تأثير نو‌زمين‌ساخت بر ژئومورفولوژي گالي‌ها در دشت‌سرهاي جنوب البرز شرقي (محدوده گرمسار- سمنان)

لندفرم­ها در نواحي داراي فعاليت­هاي تكتونيكي، حاصل تركيب پيچيده­اي از تاثيرات حركات عمودي و افقي مربوط به بلوك­هاي پوسته­اي و فرسايش يا رسوب­گذاري توسط فرايندهاي سطحي است. در اين پژوهش، گالي­هاي سطح دشت­سرهاي موجود در دامنه­هاي جنوبي البرز شرقي (منطقه گرمسار- سمنان) مورد بررسي قرار گرفته­اند. گالي­هاي موجود از مغرب به مشرق خصوصيات مورفومتري متفاوتي را نشان مي­دهند، براي پي بردن به علل اين تفاوت­ها، پس از گردآوري داده­ها و تحليل رقومي تصاوير و نقشه ­ها، قلمرو تحقيق به 4 بازه و سايت مطالعاتي تقسيم شد. در مجموع اين سايت­ها 35 گالي را پوشش دادند. در مرحله بعد پس از نمونه­برداري از ديواره تا كف گالي­ها كارهاي ژئوشيمي و دانه­سنجي بر روي آنها انجام گرفت. همچنين مورفومتري گالي­هاي هر سايت در مقاطع مشخص و فواصل مساوي انجام شد، علاوه بر اين، تراكم پوشش­گياهي و نوع آن برداشت گرديد. در مرحله بعد شواهد مورفوتكتونيك با بررسي تصاوير ماهواره­اي و شواهد زميني بررسي شد. سرانجام ارتباط مورفومتري، داده­هاي رسوبي و تاثير نوزمين­ساخت و چگونگي تأثير آن بر گالي­هاي منطقه تحليل گرديد. يافته­ها نشان مي­دهند كه داده­هاي فيزيكي رسوبات از نظر نوع و اندازه در مقياس سطحي و عمقي نتوانسته در همه سايت­ها به ­طور كامل در توسعه­يافتگي گالي­ها موثر باشد. شايد در سايت 4، توسعه­يافتگي را فقط به عامل قطر رسوب منطقه و در سايت 2، عدم توسعه­يافتگي گالي­ها را به اين عامل بتوان نسبت داد ولي در دو سايت ديگر يعني سايت 1 و 3، نمي­توان توسعه­يافتگي گالي­ها را تنها به بافت و قطر رسوب منطقه نسبت داد، بلكه بايد عامل مهم­تر تكتونيك را مد­نظر قرار داد. تفاوت شيمي رسوب در 4 سايت جزئي بود، نوع و تراكم پوشش گياهي و كاربري اراضي نيز در همه سايت­ها تقريبا يكسان است. بنابراين نتايج حاصل از تحليل عوامل تأثير­گذار و نقشه پهنه­بندي نهايي حاصل از ANP و فازي، همه نشانگر آن است كه در بين عوامل بررسي شده، نوزمين­ساخت فعال نقش اصلي را در توسعه­يافتگي گالي­ها به عهده داشته است و شيمي رسوبات نقش ثانوي را دارا بوده­است.

Introduction Landforms in the areas of tectonic activities are resulted from a complicated combination of vertical and horizontal movements related to crustal blocks, erosion, and surface sedimentation. Considering these facts, there is a great diversity of land at the Hillside of the Eastern Alborz Mountains and in the Garmsar to Semnan Range. Initially, it is impossible to provide a reason for these sudden changes, except for the tectonic factor. Because across the Garmsar fault, the gullies are concentrated only in a limited section of the fault line and have a different morphometry. On the other hand, the existence of a cone-shaped sequence along the Garmsar fault shows a clear dynamic difference in terms of the exploration process and the density between these two landforms, which has been addressed in this study. Since gullies are important landforms for identifying landform changes and sequences of landforms in geomorphology, they are important. Therefore, due to the apparent similarities of the surface of the earth, this cannot be considered effective as a single factor in different parts of gullies; this is the main issue of this research. Materials and methods In this research, we have investigated gullies on southern slopes of eastern Alborz, from Garmsar to Semnan. The gully landforms have different morphological properties from west to east. To find out the factors creating the differences, we have gathered data and processed satellite images. By the way, we have divided the region into four sites or subsections for further analysis. The samples have been taken from the walls and bottoms of the gullies for geochemical and granulometric analyses. We have also measured morphometric properties of the gullies of each site in given cross-sections and equal distances. Vegetation density and taxonomy have also been obtained through the field survey. Morpho-tectonic evidence has been examined by field observations and satellite images. Then, we have applied Analytical Network Process (ANP) and Fuzzy analysis to examine the influences of morphometry, sediment characteristics, and neo-tectonic properties on gully development in the area. Discussion and Conclusion Considering that various factors are involved in the creation and development of gullies, each of the factors considered in the region were first studied in order to determine the effect of each of them and the most important factor in this regard. The factors involved are physics and sediment chemistry, tectonic effects in the area, vegetation and land use. The results have indicated that physical characteristics of the sediments in size and type could not have similar impacts in all the subsections. In site 4, more developed landforms are related to the fine-grain textures. On the contrary, in site 2 less developed gullies are resulted from coarse grain texture. In the other two sites, the development of gullies cannot be attributed to texture and diameter of the sediments. However, the development of the landforms is more affected by it position over the walls of the fault. There are no considerable differences in vegetation, land use, and geochemical properties among the four sites. The measurements showed that the fault wall at a height of 10.5 meters at the end of site 1 and the uplift of the middle portion of the cone at site 3, at 14 meters, are the main reasons for the expansion and deepening of the gullies in these two sites. However, these conditions do not exist on the other two sites. On site 2, gullies do not develop at all, and on site 4, there is a typical development of gullies because of the type and texture of the region's sedimentation. Results Physical and chemical factors of surface sedimentation and sedimentation of gullies, as well as the study of the type and species of vegetation in the region have been studied. Also the effect of tectonics and all the evidence of tectonic landforms in the region have been investigated. The relationship between tectonics with the expansion of gullies in the region are studied. According to the final zoning map of the ANP, the largest area of the southern Alborz pediment, the low and middle erosion classes of the gully, have been allocated. Based upon the final zonation map of ANP and fuzzy, it can be concluded that two main factors of active neotectonics and deposit texture have had the most important influence on gully development.