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The continuation of the Kazerun fault system across the Sanandaj–Sirjan zone (Iran)
Journal of Asian Earth Sciences 35 (2009) 391–400
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The continuation of the Kazerun fault system across the Sanandaj–Sirjan zone (Iran) Homayon Safaei * Department of Geology, Isfahan University, Hezarjerib Ave., Isfahan, Iran
a r t i c l e
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Article history: Received 22 October 2007 Received in revised form 27 December 2008 Accepted 10 January 2009
Keywords: Sanandaj–Sirjan zone Zagros Kazerun fault Isfahan fault Isfahan Iran
a b s t r a c t The Kazerun (or Kazerun-Qatar) fault system is a north-trending dextral strike-slip fault zone in the Zagros mountain belt of Iran. It probably originated as a structure in the Panafrican basement. This fault system played an important role in the sedimentation and deformation of the Phanerozoic cover sequence and is still seismically active. No previous studies have reported the continuation of this important and ancient fault system northward across the Sanandaj–Sirjan zone. The Isfahan fault system is a north-trending dextral strike-slip fault across the Sanandaj–Sirjan zone that passes west of Isfahan city and is here recognized for the first time. This important fault system is about 220 km long and is seismically active in the basement as well as the sedimentary cover sequence. This fault system terminates to the south near the Main Zagros Thrust and to the north at the southern boundary of the Urumieh–Dokhtar zone. The Isfahan fault system is the boundary between the northern and southern parts of Sanandaj–Sirjan zone, which have fundamentally different stratigraphy, petrology, geomorphology, and geodynamic histories. Similarities in the orientations, kinematics, and geologic histories of the Isfahan and Kazerun faults and the way they affect the magnetic basement suggest that they are related. In fact, the Isfahan fault is a continuation of the Kazerun fault across the Sanandaj–Sirjan zone that has been offset by about 50 km of dextral strike-slip displacement along the Main Zagros Thrust. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The Zagros mountain belt of Iran forms a 200–350 km wide series of mountain ranges extending about 1500 km from southeastern Turkey to the Strait of Hormuz in southern Iran (Fig. 1). The Sanandaj–Sirjan zone (SSZ) is a region of polyphase deformation within greenschist and amphibolite metamorphic facies (Mohajjel and Fergusson, 2000). The latest phases of deformation record the suturing of Arabia to Eurasia during the Late Cretaceous and the subsequent southward propagation of the fold–thrust belt (Alavi, 1994). According to GPS measurements, the total convergence rate between the Arabian plate and Eurasia is estimated to range between 20 and 35 mm/yr in a NNE-trending direction (Bayer et al., 2006; Hessami et al., 2006). The Zagros is not a linear belt experiencing pure NE-SW shortening; indeed, an important component of strike-slip motion occurs along the Main Recent Fault (MRF), Fig. 1, in the northwestern part of the chain. Partitioning between the North and South Zagros is accommodated by N-trending dextral strike-slip faults that act as transfer faults along the Zagros. The largest of
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these faults is the Kazerun fault, which is responsible for a 140 km right-lateral offset of geologic features, which implies a displacement rate of 15 mm/yr since the lower Miocene (Berberian, 1981, 1995). Focal mechanism solutions of most earthquakes in the area indicate that deformation in the Zagros basement is due to shortening and thickening at numerous thrust faults (Mouthereau et al., 2006). However, observations of strike-slip faulting imply that N-S trending faults in the Zagros (e.g., the Kazerun fault), inherited from the Panafrican basement, rotated about vertical axes to accommodate the convergence between Arabia and central Iran (Hessami, 2002). The present paper documents the continuation of the Kazerun fault system (KFS), one of the most important faults in Iran, across the Sanandaj–Sirjan zone (SSZ). The KFS is a dextral strike-slip fault zone that is about 300 km long and extends from the high Zagros to the Persian Gulf plate at the Zagros front to the northeastern border of the Zagros (Baker et al., 1993; Talbot and Alavi, 1996; Sherkati and Letouzey, 2004; Sepehr and Cosgrove, 2005; Walpersdorf et al., 2006). The two fundamental questions that have been considered in this work are: (1) is the Kazerun fault system (KFS) only found within the Zagros zone and (2) how is it possible for such an important and ancient fault system to have been cut abruptly by the Main Zagros Thrust (MZT)?
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Fig. 1. Summary tectonic map of Iran. Major faults are shown as black lines. Segments of the Kazerun fault system are: (1) Dena fault, (2) Kazerun fault, and (3) Borazjan fault.
This paper presents some of the results of a structural geologic study around the city of Isfahan that mapped many previously unrecognized faults. Among these is an important north-trending transverse fault system across the SSZ that passes west of Isfahan, the Isfahan fault system. The seismicity associated with this fault system suggests that it is an active basement fault. Studies of satellite images and seismicity, supported by field observations, geomorphology, and deformation structures, suggest that this fault system is the northern continuation of the KFS across the Sanandaj–Sirjan zone.
2. Tectonic zones The NW end of the Zagros orogen is bounded by the East Anatolian left-lateral strike-slip fault, while the southeast end is bounded by the Oman Line. The Zagros orogen consists of four NW-SE trending tectonic belts parallel to the plate margin (Stocklin, 1968; Berberian and King, 1981; Alavi, 1994) (Fig. 1). From the southwest to northeast, these are: 2.1. The Mesopotamian–Persian Gulf molasse foredeep zone This zone lies west of the current Zagros deformation front near the gulf coast (Fig. 1).
2.2. The Zagros fold–thrust belt The Zagros fold–thrust belt (ZFTB) includes the High Zagros, located between the Main Zagros Thrust (MRT) and the Main Recent Fault (MRF), and the Zagros Simply Folded Belt sub zones, located between the MRF and the mountain front fault or the current Zagros deformation front (ZDF) (Fig. 1). 2.3. The Sanandaj–Sirjan metamorphic zone (SSZ) The SSZ is 150–200 km wide and lies between the towns of Sirjan in the southeast and Sanandaj in the northwest (Fig. 1). The rocks in this zone are the most strongly tectonised (deformed and metamorphosed) in the Zagros orogen. They consist of Paleozoic strata formed in an epicratonic setting that were subsequently overlain by a volcanic and sedimentary succession deposited during the Triassic opening of Neo-Tethys (e.g., Sengor, 1990). The rocks in this zone were deformed under greenschist and amphibolite type metamorphic conditions and uplifted during Late Cretaceous continental collision under dextral transpression between the Afro-Arabian continent and the Iranian micro-continent (Mohajjel and Fergusson, 2000). The SSZ can be subdivided into two parts (Eftekharnejad, 1981) to the south and north of a N-S boundary west of Isfahan: (1) the southern SSZ, consisting of rocks deformed and metamorphosed in Middle to Late Triassic and (2) the northern SSZ, which was de-
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formed in the Late Cretaceous and contains many intrusive felsic rocks (Ghasemi and Talbot, 2006). 2.4. The Urumieh–Dokhtar volcano – plutonic zone The Urumieh–Dokhtar magmatic assemblage comprises various tholeiitic, calc-alkaline, and K-rich alkaline intrusive and extrusive rocks (with associated pyroclastic and volcaniclastic successions) along the active margin of the Iranian plates (Berberian and King, 1981; Alavi, 1994).
3. Stratigraphy 3.1. Zagros The thick (up to 12 km) sedimentary sequences of the Zagros basin contain an almost continuous sequence of shelf sediments ranging in age from Infra-Cambrian to Recent. The 1–2 km thick Infra-Cambrian to early Cambrian Hormuz Salt formation were probably deposited on the northern continuation of the Panafrican basement exposed in Arabia (Talbot and Alavi, 1996). This unit was overlain by 6–10 km of platform deposits that are predominantly sandstone, shale, and dolomite (Cambrian through Triassic) and limestone with subordinate shales and evaporites (Jurassic through Lower Miocene). The mid-Miocene and younger rocks include gypsum, limestone, sandstone, shale, and conglomerate (Hessami, 2002; McQuarrie, 2004).
