نوع مقاله : مقاله پژوهشی

نویسندگان

1 پژوهشکده علوم زمین، سازمان زمین شناسی و اکتشافات معدنی کشور ، تهران، ایران

2 کرسی یونسکو در مخاطرات زمین شناختی ساحلی، پژوهشکده علوم زمین

3 سازمان زمین شناسی و اکتشافات معدنی کشور ، تهران، ایران

4 ژئوفیزیک، سازمان زمین شناسی و اکتشافات معدنی کشور ، تهران، ایران

5 چینه شناسی، سازمان زمین شناسی و اکتشافات معدنی کشور ، تهران، ایران

6 رسوب شناسی، سازمان زمین شناسی و اکتشافات معدنی کشور ، تهران، ایران

چکیده

گستره مورد مطالعه در دشت اشتهارد و در پیش بوم رشته کوه های البرز واقع شده است. اشتهارد و پیرامون آن در گستره گسل های لرزه زایی همچون گسل ایپک در جنوب و گسل شمال اشتهارد در شمال خود قرار می گیرد. در گذشته با استناد بر شواهد و پدیده های زمین ریختی و زمین لرزه های تاریخی منسوب به سگز آباد، در پهنه آبرفتی کوه جارو در دشت اشتهارد، پدیده های خطی که با پرتگاه های آشکاری همراه هستند همچون افراز جنوب اشتهارد، در شمار افراز های گسله دسته بندی می کرده اند. در این پژوهش بر اساس داده های حاصل از پارینه لرزه شناسی، ویژگی های ریخت­ زمین­ساختی، ژئوالکتریکی ارائه شده در این تحقیق، به نظر می­رسد که افراز جنوب اشتهارد نه نشانه یک گسل، بلکه حاصل یک چین خوردگی در سازند سرخ بالایی می باشد که نشان از ماهیت ریخت شناسی افراز جنوب اشتهارد است که پیش از این بعنوان گسله جنوب اشتهارد معرفی شده بود. این چین خوردگی یک خمیدگی ساده لایه ها است که به واسطه کوتاه شدگی حاکم بر این قسمت از رشته کوه های البرز، طی سنوزوئیک پسین حاصل شده است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

South Eshtehard Scarp: a fault system or a morphologic scarp

نویسندگان [English]

  • Mahmoud Noveiri 1
  • Hamid Nazari 2
  • Mohammad Ali Shukri 3
  • Mahdi Mohammadi vizheh 4
  • Reza Salamati 5
  • Morteza Sheikh 6

1 Research Institute of Earth Sciences,, Geological /survey of Iran

2 UNESCO Chair in Coastal Geological Hazards , ,Research Institute for Earth Sciences

3 Geological Survey of Iran

4 Geophysics,, Geological Survey of Iran

5 Stratigraphy,, Geological Survey of Iran

6 Sedimentology,, Geological Survey of Iran

چکیده [English]

The study area is located in Eshtehard plain and in front of the Alborz mountain range. Eshtehard and its surroundings are in the range of seismic faults such as ipak fault in the south and north Eshtehard fault in the north. In the past, based on the evidence and phenomena of earthquakes and historical earthquakes attributed to Sagzabad, in the alluvial zone of Jaroo Mountain in Eshtehard plain, linear phenomena accompanied by obvious precipices such as the scarp of the south of Eshtehard are among the fault scarp Have been categorized. In this study, based on the data obtained from paleoseismology, geomorphology, geoelectric characteristics presented in this study, it seems that the southern part of Eshtehard is not a sign of a fault, but the result of a fold in the Red Formation. It is high which shows the morphological nature of the southern part of Eshtehard, which was previously introduced as the southern Eshtehard fault. This fold is a simple curvature of the layers that has been achieved during the Late Cenozoic due to the shortening of this part of the Alborz mountain range.
Methods:
For conducting paleoseismological studies in the study area, tectonic morphological features, geophysical surveys (2 geoelectric and geomagnetic sections) were performed. Finally, a site in the east of Eshtehard was selected and 4 trenches perpendicular to the scarp were dug. Then, structural phenomena and sediment-stratigraphic deposits on the trench walls were separated and studied. In this paper, the results of trench studies are presented.
Conclosion:
The results of additional studies such as paleo-seismic studies (with excavation of 4 trenches) and geophysics (magnetometric and geoelectric), show that the layers, horizontal and without any signs of young faults in the trenches (maximum depth 4) M) that even rejects the possible lateral and hidden faults in the depths;Because if there was such a fault in the area of ​​the southern part of Eshtehard, the mapped deposits of the existing partition trenches would have to be deformed. This indicates the morphological nature of the southern part of Eshtehard, which was previously known as the South Eshtehard fault. Thus, attributing this scarp to an active fault does not seem correct. However, the question remains why and how the scarp was formed? Our field observations show that the deposits in the studied scarp are of marl genus and belong to the upper parts of the Upper Red Formation that have been exposed at the surface. The most logical justification for this is the formation of an open anticline. Geoelectric data in the west of Mokhtarabad also clearly show the existence of an anticline in the study area. Accordingly, the formation of a linear complication on marl sediments scarp can be justified by the tensions that generally occur in the outer parts of the folds.

کلیدواژه‌ها [English]

  • South Eshtehard Scarp
  • Paleoseismology
  • Fold
  • Geoelectric