نقش نوزمين‎ساخت در تحوّلات ژيومورفولوژيك مرز ساختاري البرز جنوبي ـ ايران مركزي (مطالعه‎ي موردي: حوضه‎ي حبله‎رود)

The Role of Neotectonics on Geomorphologic Evolution at Structural Boundaries in Southern Alborz- Central Iran (Case study: Habblerud basin)

رودخانه‎ي حبله‎رود، جريان دايمي حوضه‎ي آبريز كوير مركزي است كه در مرز ساختاري البرز جنوبي ـ ايران مركزي واقع شده است. به‎دليل فعّال‎بودن زمين‎ساخت البرز، لندفرم‎هاي كواترنري در اين محدوده تا اندازه‎ي زيادي تحت تاثير گسل‎هاي اصلي و فعّال منطقه قرار دارند. در اين پژوهش، براي شناسايي مناطق فعّال از نظر حركات زمين‎ساختي، شاخص‎هاي ژيومورفولوژيكي زمين‎ساختي از تصاوير ماهواره‎اي و DEM10 M استخراج شده است. اين شاخص‎ها شامل: شاخص گراديان طولي رودخانه (Sl)، عدم تقارن حوضه‎ي زهكشي (Af)، انتگرال فرازسنجي (Hi)، سينوسيتي پيشاني كوهستان (Smf)، شكل حوضه (Bs) و نسبت عرض بستر درّه به ارتفاع آن (Vf) هستند. علاوه‎بر آن، به‎دليل تراكم زياد گسل‎ها، در بررسي‎ها براي نخستين‎بار شاخص ديگري به‎نام چگالي گسل (Fd) مورد استفاده قرار گرفت. به‎دليل همگن نبودن شاخص‎ها براي تمامي بخش‎هاي منطقه، تمام متغيّر‎ها در يك جدول ماتريسي قرار گرفته و بر اساس ويژگي‎هاي منطقه، ارزش‎گذاري و پس از بي‎بُعدسازي، در مدل SAW قرار گرفت و به چهار رده‎ي بسيار فعّال، فعّال، متوسّط و فعّاليّت كم، تقسيم‎بندي شدند. به‎منظور ارزيابي نتايج به‎دست‎آمده با پژوهش ميداني و دستگاه GPS، مناطق فعّال حوضه مورد بررسي قرار گرفت و با شواهد ژيومورفولوژيكي به‎دست‎آمده مانند پادگانه‎ي رودخانه‎اي ارتفاع يافته، درّه‎هاي V شكل، توالي مخروط‎افكنه‎ها، تغيير مورفولوژي رودخانه و... نتايج به‎دست‎آمده مورد تاييد قرار گرفت. بررسي‎ها نشان مي‎دهند با وجود حضور گسل‎هاي اصلي و فعّالي همچون گسل مشا، فيروزكوه، سرخه كلوت، سيّدآباد و حصاربن، ميزان فعّاليّت در تمامي بخش‎هاي منطقه يكسان نبوده و بخش‎هاي جنوبي منطقه داراي بيشترين فعّاليّت است، بنابراين بر اساس اين پژوهش، فعّال‎ترين گسل منطقه، راندگي گرمسار معرّفي مي‎شود.