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ing (Berberian and King, 1981; Stocklin, 1968). Permian rifting of the northern edge of what became the Zagros Basin was followed along the SSZ by major asymmetric mafic (basalt, diabase, and some intermediate) volcanic activity in the Late Permian (e.g., Sengor, 1990). Some Triassic magmatism along the Southern SSZ could be considered as asymmetric magmatic activity linked to a second phase of rifting of the Nain-Baft ocean from the Triassic to the Jurassic (Ghasemi and Talbot, 2006). 3.3.2. Jurassic–Cretaceous Late Jurassic metamorphic rocks to the west of Sirjan, and lowto medium-pressure metamorphism accompanied by magmatic activity along the Southern SSZ, indicate that subduction of NeoTethys may have already commenced beneath the Southern SSZ by the Late Jurassic–Early Cretaceous (Sengor, 1990). The presence of very thick Upper Triassic–Upper Jurassic sediments along the Northern SSZ can be considered as indicative of a continuation of the Nain-Baft Ocean without the generation of any oceanic lithosphere (Ghasemi and Talbot, 2006). Later silicic to intermediate plutonic activity and further deformation occurred along the Northern SSZ during the Late Cretaceous (Berberian and King, 1981). 3.3.3. Cenozoic In the Late Cretaceous, the rate of subduction of Neo-Tethys beneath the SSZ increased (Alavi, 1994). Cretaceous and/or younger pyroclastic and/or volcaniclastic rocks are found near Nahavand and adjacent areas in Northern SSZ (Alavi, 1994).
3.2. Effects of the Kazerun fault system on lithostratigraphic units
3.4. Stratigraphy of the study area
Significant facies boundaries indicate that the Kazerun fault system was frequently active during the development of the Zagros basin. Large volumes of Cambrian Hormuz salt that formerly lay at the base of the cover sequence now extrude through >200 diapirs in the Zagros basin east of the Kazerun fault (Kent, 1979). In contrast, this salt has never been reported in wells or observed on seismic sections to the west of the Kazerun fault zone. This fault zone, or at least part of it, may have acted as the western basin bounding fault during the deposition of the Cambrian salt (Talbot and Alavi, 1996; Bahroudi and Talbot, 2003; Bahroudi and Koyi, 2004; Sepehr and Cosgrove, 2004). The Kazerun fault also acted as a boundary between different types of sedimentation during the Jurassic and most of Cretaceous (Bahroudi and Talbot, 2003; Sepehr and Cosgrove, 2004). Traditionally, the Kazerun fault divides the Gachsaran basin into two different segments. The area northwest of the Kazerun fault contains large volumes of hydrocarbons in the Oligocene–Lower Miocene Asmari limestone and is sealed by thick Gachsaran evaporites (Bahroudi and Koyi, 2004).
Most of the Isfahan area consists of Jurassic and Lower Cretaceous sediments. The Jurassic rocks include fine-grained clastic sediments interbedded with thin layers of coarse-grained clastic sediments and carbonates, or slightly metamorphosed stratigraphic units equivalent to them. The Lower Cretaceous strata lie unconformably over older Mesozoic successions. Three main units are recognizable in lower Cretaceous sediments (Zahedi, 1978):
3.3. Nature and location of division of the SSZ The exact nature and location of the division between the northern and the southern parts of the SSZ has not previously been identified (Eftekharnejad, 1981; Ghasemi and Talbot, 2006) and is the subject of this article. Table 1 summarizes the stratigraphic differences between the southern and northern parts of the Sanandaj–Sirjan zones through time, following Ghasemi and Talbot (2006). 3.3.1. Permo-Triassic Prior to the Late Paleozoic, the whole of Iran formed a relatively stable continental platform covered conformably by epeiro-continental shelf deposits, with no evidence of major magmatism or fold-
1. Lower red conglomerate with inter-layers of shale and sandstone. 2. Yellow to orange sandy dolomite, which grades into sandy limestone upwards. 3. A thick sequence of gray limestones with foraminiferal faunal assemblages and massive layering. This sequence has interbeds of shale and yellow to gray laminated limestone.
4. Geometry and structure of the Kazerun fault system (KFS) Among the N-trending transverse faults across the Zagros fold– thrust belt (ZFTB), the KFS is the best known active basement fault, and has a long history of reactivations (e.g., Baker et al., 1993; Berberian, 1995; Talbot and Alavi, 1996; Bahroudi and Talbot, 2003). The Kazerun fault displays right-lateral displacements of the Precambrian basement on aeromagnetic maps (Fig. 3) and is seismically active (Mouthereau et al., 2006). It crosses the Zagros trend, offsetting and dragging fold axes in the cover sequence in a right-lateral sense (Baker et al., 1993; Berberian, 1995; Sherkati and Letouzey, 2004; Sepehr and Cosgrove, 2005). Different styles of deformation, geomorphology, and earthquake focal mechanisms indicate that various right-lateral strikeslip faults across the ZFTB are active. This affects faulting in both the cover and basement, marking the boundary between the Fars Province in the east and Dezful Embayment to the west (Sepehr and Cosgrove, 2004; Authemayou et al., 2005).
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Fig. 2. Landsat image (TM data, RGB = 741) of the northern portion of the Kazerun fault system and the southern portion of the Isfahan fault. Table 1 Tectonic evolution of the South and North Sanandaj–Sirjan zone (summarized from Ghasemi and Talbot, 2006). Time
South Sanandaj–Sirjan zone
North Sanandaj–Sirjan zone
Middle Eocene Paleocene
Slab break-off Ophiolite emplacement along the South (SSZ) and on the Central Iranian Microcontinent – The closure of the Neo-Tethys and Naien-Baft Oceans
Slab break-off –
Late Cretaceouse Middle Cretaceouse Late Triassic Permian
Naien-Baft Ocean rifting Neo-Tethys Ocean rifting
The KFS marks the boundary between two drastically different structural domains. The simply folded belt of the ZFTB is narrow (200 km) to the west of the KFS, the Hormuz salt is absent, and earthquakes are localized on major thrust faults. In contrast, the simply folded belt of the ZFTB east of the KFS is wider (300-km) and earthquakes are widely distributed where it deformed above a decollement in the Hormuz salt (Berberian, 1995; Talbot and Alavi, 1996; Authemayou et al., 2005). Moreover, the topographic slopes are noticeably different across the KFS from West (Dezful) to East (Fars). Because it has been proposed that tectonic wedging might have been controlled by basement shortening (Mouthereau et al., 2006) the KFS appears to localize strong variations in rheological behavior at the crustal scale. Hessami et al. (2006) distinguished four areas of active crustal motion within the Zagros based on the velocities of a network of
Ophiolite emplacement along the North (SSZ) Closure of the Neo-Tethys Ocean commencing with an oceanic island arc collision – Neo-Tethys Ocean rifting
GPS stations between March 1998 and June 2001. East of the Kazerun fault, stations move SW at rates between 8 and 12 mm/yr. West of the Kazerun fault, stations move 7–12 mm/ yr with a westward component that increases westward. Tectonic and seismological observations (e.g., Berberian, 1995; Talebian and Jackson, 2004), together with the morphology, predicted these movements. These two segments of the Zagros are separated by the KFS, across which right-lateral strike-slip occurs at 2–3 mm/yr on individual fault segments, yielding a cumulative strike-slip rate of 6 ± 2 mm/yr (Walpersdorf et al., 2006). Berberian (1995) suggested right-lateral offsets across the Kazerun fault of 140–150 km and a vertical offset of about 6 km down to the west, while Authemayou et al. (2005) reported minimal offsets of 12–27 km.
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Most previous studies of the KFS have overlooked the aeromagnetic data. Modern reprocessing of measurements made in the 1970s (Yousefi, 2003) indicates the continuation of a significant magnetic lineament locally coincident with the KFS northward of the Zagros Mountains (F-4 on Fig. 3).
