تاثیر گسل‌های فعال درکواترنری برفراوانی آب‌های زیرزمینی حوضه مهارلو، زاگرس مرکزی

دراین مطالعه،ارتباط بین گسل‌های فعال در کواترنری با وضعیت فراوانی چشمه‌ها و آبدهی چاه‌ها در بخشی از زاگرس مرکزی موردبررسی واقع شده است.به این منظور،گسل‌ها، چشمه‌ها و چاه‌های منطقه مورد نظر بامطالعه دقیق نقشه‌های زمین‌شناسی،تصاویرماهواره‌ای و برداشت‌های میدانی مورد بررسی قرارگرفته و در محیط نرم افزار  gis10.1 تحت تفسیر واقع گردیده‌اند.نتایج این تحقیق حاکی ازآن است که بسیاری ازچشمه‌های منطقه ازنوع گسلی بوده ویا درشکل گیری آن‌هاگسل‌های دارای فعالیت عهد حاضرنقش اصلی را ایفا نموده‌اند. همخوانی مکانی موقعیت این چشمه‌ها با راستای گسل‌های مهمی همچون پهنه گسلی سروستان و سبزپوشان وهمچنین اختلاف فاحش آبدهی چاه‌ها در  پهنه‌های گسلی نسبت به سایرچاه‌هاازجمله شواهد این مدعا است. علاوه براین،نتایج نشان می‌دهد که ارتباط نزدیکی بین فراوانی منابع آب باعناصر ساختاری درمنطقه مورد مطالعه وجود دارد.

Quaternary active faults effect on the abundance of underground water resources in Maharlu Basin, Central Zagros

Abstract:The assessment of major faults to identify the effect of their recent activity on the tectonic evolution of the hydrogeological characteristics of drainage basins is essential.The number of springs as well as the amount of wells discharge often change corresponding to the distribution of fault zones. The 78 km long Sarvestan fault zone is the most typical case according to the distribution of springs. The fault zone, cutting across the foldthrust belt of Zagros, is dominated by strikeslip (Berberian, 1995), and has deformed some of the previously formed folds. Furthermore, The SabzPushan is an active strikeslip fault zone with length of 220 km, extended along the Zagros foldthrust belt from northwest of the Shiraz toward the southeast. Since class 1 of Iat, indicative of the most active tectonics, and class 2 of  Iat corresponding to highly active tectonics, occurs mainly in the southwestern part of the study area along the SabzPushan fault zone and also the Sarvestan fault zone, in this researchthe relation between these large scale faults which are active in quaternary and the abundance of water resources (spring and well) have been studied. Thus, using geological maps, satellite images and field studies, the faults, the springs and the wells of study area have been analyzed through GIS 10.1. The study area ,covering an area of 9732(km^2), is located along a simply folded belt of southeastern Zagros. It is underlain by Phanerozoic sedimentary sequences in elongated, doublyplunging, boxshaped anticlines, and the synclines are partly buried by younger Quaternary alluvium. The SW–NE oriented contraction that initiated in the Late Cretaceous and strengthened during the Early Miocene due to the collision of the Arabian and Eurasian plates, has led to the development of NW–SE trending, SWverging folds, and NEdipping thrusts in the Phanerozoic sedimentary strata covering the AfroArabian basement, above a detachment zone of the Infracambrian–Cambrian Hormuz evaporite (KadinskyCade and Barzangi, 1982; Alavi, 1994). In this research, Firstly, all available information for the area of interest was collected and compiled by using geological and topographic maps, satellite images as well as fieldwork. Then, all data were converted to digital format and consequently different layers were created, such as tectonic elements, lithology, slope, elevation and drainage density. The relationship between the number of springs and distance from tectonics elements is generated by cross operation between springs layer and tectonics elements layer using buffer and distance operation in the GIS environment. As it is shown, there is a tendency for springs to occur at short distances from tectonics elements. It can be concluded that many of tectonic elements, are conduits of water. However, the water surfaces not necessarily on the element itself. The springs at large distance from the main tectonic elements can be explained by local geologic conditions, including fracturing not mapped at the scale used. In order to identify the relationship between the amount of slope and the number of springs, the topospring layer was crossed with the slope map in the GIS. The result is showing that there is a good correlation between slope class and the frequency of springs located in each class and represent that the number of springs is a function of slope. Springs layer was, also, crossed with 400 m altitude zones of a DEM (digital elevation model) to present the relationship between the elevation levels and the number of springs. The output clears that the vast majority of springs are in 14002200 meter. Springs at high altitude indicate local geologic control. Drainage density has been measured as total stream length per unit area of each basin. The results reveal that high drainage density is correspondent with impermeable sub surface units and mountainous relief, whereas, the low drainage density reveals that the subsurface material are permeable and low relief which results in more infiltration capacity in the basins. According to the results, most of the springs are either faulted ones or identified with major faults through the study area. It has been proved not only by correspondence between the location of springs and the trend of main faults such as Sabz Pushan and Sarvestan, but also by the huge differences in the amount of wells’ discharge. Furthermore, the results indicate that there exists an entwined relationship between the abundance of water resources and structural elements.