پهنه‌بندي و واكاوي فرايندهاي هوازدگي در غرب دشت مركزي - زاگرس

Zoning and Analysis of Weathering Processes in West of Central Plains and Zagros

مطالعه هوازدگی ازآن‌جهت كه این فرایند باعث از هم پاشیدن و تجزیه سنگ‌ها، تشدید فرسایش (شامل حمل آن‌ها توسط آب، باد و یخ و برف)، فروریزش تحت نیروی جاذبه، ایجاد اشكال ناهمواری مختلف و حتی ایجاد تمركز و تشكیل مواد معدنی و تشكیل خاك می‌شود، دارای اهمیت است. این تحقیق به بررسی وضعیت هوازدگی در بخش‌هایی از رشته‌كوه‌های زاگرس و غرب دشت مركزی ایران پرداخته است. مبنای تعیین مناطق هوازدگی مدل‌هایی است كه لوئیز پلتیر ارائه كرده است. ازآنجایی‌كه دو عنصر اقلیمی دما و بارش سالانه در مدل‌های ارائه‌شده تعیین‌كننده نوع هوازدگی هستند؛ در ابتدا با استفاده از داده‌های 22 ایستگاه هواشناسی اقدام به پهنه‌بندی بارش با استفاده از میانیابی به روش IDW در سامانه اطلاعات جغرافیایی و دمای سالانه به روش PRIMS شد. سپس این دو نقشه با استفاده از نرم‌افزار ENVI به یك تصویر دو باندی تبدیل و با استفاده از نرم‌افزار Matlab به پیكسل‌های تشكیل‌دهنده آن تجزیه و یك خروجی از آن با فرمت Text تهیه گردید. با فراخوانی این داده‌ها و همچنین اشكال هوازدگی پلتیر در سامانه اطلاعات جغرافیایی موقعیت این پیكسل‌ها را نسبت به اشكال هوازدگی پلتیر مشخص و اقدام به تعیین نوع هوازدگی هر پیكسل و در نهایت نقشه هوازدگی منطقه گردید. نتایج نشان داد غرب دشت مركزی جزو مناطق مورفوكلیماتیك خشك و نیمه‌خشك است كه هوازدگی شیمیایی ضعیفی در آن حاكم است و كوه‌های زاگرس اغلب جزو مناطق ساوان و معتدل است كه دارای هوازدگی شیمیایی متوسط است و فرسایش جریانی و حركات توده‌ای بیشتری را نسبت به غرب دشت مركزی دارد.

. Introduction Weathering is crushing, decomposition and decaying the rocks of outer Earth 's crust by natural processes such as; running waters, wind blowing and ice creeping and biological actions and so on. The study of weathering is important because this process lead to formation of all known of soil types in outer part of earth crust finally. Climate is the most important parameter in creation of weathering and crops of it. Two climatic variables that have a clear role in weathering is air temperature and precipitation. Louise Peltier by using of these two variables defined some variety of weathering realm in several graphs. Weathering zones used in this paper are based on models that Peltier has offered. 2. Study area The study area includes a non-regular quadrilateral limited to cities of Khorramabad (48o 17 ' E, 33 o 26 ' N), Kashan (51o 27 'E, 33o 59 ' N), Yazd (54o 17 ' E, 31o 54 ' N) and Yasooj (51o 41 E, 30 o 50 ' N). Area total of 88711 km2, which is 5.38% of the total area of the Iran country. The area selected so that covers the part of Zagros mountain and west central plains between Kashan and Yazd. 3. Material and Methods Because the importance of air temperature and precipitation on weathering, at first attempt to produce map of average annual temperature and precipitation of study region. By using data of 22 meteorological stations through IDW interpolation method and then PRIMS method rainfall and air temperature zoning was performed by GIS. Then two mentioned maps convert to a dual-band image using ENVI software and by using MATLAB program the constituent pixels was analyzed and was extracted with TEXT format. By importing above mentioned data and Peltier graphs coordinates in GIS, and by the pixel positioning relative to the defined area in Peltier graphs, weathering character of any pixel identified and the weathering map of study area were made. 4. Results and Discussion In this study, mean air temperature and precipitation have used for preparing the weathering zones map for a study area based on the original graphs of Peltier. These maps are including chemical weathering, freezing action, composite weathering, pluvial erosion, mass movements, wind action and morphogenetic regions. Since the study area is only 5.38% of the total area of Iran, the 9 categories specified by Peltier graph, there are 6 types in study area. The results show that the west of central plains belongs to arid and semiarid morphogenetic regions, where there is a weak chemical weathering and in contrast wind action has a major factor for weathering. The realm of Savanna and temperate regions are often belonged to Zagros Mountains that marked by moderate chemical weathering and pluvial erosion, and mass movements in Zagros is more than the eastern parts of study area. In very small mountainous area near the Shahrekord and Yasooj with 28 Km2, freezing action has modest role in the destruction of the rocks and the rest of the Zagros mountains have weak freezing. But west of central plains does not have freezing action. 5. Conclusion The applicable results show that if maps being fragmented to smallest components or pixels then determining of each pixel weathering feature lead to better map production with good accuracy. Also results show that in all of the morphogenetic and weathering maps, the role of Zagros valleys and ridges is quite evident for variety in morphogenetic and weathering areas.