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The KFS consists of three north-trending fault zones about 100 km long (Sepehr and Cosgrove, 2005) (Fig. 1). These three fault zones (the Dena, Kazerun, and Borazjan faults) are transfer faults (Authemayou et al., 2005). Isopach maps of the Jurassic and Cretaceous reveal that the KZF was activated as transfer faults during the Tethyan tectonics (Sepehr and Cosgrove, 2004).
Fig. 3. Magnetic lineament map of Iran on the basis of airborne magnetic data surveyed in the 1970s (from Yousefi, 2003).
Fig. 4. Landsat image (TM data, RGB = 741) of the Isfahan area: (a) without faults and (b) with faults.
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5. Geometry and structure of the Isfahan fault system (IFS) Analysis of satellite data (Landsat TM, SPOT, and IRS), followed by field studies, has allowed the recognition of many previously unmapped faults in the SSZ near Isfahan (Fig. 4). The present study documents two major north-trending faults to the west of the city of Isfahan. These faults are parallel and are about 10 km apart. It is likely that this suite of major and minor parallel faults on the surface overlies a major north-trending fault in the basement. Urban and/or agricultural developments locally obscure the traces of these faults, so they are therefore not shown as continuous lines on the maps and satellite images (Figs. 4 and 7). The western fault is here named the Isfahan fault. It is the better exposed of the two faults, and is 220 km long. The eastern fault is 160 km long and is named the Khomeynishahr fault (Fig. 4). 5.1. Isfahan fault The N-S trending Isfahan fault (IF) can be traced discontinuously for 220 km from the MZT in the south to at least the southern boundary of the Urumieh–Dokhtar zone in the north. It consists of three segments: Southern, Central, and Northern (Fig. 4). 1. The Southern segment of the IF is just over 40 km long. Its southern terminus is located about 12 km northwest of Semirom, south of where it offsets the MZT by about 2 km (IF1 in Figs. 2 and 4). The trace of the Southern segment of the Isfahan fault can be followed in the field to about 40 km to the north (Fig. 2). Some parts of this segment are exposed at higher elevation. The average strike of Hilltops and faults at west of Isfahan fault are 120°N, but in east of this fault are 140°N. Thus, it seams like that the geomorphological Hilltops have been rotated (?) by about 20° clockwise around a vertical axis (Fig. 4 and northeast of Fig. 2). 2. The Central segment is separated from the southern segment by a gap of about 7.5 km, and can be traced for about 30 km further north (IF2 in Fig. 4). The southern part of the central segment
offsets Cretaceous limestones and Pliocene conglomerates by a dextral strike-slip displacement of about 5 km. Fault scarps in recent alluvium along the northern part of the central segment, as suggested by satellite images, suggest that the fault reaches the surface and may be is currently active. 3. The northern segment of the IF is about 100 km long and is separated from the central segment by a gap of about 50 km, where its trace may be obscured by the agriculture developments southwest of Isfahan (IF3 in Fig. 4). The southern part of the northern segment is exposed in Jurassic shales and Lower Cretaceous limestones near the hilltops. Here, the Isfahan fault and associated minor parallel normal faults dip between 45° east and subvertical. Displacements in recent alluvium along the central part of the northern segment of the IF may indicate recent activity. The northern end of the Isfahan fault is concealed under a large alluvial fan fed by a deep N-trending valley in the Urumieh–Dokhtar volcanic zone that may be eroded along a northern continuation of the IF (Figs. 4 and 5). 5.2. Khomeynishahr fault This fault is 160 km long and consists of four segments that are parallel to and about 10 km east of the Isfahan fault (Fig. 4). 1. The southern segment is less than 25 km long, and both its ends terminate at faults that strike approximately east-west (KF1 in Fig. 4). The central and southern parts of this segment offset Cretaceous limestones about 1 km dextral strike-slip and its northern segment displaces alluvium. 2. The central segment is separated from the southern segment by a 20 km gap where the trace of the Khomeynishahr fault is concealed beneath alluvium. The central segment is over 55 km long and passes through the cities of Isfahan and Khomeynishahr (KF2 in Fig. 4). The southern part of the central segment crosses the hills as minor parallel faults that strike 175°N with variable dips near 60° to the ENE (Fig. 6a). Equivalent some minor parallel faults near
Fig. 5. Landsat image (TM data, RGB = 741) of the apparent northern termination of the Isfahan fault near the southern boundary of the Urumieh–Dokhtar volcanic zone.
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Fig. 6. (a) Scarps in exposures of Cretaceous limestone, close to the central segment of the Khomeynishahr fault that passes in front of the hills. The trace of other faults, most with E-W strikes, can also be seen in the hills (view to the east). (b) The effect of minor N-S faults parallel to the central segment of the Khomeynishahr fault in the Iran-Kuh hills (view to the north). (c) Scarps and displacements in the Iran-Kuh hills due to the effects of minor N-S faults parallel to the central segment of the Khomeynishahr fault and a major N-S fault a few hundred meters away from figure (b) (view to the north). (d and e) Slickensides on a minor fault parallel to the Khomeynishahr fault that show both dip slip and strike-slip motion.
the center of this segment strike 05°N and dip 50° to the ESE (Fig. 6b and c). Slickenlines on these minor faults indicate that some have undergone normal movement while others have undergone strike-slip motion (Fig. 6d and e). The northern termination of this segment is located in the hills overlooking the city of Isfahan. 3 and 4. The northern segment of the Khomeynishahr fault consists of two short segments which are about 10 and 20 km long (KF3 and KF4 in Fig. 4). Both segments displace Jurassic shales and Cretaceous limestones and have local scarps in recent alluvium. The Isfahan and Khomeynishahr faults observed at the surface are considered here to be overlying an active basement fault system named the Isfahan fault system, which has experienced both normal and strike-slip components of motion.
the dominant trend is N130E, whereas to the east the dominant trend has changed to N150E. This obvious and sudden change in the trends of structures is related to right-lateral strike-slip motion along the north-trending Isfahan fault system. The trace of the northern part of the Isfahan fault system is most obvious north of Isfahan city, where it juxtaposes high outcrops of Cretaceous limestone and Jurassic shales in the west against large depressions in the east, including the Gavkhuni, Abarkuh, and Sirjan salt desert depressions. These depressions are easily recognizable on geological or topographical maps of the South Sanandaj– Sirjan zone and reach a total length of nearly 700 km.
5.3. Geomorphology
6.1. Zagros
Most of the faults and fold axes in the simply folded zone of the Zagros and the SSZ trend northwest-southeast. Structures in the study area follow this general rule except for those within about 120 km of the trace of the Isfahan fault system, and especially southwest of Isfahan city, where the fold axes, faults, and the geomorphological highs have all been rotated (?) clockwise by about 20° about a vertical axis (Fig. 4). West of the Isfahan fault system,
The focal mechanism solutions of earthquakes in the Zagros Mountains imply that most of the seismicity is associated with thrust faulting (e.g., Jackson et al., 1995). Most of the seismicity lies along a linear zone along the ZFTB. The lateral offset of this zone indicates segmentation of the ZFTB along N-S transfer faults that root to deep-seated strike-slip faults in the basement. The dominant NW-SE trending features of the belt have undergone repeated
6. Seismicity
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6.2. The Sanandaj–Sirjan zone One of the characteristic features of the SSZ is its relatively low seismic activity. Because no significant earthquakes have been recorded in this zone, the SSZ has been the focus of very few studies. Nonetheless, the occurrence of historical earthquakes indicates that this zone is active. For instance, the Isfahan earthquake of 1344 killed about 20 people and destroyed the city wall and some houses (Ambraseys and Melville, 1982). Earthquakes in the Isfahan area have been documented by two local seismological networks over two different time intervals (Mohajer Ashjaee, 1981). 6.2.1. Seismic data recorded from 1976 to 1978 The first study of the seismicity of the Isfahan area was carried out by the seismography department of the Iranian Atomic Energy Organization prior to establishing the nuclear technology center of Isfahan (Mohajer Ashjaee, 1981). A network of seven portable seismographs located 169 earthquakes (up to magnitudes 4.8 ML) and microearthquakes (down to magnitudes 1.5 ML) in and around Isfahan between August 1976 and July 1978 (Fig. 7). The results of this study led to the conclusion that the seismicity of the Isfahan area is dispersed down to depths of 32 km, with only some of the shocks occurring on the active faults that have been mapped in the area. The earthquakes in this data set clustered in two areas, one 20– 40 km southwest of Isfahan and the other where the northern part of the Kazerun fault crosses the MZT (Fig. 7). Focal mechanism solutions of the earthquakes have been characterized by Mohajer Ashjaee (1981), who concluded that:
Fig. 7. Earthquakes epicenters during 1976–1978 (blue circles are for color figure and black circles are for B&W figure) and 2000–2003 (green circles are for color figure and white circles are for B&W figure), superimposed on SRTM data and fault map of the Isfahan area.
