تاثير فعاليت‌هاي تكتونيكي بر مورفولوژي مخروط افكنه‌هاي شمال بيرجند

Effect of the tectonic activities on the north Birjand alluvial fans morphology

زمين­ ساخت منطقه شمال بيرجند عمدتا تحت تاثير عملكرد پهنه گسلي شكرآب قرار دارد. راندگي شكرآب، داراي روند باختري- خاوري با شيب 60 درجه شمالي، در فاصله 4 كيلومتري از شمال شهر بيرجند قرار دارد كه يك چشمه لرزه­اي براي شهر بيرجند است. بدليل اينكه گسل شكرآب از قطعات مختلف تشكيل شده است، شاخص ­هاي مرفومتريكي مخروطه افكنه ­ها جهت تعيين پويايي زمين ساختي در سه بخش شرقي، مركزي و غربي محاسبه گرديد. مقادير بدست آمده از شاخص خميدگي مخروطه افكنه (β) در سراسر پهنه گسلي نشان مي­دهد كه تمامي مخروطه افكنه­ هاي مورد مطالعه، بيضي شكل هستند و فعاليت بالاي تكتونيكي را نشان مي­ دهند. ميانگين درصد مخروط گرايي در مخروطه افكنه­ هاي بخش غربي، مركزي و شرقي گسل شكرآب به ترتيب برابر با 86 % ، 92 % و 90 % و همچنين ميانگين ضريب كشيدگي(BS) به ترتيب برابر با 3.2 ، 2.33 و 2.92 است. شيب سطحي مخروطه افكنه­ ها اكثرا بيشتر از 5 درجه است و نيمرخ ارتفاعي مخروط افكنه­­ ها در اكثر قريب به اتفاق آنها، مقعر است كه گوياي فعاليت كواترنري ناشي از عملكرد سيستم گسلي شكرآب است. شواهد مربوط به تاثير گسل بر مخروطه افكنه ­ها از جمله بريدگي و جابجايي در مخروطه افكنه­ ها، خندق­هاي فرسايشي، مخروطه افكنه­ هاي نامتقارن و خميده، موقعيت مخروطه افكنه­ هاي جديد نسبت به پيشاني كوهستان، حاكي از فعاليت زمين ساختي بالا در منطقه و نيز بيشتر بودن فعاليت در بخش هاي غربي و شرقي نسبت به بخش مركزي است. هم چنين در بخش­هاي غربي و شرقي گسل شكرآب كه طول­ قطعات گسلي فعال بيشتر از بخش مركزي مي ­باشد، وسعت مخروطه افكنه ايجاد شده بيشترمي ­باشند.

The alluvial fans are the condensed forms of the Quaternary and the covenant period, and their examination can provide researchers with useful information about the neotectonic regional activities in which they were formed. Geomorphic indexs are useful and reliable tools for the evaluation of tectonic activities because they can easily be identified with areas that have experienced past tectonic or fast-moving activities. The tectonic movements change the surface of erosion and affect the active processes at the surface of the alluvial fans and their evolution. On this basis, we can use the alluvial fans surfaces to study the history and movements of faults and the occurrence of earthquakes. The occurrence of tectonic events, faults, folding, subsidence or uplift, results in the reaction of alluvial fans, so that the reaction can be recorded in the type, size, shape of the sediment and morphology of the alluvial fans. The slope of the alluvial fans surface is also controlled by tectonics, and so the morphological and geometric characteristics of the surface of the alluvial fans can be considered as tectonic activity. The purpose of this paper is to investigate the morphometric parameters of the alluvial fans located in the north of Birjand city and the relationship between the values of the geometric indices of the alluvial fans and the level of neotectonic activity of the Shekarab fault system. First, 22 alluvial fans were selected to examine the activity of northern Birjand which are related to the activity of the Shekarab fault segments . The domains of these alluvial fans was determined using topographic maps of 1: 25000 Birjand and Khosf, and ER Maper and Google Earth software and were digitized in the ARC GIS 10.3 software environment. Quantitative data such as area, slope, length, width and height of the fans were calculated from 1: 10000 geological maps of Birjand and Khosf. Topographic maps and DEM data were used to calculate the mean height and longitudinal profiles of the alluvial fans. In addition, field surveys were carried out to map faults in the region and identify their mechanism. In identifying the different levels of the alluvial fans, satellite images were interpreted. It was also used to investigate the effect of fault movement on the development and change of pattern and displacement of the alluvial fans. Morphometric indices of alluvial fans including bending coefficient (β), cone coefficient, elongation coefficient, altitude variation and longitudinal profile of alluvial fans are calculated and analyzed in this study. Due to the Shekarab fault is composed of different parts, the morphometric indexes of the alluvial fans were calculated to determine the tectonic activities in the three eastern, central and western parts. Based on the results obtained from the calculation of alluvial fans indexs and their integration together, as well as the comparison with the results of previous studies (Hamdouni, 2008 & Ioannis, 2006 & Blair, 1994), can be explained the relationship between the morphology of the alluvial fans area tectonics activities. North Birjand Area Tectonics is mainly affected by performance of the Shekarab fault Zone. Shekarab thrust has the east - west trend with 60 degree dip to north located in 4 km north of the Birjand city. There is a seismic source for the Birjand city. Because the Shekarab fault is composed of different parts also for determining the tectonic activity, alluvial fans indexes are calculated in eastern, central and western parts. Calcluted numbers for alluvial fans bending index (β) in the entire zone indicate that all of the alluvial fans are ellipsoid shape and showing high tectonic activity. Average of spire coefficients in western, central and eastern alluvial fans respectively are 86 %, 92 % and 90 % and also Average of extentional coefficients respectively are 3.2, 2.33 and 2.92. Alluvial fans surface slope are more than 5ᴼ and elevational profils in all of them are concave showing quaternary activity resulted from performance of the Shekarab fault system. The indicators of the fault effect in alluvial fans such as scrape and displacement in alluvial fans, erosional canals, unsymetric and bended alluvial fans, the location of new alluvial fans related to mountains front are showing high tectonic activity in the reagion and also the more activity in the eastern and western parts than the Central part. Also in the western and eastern parts of Shrkarab fault that the fault segments are longer than central part, the volume of alluvial fans are more.