Iran is located in the southern margin of the Eurasian plate at longitude between 44° 5' and 63° 18', and latitude between 25° 3' and 39° 47'. This country, which has a hot and dry climate, is a platform-shaped land. Its oldest rocks, which in fact form the crystalline basement of Iran, are around 1000 to 1500 million years old and are very young compared to very old ones in other parts of the world-with about 3811 million years old (Rahimi, Zamani, & Ghotbi, 2022).
The Zagros Orogenic Belt is part of the Alpine-Himalayan mountain range created by the closure of the Neotethys Ocean. This orogenic belt has a northwest-southeast trend and has three tectonic zones. These zones are parallel to each other and from northeast to southwest, respectively are: (1) the Urmieh-Dokhtar zone, (2) the Sanandaj-Sirjan zone, and (3) the Zagros Fold-Thrust (Alavi, 1994; Berberian & King, 1981b; Ghasemi & Talbot, 2006; Mohajjel, Fergusson, & Sahandi, 2003).
The Urumieh-Dokhtar Magmatic Zone:
This zone, with a width of 50-80 km, consists of inner and outer volcanic rocks of the Cenozoic Andean type. According to researchers, this zone is the outcome of the collision of the Arabian and Eurasian plates and the subduction of the Neotethys ocean crust under the Iranian crust in the middle to late Miocene era. Volcanic activity in this area has extended since the Cretaceous. In recent years, some researchers proposed a rift model for the interpretation of the origin of magmatism in the UDMA (Amidi, Emami, & Michel, 1984; Amidi & Michel, 1985; Lescuyer & Riou, 1976; Sabzehei, 1974) and some others emphasized the subduction of Neo-Tethyan beneath central Iran (e.g.,(Dargahi, Arvin, Pan, & Babaei, 2010; Kananian, Sarjoughian, Nadimi, Ahmadian, & Ling, 2014; Rezaei-Kahkhaei, Galindo, Pankhurst, & Esmaeily, 2011; Sarjoughian et al., 2012)).
The Sanandaj-Sirjan Metamorphic Zone:
The Sanandaj–Sirjan Zone contains the metamorphic core of the Zagros continental collision zone in western Iran. The zone has been subdivided into the following from southwest to northeast: an outer belt of imbricate thrust slices (radiolarite, Bisotun, ophiolite and marginal sub-zones, which consist of Mesozoic deep-marine sediments, shallow-marine carbonates, oceanic crust and volcanic arc, respectively) and an inner complexly deformed sub-zone (late Palaeozoic–Mesozoic passive margin succession) (Mohajjel et al., 2003).
The Zagros Fold-Thrust Belt:
The Zagros fold-thrust belt in Iran forms the external part of the Zagros active orogenic wedge. It includes a sequence of heterogeneous latest Neoproterozoic–Phanerozoic sedimentary cover strata, ∼7 to 12 km thick and composed of alternating incompetent and competent layers, overlying Precambrian crystalline basement with a complex pre-Zagros structural fabric Alavi, 2007. This zone, with an estimated length of 1800 km, is placed in the foreland of the collision between the Arabian plate and the Eurasian plate. The Arabic plate moves towards the north and northeast at a rate of around 3 cm per year. Its collision with the Eurasian plate prompted a shortening of approximately 65 to 78 km in the Zagros sedimentary basin. Part of the unconformities caused by this collision, which occurred in the Oligocene and Late Eocene, also covers the Alborz, Caucasus Mountains, and Kopeh Dagh in northern Iran. Unconformities in the lower and upper Asmari Formation in the Late Eocene and Early Miocene, in the lower part of Aghajari Formation in the middle Miocene (mid-Miocene), and the lower part of the Bakhtiari Formation in the Late Pliocene and mid-Pleistocene indicates orogenic activities in the Zagros zone. Zagros orogeny began from the late Eocene (late Eocene) with the closure of the Neo-Tethys ocean and has maintained to this day. The tectonic structures resulting from this orogeny host large sources of the world's hydrocarbon resources. In addition to the copious folds, which chiefly have a northwest-southeast trend, several zones have controlled the basement strike-slip fault of tectonic structures and the stratigraphic sequence of the sedimentary cover of the region. These have performed a significant role in building huge hydrocarbon reserves and seismicity in the area. Meantime, strike-slip fault zones mainly have a north-south trend and their mechanism changes from northwest to southeast - including fault zones of Kazerun, Borazjan, Kareh-Bas, sabzpooshan, Sarvestan. The most prominent thrust Dip-slip fault zones are the mountain front fault and the Main Zagros Reverse Fault, also known as the High Fault.
In this article, several stratigraphic columns of the region have been analyzed. Also, by examining facies changes and thickness of Asmari, Razak, and Gachsaran formations, the role of basement fault zones in controlling the situation and stratigraphic sequence of these formations has been recognized.
