The Zagros fold-thrust belt in SW Iran is a part of the Alpine-Himalayan system which consists of a variety of structures with different sizes or geometries. Morphology of the Zagros Mountains is too complicated, because of impaction of different basement faults during the orogeny. The Izeh fault (N-S trending) is one of these deep-seated faults which its movements had been modified sedimentation patterns and deformation styles of geological units in the Central Zagros. Moreover, it's expected that hydrocarbon migration and accumulation in SW Iran had been affected by this fault system.
This paper presents new findings concerning deformation style and mechanism of the Izeh fault. Inasmuch as the RS (Remote Sensing) imagery is one of the valuable means available to geologists for locating geological/ geomorphological features expressing regional fault or fracture systems, therefore, the satellite images were used for structural analysis of the Izeh fault. The study area lies between latitude 31° 30 ? to 31° 52 ? N, and longitude 49° 30 ? to 49° 53 ? E. The geological setting of the area is just between the Izeh Zone and the Dezful Embayment. It has rugged topography and elevation ranged between 548 m to 1499 m (from sea level).
Materials and Methods
In this research, the ASTER image (with high spectral resolution), IRS-PAN images (with high spatial resolution), and Tectonic & Geological maps covering the study area have been used for structural analysis of the Izeh fault system. Furthermore, in order to 3D analysis of geological structures in the area, a detailed Digital Elevation Model (DEM) has been constructed, using digital Topographical maps (with scale 1:25,000). Geometrical correction and digital image processing techniques have been carried out by ER Mapper 6.4 software. The enhancing techniques (e.g. Data Fusion, Spatial Filtering, etc.) were applied for modification satellite images to highlight structural elements and deformational features of the Izeh fault. Moreover, the Sun angle (Directional) filters were applied for increasing geometrical parameters of structural lineaments (e.g. fracture traces) on the images. Also, to increase structural contrast on the DEM and improving visual description of fracture edges, Color dropped images and Shadow images (i.e. 2D DEM) of the area have been prepared. As well, by overlaying the DEM with satellite images and geological maps, 3D Models of the area have been constructed. Then, based on these models, the deformation geometries of the rock units were interpreted and mapped precisely. At last, in order to produce the digital Fracture Trace Map of the study area, all the acquired fracture traces data were integrated in Geographic Information System (GIS) environment by using ArcView 3.2a software.
Results and Discussion
RS observations show that morphotectonic of the central Zagros has been modified by the Izeh fault system. Throughout the study area, a few Asmari formation outcrops (e.g. in Kamar-Deraz, Tanowsh, and Tukak anticlines) have been dragged and rotated in a way which is implied on right-lateral displacement of the Izeh fault. Moreover, the movements of the Izeh fault system had been associated with rupturing and/ or displacement of the Fars group (Mio-Pliocene) and the Bakhtiari formation (Pleistocene), too.
The Fracture Trace Map of the study area is suggested that the Izeh fault had a great force on fracturing. For investigating about the Izeh fault effects on fracture patterns of geological units, the study area has been divided in 9 subzones. In each subzone, the structural data (e.g. azimuth, length) of the fracture traces were taken from the Fracture Trace Map. This can be done by using Distance/ Azimuth Tools v. 1.6 extension of ArcView 3.2a software. Then, in order to clarify the main fracture trends in each subzone, the Rose Diagrams of orientation data of fracture traces were calculated and plotted by using Rockworks 2006 software. In this matter, to compare the relationships between Number and Length of fracture sets, rose diagrams of orientation data have been plotted based on both Frequency and Cumulative length of fractures.
The structural analysis of the fracture trace data are shown several fracture systems with varied lengths and orientations. The rose diagram plots indicated that fracture distributions in different subzones were not the same. Generally, throughout the area there are two main fracture sets (NW-SE and NE-SW trending) which are comparable to fold-related fracture system. However, in each subzone, there are few fracture sets which seem to be independent from folding. These fracture sets usually crossed through rocks with different ages and/or structural settings. Structural evidences suggested that these sets were related to reactivation of the Izeh fault during the Zagros orogeny.
The RS observations and surveys in the Central Zagros show that the Izeh fault system is associated with complexity in the structural styles of the deformed rocks. The modification of geological units - with different ages and/ or structural settings - had been occurred through an extensive zone. From a morphotectonic point of view, the Izeh fault zone has been marked by right-lateral dragging and rotation of fold axes, and rupturing and/ or displacement of sedimentary strata, too. Moreover, the Izeh fault movements had been modified fracture patterns of the geological units. Throughout the study area, there are main fracture sets which had been associated with many subsidiary fracture sets. Structural analysis of the fractures suggests that some fracture sets are independent from folding mechanisms, and are related to reactivation of the Izeh fault during the orogeny.
Consequently, morphotectonic and structural evidences are shown that throughout the Izeh fault zone, there are many structural features which have been created by different mechanisms, along with different stages of the Zagros orogeny. Furthermore, some evidences implied that this fault has been reactivated during juvenile phases of the alpine orogeny.