horizontal offsets along these transverse basement faults (Hessami, 2002). The KFS is one of these important transverse basement faults and has earthquake focal mechanisms consistent with right-lateral strike-slip motion (Baker et al., 1993; Berberian, 1995). Fault slipvector inversions indicate a right-lateral strike-slip regime along the KFS associated with a N35–40°E trending r1 and a thrust-faulting regime around the bent splay fault terminations (Authemayou et al., 2005).
1. The composite focal mechanism solution for earthquakes along the northern part of the Kazerun fault indicates a dextral normal fault with a N-NW strike consistent with the strike of the Kazerun fault but with a dip of only 38° to the East (Fig. 8A). 2. The composite focal mechanism solution of the earthquakes in the area 20 to 40 km southwest of Isfahan city indicates a dextral normal fault with an N-NW strike but a dip of 26° to the ENE (Fig. 8B).
6.2.2. Seismic data recorded from 2000 to 2003 The second study of seismic activity in the Isfahan area was recorded by the four recording stations of the Isfahan seismographical network (Iranian Seismological Center Online Databank, http:// irsc.ut.ac.ir/). The data used here were recorded from 2000 to 2003. This network resumed recording in 2003, following an interruption of about two years.
Fig. 8. Characteristic composite focal mechanisms for earthquakes in Fig. 7: (A) northern part of the Kazerun fault and (B) near the Isfahan fault, 20–40 km southwest of Isfahan city.
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The two sets of earthquake data are analyzed together here, as there is considerable overlap (Fig. 7). Many of the earthquakes recorded in these two studies were located near the previously unmapped faults identified in this study (e.g., Fig. 4) and indicate that some of these are still active Most of the epicenters of earthquakes with magnitudes between 1.5 and 4.8 (ML) define lineaments along the Isfahan and Khomeynishahr faults, especially where they cross other active faults (Fig. 7). There is an obvious concentration of earthquakes close to where the active faults with the typical Zagros trend intersect the Isfahan and Khomeynishahr faults. Ten earthquakes with magnitudes between 3 and 4 (ML) and three with magnitudes between 4 and 5 (ML) occurred southwest of Isfahan city. Moreover, nine of the earthquakes along or near the Khomeynishahr fault had magnitudes between 3 and 4 (ML). Many earthquake epicenters (Fig. 7) allow the traces of the newly mapped fault segments to be extrapolated through areas where they are obscured by agricultural and urban development. These earthquakes were up to 30 km deep, indicating that the Isfahan fault system is active within the basement.
7. Discussion and conclusion 1. For many years, the Kazerun fault system has been identified as a major fault in the basement of the Zagros Mountains. Reactivation of this fault system defined sedimentary basins from the Infracambrian to Tertiary times, and its seismicity indicates that it is still active. The different stratigraphies of the cover sequences on either side of the KFS also influenced the different structural styles on either side. An important question raised by this work is: how was it possible for this important and ancient fault system to be cut abruptly by the MZT? 2. It is my contention that the 220 km long Isfahan fault and the adjacent 160 km long Khomeynishahr fault both overlie an active basement fault that has both normal and strikeslip components of motion. 3. The concentration of more than 100 earthquakes near the Isfahan and the Khomeynishahr faults in the period from 1976 to 1978 and 2000 to 2003 indicate that these two faults are active. The common focal mechanism solutions of these earthquakes are consistent with the strikes of these faults southwest of Isfahan, and indicate a normal and strike-slip fault with an N-NW strike, although the 26° dip to NE (Mohajer Ashjaee, 1981) is unexpected (due to low dip angle of fault plane). 4. Earlier stratigraphic, petrologic, and structural studies (e.g., Ghasemi and Talbot, 2006) emphasized the fundamental differences between the northern and southern segments of the SSZ. However, none of these studies defined the precise boundary between these two segments, but rather considered it to be somewhere in the western parts of Isfahan city. In this study, the Isfahan fault system, with its higher elevations to the west and low elevations to the east, is proposed as the precise boundary between the northern and southern segments of the SSZ. 5. The KFS and the Isfahan fault systems are both approximately north-trending dextral strike-slip fault systems. The Isfahan fault system can therefore be considered as the continuation of the KFS across the SSZ. Because of dextral strikeslip of the Main Zagros thrust and other parallel faults these two faults are currently offset by 52 km. 6. Additional research is required in this area, particularly to establish whether the Isfahan fault continues into or through the Urumieh–Dokhtar zone.
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7. Although the KFS has been recognized for many years, this is the first suggestion that this fault system continues to the north as the Isfahan fault system. The clarification here of the Isfahan fault system as the boundary between the northern and southern segments of the SSZ is a step forward in understanding the relationships between this zone and the Zagros orogen. 8. The relationship between the Kazerun fault and the Isfahan fault can be explained with two possible scenarios. a. One scenario is that the Isfahan fault system post-dates the suturing of the Iranian and Arabian plates in the Zagros. The Isfahan fault probably formed in the Late Cretaceous–Paleocene soon after suturing as a result of renewed activity along the Kazerun fault zone. Because of dextral strike-slip of the Main Zagros thrust these two faults are currently offset by 52 km. This model assumes that the MZT represents the suture between the two plates. b. The second scenario considers that the Kazerun fault and Isfahan fault systems are N-S trending Panafrican basement faults. These faults were formally part of the same fault but have subsequently separated by 52 km of dextral strike-slip movement along the MZT. If this scenario is correct then the suture would not coincide with the MZT as both sides of this fault would have originally formed part of the same (i.e. Arabian) plate. This scenario agrees with models considering that the suturing of the Iranian and Arabian plates occurred along the Sanandaj– Sirjan and Urumieh–Dokhtar zones (e.g., Alavi, 1994). For confirmation of the mechanism, however, each scenario requires further work.
Acknowledgements I would like to express my appreciation to all those whose cooperation and support allowed my studies in the Isfahan area. Particular thanks are due to Prof. Christopher Talbot and Dr. Frédéric Mouthereau for critically reading and helping with the text and for good advice. I have benefited greatly from discussions with them. I thank Mansor Shishehfrosh of the Isfahan Head of Province Office for his valuable support. This study was financed by the Isfahan University Research Council (under Project Nos. 800311 and 7-3486). I appreciate their fundamental support and patience. References Alavi, M., 1994. Tectonics of the Zagros Orogenic belt of Iran, new data and interpretations. Tectonophysics 229, 211–238. Ambraseys, N.N., Melville, C.P., 1982. A History of Persian Earthquakes. Cambridge University Press, New York. 219pp. Authemayou, C., Bellier, O., Chardon, D., Malekzade, Z., Abbassi, M., 2005. Role of the Kazerun fault system in active deformation of the Zagros fold-and-thrust belt (Iran). C.R. Geoscience 337, 539–545. Bahroudi, A., Talbot, C.J., 2003. The configuration of the basement beneath the Zagros basin. J. Petrol. Geol. 26 (3), 257–282. Bahroudi, A., Koyi, H.A., 2004. Tectono-sedimentary framework of the Gachsaran Formation in the Zagros foreland basin. Mar. Petrol. Geol. 21, 1295–1310. Baker, C., Jackson, J., Priestley, K., 1993. Earthquakes on the Kazerun line in the zagros Mountains of Iran: strike-slip faulting within a fold and thrust belt. Geophys. J. Int. 115, 41–61. Bayer, R., Chery, J., Tatar, M., Vernant, Ph., Abbassi, M., Masson, F., Nilforoushan, F., Doerflinger, E., Regard, V., Bellier, O., 2006. Active deformation in Zagros– Makran transition zone inferred from GPS measurements. Geophys. J. Int. 165, 373. doi:10.1111/j.1365-246X.2006.02879.x. Berberian, M., 1981. Active faulting and tectonics of Iran. In: Gupta, H.K., Delany, F.M. (Eds.), Zagros-Hindu Kush-Himalaya Geodynamic Evolution, Geodynamics Series, vol. 3. American Geophysical Union, pp. 33–69. Berberian, M., 1995. Master blind thrust faults hidden under the Zagros folds: active basement tectonics and surface morphotectonics. Tectonophysics 241, 193– 224.
