بررسي ساختار گسل مشا (جنوب البرز مركزي) با استفاده از روش مگنتوتلوريك

Study of Mosha fault structure (South of the Central Alborz) using magnetotelluric method

بررسي پديده‌‌هاي زمين‌‌شناسي و زمين‌ساختي البرز و تحليل چگونگي روند و منشا اين پديده‌‌ها همواره به‌منزله يكي از جذابيت‌هاي البرز مورد ‌توجه بوده است. البرز مركزي به دليل برهم‌كنش‌هاي متعدد ساختاري و پيچيدگي‌هاي زمين‌ساختي، چالش‌هاي بيشتري را دربر داشته است. با بررسي‌هاي جديد در مورد روند و عملكرد بخش‌‌هايي از گسل‌‌هاي بزرگ و توانمند اين منطقه ابهامات فراواني به‌وجود آمده است. البرز مركزي به دليل فشارش مداوم صفحه عربي به صفحه ايران با جهت N10-20E داراي ساختارهاي شديداً گُسليده فشاري و چين‌خورده تقريباً شرقي- غربي و با شيب رو به شمال است. اخيراً در مورد برخي از گسل‌ها روشن شده است كه داراي حركات كششي با شيب رو به جنوب هستند. در اين تحقيق با استفاده از روش مگنتوتلوريك، در امتداد يك نيم‌رخ‌‌ 5/2 كيلومتري عمود بر گسل مشا و در جنوب شرق آتشفشان دماوند محل دقيق گسل، منطبق با نقشه زمين‌‌شناسي شناسايي شده است. از سوي ديگر تغييرات شيب گسل از 75 تا80 درجه به سمت شمال در اعماق، به حالت تقريباً قايم در نزديكي سطح رسيده است. كه با توجه به تغيير روند مشهود در مدل‌‌هاي حاصل از وارون‌سازي، عملكرد گسل در جابه‌جايي واحدهاي داراي مقاومت يكسان با در نظر گرفتن شيب گسل به طرف شمال، نرمال است. عملكرد شاخه‌‌هاي فرعي گسل و همچنين آبراهه‌‌هاي گسلي نيز در اين تحقيق در زير بعضي از ايستگاه‌ها شناسايي شد كه نشان از وجود زون‌‌هاي خُرد شده با مقاومت كم دارد. تغيير شيب صفحه اصلي گسل از نتايج اساسي و بحث‌انگيز تحقيق حاضر و با توجه به پيشينه زمين‌ساختي البرز مركزي طراحي نيم‌رخ‌‌‌‌هاي موازي و عمود بر امتداد گسل در اين منطقه براي تاييد صحت نتايج فوق پيشنهاد مي‌شود.

Magnetotellurics (MT) is an electromagnetic geophysical method of imaging the earthʹs subsurface by measuring natural variations of the electrical and magnetic fields at the Earthʹs surface. Investigation depth ranges from 300m below ground by recording higher frequencies down to 10,000m or deeper with long-period soundings. Research applications include experimentation to further develop the MT technique, long-period deep crustal exploration, and earthquake precursor prediction research. Also, Magnetotelluric method is a powerful tool for deep crustal studies of tectonically active mountainous regions such as the Himalaya, where logistic constraints severely limit the use of other artificial source electrical and electromagnetic methods. Topographic variations in mountainous regions distort apparent resistivity curves and thus lead to artifacts in interpreted models. The results of the magnetotelluric soundings provide new insights on the geological structure and tectonics of the mountainous areas. Magnetotelluric method (MT) offers opportunity to detect crustal fluids along faults due to their high conductivity anomaly. Supposing that fluids deposited minerals in the conductive fractures (faults, dykes) decreasing the resistivity, the high seismicity in the area can be explained by the presence of the fluids. Mosha Fault in northern part of Iran is located in an E-W trend and act as a separator for Alborz mountain chain from Central Iran. This fault is interesting to investigate, because Tehran city (Capital of Iran) has a risk of an earthquake occurrence so that understanding the fault characteristics is very important. Mosha Fault Zone is also an interesting structure due to its key character of mysterious structural relation to the mechanism of the fault zone. Mosha fault has the most complicated tectonic structure in Alborz zone due to big changes that have affected the region basically. These ambiguities come from the increase with new studies about process and work parts of great faults in the area. Central Alborz inclusive structures are reverse faults with nearly East- West crackled dipping towards north. This can be a result due to continuous compression of the Arabian plate to the Iranian plate with N10-20E direction. Latter studies showed that some of the faults recently show tensional behavior or the dip is changing towards to the south, generally. Based on the seismic and GPS studies, the Uplift, south Caspian subsidence and subsequent folding, reversal of Alborz strike-slip (from dextral to sinistral) and eastward extrusion of central Iran, coarse Zagros molasse deposition, Dead Sea transform reorganization, Red Sea oceanic spreading, and North and East Anatolian fault slip, all apparently began ca. 5 ± 2 Ma, suggesting a widespread tectonic event that was a response to buoyant Arabian lithosphere choking the Neo-Tethyan subduction zone. Basically, some researchers believe that the westward rotation of the stable Caspian block at this time is started and it cause the change of the dextral strike-slip faults mechanism to the sinistral strike-slip one which cause to making pull-apart system in some part and also in the development of new normal faults with a change in dip direction and block motion of pre-existing faults. In this study, MT measurements were carried out in the period range 0.001- 420 s crossing Mosha fault along a 2.5k m profile with 8 MT sites. In these work we detect geo-electrical resistivity anomalies of the Earth’s crust and link them to local seismic activities. And also the fault location on the geological map is investigated. Surprisingly, fault dip changes from 75-80 degrees in depths to nearly vertical (90 degrees), at the surface. With attention to the geological map and occurrence of this kind of variations in resistive parts, it is not as a sign of geological units differences. We strongly believe that the fault plan is rotating from north toward south. It means that the new block movement would be a normal displacement and not reverse type. For completion and supporting up the results, more profiles in both directions -perpendicular and parallel to the fault tenor- would be necessary to be done. The place of two remarkable drainages system in the studied area and along the profile is compatible with two linear near surface trend of low resistance anomalies. The root of Karaj formation is distinguished at depth of 2500m with having 2.2 to 2.4 ohm meter resistivity. It is shown geophysically below the number four completely and in corner of stations number 3 and 5. It seems that in depth, Mosha fault and Karaj formation have the same trend and dip direction. It would be a key point to understand and decipher the mysterious about the place and circumstance of Karaj pyroclastics emission. We suggest that they are emitted from faulted structures during Eocene. From this point, we will expand our study to a three-dimensional analysis including the complete data set in order to reveal the detailed features of the electrical structure around the focal regions where the great earthquakes maybe threaten the metropolis.