شواهد مورفولوژيكي و مورفومتريكي تاثير تكتونيك فعّال بر مخروط‎افكنه‎هاي شمال دامغان

Morphologic and Morphometric Evidence for Active Tectonic Effects on Alluvial Fans in North Damghan

اين پژوهش به بررسي شواهد و آثار گسل هاي فعّال (گسل دامغان و گسل تزره) بر مورفولوژي و مورفومتري 16 مخروط افكنه، واقع در بخش شمالي دامغان مي پردازد. روش كار برپايه‎ي تحليل هاي كمّي و كيفي حاصل از تصاوير ماهواره اي و مدل هاي رقومي ارتفاعي، ويژگي هاي مورفومتري مخروط ها، اندازه‎گيري ميزان جابه‎جايي آبراهه ها، ميزان بالاآمدگي رسوبات و تحليل شيب و نيمرخ هاي طولي و عرضي است. نقشه هاي توپوگرافي 1:25000، عكس هاي هوايي 1:55000، تصاوير ماهواره اي ETM و نقشه هاي زمين شناسي 1:100000 و داده هاي ارتفاعي رقومي، ابزارهاي اصلي پژوهش را تشكيل داده اند. همچنين پيمايش هاي ميداني براي بررسي و اندازه گيري شواهد فعّاليّت گسل ها در دو مرحله انجام گرفته است. نتايج نشان مي-دهد كه تاثير عمده‎ي گسل تزره، سبب جابه‎جايي شبكه‎ي اصلي در راس مخروط و تحت تاثير آن، تغيير موقعيّت رسوب‎گذاري مخروط افكنه شده است. همچنين فعّاليّت گسل مذكور، سبب بالا آمدن رسوبات مخروط افكنه اي و رسوبات نيوژن زيرين شده است، در حالي‎كه شواهد فعّاليّت گسل دامغان چيره تر بوده و سبب ايجاد سطوح مختلف بالاآمده، متروك‎ماندن سطح مخروط ها، جابه‎جايي نقطه‎ي تقطيع آبراهه، بالاآمدگي رسوبات مخروط افكنه اي و جابه‎جايي افقي شبكه‎ي زهكشي سطح مخروط‎افكنه‎ها و همين‎طور تاثير بر فضاي قابل دسترس مخروط‎افكنه‎ها شده است. حركت گسل دامغان آثار مشخّصي بر جابه‎جايي رسوبات مخروط-افكنه اي داشته است. مولّفه‎ي قايم گسل دامغان، سبب شكل گيري سطوح قديمي و جديد (فعّال و غيرفعّال) در سطح اغلب مخروط‎افكنه‎ها شده است. بررسي آماري متغيّرها نشان مي دهد كه بين برخي روابط مورفومتريكي مخروط‎افكنه‎ها، مانند مساحت حوضه آبريز و مساحت مخروط‎افكنه‎ها، رابطه و همبستگي آماري مناسبي وجود دارد. اين در حالي است كه مابين متغيّرهايي چون، ارتفاع متوسّط مخروط‎افكنه‎ها و شيب متوسّط آنها، همبستگي ضعيفي وجود دارد.

Introduction When a river reaches from high gradient mountains to a low gradient plain, aggradations phenomenon is occurred and alluvial fans are built. At least five factors influence fan processes, including catchment bedrock lithology, catchment shape, neighboring environments, climate, and tectonics. Rocks of differing lithology yield contrasting sediment suites and volumes due to their variable response to weathering. Bedrock in desert settings is optimal for fan development, especially tectonically maintained mountain fronts, yields sediment in varying size.thevolume depends on: (a) the style of fracturing in proximity to faults, (b) the presence or absence of internal discontinuities such as bedding planes or foliation planes, and (c) the reaction to chemical weathering and non-tectonic types of physical weathering. Aeolian, fluvial, volcanic, lacustrine, or marine environments that border alluvial fans can impact fan processes by modifying the conditions of deposition. Fluvial environments, usually in the form of longitudinally oriented rivers, may affect fans by eroding their distal margins. Climate and its fluctuations affect water supply and vegetation cover. Without continued tectonics, fans may become minor or short-lived features. In this study, the evidence and impacts of the active faults are investigated in 16 alluvial fans morphometry and morphology located at the north part of the city of Damghan. Methodology The method is based on the obtained qualitative and quantitative data. The quantitative data includes satellite image interpretation and digital elevation models, alluvial fan morphometry, channel displacement and rate of sediment uplift. Super imposed profiles techniques, longitudinal and cross profile and gradient analysis were used to interpret the active fault effects on alluvial fans. Topography maps (1:25000), aerial photos (1:55000), ETM satellite image with 30 m spatial resolution, 8 bands, geology maps (1:100000) and digital elevation models (50m pixel resolution) were utilized in this study. Also, field work for investigation of evidence of faults activities were performed in two stages. Results and Discussion This area is located between two geo-structural zones. Alborz zone in the north and central Iran zone in the south. There are two main faults in this area. Tazareh fault is located at Alborz mountain front and affects the apex of alluvial fans, whereas Damghan fault is located far from mountain front. Distance between these faults is about 5 km. Tazareh fault has caused the displacement of the main channel in fan apex, and therefore has changed the sedimentation position; while evidence of Damghan fault activities are more, and has caused uplifting surfaces, derelict of fan surface, change of intersection point, uplift of fan sediment and lateral change of fan surface channels. The slope of most alluvial fans is 2-5 degree, except for fan No. 7 that is less than 2 degree. This fan is the largest fan in the study area. The lower gradient may be contributed to its large drainage basin. Damghan fault activities have caused alluvial fan deposits uplifted about 10 m. Conclusion The findings showed that Tazareh and Damghan faults were active in quaternary epoch and position of alluvial fans was affected by activities of these faults. Faults have had either lateral or vertical displacements. Findings showed that the slope of all alluvialfans (except for fan number 7) increased suddenly in the apex of fans. Therefore the longitudinal profiles of alluvial fans are in a concaved shape. This means that tectonic activities affect the evolution of alluvial fans in quaternary epoch. Investigations show that there is no statistical correlation among the variables that affect the alluvial fans. Generally, tectonic activities disrupt natural evolution of alluvial fans. Each fault has a different effect on alluvial fan evolution. Tazareh fault has caused the displacement of the main channel to the east of fan apex and has increased slope of this part. Damghan fault has caused uplift of fan deposits, change the intersection point and reconstruction of new alluvial fans in the lower of this point. Accommodation space of alluvial fans from Siahkooh to Roodbar has affected by Damghn fault activities. Indeed, uplift of this part of fault has limited accommodation space for alluvial fans growth. There are a correlation between number of uplifted surface of alluvial fans, number of uplifted playa sediments (clay and silt) and number of channel displacements; this means that Damghan fault experienced three separate activities in quaternary epoch.