تاثیر نو‌زمین‌ساخت بر ژئومورفولوژی گالی‌ها در دشت‌سرهای جنوب البرز شرقی (محدوده گرمسار- سمنان)

لندفرم ها در نواحی دارای فعالیت های تکتونیکی، حاصل ترکیب پیچیده ای از تاثیرات حرکات عمودی و افقی مربوط به بلوک های پوسته ای و فرسایش یا رسوب گذاری توسط فرایندهای سطحی است. در این پژوهش، گالی های سطح دشت سرهای موجود در دامنه های جنوبی البرز شرقی (منطقه گرمسار -سمنان) مورد بررسی قرار گرفته اند. گالی های موجود از مغرب به مشرق خصوصیات مورفومتری متفاوتی را نشان می دهند، برای پی بردن به علل این تفاوت ها، پس از گردآوری داده ها و تحلیل رقومی تصاویر و نقشه ها، قلمرو تحقیق به 4 بازه و سایت مطالعاتی تقسیم شد. در مجموع این سایت ها 35 گالی را پوشش دادند. در مرحله بعد پس از نمونه برداری از دیواره تا کف گالی ها کارهای ژئوشیمی و دانه سنجی بر روی آنها انجام گرفت. همچنین مورفومتری گالی های هر سایت در مقاطع مشخص و فواصل مساوی انجام شد، علاوه بر این، تراکم پوشش گیاهی و نوع آن برداشت گردید. در مرحله بعد شواهد مورفوتکتونیک با بررسی تصاویر ماهواره ای و شواهد زمینی بررسی شد. سرانجام ارتباط مورفومتری، داده های رسوبی و تاثیر نوزمین ساخت و چگونگی تاثیر آن بر گالی های منطقه تحلیل گردید. یافته ها نشان می دهند که داده‌های فیزیکی رسوبات از نظر نوع و اندازه در مقیاس سطحی و عمقی نتوانسته در همه سایت‌ها به طور کامل در توسعه‌یافتگی گالی ها موثر باشد. شاید در سایت 4، توسعه یافتگی را فقط به عامل قطر رسوب منطقه و در سایت 2، عدم توسعه یافتگی گالی ها را به این عامل بتوان نسبت داد ولی در دو سایت دیگر یعنی سایت 1 و 3، نمی توان توسعه یافتگی گالی ها را تنها به بافت و قطر رسوب منطقه نسبت داد، بلکه باید عامل مهم تر تکتونیک را مدنظر قرار داد. تفاوت شیمی رسوب در 4 سایت جزئی بود، نوع و تراکم پوشش گیاهی و کاربری اراضی نیز در همه سایت ها تقریبا یکسان است. بنابراین نتایج حاصل از تحلیل عوامل تاثیرگذار و نقشه پهنه بندی نهایی حاصل ازanp و فازی، همه نشانگر آن است که در بین عوامل بررسی شده، نوزمین ساخت فعال نقش اصلی را در توسعه‌یافتگی گالی ها به عهده داشته است و شیمی رسوبات نقش ثانوی را دارا بوده است.

Neotectonic influence on geomorphology of gallies on plains of western Alborz mountains (from Garmsar to Semnan)

Extended abstractIntroductionLandforms 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 coneshaped 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 methodsIn 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 crosssections and equal distances. Vegetation density and taxonomy have also been obtained through the field survey. Morphotectonic 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 neotectonic properties on gully development in the area.Discussion and ConclusionConsidering 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 finegrain 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.