Iran Faults:
Zagros fault in the northeastern section of Zagros with the northwest-southeast trend and abrupt slope is sometimes identified vertically, acting as a reverse fault. This fault extends especially in the areas from Lorestan to Marivan and after entering Iraq, it returns to Iran and reaches the Sardasht district (Nabavi, 1976). The sequence of this fault passes along the southwest of Fars (Neyriz) so that its entire length is approximately 1350 km. In Fars areas, the Zagros fault has a lower incline and is often seen as an overthrust. The movement of this fault is rightward and it has set the evaporative sediments of the former Cambrian, which should have been created in the Qatar-Kazerun evaporation basin, 200 to 300 km away (in Zardkooh). Reportedly, the time of the initial movement of the Zagros fault belongs to the Precambrian. This movement has continued in later periods. It is clear that some major fault systems have played a continuous but varied role from the Precambrian until the present, and whatever controlled the original fold orientation at the onset of continental compression (65 Ma) apparently still controls the orientation of contemporary folding (Hoseinzadeh, Daneshian, Moallemi, & Solgi, 2015).
Kazerun Fault:
This fault has a length of around 235 km and extends in the north to the Dena fault and in the south to the Persian Gulf. Kazerun fault has a variable north-south direction and between 25° to 341° Azimuth and accordingly can be split into several parts (Berberian & King, 1981a).
This fault is one of the largest lateral sinistral strike-slip faults. Kazerun fault is a fundamental fault that while controlling the western border of the Hormoz salt basin, has further affected the Zagros sediments. Such that it has bent and moved the structures of Zagros in a sinistral direction, particularly significant in the Kazerun and Borazjan zones.
Kazerun fundamental rupture is one of the active faults. Other than its effects on Quaternary young sediments, a lot of seismic data indicate the occurrence of movement at the slip level of this rupture.
Kareh-Bas Fault:
This fault is one of the transverse faults with north-south direction, the southern extension of Ardakan rupture. It also consists of several fault segments. Its northern part (F16) with a length of 25 km and a 325° Azimuth with a sinistral strike-slip fault mechanism, causes the axis of folded buildings to deviate. Kareh Bas Transcurrent Fault System with a total length of 200 Km is situated about 80 Km east of the Borazjan segment (a part of Kazerun fault zone) and 45 km west of Shiraz, in the Koohmareh region. It is a nearly N-S trending right-lateral linked strike-slip fault system, and several anticlinal axes have been displaced by it. Strike separation (109 Km) of Mountain Front Fault/Flexure (MFF) of Zagros is the most important function of Kareh Bas Transcurrent Fault System (Arian & Noroozpour, 2015).
Sabzpooshan fault:
This rupture has affected the western parts of the Shiraz plain. This rupture is a continuation of the southeastern branch of Ardakan fault named of Goim fault and Zarghan fault. By changing the trend of the mentioned faults to the north-south direction with 351° Azimut, this rupture has been created. This rupture is of the sinistral strike-slip fault type.
This rupture in the form of faults set has caused the deviation of the axis and bending of the promontory of Drak Mountain with sinistral mechanism and intensified the subsidence process of the seismic cement factory (west of Shiraz plain). Next, the deviation of the Sabzpooshan fault axis in the eastern parts of Ghir and Heram is presented in the geological map of 1: 100,000 Ghir (Yousefi et al., 2002, Geological map of Ghirokarzin with a scale of 1: 100000, Geological Survey of Iran).
Faults in line with the Zagros event:
The frequency of these ruptures in the Zagros region is high and by moving from the northeast to the southwest, their frequency and displacement reduce, and in the southwestern region of Nurabad (Dezful embayment zone) the emergence of faults is low. The reasons for this are the thick sedimentary cover (about 13 km) and the unstable outcrops of the Gachsaran, Mishan, and Aghajari formations, which are suitable conditions for camouflaging subsurface abnormalities. Apart from the above, a folded building rarely reaches the surface in the Zagros event, which is not affected by the Northwest-Southeast faults. Tectonic changes in a zone can perform an essential role in changing sediment deposition conditions in the progressing and regressing sea sedimentary basins, discontinuities, and changes in pressure and temperature applied to rocks in the area. In other words, tectonic events prompt rock facies changes.
Hence, the study of sedimentary basins is essential in recognizing effective tectonic events during their expansion (Pedram Razi, Piroozmand, Zolfaghari, Kazemnejad, & Firoozbakhsh, 2013). In this article, by studying the stratigraphy of the subject area and explaining the facies of the formations of these areas, an attempt has been made to study the role of basement faults in controlling fossils in the formations of the region. One of the methods to accomplish this goal is to study stratigraphic segments and compare the change in thickness and material of the formation in adjoining areas. By studying stratigraphic sections, the changes in water level and depth of the basin, and as a result, the advance and recession of the sea and the occurrence of discontinuities and subsidence of the sedimentary basin is determined.