros is 2719 meters. In total, ELA of the Nesar slopes was 2561 m and the Negar slopes were 2822 m (261 m height difference). The analysis of the effect of the slope and height of the peak on ELA indicates that the effect of these factors on Negar slopes is greater than the Nesar slopes.4ConclusionIn the Zagros geomorphic unit, 64 percent of the cirques are in the northeast direction and 36 percent in the southwest direction. The proposed methods for ELA in Zagros are not the same, and this difference is due to differences in the parameters of the methods. Among the methods used to estimate ELA, a method has been introduced as a suitable method that, in addition to the acceptable elevation, the ELA difference is reflected in different slopes well. Usually, ELA in the Negar slopes (south, southwest, west) is greater than the Nesar slopes (north, northeast, east), but the ELA which was estimated in some of the methods applied in this unit (Wright’s methods, Cirquefloor altitude, TerminustoHead Altitude Ratios) have shown the inverse result. Therefore, according to the results, they cannot be used to estimate the ELA. The estimates of ELA by Altitude Ratios method, same about the amount and same of the ELA difference in different directions seem more realistic and reasonable. Analyzing ELA estimated by different methods in the basins of the Zagros indicates that ELA is estimated by Altitude Ratios methods to be more consistent with the environmental conditions introducing the best method to estimate the ELA for this geomorphic unit. Based on this method, the average of ELA the whole unit is 2719 meters, and the height difference between the Nesar slopes and the Negar slopes is 261 meters. Existing differences can also be attributed to the views of researchers at the time of identifying past cirques and the method of estimating the ELA. It can also result from the influence of environmental factors such as the difference and direction of the slope and the highest peak in the region.