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Berberian, M., King, G.C.P., 1981. Towards a paleography and tectonic evolution of Iran. Canadian J. Earth Sci. 18 (2), 210–265. Eftekharnejad, J., 1981. Tectonic division of Iran with respect to sedimentary basins. J. Iran. Petrol. Soci. 82, 19–28 (in Farsi). Ghasemi, A., Talbot, C.J., 2006. A new tectonic scenario for the Sanandaj–Sirjan zone (Iran). J. Asian Earth Sci. 26, 683–693. Hessami, K., 2002. Tectonic history and present-day deformation in the Zagros fold– thrust belt, PhD Thesis. Uppsala University. ISBN: 91-554-5285-5.. Hessami, K., Nilforoushan, F., Talbot, C.J., 2006. Active deformation within the Zagros Mountains deduced from GPS measurement. J. Geol. Soci. London 163 (1), 143–148. Iranian Seismological Center Online Databank.
Sengor, A.M.C., 1990. A new model for the Late Paleozoic–Mesozoic tectonic evolution of Iran and implications for Oman. In: Robertson, A.H.F., Searle, M.P., Ries, A.C. (Eds.), The Geology and Tectonics of the Oman Region. Geological Society, London, pp. 797–831 (Special Publication 49). Sepehr, M., Cosgrove, J.W., 2004. Structural framework of the Zagros fold–thrust belt, Iran. Mar. Petrol. Geol. 21, 829–843. Sepehr, M., Cosgrove, J.W., 2005. Role of the Kazerun fault zone in the formation and deformation of the Zagros fold–thrust belt, Iran. Tectonics, vol. 24, pp. TC50051–TC5005-13. ISSN: 0278-7407. Sherkati, S., Letouzey, J., 2004. Variation of structural style and basin evolution in the central Zagros (Izeh zone and Dezful Embayment), Iran. Mar. Petrol. Geol. 21 (5), 535–554. Stocklin, J., 1968. Structural history and tectonics of Iran: a review. Am. Ass. Petrol. Geol. Bull. 52, 1229–1258. Talbot, C.J., Alavi, M., 1996. The Past of a Future Syntaxis Across the Zagros. Geological Society of London. pp. 89–109 (Special Publication 100). Talebian, M., Jackson, J., 2004. A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran. Geophys. J. Int. 156, 506– 526. Walpersdorf, A., Hatzfeld, D., Nankali, H., Tavakoli, F., Nilforoushan, F., Tatar, M., Vernant, P., Chery, J., Masson, F., 2006. Difference in the GPS deformation pattern of North and Central Zagros (Iran). Geophys. J. Int. 167, 1077– 1088. Yousefi, E.Q., 2003. The 2500k magnetic lineament map of Iran. Already published by National Geoscience Database of Iran. Zahedi, M., 1978. Geological Map of Isfahan, no. F8, Scale 1/250 000, G.S.I., Tehran.
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The continuation of the Kazerun fault system across the Sanandaj–Sirjan zone (Iran) Homayon Safaei * Department of Geology, Isfahan University, Hezarjerib Ave., Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 22 October 2007 Received in revised form 27 December 2008 Accepted 10 January 2009
Keywords: Sanandaj–Sirjan zone Zagros Kazerun fault Isfahan fault Isfahan Iran
a b s t r a c t The Kazerun (or Kazerun-Qatar) fault system is a north-trending dextral strike-slip fault zone in the Zagros mountain belt of Iran. It probably originated as a structure in the Panafrican basement. This fault system played an important role in the sedimentation and deformation of the Phanerozoic cover sequence and is still seismically active. No previous studies have reported the continuation of this important and ancient fault system northward across the Sanandaj–Sirjan zone. The Isfahan fault system is a north-trending dextral strike-slip fault across the Sanandaj–Sirjan zone that passes west of Isfahan city and is here recognized for the first time. This important fault system is about 220 km long and is seismically active in the basement as well as the sedimentary cover sequence. This fault system terminates to the south near the Main Zagros Thrust and to the north at the southern boundary of the Urumieh–Dokhtar zone. The Isfahan fault system is the boundary between the northern and southern parts of Sanandaj–Sirjan zone, which have fundamentally different stratigraphy, petrology, geomorphology, and geodynamic histories. Similarities in the orientations, kinematics, and geologic histories of the Isfahan and Kazerun faults and the way they affect the magnetic basement suggest that they are related. In fact, the Isfahan fault is a continuation of the Kazerun fault across the Sanandaj–Sirjan zone that has been offset by about 50 km of dextral strike-slip displacement along the Main Zagros Thrust. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The Zagros mountain belt of Iran forms a 200–350 km wide series of mountain ranges extending about 1500 km from southeastern Turkey to the Strait of Hormuz in southern Iran (Fig. 1). The Sanandaj–Sirjan zone (SSZ) is a region of polyphase deformation within greenschist and amphibolite metamorphic facies (Mohajjel and Fergusson, 2000). The latest phases of deformation record the suturing of Arabia to Eurasia during the Late Cretaceous and the subsequent southward propagation of the fold–thrust belt (Alavi, 1994). According to GPS measurements, the total convergence rate between the Arabian plate and Eurasia is estimated to range between 20 and 35 mm/yr in a NNE-trending direction (Bayer et al., 2006; Hessami et al., 2006). The Zagros is not a linear belt experiencing pure NE-SW shortening; indeed, an important component of strike-slip motion occurs along the Main Recent Fault (MRF), Fig. 1, in the northwestern part of the chain. Partitioning between the North and South Zagros is accommodated by N-trending dextral strike-slip faults that act as transfer faults along the Zagros. The largest of
* Tel.: +98 311 7932153; fax: +98 311 7932152. E-mail address: [email protected]. 1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2009.01.007
these faults is the Kazerun fault, which is responsible for a 140 km right-lateral offset of geologic features, which implies a displacement rate of 15 mm/yr since the lower Miocene (Berberian, 1981, 1995). Focal mechanism solutions of most earthquakes in the area indicate that deformation in the Zagros basement is due to shortening and thickening at numerous thrust faults (Mouthereau et al., 2006). However, observations of strike-slip faulting imply that N-S trending faults in the Zagros (e.g., the Kazerun fault), inherited from the Panafrican basement, rotated about vertical axes to accommodate the convergence between Arabia and central Iran (Hessami, 2002). The present paper documents the continuation of the Kazerun fault system (KFS), one of the most important faults in Iran, across the Sanandaj–Sirjan zone (SSZ). The KFS is a dextral strike-slip fault zone that is about 300 km long and extends from the high Zagros to the Persian Gulf plate at the Zagros front to the northeastern border of the Zagros (Baker et al., 1993; Talbot and Alavi, 1996; Sherkati and Letouzey, 2004; Sepehr and Cosgrove, 2005; Walpersdorf et al., 2006). The two fundamental questions that have been considered in this work are: (1) is the Kazerun fault system (KFS) only found within the Zagros zone and (2) how is it possible for such an important and ancient fault system to have been cut abruptly by the Main Zagros Thrust (MZT)?
