Investigation of Morphological Features and Faulting Mechanisms of NW-Striking Faults Affecting Alluvial Deposits in Tehran's Piedmont

Tehran lies on the southern flank of the Central Alborz, an active mountain belt characterized by many historical earthquakes, some of which have affected Tehran itself. It is an arcuate fold and thrust belt, consisting of two major distinct structural trends: a NW-SE trend characterizing the west-central Alborz, and a NE-SW trend marking the east-central Alborz. The steep slopes of the Alborz on its southern flank adjoin the piedmont, which is covered by various units of post-orogenic alluvium. The border between the Alborz Mountain and the Tehran's piedmont (northern part of Tehran city) is marked by the North Tehran Fault (NTF) [1], dividing the Eocene rock formation from the alluvial units of different ages (Early Pleistocene to the recent alluvium). The oldest alluvial formation, named Hezardarreh is a folded and faulted conglomerate forming hills parallel to the mountain front. The age of this formation is difficult to determine accurately, due to the lack of datable features such as pollen, fossils or lithic industry [2]; however, it is mostly assumed to be Plio-Quaternary (the bottom of formation) to Early Quaternary (the upper part of formation). The formation overlying the eroded Hezardarreh Formation is very heterogeneous in composition, in particular to the north of Tehran and has been considered to be Mid-Pleistocene. The next following formation is widespread deposits named the “Tehran alluvium” have been assigned to Late Pleistocene [3]. The youngest mapped unit in Tehran City overlaying the Late Pleistocene alluvium is Holocene deposit, which is divided in younger part (4000 to 5000 years BP) and an older unit in age of 12000 years (BP) [4]. The assessment of seismic hazard depends upon the understanding of activity, mechanism and trends of faults affecting an area. The rapid urbanization of Tehran with remarkable concentration of people needs to be studied by new studies considering all possible active fault trends. The E-W-trending active faulting in Tehran is delineated by their morphological features recognized on aerial photos of 1955. This study examines the NW-trending faults as a possible seismic source. The measurement of 56 fault planes in 13 outcrops have shown that the morphological features associated with NW-directed faults are preserved in middle and late Pleistocene deposit of Tehran's piedmont. The length of these faults varies between 2.6 to 6.5 Km arranged in an en-echelon manner. The compressive faulting mechanisms of these trends are in accordance with the NE-directed present day stress direction. Morphological features of the NW-directed faults are controversial, because they appear as normal and compressive on aerial photos and in the outcrops. It means that the faulting mechanism in Tehran's piedmont cannot be explained by a single stress direction. In fact, two different stress directions have affected the alluvial plain, namely an older NW-directed prior to the NE-directed one, which explains the normal faulting mechanism along the NW-striking faults and was mapped as old normal fault, e.g. [5]. Thus the NW-trending faults are inherited fault affecting the Early and Mid-Pleistocene deposit as normal faults. According to the present day stress prevailing in the South Central Alborz obtained by P-axis of focal mechanisms and fault slip data in relative young alluvial deposits [6-7] a NE-directed stress explains the faulting mechanisms of active faults in this area. Although there is now field evidences proving the fault activity of NW-striking faults but the present day stress (NE-directed), it is reasonable to assume seismic activity along these faults.