Recent tectonic activity in Qom- Zefreh fault zone, Central Iran

The Qom- Zefreh fault zone is located in the Central Iran. This fault considered as one of the fundamental faults of the Central Iran which is active in quaternary. Travertine deposits has been affected by the Qom- Zefreh fault zone. The Qom- Zefreh fault zone has been known as a quaternary fault in the western part of central Iranian plate and it’s recently of movement is considered early. The values of geomorphic indices as well as Iat often change corresponding to the distribution of this fault zone. Therefore, estimation of the morphometric parameters to identify the effect of its recent activity on the tectonic evolution of the drainage basins of the study area is essential. In order to assess the relative tectonic activity through the Qom- Zefreh fault zone, subbasins and stream network were extracted by using Arc Hydro Tools software (an extension of Arc GIS software, ESRI) based on the DEM and in turn, 103 subbasins have been resulted. In this study, considering the diversity of the morphotectonic features, six morphometric indices relevant to the river channel, drainage basin, and mountain fronts were computed for every catchment, and consequently, a single index (Iat) was calculated from these indices for each of 103 subbasins to define the degree of active tectonics. The values for each basin were classified into 3 classes and the result of the classification for each subbasin is shown in maps and Tables. Thus, this study is addressed to analyze of six important Morphotectonic indices such as stream length gradient (SL), drainage basin asymmetry (Af), hypsometric integral(Hi), drainage basin shape (Bs), ratio of valley floor width to valley height (Vf), and mountain-front Sinuosity (J). Finally, the Index of the relative tectonic activity (Iat) was calculated through which the Qom- Zefreh fault zone is classified into four tectonic activity classes, from very high to low; 1—very high (1.0≤Iatb1.5); 2—high (1.5≤Iatb2.0); 3—moderate (2.0≤Iatb2.5); and 4—low (2.5≤Iat) (El Hamdouni et al., 2007). The distribution of the four classes of Iat is presented in a well classified map. The indices represent a quantitative approach to differential geomorphic analysis related to erosion and depositional processes which include the river channel and valley morphology as well as tectonically derived features, such as fault scarps. We also evaluated the outputs of the morphometric analyses based on field-based geomorphological observations. Thus, these results are proved to be extremely beneficial to evaluate relative rates of active tectonics of this region. The Iat index classes computed within the study area, about 14.39% (904.13 Km2) is class 1 (very high relative tectonic activity); 52.11% (3275 Km2) is class 2 (High Relative Tectonic Activity); 27.29% (1715 Km2) shows moderate values of tectonic activity (class 3) and 6.21% (391 Km2) has the lowest values of relative tectonic activity (class 4) in the study area. In the study area, the highest level of tectonic activity in the southeastern, northwestern and eastern borders in the study area is assigned to activity of the Qom- Zefreh fault zone. The western part of study area in the Qom- Zefreh fault zone in central Iran represents low values of tectonic activity. The travertine deposits in the study area (late Quaternary deposits) has been affected by recent activity of Qom- Zefreh fault zone in central Iranian plate. The geomorphological and geological evaluations, performed in the study area indicate that the Qtr surface (travertine deposits) is the Pleistocene deposit affected by the NW-SE trending faults in the southeast, northwest and east study area in the Qom- Zefreh fault zone. Aforementioned evidence corresponds well with the distribution of relative tectonic activity. Partially in the west part of the study area almost with no significant fault indicates the low class of the tectonic activity. Iat is mostly high and partially very high almost throughout the east, southeast and northwest part of the study area which corresponds to a straight mountain front, asymmetric drainage basins, gorges, triangular facets, faults and fracturing in the recent travertine deposits, knickpoints, V shaped valleys and deflected stream along the Qom- Zefreh fault zone. Nevertheless, the lowest Iat values (class 4) mainly occur in the western part of the study area in Qom- Zefreh region where all geomorphic indices suggest low tectonic activity.