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Fig. 1. Summary tectonic map of Iran. Major faults are shown as black lines. Segments of the Kazerun fault system are: (1) Dena fault, (2) Kazerun fault, and (3) Borazjan fault.
This paper presents some of the results of a structural geologic study around the city of Isfahan that mapped many previously unrecognized faults. Among these is an important north-trending transverse fault system across the SSZ that passes west of Isfahan, the Isfahan fault system. The seismicity associated with this fault system suggests that it is an active basement fault. Studies of satellite images and seismicity, supported by field observations, geomorphology, and deformation structures, suggest that this fault system is the northern continuation of the KFS across the Sanandaj–Sirjan zone.
2. Tectonic zones The NW end of the Zagros orogen is bounded by the East Anatolian left-lateral strike-slip fault, while the southeast end is bounded by the Oman Line. The Zagros orogen consists of four NW-SE trending tectonic belts parallel to the plate margin (Stocklin, 1968; Berberian and King, 1981; Alavi, 1994) (Fig. 1). From the southwest to northeast, these are: 2.1. The Mesopotamian–Persian Gulf molasse foredeep zone This zone lies west of the current Zagros deformation front near the gulf coast (Fig. 1).
2.2. The Zagros fold–thrust belt The Zagros fold–thrust belt (ZFTB) includes the High Zagros, located between the Main Zagros Thrust (MRT) and the Main Recent Fault (MRF), and the Zagros Simply Folded Belt sub zones, located between the MRF and the mountain front fault or the current Zagros deformation front (ZDF) (Fig. 1). 2.3. The Sanandaj–Sirjan metamorphic zone (SSZ) The SSZ is 150–200 km wide and lies between the towns of Sirjan in the southeast and Sanandaj in the northwest (Fig. 1). The rocks in this zone are the most strongly tectonised (deformed and metamorphosed) in the Zagros orogen. They consist of Paleozoic strata formed in an epicratonic setting that were subsequently overlain by a volcanic and sedimentary succession deposited during the Triassic opening of Neo-Tethys (e.g., Sengor, 1990). The rocks in this zone were deformed under greenschist and amphibolite type metamorphic conditions and uplifted during Late Cretaceous continental collision under dextral transpression between the Afro-Arabian continent and the Iranian micro-continent (Mohajjel and Fergusson, 2000). The SSZ can be subdivided into two parts (Eftekharnejad, 1981) to the south and north of a N-S boundary west of Isfahan: (1) the southern SSZ, consisting of rocks deformed and metamorphosed in Middle to Late Triassic and (2) the northern SSZ, which was de-
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formed in the Late Cretaceous and contains many intrusive felsic rocks (Ghasemi and Talbot, 2006). 2.4. The Urumieh–Dokhtar volcano – plutonic zone The Urumieh–Dokhtar magmatic assemblage comprises various tholeiitic, calc-alkaline, and K-rich alkaline intrusive and extrusive rocks (with associated pyroclastic and volcaniclastic successions) along the active margin of the Iranian plates (Berberian and King, 1981; Alavi, 1994).
3. Stratigraphy 3.1. Zagros The thick (up to 12 km) sedimentary sequences of the Zagros basin contain an almost continuous sequence of shelf sediments ranging in age from Infra-Cambrian to Recent. The 1–2 km thick Infra-Cambrian to early Cambrian Hormuz Salt formation were probably deposited on the northern continuation of the Panafrican basement exposed in Arabia (Talbot and Alavi, 1996). This unit was overlain by 6–10 km of platform deposits that are predominantly sandstone, shale, and dolomite (Cambrian through Triassic) and limestone with subordinate shales and evaporites (Jurassic through Lower Miocene). The mid-Miocene and younger rocks include gypsum, limestone, sandstone, shale, and conglomerate (Hessami, 2002; McQuarrie, 2004).
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ing (Berberian and King, 1981; Stocklin, 1968). Permian rifting of the northern edge of what became the Zagros Basin was followed along the SSZ by major asymmetric mafic (basalt, diabase, and some intermediate) volcanic activity in the Late Permian (e.g., Sengor, 1990). Some Triassic magmatism along the Southern SSZ could be considered as asymmetric magmatic activity linked to a second phase of rifting of the Nain-Baft ocean from the Triassic to the Jurassic (Ghasemi and Talbot, 2006). 3.3.2. Jurassic–Cretaceous Late Jurassic metamorphic rocks to the west of Sirjan, and lowto medium-pressure metamorphism accompanied by magmatic activity along the Southern SSZ, indicate that subduction of NeoTethys may have already commenced beneath the Southern SSZ by the Late Jurassic–Early Cretaceous (Sengor, 1990). The presence of very thick Upper Triassic–Upper Jurassic sediments along the Northern SSZ can be considered as indicative of a continuation of the Nain-Baft Ocean without the generation of any oceanic lithosphere (Ghasemi and Talbot, 2006). Later silicic to intermediate plutonic activity and further deformation occurred along the Northern SSZ during the Late Cretaceous (Berberian and King, 1981). 3.3.3. Cenozoic In the Late Cretaceous, the rate of subduction of Neo-Tethys beneath the SSZ increased (Alavi, 1994). Cretaceous and/or younger pyroclastic and/or volcaniclastic rocks are found near Nahavand and adjacent areas in Northern SSZ (Alavi, 1994).
3.2. Effects of the Kazerun fault system on lithostratigraphic units
3.4. Stratigraphy of the study area
Significant facies boundaries indicate that the Kazerun fault system was frequently active during the development of the Zagros basin. Large volumes of Cambrian Hormuz salt that formerly lay at the base of the cover sequence now extrude through >200 diapirs in the Zagros basin east of the Kazerun fault (Kent, 1979). In contrast, this salt has never been reported in wells or observed on seismic sections to the west of the Kazerun fault zone. This fault zone, or at least part of it, may have acted as the western basin bounding fault during the deposition of the Cambrian salt (Talbot and Alavi, 1996; Bahroudi and Talbot, 2003; Bahroudi and Koyi, 2004; Sepehr and Cosgrove, 2004). The Kazerun fault also acted as a boundary between different types of sedimentation during the Jurassic and most of Cretaceous (Bahroudi and Talbot, 2003; Sepehr and Cosgrove, 2004). Traditionally, the Kazerun fault divides the Gachsaran basin into two different segments. The area northwest of the Kazerun fault contains large volumes of hydrocarbons in the Oligocene–Lower Miocene Asmari limestone and is sealed by thick Gachsaran evaporites (Bahroudi and Koyi, 2004).
Most of the Isfahan area consists of Jurassic and Lower Cretaceous sediments. The Jurassic rocks include fine-grained clastic sediments interbedded with thin layers of coarse-grained clastic sediments and carbonates, or slightly metamorphosed stratigraphic units equivalent to them. The Lower Cretaceous strata lie unconformably over older Mesozoic successions. Three main units are recognizable in lower Cretaceous sediments (Zahedi, 1978):
3.3. Nature and location of division of the SSZ The exact nature and location of the division between the northern and the southern parts of the SSZ has not previously been identified (Eftekharnejad, 1981; Ghasemi and Talbot, 2006) and is the subject of this article. Table 1 summarizes the stratigraphic differences between the southern and northern parts of the Sanandaj–Sirjan zones through time, following Ghasemi and Talbot (2006). 3.3.1. Permo-Triassic Prior to the Late Paleozoic, the whole of Iran formed a relatively stable continental platform covered conformably by epeiro-continental shelf deposits, with no evidence of major magmatism or fold-
1. Lower red conglomerate with inter-layers of shale and sandstone. 2. Yellow to orange sandy dolomite, which grades into sandy limestone upwards. 3. A thick sequence of gray limestones with foraminiferal faunal assemblages and massive layering. This sequence has interbeds of shale and yellow to gray laminated limestone.
