شواهد تكتونيك فعال مرتبط با پهنه گسلي جوان اصلي در منطقه سروآباد، شمال‌باختر ايران

Active tectonic evidences related to the Main Recent Fault in the Sarvabad region, NW Iran

هدف از اين مطالعه بررسي شواهد تكتونيك فعال مرتبط با گسل جوان اصلي زاگرس (MRF) در منطقه سروآباد در شمال‌باختري ايران بود؛ كه مطالعه‌اي در مورد ريخت‌زمين‌ساخت و ساختار اين منطقه تا قبل از اين پژوهش صورت نگرفته است. اين گسل با حركت امتداد لغز راست ‌بر يكي از ساختارهاي مهم لرزه خيز در باختر ايران است كه مسبب زلزله هاي بزرگي بوده است. در اين مطالعه، با استفاده از شاخص هاي ريخت زمين‌ساختي، فعاليت تكتونيكي نسبي در راستاي اين گسل بررسي شد. نتايج به‌دست‌آمده از شاخص هاي ريخت زمين‌ساختي بر اساس تحليل هاي سلسله‌مراتبي وزن دهي‌شده و وزن هر شاخص در نقشه مربوطه آمده است. براي يكسان سازي فاكتورها با استفاده از منطق فازي هر نقشه به بازه بين 0 تا 1 تبديل و با همپوشاني همه نقشه هاي فازي نقشه نهايي فعاليت نسبي تكتونيكي تهيه شد. با توجه به نقشه نهايي بخش هاي جنوبي و جنوب‌خاوري منطقه مورد مطالعه با راستاي شمال باختري ـ جنوب خاوري و منطبق با MRF فعاليت نسبي بالايي دارد. شواهد صحرايي نشان مي دهد حركت امتدادلغز راست‌بر اين گسل در منطقه سروآباد همراه مؤلفه فشارشي مثل گسل هاي معكوس و ساختارهاي گلمثبت (جنوب باختري گسل) و كششي مثل گسل هاي نرمال (شمال باختري گسل) است و به صورت دره هاي خطي گسلي قابل مشاهده است.

-Introduction The Main Recent Fault (MRF) is a seismic structure on the northwestern boundary of the Zagros belt and southern border of the Sanandaj-Sirjan belt. This fault as a major strike-slip fault consists of several segments in the Zagros collision zone. The seismic activity of the southwestern segments is greater than the northwestern segments. Several earthquake events have occurred along the MRF zone in the past decade, as the largest was the 1909 Silakhor earthquake with 7.4 magnitude. Several structural studies have been done along the MRF from the southwest segment (Dorud fault) to northwestern segments (Marivan and Piranshahr segments), because of the seismicity importance of this fault zone. However, there is no any structural and morphotectonics studies along this fault in the Sarvabad region. Therefore, in this research, the relative tectonic activity of the MRF in the Sarvabad region has been investigated based on field studies and measurements of the morphometric indicators. 2-Materials and methods In this study, six morphometric indicators were measured in the basins of the study area, to assessment of the tectonic activity related to the MRF fault zone in the Sarvabad region. These indicators include: stream length-gradient index (SL), asymmetry factor index (Af), basin shape index (Bs), hypersometric integral index (Hi), valley height-width index (Vf) and topographic symmetry factor index (T). These indicators have been extracted using digital elevation model (DEM) and geological maps of the study area. Then, for a detailed study of the relative active tectonic using analytical hierarchy process (AHP), we first considered weight of each index based on its importance, and finally, weighted average of indicators has been analyzed for data standardization using fuzzy logic. 3-Results and discussion In the northwest basins of the study area and along the MRF zone, SL index indicates high values, according to the SL map and related graphs. According to the Hi index diagrams, the basins of the study area are classified in three categories. Category 1 (young topographic and convex curve shape), category 2 (sigmoid curve shape and mature basin) and category 2 (concave curved shape) and nine measured basins are classified in the first category. Three categories for the Af index are considered in the study area. The basins of the central part of the study area indicate the highest asymmetry, because of the strike-slip activity of the MRF. Almost half of the basins of the study area are classified in class 1 and 2, according to Bs index classification map. The values of the Vf index is decreased in the basins along the active strike-slip fault in the study area. The T index in the most basins of the study area shows 1 and 2 classes, indicating an asymmetric region in the central part and along the active strike-slip fault systems. In this study, we used a weighting system for morphotectonic indices to calculate the exact relative tectonic activity. morphotectonic indicators affected by tectonic structures have the most weights and indices controlled by topography and mineralogy are less weights. The weight of each indices is multiplied in the measured map of each basin, after weighing the indices in the studied area. Then the finalized weight maps are shown separately for the indices. All values of the weighted map have been converted into the same range from zero to one using the fuzzy method, in order to standardize the values of the indices. Then, six index maps was combined using a simple additive weighting (SAW) model. Then the final map and, in fact, the output of the SAW model, are classified into 3 classes of relative tectonic activity from high to low. Based on this model, the Southeastern and West parts of the region show a higher relative tectonic activity related to the Northwestrn parts. 4- Conclusion Based on final map obtained from the SAW model Basins with high relative tectonic activity is located in the Southeastern and West parts and in northwest-southeast line trend along the MRF zone. Field evidence indicate that the recent tectonic activities in the study area are visible as V-shaped valleys, linear fault valleys, and fault planes. In the northern part of the MRF zone in the Sarvabad region, the strike-slip movement of the fault indicates an extensional component. This extensional component is visible as normal faults and the formation of veins. In the southwestern part of the study area, the strike-slip fault movements indicates compressive component, as reverse faults and flower structures.