Investigation of two interpretations of a fault in northern Tehran

Document Type : Research Article

Author

Associate Professor International Institute of Earthquake Engineering and Seismology Tehran, Iran

Abstract

The abrupt topographical change in northern Tehran divides the Eocene rock formation from Quaternary and Plio-Quaternary alluvial deposits situated in the piedmont and the plain. This phenomenon has been interpreted differently by two Geologists in the 70's and 80's. The term North Tehran Fault (NTF) is the first interpretation of rock-alluvium boundary, coined by Tchalenco (1975). According to this interpretation consist the NTF of the fault system, arranged in an en-echelon manner, not necessarily forming the rock-alluvium boundary in northern Tehran. A later interpretation, by joining all the fault systems, as a single line, called North Tehran Thrust (NTT). According to this interpretation forms (NTT) a single line, defining the boundary of rock-alluvium in northern Tehran (Berberian et al., 1983), not confusing with North Tehran Fault (NTF). The mentioned two interpretations of faulting in northern Tehran include two different faulting mechanisms opposing each other diagonally. By time are the two interpretations wrongly melted together as (NTF) and were used by several authors without paying attention to its original meaning interpreted by Tchalenko (1975).
In later works (Langraf et al., 2009; Ritz et al., 2012) is (NTF) the main structure responsible for the rising of highly elevated rock formation in the hanging wall of North Tehran Thrust (NTT).
The present study deals strictly with the boundary of rock and alluvium in northern Tehran, which was called (NTT). Kinematic study along the contact of rock and alluvium revealed two characteristic features: 1- Rock-Alluvium boundary occurs not along with a single faulting trend, it is rather arranged along NW-, N-S, E-W, and NE-striking faults. 2- Obtained stress direction associated with fault plane solution show different directions.
In many places is the NTT covered by rock slides, obscuring the trace of the rock-alluvium boundary. Older rockslides are thrusted over alluvium units of different ages. The contact of such boundaries shows striations compatible with present-day stress direction. The slow deformation rate in northern Tehran is not concentrated along a single fault. Therefore, it seems that absorbed deformation in northern Tehran is distributed over a wide range adjacent to the rock-alluvium boundary. These observations suggest an unrecognized fault, which needs more careful geological and seismological study.
Considering the three different trends of NTT, namely NW-, E-W- and NE-trending, no fold axis run parallel to those trends. The results obtained in this study suggest the NTT is not a major fault and in addition, it could not be regarded as a single fault responsible for the rising of the rock formation on the hanging wall of NTT in northern Tehran.

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References

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Iranian Journal of Geophysics
Volume 15, Issue 2
August 2021
Pages 19-33

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