4. Geometry and structure of the Kazerun fault system (KFS) Among the N-trending transverse faults across the Zagros fold– thrust belt (ZFTB), the KFS is the best known active basement fault, and has a long history of reactivations (e.g., Baker et al., 1993; Berberian, 1995; Talbot and Alavi, 1996; Bahroudi and Talbot, 2003). The Kazerun fault displays right-lateral displacements of the Precambrian basement on aeromagnetic maps (Fig. 3) and is seismically active (Mouthereau et al., 2006). It crosses the Zagros trend, offsetting and dragging fold axes in the cover sequence in a right-lateral sense (Baker et al., 1993; Berberian, 1995; Sherkati and Letouzey, 2004; Sepehr and Cosgrove, 2005). Different styles of deformation, geomorphology, and earthquake focal mechanisms indicate that various right-lateral strikeslip faults across the ZFTB are active. This affects faulting in both the cover and basement, marking the boundary between the Fars Province in the east and Dezful Embayment to the west (Sepehr and Cosgrove, 2004; Authemayou et al., 2005).
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Fig. 2. Landsat image (TM data, RGB = 741) of the northern portion of the Kazerun fault system and the southern portion of the Isfahan fault. Table 1 Tectonic evolution of the South and North Sanandaj–Sirjan zone (summarized from Ghasemi and Talbot, 2006). Time
South Sanandaj–Sirjan zone
North Sanandaj–Sirjan zone
Middle Eocene Paleocene
Slab break-off Ophiolite emplacement along the South (SSZ) and on the Central Iranian Microcontinent – The closure of the Neo-Tethys and Naien-Baft Oceans
Slab break-off –
Late Cretaceouse Middle Cretaceouse Late Triassic Permian
Naien-Baft Ocean rifting Neo-Tethys Ocean rifting
The KFS marks the boundary between two drastically different structural domains. The simply folded belt of the ZFTB is narrow (200 km) to the west of the KFS, the Hormuz salt is absent, and earthquakes are localized on major thrust faults. In contrast, the simply folded belt of the ZFTB east of the KFS is wider (300-km) and earthquakes are widely distributed where it deformed above a decollement in the Hormuz salt (Berberian, 1995; Talbot and Alavi, 1996; Authemayou et al., 2005). Moreover, the topographic slopes are noticeably different across the KFS from West (Dezful) to East (Fars). Because it has been proposed that tectonic wedging might have been controlled by basement shortening (Mouthereau et al., 2006) the KFS appears to localize strong variations in rheological behavior at the crustal scale. Hessami et al. (2006) distinguished four areas of active crustal motion within the Zagros based on the velocities of a network of
Ophiolite emplacement along the North (SSZ) Closure of the Neo-Tethys Ocean commencing with an oceanic island arc collision – Neo-Tethys Ocean rifting
GPS stations between March 1998 and June 2001. East of the Kazerun fault, stations move SW at rates between 8 and 12 mm/yr. West of the Kazerun fault, stations move 7–12 mm/ yr with a westward component that increases westward. Tectonic and seismological observations (e.g., Berberian, 1995; Talebian and Jackson, 2004), together with the morphology, predicted these movements. These two segments of the Zagros are separated by the KFS, across which right-lateral strike-slip occurs at 2–3 mm/yr on individual fault segments, yielding a cumulative strike-slip rate of 6 ± 2 mm/yr (Walpersdorf et al., 2006). Berberian (1995) suggested right-lateral offsets across the Kazerun fault of 140–150 km and a vertical offset of about 6 km down to the west, while Authemayou et al. (2005) reported minimal offsets of 12–27 km.
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Most previous studies of the KFS have overlooked the aeromagnetic data. Modern reprocessing of measurements made in the 1970s (Yousefi, 2003) indicates the continuation of a significant magnetic lineament locally coincident with the KFS northward of the Zagros Mountains (F-4 on Fig. 3).
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The KFS consists of three north-trending fault zones about 100 km long (Sepehr and Cosgrove, 2005) (Fig. 1). These three fault zones (the Dena, Kazerun, and Borazjan faults) are transfer faults (Authemayou et al., 2005). Isopach maps of the Jurassic and Cretaceous reveal that the KZF was activated as transfer faults during the Tethyan tectonics (Sepehr and Cosgrove, 2004).
Fig. 3. Magnetic lineament map of Iran on the basis of airborne magnetic data surveyed in the 1970s (from Yousefi, 2003).
Fig. 4. Landsat image (TM data, RGB = 741) of the Isfahan area: (a) without faults and (b) with faults.
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5. Geometry and structure of the Isfahan fault system (IFS) Analysis of satellite data (Landsat TM, SPOT, and IRS), followed by field studies, has allowed the recognition of many previously unmapped faults in the SSZ near Isfahan (Fig. 4). The present study documents two major north-trending faults to the west of the city of Isfahan. These faults are parallel and are about 10 km apart. It is likely that this suite of major and minor parallel faults on the surface overlies a major north-trending fault in the basement. Urban and/or agricultural developments locally obscure the traces of these faults, so they are therefore not shown as continuous lines on the maps and satellite images (Figs. 4 and 7). The western fault is here named the Isfahan fault. It is the better exposed of the two faults, and is 220 km long. The eastern fault is 160 km long and is named the Khomeynishahr fault (Fig. 4). 5.1. Isfahan fault The N-S trending Isfahan fault (IF) can be traced discontinuously for 220 km from the MZT in the south to at least the southern boundary of the Urumieh–Dokhtar zone in the north. It consists of three segments: Southern, Central, and Northern (Fig. 4). 1. The Southern segment of the IF is just over 40 km long. Its southern terminus is located about 12 km northwest of Semirom, south of where it offsets the MZT by about 2 km (IF1 in Figs. 2 and 4). The trace of the Southern segment of the Isfahan fault can be followed in the field to about 40 km to the north (Fig. 2). Some parts of this segment are exposed at higher elevation. The average strike of Hilltops and faults at west of Isfahan fault are 120°N, but in east of this fault are 140°N. Thus, it seams like that the geomorphological Hilltops have been rotated (?) by about 20° clockwise around a vertical axis (Fig. 4 and northeast of Fig. 2). 2. The Central segment is separated from the southern segment by a gap of about 7.5 km, and can be traced for about 30 km further north (IF2 in Fig. 4). The southern part of the central segment
offsets Cretaceous limestones and Pliocene conglomerates by a dextral strike-slip displacement of about 5 km. Fault scarps in recent alluvium along the northern part of the central segment, as suggested by satellite images, suggest that the fault reaches the surface and may be is currently active. 3. The northern segment of the IF is about 100 km long and is separated from the central segment by a gap of about 50 km, where its trace may be obscured by the agriculture developments southwest of Isfahan (IF3 in Fig. 4). The southern part of the northern segment is exposed in Jurassic shales and Lower Cretaceous limestones near the hilltops. Here, the Isfahan fault and associated minor parallel normal faults dip between 45° east and subvertical. Displacements in recent alluvium along the central part of the northern segment of the IF may indicate recent activity. The northern end of the Isfahan fault is concealed under a large alluvial fan fed by a deep N-trending valley in the Urumieh–Dokhtar volcanic zone that may be eroded along a northern continuation of the IF (Figs. 4 and 5). 5.2. Khomeynishahr fault This fault is 160 km long and consists of four segments that are parallel to and about 10 km east of the Isfahan fault (Fig. 4). 1. The southern segment is less than 25 km long, and both its ends terminate at faults that strike approximately east-west (KF1 in Fig. 4). The central and southern parts of this segment offset Cretaceous limestones about 1 km dextral strike-slip and its northern segment displaces alluvium. 2. The central segment is separated from the southern segment by a 20 km gap where the trace of the Khomeynishahr fault is concealed beneath alluvium. The central segment is over 55 km long and passes through the cities of Isfahan and Khomeynishahr (KF2 in Fig. 4). The southern part of the central segment crosses the hills as minor parallel faults that strike 175°N with variable dips near 60° to the ENE (Fig. 6a). Equivalent some minor parallel faults near
Fig. 5. Landsat image (TM data, RGB = 741) of the apparent northern termination of the Isfahan fault near the southern boundary of the Urumieh–Dokhtar volcanic zone.
