نوع مقاله : مقاله پژوهشی

نویسندگان

1 تکتونیک، گروه زمین شناسی، دانشکده علوم زمین، دانشگاه خوارزمی، تهران

2 دانشکده علوم زمین، دانشگاه آزاد اسلامی، واحد تهران شمال

چکیده

گستره مورد مطالعه، حوضه قم-ساوه، حاشیه شمالی ایران مرکزی و پهنه آتشفشانی ارومیه-دختر، بین گسل­ چپ‌بر کوشک-نصرت در شمال و شمال خاوری و گسل تلخاب در جنوب باختری قرار گرفته است.  مهمترین گسل‌های پهنه مورد مطالعه، بر اساس نقشه‌‌های زمین­شناسی 1:100000 منطقه، شامل کوشک-نصرت، ساوه، ایندس، تفرش، نوبران، تلخاب، البرز و پهنه گسله کوه قرمز  است. در پژوهش حاضر سعی شده میزان فعالیت زمین­ساختی این پهنه، با استفاده از روش­های ریخت­زمین­ساختی مورد بررسی قرار گیرد و برای مستند­سازی داده­های به دست آمده، از شواهد صحرایی و ریخت­ساخت­های موجود در منطقه مورد استفاده قرار گیرد.  به منظور ارزیابی میزان فعالیت زمین­ساختی گستره مورد مطالعه از اندازه­گیری شاخص­های ریخت سنجی و انطباق آنها با محل گسل­های اصلی بهره گرفته شده است. سه شاخص ریخت سنجی (Morphotectonic) شامل، گرادیان طولی (SL)، سینوسیتی پیشانی کوهستان (Smf) و نسبت پهنای کف دره به ارتفاع دره (Vf) مورد توجه قرار گرفته که در محیط Arc GIS 10.1 و با کمک نرم افزار Arc hydro اندازه­گیری شده­اند. تفسیر نتایج به دست آمده، نشان دهنده فعالیت زمین ساختی بالا در نزدیکی گسل­های کوشک-نصرت، نوبران، ایندس، پهنه گسله کوه قرمز و تفرش بوده که این موضوع با شواهد برداشت­های صحرایی از منطقه منطبق است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Morphotectonic analysis and dynamics of north_western Central iran, the Qom-Saveh Basin

نویسندگان [English]

  • Elham Ghorbani 1
  • Asghar Dowlati 1
  • Mohsen Pour Kermani 2

1 Tectonics, Department of Geology, Faculty of Earth Sciences, Kharazmi University, Tehran

2 Faculty of Earth Sciences, Islamic Azad University, North Tehran Branch

چکیده [English]

The study area with almost 13000 km2, Qom-Saveh basin, is located in most northern boundary of the Central Iran, where Eocene-Oligocene volcanic rocks and associated sediments as well as Qom Formation crops out between the dextral Koshk-Nosrat fault in the north and the Talkhab fault in the south. The Koshk-Nosrat, Saveh, Indes, Tafresh, Nobaran, Talkhab, Alborza and Koh Qermez fault zone are the most important faults, are delineated in geological maps with 1:100,000 Scale. The faults are covered by the Quaternary alluvium in the major part of its length, so morphotectonical characteristics gives more evidences about the regional activity.
Materials and Methods
Three morphotectonic indices, Stream Length-Gradient (SL), Mountain Front Sinuosity (Smf) and Valley height ratio (Vf), are used to determine the tectonic activities of 175 adjacent catchment draining in the study area, which are determined by ArcGIS 10.1 and Arc hydro software. The SL index depended to the gradient of the rivers and high value indicate higher exhumation in the basin so it can confirm activities of faults with higher dip displacement than lateral component. Smf index related to activities of faults in front of mountains and lower value indicate higher activities of faults than erosional process. The Vf index depended to of dip slip fault movements, therefore less Vf value shows higher uplift than erosion and makes V shape valley in the area.
Results and Discussion
The morphotectonic analysis consist of high SL value (>950) in south of Indes and Tafresh faults, which indicate a high exhumation in these area. Measurements on 396 mountain front for calculation of smf index, indicate low value of the index on main part of Nobaran, Indes, Alborz, Saveh, Koh Qermez, Tafresh and Kosh-Nosrat faults (<1.10), which points to high activities of the faults. Usually, faults in southern part of the study area such as Tafresh does not show such a value and activities. 40 valley, distributed all over the study area, are chosen to calculate vf index. The calculation indicate valleys related to Koh Qermez fault zone, and all over the Indes fault (whithin the study area) have lowest value of vf (<0.5), which points to high uplifting in these basins.  
Fieldwork studies and satellite image processing are done to control the results obtained from the morphotectonic indices. High resolution satellite images of Google Earth and Bing have used to determine deflection and offsets of streams and rivers. Usually, main rivers near to all of the main faults of the study area show a systematic right-deflection and offsets, which suggest right lateral component activities of the faults. The right-deflection are observed on fieldworks and minimum offsets of the streams and Quaternary sediments are about 2m. Usually, Small streams and youngest Quaternary alluvial near to the Koshk-Nosrat and Nobaran Faults (NW segment of Indes fault) show a small left-deflection and offsets, which indicate the last activities of the faults are sinistral in these faults.
South of Koshk-Nosrat fault, in hanging wall, several NW-SE reverse faults are recorded in Quaternary terraces with total displacements of 5 to 10 cm. since the strike of the Quaternary faults are parallel to the Koshk-nosrat fault and have same dip directions (to south) so most probably the Quaternary faults are splayed out from the Kosh-Nosrat fault.
A clear WNW-ESE lineation with streams deflection and effected on morphology of Quaternary sediments is visible in north of Nobaran Fault in satellite images. Just adjust to the lineation several almost N-S reverse faults with dipping 30ºE with total displacements of 20 to 30cm is recorded in Quaternary terraces. Normal faults, almost parallel to the Quaternary reverse faults but with opposite dips (52W), are extended in hanging wall of the reverse faults. The total displacement of the normal faults are much less than the reverse faults (5-7cm). The reverse faults are created by activities of the lateral component of a branch of the Nobaran fault. The normal faults should form by activities of reverse fault by moving material toward foreland (west) and so breed normal faults by collapse in soft Quaternary sediments.
Conclusion
In general based on morphotectonic indices and fieldwork data and observations all the study area but south eastern part (fault zone around Qom city) and Talkhab fault, are active. The most activities of faults are seen in Koshk-Nosrat, where reverse faults cuts Quaternary sediments, Koh Qermez fault zone, Indes and its NW continuation (Nobaran fault), where reverse and normal faults cuts Quaternary sediments clearly, Saveh and Tafresh in order.

کلیدواژه‌ها [English]

  • Morphotectonic indices
  • Indes Fault
  • Quaternary Fault
  • Koshk-Nosrat Fault