شواهد زمین‌شناسی و زمین‌ریخت‌شناسی مبنی بر فعالیت گسله‌های بخش شمالی ساوه (ایران مرکزی)

نوع مقاله : Articles

نویسندگان

پژوهشکده زلزله‌شناسی، پژوهشگاه بین المللی زلزله‌شناسی و مهندسی زلزله، تهران

چکیده

شهرستان‌های قم و ساوه از دیدگاه زمین‌شناسی در محل پیوند دو ایالت لرزه زمین‌ساختی ایران مرکزی و البرز قرار دارد. از گسله‌های فعالی که در این منطقه معرفی شده‌اند می‌توان به گسله کوشک نصرت اشاره کرد. در راستای این گسله تاکنون زمین‌لرزه دستگاهی و یا تاریخی گزارش نشده است. این گسله از دو روند نزدیک به خاوری- باختری و شمال باختری شکل گرفته است و بیشترین بخش آن از میان توده‌های سنگی با سن‌های مزوزوئیک و ائوسن می‌گذرد. بخش کوچکی از این گسله آبرفت‌های جوان (پلایستوسن بالایی) را متأثر می‌کند. برای مشخص شدن فعالیت زمین‌ساختی این گسله برای محاسبات خطر، افزون بر پیمایش‌های میدانی روش‌های دیگری به‌کار گرفته شده‌اند. بر اساس مطالعات وضعیت تنش به روش زمین‌ساختی و لرزه‌ای شاهدی بر فعالیت بخش خاوری- باختری گسله به دست نمی‌آید. برداشت‌های میدانی و مطالعه زهکش‌ها به‌طور مشخص نشان‌دهنده‌ی راستالغز بودن این گسله با مؤلفه راست‌بر است که با جهت تنش به‌دست‌آمده همخوانی ندارد. روش‌های دیگری چون شاخص‌های مورفومتریک (شاخص هیپسومتری و شیب رودخانه) نیز یافته بالا را تأیید می‌کنند. پرسش بعدی که در ارتباط به بخش خاوری- باختری گسله کوشک نصرت مطرح می‌شود ارتباط احتمالی آن با پایانه شمال باختری گسله سیاه کوه است که در جنوب شهر تهران قرار می‌گیرد. با تکیه بر یافته‌های مورفوتکتونیکی و عکس‌های ماهواره‌ای به نظر می‌رسد که پیوندی میان این دو گسله وجود داشته باشد؛ هرچند که پیگیری این گسله به‌صورت یک خط واحد بر روی عکس‌های ماهواره‌ای مشخص نیست اما می‌تواند گسله از خطوط ناپیوسته همراه با تداخل دو روند گسلی خاوری- باختری و شمال باختری از گسله کوشک نصرت (بخش خاوری- باختری آن) تا گسله سیاه‌کوه تشکیل شده باشد.

کلیدواژه‌ها


عنوان مقاله [English]

Geological and Geomorphological Evidences of Fault Activities in Area of the North Saveh (Central Iran)

نویسندگان [English]

  • Mohammad Reza Abbassi
  • Leyla Mahshadnia
International Institute of Earthquake Engineering and Seismology, Tehran, Iran
چکیده [English]

The studied area lies in the Central Iranian plate. Two reasons for the lack of knowledge concerning the recognition of active faulting in this region could be summarized as following: 1- faults with pure vertical motion are difficult to recognize on aerial photos, 2- especially those cutting through rock units.
The mapped active fault, Kushk-e Nosrat is an example of uncertain knowledge of its activity. The existing evidence to prove its activity such as seismicity and geologic data are yet unclear, and concerning morphological aspects it contradicts the present-day stress direction. The present-day stress direction dictates a left lateral deflection of drainages, however the drainage pattern is in accordance with NW-directed paleostress. Therefore, it was necessary to examine the activity of Kushk-e Nosrat fault.
The current investigation focuses on two specific issues: 1- The characterization of changes in the Plio–Quaternary state of stress using inversions of slip vectors measured on fault planes with a broad range of displacement values; 2- The identification of present- day stress field by inversion of seismically determined by fault slip vectors (focal mechanism of earthquakes); and 3- The verification of the coherence and mechanical compatibility of the inversion results with the geological and structural characteristics of the region illustrating the kinematic significance of each fault system.
The central part of Kushk-e Nosrat trending (N100E) is a dormant fault. The evidence for the inactivity are reflected in prevailing stress direction, drainage pattern and Quaternary deposits. The stress direction measured directly on several station on this fault show a NW-directed stress (σ1), which explains the right lateral deflection of drainage pattern crossing this fault. The last evidence of inactivity of these faults is delivered by mapping of geomorphic surfaces. The young alluvial deposits such as Holocene and late Pleistocene in age are not affected by Kushk-e Nosrat Fault.
The deflection of drainage pattern crossing the fault is compatible with a NW-directed stress direction. The inversions of slip vectors measured on fault planes has shown the states of paleostress existing on the faults and the inversion of focal mechanisms revealed the present day stress direction as following:

The present day state of stress deduced form focal mechanisms is characterized by a transpressional tectonic regime consistent with the ongoing stress state (N20°E trending σ1);
The paleostress field was characterized by a regional mean of N140±10°E trending horizontal compression (σ1), and a transtensional tectonic regime. The change from paleostress to modern stress states has occurred through an intermediate stress state.

Characterized by a mean regional N-S trending σ1.
According to the obtained results, the assessment of earthquake hazard based on Kushk-e Nosrat fault must be revised in term of considering them as active faults; therefore, caution must be paid by taking them into any calculation. Under the influence of NW-directed paleostress, the faults trending nearly E-W acted as dextral faults, thus the present landscape must be interpreted as the result of a relatively recent reversal of stress direction.

کلیدواژه‌ها [English]

  • Active Faulting
  • Central Iran
  • Geomorphology
  • Present-Day Stress
  • Paleostress
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