مدل‌سازي آزمايشگاهي برهم‌كنش گسل‌ها در تكامل ساختاري تاقديس‌هاي رگ سفيد و تنگو (جنوب غرب ايران)

در فروبار دزفول جنوبي با توجه به توالي فاز­هاي دگرشكلي زاگرس و رخداد وارونگي تكتونيكي در گسل­ راندگي رگ سفيد و همچنين تجديد فعاليت گسل­ پي سنگي هنديجان، بر هم­كنش عمودي بين گسل­ها در پيش بوم زاگرس اتفاق افتاده است؛ كه اين اثر متقابل تاثيراتي مهم بر الگوي ساختاري و تشكيل تاقديس ­هاي رگ سفيد و تنگو دارد. فاز چين خوردگي اصلي در پيش بوم زاگرس و در تاقديس­ رگ سفيد در پليوسن رخ داده است. جهت مدل­سازي آزمايشگاهي تكامل تكتونيكي، با توجه به محاسبه ميزان كوتاه­شدگي عمود بر بخش مركزي تاقديس رگ سفيد، با اعمال فشردگي به ميزان 16 درصد و با حركت فك­هاي متحرك دستگاه برش با سرعت ثابت، وارونگي تكتونيكي و تبديل حركت از كشش به راندگي در گسل رگ سفيد سبب مي­شود توالي لايه­ها دگرشكل شوند و چين خوردگي وابسته به گسل در تاقديس رگ سفيد تشكيل گردد. با ورود جبهه دگرشكلي زاگرس به پيش بوم منطقه كوهزايي در پليوسن، علاوه بر برخاستگي تاقديس تنگو در راس گسل هنديجان، محور تاقديس رگ سفيد به علت حركت امتداد لغز راستگرد در گسل هنديجان، به ميزان 30 درجه به صورت ساعتگرد مي­چرخد. مدل ­سازي ما نشان مي­دهد كه دليل طول موج بيشتر بخش باختري تاقديس رگ سفيد به علت ضخامت زياد لايه نمكي سازند گچساران و همچنين مقدار بالاي نسبت ضخامت لايه نامقاوم به لايه مقاوم است كه در اثر واقع شدن بخش باختري تاقديس رگ سفيد در يك پهنه گسلي راستگرد و چرخش ساختاري حاصل از آن، پهن شدگي و افزايش طول موج تاقديس تسهيل شده است.

Introduction The subduction systems are located in the continental collision phase. Due to the tectonic regime reversion from the tensile phase to the compressive phase and due to the reactivation of the normal and transverse basement faults, constitute folded belts that are in their tectonic evolution, have experienced multiple fault interactions. Therefore, based on the angle of post-collision shortening axis, relative to each of these old basement faults, their reactivation in the form of thrust or strike-slip components and their effects on the structural pattern of the folds, are justified. In the Zagros foreland, there are several fault lineaments with different trends and mechanisms. They have been reactivated during the collision phase of the Iranian and Arabian sheets. The present study tries to show the effect of the Zagros thrust faults and the Arabian strike slip faults interaction on the development and structural evolution of the Rag Sefid and Tango anticlines using analog modeling. Materials and Methods In order to model the interaction of basement faults in the South Dezful embayment, first, in accordance with the structural realities of the region, the topography of the basement is simulated using the cutting of wooden boards. The arrangement and cutting of the boards are according to the slope of the Rag Sefid thrust fault. First the wooden board is cut at an angle of 47 degrees. Also, considering that about one third of the Rag Sefid anticline has an axial curvature, and due to the slope of approximately 80 degrees at Hendijan fault, which is perpendicular to the Rag Sefid fault, the wooden board is cut into two unequal parts with an 80-degree slope. According to the interpretation of reflective seismic sections and drilled well data, the stratigraphic sequences detectable in the southern Dezful embayment are Aghajari, mobile Gachsaran formation and Middle Resistance Group between Asmari to Gadvan, respectively. Therefore, in order to model these stone units, clay with a thickness of 3 and 2 cm was used for upper and middle resistant units, respectively. Also, in order to simulate the Gachsaran moving formation, a combination of sifted rock powder and 3 cm thick engine oil is used at the beginning of modeling. Results and discussion The northwest-southeast trend of Rag Sefid anticline located in Zagros foredeep, has been raised due to the Zagros orogenic phase in the Pliocene. After the collision of the plates from the Late Eocene onwards, in addition to folding on the northwest-southeast faults, the north-south basement faults during the Late Cenozoic have been reactivated by the entry of the Zagros deformation front into the study area. The reactivation of these faults has caused changes in sedimentary cover, such as facies change and sediment thickness as well as changes along the axis of surface anticlines. Oblique convergence after collision between Iran and the Arabian Plate has shortened the succession of the Zagros basement; so that the northwest-southeast longitudinal faults that were extensional at the time of the rift formation are now basement thrusts in this belt. The north-south faults, which have trends similar to those of the north-south basement fault in the eastern Arabian Block, are reactivated as a result of this compression. Conclusion Due to the general compression trend of N22E in southwestern Iran and the trend of the southern part of the Hendijan fault (N20E), the small-scale fold at the Tango anticline located on the Hendijan fault is due to the parallelism of this transverse fault and the overall compression direction. In contrast to the Rag Sefid anticline, where the trend of the main fault is approximately perpendicular to the direction of total compression (N100), a clear fold with large structural dimensions is created in the Rag Sefid anticline. Our results show that with the entry of the Zagros deformation front into the foreland of the orogenic region in the Pliocene, in addition to the Tango anticline rising at the top of the Hendijan fault, the Rag Sefid anticline has rotated axially by 30 degrees clockwise due to the movement of the right-hand slip in the Hendijan fault. Our modeling shows that the reason for the higher wavelength of the western part of the Rag Sefid anticline is due to the high thickness of the salt layer and also the high value of the ratio number. Due to the location of the western part of the Rag Sefid anticline in a right-sided fault zone and the resulting structural rotation, the widening and increasing wavelength of the anticline is facilitated.