مطالعه ساختار لیتوسفر شمال‌غرب ایران با استفاده از تابع گیرنده S

نوع مقاله : پژوهشی

نویسندگان

1 دانش‌آموخته کارشناسی ارشد، دانشکده علوم زمین، دانشگاه تحصیلات تکمیلی علوم پایه، زنجان، ایران

2 استادیار، دانشکده علوم زمین، دانشگاه تحصیلات تکمیلی علوم پایه، زنجان، ایران

3 دانشیار، دانشکده علوم زمین، دانشگاه تحصیلات تکمیلی علوم پایه، زنجان، ایران

چکیده

در این مقاله نتایج تحلیل و مهاجرت به عمق توابع گیرنده S در شمال‌غرب ایران ارائه می­شود. شبکه لرزه­نگاری مورد استفاده در این مطالعه شامل 23 ایستگاه باند پهن و متوسط است که از ساحل غربی دریای خزر آغاز شده و تا ساحل شرقی دریاچه ارومیه در امتداد یک خط تقریباً مستقیم با جهت­گیری شرقی-غربی و عمود بر رشته­کوه­های تالش ادامه می­یابد و از مجاورت آتش‌فشان­های سهند و سبلان می­گذرد. زلزله­های دورلرز ثبت شده از زلزله­های رخداده در فاصله رومرکزی 55 درجه تا 85 درجه و بزرگای بیش از 5/5 که از بانک داده جمع­آوری شده در بازه زمانی آگوست 2008 تا جولای 2012 استخراج شدند و توابع گیرنده S با واهمامیخت مؤلفه شعاعی از مؤلفه قائم به‌دست آمد. نتایج حاصل از بر­هم­انبارش این توابع گیرنده S نشان می­دهد که شمال­غرب ایران دارای لیتوسفر نسبتا نازکی است که ضخامت آن در بیشتر مناطق برابر با 10±80 کیلومتر است و در زیر سبلان به 10±120 کیلومتر افزایش می­یابد. به‌نظر می­رسد نازک بودن لیتوسفر تطابق نزدیکی با وجود بی­هنجاری­های دمایی در شمال­غرب ایران دارد. پوسته و لیتوسفر در زیر آتش‌فشان سبلان نسبت به نواحی اطراف ضخیم­تر است که می­تواند ناشی از همگرایی بلوک خزر جنوبی و لیتوسفر قاره­ای شمال­غرب ایران باشد. به‌علاوه نتایج ما یک پله در مرز موهو در زیر کوه‌های تالش را آشکار می­کند که به‌نظر می­رسد به‌علت مجاورت پوسته خزر جنوبی و پوسته قاره­ای شمال‌غرب ایران در این منطقه باشد.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Lithospheric Structure of the NW Iran revealed by S Receiver Functions

نویسندگان [English]

  • Vahideh Ghorbanalizadeh 1
  • Seyed Khalil Motaghi 2
  • Farhad Sobouti 3
1 M.Sc. Graduated, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran
2 Assistant Professor, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran
3 Associate Professor, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran
چکیده [English]

Iran Plateau is part of the Alpine-Himalayan orogenic system in western Asia. It is located in a seismically active region affected by a transpressional tectonic regime of oblique convergence, generated by the convergence of the Arabian plate toward Eurasia. The current morphology of Iranian plateau had been dominated by opening and closing of the Paleo-Tethys and Neo-Tethys oceans in the past. Current lithospheric deformation in the NW Iran is shaped by the convergence of Arabia and Eurasia and the westward motion of the rigid South Caspian Basin. The South Caspian Basin is a relatively aseismic rigid basement block and has affected the deformation history of its surrounding continental regions. The South Caspian Basin and the Kura depression to its west are thought to be a relic back-arc of the Tethyan Mesozoic subduction caught up in a continental collision zone similar to the Black Sea and the eastern Mediterranean. The South Caspian Basin is a piece of unusually-thick oceanic-like crust, because of its low elevation and its west and southward motion relative to central Iran.
Here we present results of a S-receiver function analysis for a 290 km long temporary seismic network in the NW Iran. The network is a linear array stretching from the western coast of the South Caspian Basin to the Lake Urumieh. We computed the individual S receiver functions for 23 broad-band seismic stations and then we stacked them based on their piercing points at depth of 80 km. To calculate S receiver functions, the teleseismic S waveforms were cut from 200 s before to 100 s after the theoretical S wave onset. ZNE-component waveforms were rotated into the ZRT coordinate system and the R component was deconvolved from the Z component. To make S receiver functions similar to P receiver functions, we reversed the time axis and the polarity of S receiver function time series. The Gaussian smoothing factor was selected equal to 1.0 for both data sets, which is equivalent to the application of a Gaussian low pass filter with a corner frequency of ~0.2 Hz to the receiver functions.
Piercing points at north and south of the linear profile were separated into two different data sets to avoid the stacking of 3D heterogeneities. Stacking receiver functions confirms a thin crust east of the Talesh Mountains juxtaposing with a thick continental crust beneath the NW Iran. We interpret the thin crust as an oceanic-like crust belonging to the South Caspian Basin. The detected Lithosphere-Asthenosphere boundary is almost shallow with an average depth of ~100 km. However, its variations across the profiles are different in each data set. Variations beneath the region south of the profile are minor revealing a flat, thin lithosphere beneath the region. The lithosphere (as well as crust), however, becomes thick beneath Sabalan volcano, north of the profile, probably due to convergence of the NW Iran and the South Caspian basin above the North Tabriz Fault. This interpretation implies that the North Tabriz Fault is a continental suture between the NW Iran and Central Iran plateau.

کلیدواژه‌ها [English]

  • NW Iran
  • S receiver function
  • Moho boundary
  • Lithospheric thickness
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