Introduction Tectonic geomorphology is a relatively new and interdisciplinary field between structural geology/tectonics and Earth surface processes. The most common goal of tectonic geomorphology research is to use Quaternary landforms and stratigraphy to infer the nature, patterns, rates, and history of near-surface tectonic processes. Tectonic geomorphology provides a whole set of tools for deciphering the most recent activity on live structures (Keller and Pinter 2002; Pinter 1996). These tools kit is a quantitative measure of neotectonics landscapes giving useful information about neotectonic activities. In this work, Geomorphologic indices include : stream length-gradient index (SL), drainage basin asymmetry (Af), hypsometric integral (Hi), ratio of valley-floor width to valley height (Vf), index of drainage basin shape (Bs), and index of mountain front sinuosity (Smf) . In this study fault density (Fd) index is investigated as one of the most important geomorphologic indices. The study area is located between the southern central Alborz and northern central Iran structural zone boundaries, two different tectonic active zones which are an ideal location to test new model (SAW). Methodology In this research, in order to increase the accuracy of calculating geomorphological indices in Habble Rud basin extract digital elevation model (DEM) by resolution and accuracy of 10 meters, from topographic maps at the scale of 1:25000 was used. Also, we used geologic maps of Tehran and Semnan (1:250000), Garmsar and Damavand (1:100000), satellite imagery include ASTER (2007), Landsat ETM (2002), TM (1988) and topographic maps (1:50000), 1:25000 shape files. The various indices were divided into three classes: form class one being high activity to class three being low activity. We set geomorphic indices in a matrix table and got values based on characteristics and field observations. The approach of this paper is a quantitative method based on local characteristics concentrating to distinguish the role of neotectonics on evaluation of quaternary landforms, so we used a simple additive weighting model (SAW) to accumulate variables and they are ranked. Results and Discussion Some studies used a combination of two indexes (Smf and Vf) or Length gradient river to provide semi-quantitative information of relative degree of active tectonics. El Hamduni(2008) studied Izbor River basin using 6 indices including : stream length-gradient index (SL), drainage basin asymmetry (Af), hypsometric integral (Hi), ratio of valley-floor width to valley height (Vf), index of drainage basin shape (Bs) and index of mountain front sinuosity (Smf). He divides them into four classes from relatively low to highest tectonic activity. Due to the multitude variables, the technique in this study is a simple model (SAW) through GIS technique. In this method, Habble Rud basin was selected and divided into 22 sub basins, after calculating and classification of 7 tectonic geomorphology indices, in first place, all indexes were ranked and set in matrix table giving values to the index. Faults are one of the important tectonics phenomena; therefore we calculate the fault density as an index in the model. The classification used in this paper for each geomorphic index is calculated from El Hamdouniʹs method. Within the study area, about 1% (378 km2) belong to class 1 (very high relative tectonic activity) as measured by SAW; 48.3% (1683.6km2) shows high relative tectonic activity as measured by SAW (class 2); 37.4% (1300.9km2) moderate values of tectonic activity in terms of SAW (class 3); and 3.4% (119.6km2) belong to the lowest values of relative tectonic activity (class 4) based upon SAW. Thus, we can conclude that 1/3 of the study area is classified into classes 2 or 1 of high to very high tectonic activity in terms of the apparent geomorphic response. Conclusion The study area is located between the southern Alborz and Central Iran structural zones, characterizing by active structural zone. Because of the lack of chronology of Quaternary units, Vertical rates of active tectonics in southern Alborz are unknown. N-S shortening across central Alborz has been evaluated to 5± 2mm/yr using GPS measurements (Vernant et al. 2004). The Mosha fault as one of the most active faults in this region causes several earthquakes greater than MS = 6.5 (Berberian and Yeats 2001). One of the greatest shocks (Ms~ 7.6) occurred in the Garmsar fault during the third century BC and in 743AD. Ashtari (2005) indicated a concentration of earthquake activities located north of the Garmsar and Mosha faults. Finally, the result of this method has been linked to seismic map showing concentration of seismic activity around of Garmsar fault. The descriptions of tectonic landforms corresponded to the classification of SAW of moderate to high tectonic activity very well. As an advantage of this method relative to the IAT, those indices is adapted to the characteristics of the studied area. Geomorphic indices of active tectonics are useful tools to analyze the influence of active tectonics. These indices have the advantage of being calculated using GIS and remote sensing packages over large areas as accurate tool to identify geomorphic anomalies related to active tectonics. The SAW result showed that the most active part of Habble Rud is Garmsar fault. Since the weighting in this model is based on natural characteristics, so it can be applied in different areas with different characteristics.