بررسي تكامل ساختاري كوههاي شمال بيرجند با استفاده از ترسيم مقاطع عرضي

Investigating of structural evolution of northern Birjand Mountains using cross section drowing

گسل ها از ساختارهاي اصلي كنترل كننده‌ي هندسه و كينماتيك ساختاري در كوهزادها به شمار مي‌آيند. منطقه‌ي مورد مطالعه (كوه‌هاي شكراب) در استان خراسان جنوبي و قسمت شمالي شهرستان بيرجند قرار دارد. هدف از اين پژوهش بررسي تاثير گسل‌ها بر تكامل ريخت‌زمين‌ساختي منطقه‌ي مورد مطالعه است. در اين پژوهش تحليل ساختاري كوه‌هاي شكراب واقع در شمال بيرجند با استفاده از عمليات صحرايي، ترسيم مقاطع عرضي و شاخص‌ ژئومورفيك طول- شيب‌ رودخانه (SL) انجام شد. براي تحليل ساختاري و مشخص نمودن تاثير گسل‌ها بر تكامل ساختاري ابتدا با استفاده از عمليات صحرايي گسل‌ها شناسايي گرديد و سپس با استفاده از DEM و داده‌هاي مربوط به گسل‌هاي منطقه مقاطع عرضي ترسيم شد. مقاطع عرضي عمود بر ساختارهاي منطقه (گسل‌هاي طولي و عرضي) رسم گرديد. ترسيم مقاطع عرضي در جهت عمود بر گسل‌ها نشان‌مي‌دهد كه در مقطع CD كه در قسمت غربي منطقه‌ي مورد مطالعه واقع شده است. شاهد كاهش شيب گسل‌ها از شمال به سمت جنوب هستيم، بنابراين مقطع CD فعال‌ترين مقطع مي‌باشد. محاسبه‌ شاخص SL نشان مي‌دهد كه بيشترين فعاليت تكتونيكي و بيشترين مقدار شاخص ‌ SLمربوط به قسمت غربي كوه‌هاي شكراب است كه دليل آن عملكرد اخير گسل‌هاي راندگي مي‌باشد. بيشترين تراكم گسل‌هاي راندگي مربوط به بخش غربي منطقه‌ي مورد مطالعه است و در مناطقي از كوه-هاي شكراب كه بيشترين تراكم گسل‌هاي راندگي وجود دارد بالاآمدگي تكتونيكي و فعاليت تكتونيكي نيز افزايش يافته است. نتايج اين پژوهش نشان مي‌دهند كه با حركت در روند E-W كوهستان شاهد بيشترين فعاليت تكتونيكي در قسمت غربي مي‌باشيم.

The drainage pattern in tectonically active regions is very sensitive to processes such as upliftment, folding, faulting, and tilting which are responsible for river incision, basin asymmetry, drainage geometry, and river deflection. Shekarab Mountain is located in the East of Iran and north of sistan suture zone. Aim of this research is investigating effect of faults mechanism on structural evolution of Shekarab Mountain. In this study for investigating of structural evolution cross section drowing were used. In this research for evaluation rate of tectonic activity morphometric indices such as stream-gradient index SL were used. The highest amount of SL index is related to western part of Shekarab Mountain that is due to recent operation of trust faults. Field data and geomorphic indices shows that in areas of Shekarab Mountains that have the highest density of thrust faults under pressure operation of trust faults tectonic uplift and tectonic activity also increased. Methodology Faults and fractures of the study area are extracted from field operations and cross sections were drawed using Digital Elevation Model (DEM) and field data. Structural map of the studied brittle structures (faults and folds) were draw using field operations data. In this research to study the structural evolution of the Shekarab Mountains several investigations used on E-W trend geological cross section and morphometric index such as SL were used. In order to evaluate rate of tectonic activity stream length-gradient index (SL) were calculated. The SL index is very sensitive to change in channel slope, and this sensitivity allows the evaluation of relationship among possible tectonic activity, rock resistance, and topography. Result and Discussion Shekarab Mountains is located in eastern Iran and is placed in geographical position of 58˚ 37́E to 59˚ 16́E and 32˚ 50́N to 33˚ 09́N. Shekarab Mountains is a terminal splay of Nehbandan fault that is located in the Sistan suture zone. Iran lies on the Alpine earthquake belt which runs west-East from the Mediterranean to Asia. The present tectonics of Iran results from the north-south convergence between the plate of Arabia to the southwest and Eurasia to the northeast. This convergence is accommodated across the Iranian Plateau and adjacent deformed zones, and the deformation, as defined by seismicity and geology is not uniformly distributed. Much of the deformation is concentrated in the Zagros active thin-skinned Fold and Thrust Belt in the southwest, Alborz Thrust Belt bordering oceanic crust of the south Caspian depression, Kopeh-Dagh active thin skinned Fold Belt in the north east, and in East-Central Iranian thick-skinned range and basin province. Results from a regional GPS network indicate that the total convergence across Iran is 25 mm/yr in eastern Iran. Sistan structural zone is a north-south trend and represents the suture between Lut block and Afghan block. The existence of Nehbandan fault system in the border between Sistan suture zone and Lut block has caused several rock units in the margins and within the structural state of Sistan. Nehbandan fault system with strike-slip mechanism by north-south general trend has sub-branches in the North and South terminals. Northern terminal of Nehbandan fault has rotated toward west and its southern terminal toward east. Geometric and kinematic position of identified faults in the studied area show that most faults of study area have a reverse component such as fault F17 Reverse with sinistral strike slip component, fault F6 Sinistral strike slip with reverse component, fault F9 Reverse with dextral strike slip component. Conclusion Faults and folds are the most important structures in the Shekarab Mountains. For identification of faults and role of faults in structural evolution of Shekarab Mountain field operations were used. After completion field operations of study area Structural map of brittle structures (faults and folds) was prepared. Geometric and kinematic analysis of identified faults indicate that mechanism of most study areas faults are reverse with dextral strike slip component, which indicated the overcoming of compressional stress in Shekarab Mountains. Northern terminal of Nehbandan fault has rotated toward west and its southern terminal toward east. Geometric and kinematic position of identified faults in the studied area show that most faults of study area have a reverse component such as fault F17 Reverse with sinistral strike slip component, fault F6 Sinistral strike slip with reverse component, fault F9 Reverse with dextral strike slip component.The highest amount of SL index is related to western part of Shekarab Mountain that is due to recent operation of trust faults. Field data and geomorphic indices shows that in areas of Shekarab Mountains that have the highest density of thrust faults under pressure operation of trust faults tectonic activity also increased.