Active tectonics assessment of the southeastern faults of the Hamadan city

Introuduction:

The Iranian plateau is located in the middle part of the Alpine-Himalayan orogenic belt, and it is one of the active regions of the world (Agard et al., 2005; Allen et al., 2004; Berberian and King, 1981). The Zagros orogeny was formed in the western part of the Iranian plateau by the closure of the Neo-Tethys Ocean and collision of the Arabian and Eurasia plates (Agard et al., 2011; Jackson, 2011; Berberian, 1983). The Zagros orogeny is subdivided into four major zones from the NE to the SW: the Urumieh-Dokhtar Magmatic Arc (UDMA); Sanandaj-Sirjan Zone (SSZ); High Zagros Belt (HZB) and the Simply Folded Belt (SFB) (Mohajjel and Fergusson, 2013). The study area around the Hamedan city is located in the Sanandaj-Sirjan zone, and the perpendicular component to the Zagros trend has caused the formation of major reverse faults.There are many faults in the south-eastern part of the Hamedan city and they generally have a reverse mechanism and some of them have entered the city at the NW termination. These faults include Siahkamar-Alavi, Yalfan-Arzanfood, Keshin-Simin, Tafrijan-Mangawi and Anglas-Varkaneh faults. Generally, no detailed structural and geomorphological study has been done on these faults. Therefore, the aim of this study is to investigate the morphotectonics evidences related to the southeast faults of the Hamedan city using the fractal pattern of the faults and measuring the morphometric indices and field observations.

The data used in this study, to calculate the morphometric indices and fractal dimensions of faults, were the geological maps, digital elevation model (DEM) and field observations. To investigate the active tectonic associated with the south-eastern part of the Hamedan city, the ASTER digital elevation model (30-m resolution) have been used to drainage and basins extraction. Then, the study area was divided into 40 basins for calculating the morphometric indices. Then, the stream length gradient index (SL), hypsometric integral index (Hi), basin shape index (Bs), drainage basin asymmetry factor (Af) and transverse topographic symmetry index (T) have been calculated and the corresponding map were constructed. Then, these maps are combined using hierarchical analysis process (AHP) to construction a final map of relative tectonic activity.

Fractal dimension diagrams of faults in four box of the study area are calculated as: Da=1.5724, Db=1.5428, Dc=1.6551, Dd=1.6864, De=1.8088, Df=1.6436. As a result, the fractal dimension related to box b shows the minimum value (1.5428) and the fractal value of box e shows the maximum value (1.8088). Therefore, it can be concluded that the amount of fractures has increased from the NE to the SW and a high density of faults is observed in the south and SE parts of the Hamadan city. According to the values obtained from Af index, 12 basins are classified in class one, 6 basins in class two and 22 basins in class three. Several basins in the eastern part and two relatively large basins in the northern and southern parts of the studied area are also classified in class one. According to the values obtained from Bs index, three basins are in class 1, five basins are in class 2 and 32 basins are in class 3, and the lithology of the region (slate and phyllite) may be effective in this classification. According to the values obtained from Hi index, in the study area, most of the basins, except for two basins in the northeastern part of the studied area, show low relative tectonic activity. Results of T index indicate that 13 basins are classified in the first class, 26 basins in the second class, and two basins in the third class. And most of the basins, especially in the central part, have high to moderate tectonic activity. The western basins of the region have high values of SL index, which shows the high tectonic activity of the region. Ten basins in the central and south-western parts of the study area are classified in class one, and most of the basins in the eastern half of the studied area are classified in class three with low relative tectonic activity. Based on the final map of relative tectonic activity, the eastern half and also the northern part of the study area show moderate to low relative tectonic activity and these parts are divided into class two and three. Most of the eastern half parts of the study area are classified in class 3 with low relative tectonic activity. The western half of the study area and especially the central, southern and southwestern parts are classified in class 1 with high relative tectonic activity.

There are several faults with a general NW-SE trend in the southern and southeastern parts of the Hamedan city, which have caused the deformation of the rocks in the study area. The density of fractures increased from the northeast to the southwest of the study area, and the highest density of faults is observed in the southern and southeastern parts of the Hamadan city. In the northern and northeastern parts, due to the presence of recent deposits, the density of fractures is low, indicating low tectonic activity. The field evidence of these faults and fractures are observed as the fault zones and numerous fractures. Also, the formation of asymmetric basins and thrusting of different units are other geological effects related to these faults in field observations. The Yalfan-Arzanfood fault with the reverse mechanism has caused thrusting of the andalusite-schist units and Cretaceous limestone over the slates. The NW termination of this fault has passed through the north-eastern part of the Ekbatan Dam and has caused deformation of the adjacent units of the dam. The Keshin-Simin fault has created numerous fractures with a relatively parallel pattern in the schist units. These fracture zones are more than one meter in some parts and reach several centimeters close to the main fractures. The north-western termination of this fault cuts the Hamedan city from the south-eastern side and there is a large concentration of population along this fault.