ارزيابي ميزان جنبايي زمين ساختي و پويايي گسل هاي شمال باختري گستره ايران مركزي، پهنه قم-ساوه

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

گستره مورد مطالعه، حوضه قم - ساوه، حاشيه شمالي ايران مركزي و پهنه آتشفشاني اروميه - دختر، بين گسل چپ‌بر كوشكنصرت در شمال و شمال خاوري و گسل تلخاب در جنوب باختري قرار گرفته است.  مهمترين گسل‌هاي پهنه مورد مطالعه، بر اساس نقشه‌‌هاي زمين شناسي 1:100000 منطقه، شامل كوشك - نصرت، ساوه، ايندس، تفرش، نوبران، تلخاب، البرز و پهنه گسله كوه قرمز است. در پژوهش حاضر سعي شده ميزان فعاليت زمين ساختي اين پهنه، با استفاده از روش هاي ريخت زمين ساختي مورد بررسي قرار گيرد و براي مستندسازي داده هاي به دست آمده، از شواهد صحرايي و ريخت ساخت هاي موجود در منطقه مورد استفاده قرار گيرد.  به منظور ارزيابي ميزان فعاليت زمين ساختي گستره مورد مطالعه از اندازه گيري شاخص هاي ريخت سنجي و انطباق آنها با محل گسل هاي اصلي بهره گرفته شده است. سه شاخص ريخت سنجي (Morphotectonic) شامل، گراديان طولي (SL)، سينوسيتي پيشاني كوهستان (Smf) و نسبت پهناي كف دره به ارتفاع دره (Vf) مورد توجه قرار گرفته كه در محيط Arc GIS 10.1و با كمك نرم افزارArc hydro اندازه گيري شده اند. تفسير نتايج به دست آمده، نشان دهنده فعاليت زمين ساختي بالا در نزديكي گسل هاي كوشكنصرت، نوبران، ايندس، پهنه گسله كوه قرمز و تفرش بوده كه اين موضوع با شواهد برداشت هاي صحرايي از منطقه منطبق است.

The study area with almost 13000 km2, QomSaveh basin, is located in most northern boundary of the Central Iran, where EoceneOligocene volcanic rocks and associated sediments as well as Qom Formation crops out between the dextral KoshkNosrat fault in the north and the Talkhab fault in the south. The KoshkNosrat, 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. Three morphotectonic indices, Stream LengthGradient (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. 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 KoshNosrat faults ( 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 rightdeflection and offsets, which suggest right lateral component activities of the faults. The rightdeflection 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 KoshkNosrat and Nobaran Faults (NW segment of Indes fault) show a small leftdeflection and offsets, which indicate the last activities of the faults are sinistral in these faults. South of KoshkNosrat fault, in hanging wall, several NWSE 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 Koshknosrat fault and have same dip directions (to south) so most probably the Quaternary faults are splayed out from the KoshNosrat fault. A clear WNWESE 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 NS 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 (57cm). 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. 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 KoshkNosrat, Koh Qermez fault zone, Indes and its NW continuation (Nobaran fault), Saveh and Tafresh in order.