ساختار سرعت گوه برافزایشی مکران مرکزی و پیامدهای آن در اکتشاف منابع هیدروکربنی و خطر زمین‌لرزه

نوع مقاله : مقاله پژوهشی‌

نویسندگان

1 دانشجوی دکتری زلزله‌شناسی، دانشکده علوم زمین، دانشگاه تحصیلات تکمیلی در علوم پایه، زنجان، ایران

2 استاد ژئوفیزیک، دانشکده علوم زمین، دانشگاه تحصیلات تکمیلی در علوم پایه، زنجان، ایران

چکیده

فرورانش مکران منشور برافزایشی بسیار ضخیم و عریضی دارد که دو­سوم آن در خشکی واقع است. بخش خشکی وسیع این منشور برافزایشی، فرصت بی‌نظیری را برای مطالعه ساختار سرعتی یک منشور برافزایشی فراهم آورده است. با وجود امکان وقوع زمین‌لرزه‌های بزرگ ابرراندگی در منطقه و قرار گرفتن بنادر مهم تجاری مانند چابهار و کنارک روی این منشور، منطقه مکران از مناطق کمتر مطالعه­شده در ایران است. همچنین وجود رسوبات ضخیم جوان و نشتی‌های هیدروکربنی، گوه برافزایشی مکران را به یکی از مناطق مستعد وجود ذخایر نفتی تبدیل کرده است. در این مطالعه با استفاده از داده‌های نوفه زمینه ثبت‌شده در شبکه محلی لرزه‌نگاری متراکم نصب­شده در این منطقه (2020-2016) و با استفاده از روش توموگرافی الحاقی نوفه زمینه به بررسی ساختار منشور برافزایشی در ناحیه اطراف مرز مکران غربی و شرقی واقع در قسمت ایرانی مکران پرداخته شده است. نتایج نشان می‌دهد منشور برافزایشی مکران به دو بخش پیشانی و دیرینه تقسیم می‌شود که سرعت موج برشی در قسمت دیرینه نسبت به قسمت پیشانی به‌طور متوسط ۴/۰ کیلومتر بر ثانیه بیشتر است. قسمت پیشانی منشور برافزایشی که در جنوب منشور قرار دارد، کمتر از نصف عرض منشور را در قسمت خشکی دربرمی‌گیرد. سرعت زیاد موج برشی در بخش دیرینه منشور برافزایشی مکران بیانگر درهم‌تنیدگی ساختارهای داخل گوه و وجود تکه‌های اقیانوسی کنده­شده از پوسته اقیانوسی در حال فرورانش است که شرایط را برای وجود ذخایر هیدروکربنی بسیار نامحتمل‌ می‌کند. بر این اساس پیشنهاد می‌شود مراحل اکتشاف ذخایر هیدروکربنی در مکران ایران ابتدا در قسمت پیشانی منشور برافزایشی متمرکز شود. سرعت کمتر موج برشی در پیشانی منشور برافزایشی می‌تواند باعث به‌وجود­آمدن اثر حوضه رسوبی و افزایش دامنه امواج زمین‌لرزه رسیده به این منطقه و ناگزیر، سبب افزایش خطر زمین‌لرزه در این منطقه شود.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

The structure of the central Makran accretionary prism and its implications for hydrocarbon exploration and seismic hazard

نویسندگان [English]

  • Mohammad Enayat kovarchin ghaleh 1
  • Abdolreza Ghods 2
1 Ph.D. student, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran
2 Professor of Geophysics, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran
چکیده [English]

The Makran subduction with an approximate length of 900 kilometers has a thick and wide accretionary prism. Two-thirds of the accretionary prism width (around 200 kilometers) is onshore. The wide onshore part of the Makran accretionary prism provides a unique opportunity to study the structure of an accretionary prism. Despite the potential occurrence of large megathrust earthquakes in the region and the presence of major commercial ports like Chabahar and Konarak on the accretionary prism, the Makran region remains one of the less explored areas in Iran. Moreover, the presence of thick and young sedimentary deposits and hydrocarbon seepages has transformed the Makran accretionary prism into one of the promising regions for potential oil reserves.
    In this study, we have focused on investigating the structure of the accretionary prism in the border of western and eastern Makran (i.e., Central Makran) lying within the Iranian part of Makran, using ambient noise data recorded by a dense local seismic network installed from 2016 to 2020 and ambient noise adjoint tomography method. By improving the initial used shear wave velocity model, we could reduce the misfit between observed and synthetic forward waveform field by 80%. The final velocity model covers the accretionary prism of Central Makran up to the depth of 60 km. The results reveal that the Makran accretionary prism can be divided into two frontal- and paleo-prism segments. The paleo-prism segment exhibits a higher shear wave velocity (averaging 4.0 km/s) compared to the frontal-prism segment. The frontal-prism segment, located in the southern part of the accretionary prism, covers less than half of the onshore accretionary prism and its average thickness is about 22 km. The higher shear wave velocity in the paleo-prism segment implies the presence of convoluted structures mixed with pieces of oceanic crust scrapped from the top of the subducting oceanic lithosphere. The thickness of the sedimentary cover within the paleo-prism segment varies between 22 to 30 km but the depth to magnetic basement is less than 8 km over the segment implying the presence of magnetized igneous rocks within the paleo-prism segment. These conditions make the existence of hydrocarbon reservoirs in the paleo-prism segment of the accretionary prism highly unlikely. Based on these findings, it is recommended to initially concentrate the exploration stages for hydrocarbon resources in the frontal-prism segment. The lower shear wave velocity in the frontal-prism segment can potentially lead to the development of a sedimentary basin effect and an increase in the amplitude of seismic waves reaching this region, ultimately increasing the seismic hazard in the frontal-prism segment.
 

کلیدواژه‌ها [English]

  • Makran subduction
  • adjoint tomography
  • ambient noise
  • exploration of hydrocarbon resources
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