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Fig. 6. (a) Scarps in exposures of Cretaceous limestone, close to the central segment of the Khomeynishahr fault that passes in front of the hills. The trace of other faults, most with E-W strikes, can also be seen in the hills (view to the east). (b) The effect of minor N-S faults parallel to the central segment of the Khomeynishahr fault in the Iran-Kuh hills (view to the north). (c) Scarps and displacements in the Iran-Kuh hills due to the effects of minor N-S faults parallel to the central segment of the Khomeynishahr fault and a major N-S fault a few hundred meters away from figure (b) (view to the north). (d and e) Slickensides on a minor fault parallel to the Khomeynishahr fault that show both dip slip and strike-slip motion.
the center of this segment strike 05°N and dip 50° to the ESE (Fig. 6b and c). Slickenlines on these minor faults indicate that some have undergone normal movement while others have undergone strike-slip motion (Fig. 6d and e). The northern termination of this segment is located in the hills overlooking the city of Isfahan. 3 and 4. The northern segment of the Khomeynishahr fault consists of two short segments which are about 10 and 20 km long (KF3 and KF4 in Fig. 4). Both segments displace Jurassic shales and Cretaceous limestones and have local scarps in recent alluvium. The Isfahan and Khomeynishahr faults observed at the surface are considered here to be overlying an active basement fault system named the Isfahan fault system, which has experienced both normal and strike-slip components of motion.
the dominant trend is N130E, whereas to the east the dominant trend has changed to N150E. This obvious and sudden change in the trends of structures is related to right-lateral strike-slip motion along the north-trending Isfahan fault system. The trace of the northern part of the Isfahan fault system is most obvious north of Isfahan city, where it juxtaposes high outcrops of Cretaceous limestone and Jurassic shales in the west against large depressions in the east, including the Gavkhuni, Abarkuh, and Sirjan salt desert depressions. These depressions are easily recognizable on geological or topographical maps of the South Sanandaj– Sirjan zone and reach a total length of nearly 700 km.
5.3. Geomorphology
6.1. Zagros
Most of the faults and fold axes in the simply folded zone of the Zagros and the SSZ trend northwest-southeast. Structures in the study area follow this general rule except for those within about 120 km of the trace of the Isfahan fault system, and especially southwest of Isfahan city, where the fold axes, faults, and the geomorphological highs have all been rotated (?) clockwise by about 20° about a vertical axis (Fig. 4). West of the Isfahan fault system,
The focal mechanism solutions of earthquakes in the Zagros Mountains imply that most of the seismicity is associated with thrust faulting (e.g., Jackson et al., 1995). Most of the seismicity lies along a linear zone along the ZFTB. The lateral offset of this zone indicates segmentation of the ZFTB along N-S transfer faults that root to deep-seated strike-slip faults in the basement. The dominant NW-SE trending features of the belt have undergone repeated
6. Seismicity
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6.2. The Sanandaj–Sirjan zone One of the characteristic features of the SSZ is its relatively low seismic activity. Because no significant earthquakes have been recorded in this zone, the SSZ has been the focus of very few studies. Nonetheless, the occurrence of historical earthquakes indicates that this zone is active. For instance, the Isfahan earthquake of 1344 killed about 20 people and destroyed the city wall and some houses (Ambraseys and Melville, 1982). Earthquakes in the Isfahan area have been documented by two local seismological networks over two different time intervals (Mohajer Ashjaee, 1981). 6.2.1. Seismic data recorded from 1976 to 1978 The first study of the seismicity of the Isfahan area was carried out by the seismography department of the Iranian Atomic Energy Organization prior to establishing the nuclear technology center of Isfahan (Mohajer Ashjaee, 1981). A network of seven portable seismographs located 169 earthquakes (up to magnitudes 4.8 ML) and microearthquakes (down to magnitudes 1.5 ML) in and around Isfahan between August 1976 and July 1978 (Fig. 7). The results of this study led to the conclusion that the seismicity of the Isfahan area is dispersed down to depths of 32 km, with only some of the shocks occurring on the active faults that have been mapped in the area. The earthquakes in this data set clustered in two areas, one 20– 40 km southwest of Isfahan and the other where the northern part of the Kazerun fault crosses the MZT (Fig. 7). Focal mechanism solutions of the earthquakes have been characterized by Mohajer Ashjaee (1981), who concluded that:
Fig. 7. Earthquakes epicenters during 1976–1978 (blue circles are for color figure and black circles are for B&W figure) and 2000–2003 (green circles are for color figure and white circles are for B&W figure), superimposed on SRTM data and fault map of the Isfahan area.
horizontal offsets along these transverse basement faults (Hessami, 2002). The KFS is one of these important transverse basement faults and has earthquake focal mechanisms consistent with right-lateral strike-slip motion (Baker et al., 1993; Berberian, 1995). Fault slipvector inversions indicate a right-lateral strike-slip regime along the KFS associated with a N35–40°E trending r1 and a thrust-faulting regime around the bent splay fault terminations (Authemayou et al., 2005).
1. The composite focal mechanism solution for earthquakes along the northern part of the Kazerun fault indicates a dextral normal fault with a N-NW strike consistent with the strike of the Kazerun fault but with a dip of only 38° to the East (Fig. 8A). 2. The composite focal mechanism solution of the earthquakes in the area 20 to 40 km southwest of Isfahan city indicates a dextral normal fault with an N-NW strike but a dip of 26° to the ENE (Fig. 8B).
6.2.2. Seismic data recorded from 2000 to 2003 The second study of seismic activity in the Isfahan area was recorded by the four recording stations of the Isfahan seismographical network (Iranian Seismological Center Online Databank, http:// irsc.ut.ac.ir/). The data used here were recorded from 2000 to 2003. This network resumed recording in 2003, following an interruption of about two years.
Fig. 8. Characteristic composite focal mechanisms for earthquakes in Fig. 7: (A) northern part of the Kazerun fault and (B) near the Isfahan fault, 20–40 km southwest of Isfahan city.
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The two sets of earthquake data are analyzed together here, as there is considerable overlap (Fig. 7). Many of the earthquakes recorded in these two studies were located near the previously unmapped faults identified in this study (e.g., Fig. 4) and indicate that some of these are still active Most of the epicenters of earthquakes with magnitudes between 1.5 and 4.8 (ML) define lineaments along the Isfahan and Khomeynishahr faults, especially where they cross other active faults (Fig. 7). There is an obvious concentration of earthquakes close to where the active faults with the typical Zagros trend intersect the Isfahan and Khomeynishahr faults. Ten earthquakes with magnitudes between 3 and 4 (ML) and three with magnitudes between 4 and 5 (ML) occurred southwest of Isfahan city. Moreover, nine of the earthquakes along or near the Khomeynishahr fault had magnitudes between 3 and 4 (ML). Many earthquake epicenters (Fig. 7) allow the traces of the newly mapped fault segments to be extrapolated through areas where they are obscured by agricultural and urban development. These earthquakes were up to 30 km deep, indicating that the Isfahan fault system is active within the basement.
7. Discussion and conclusion 1. For many years, the Kazerun fault system has been identified as a major fault in the basement of the Zagros Mountains. Reactivation of this fault system defined sedimentary basins from the Infracambrian to Tertiary times, and its seismicity indicates that it is still active. The different stratigraphies of the cover sequences on either side of the KFS also influenced the different structural styles on either side. An important question raised by this work is: how was it possible for this important and ancient fault system to be cut abruptly by the MZT? 2. It is my contention that the 220 km long Isfahan fault and the adjacent 160 km long Khomeynishahr fault both overlie an active basement fault that has both normal and strikeslip components of motion. 3. The concentration of more than 100 earthquakes near the Isfahan and the Khomeynishahr faults in the period from 1976 to 1978 and 2000 to 2003 indicate that these two faults are active. The common focal mechanism solutions of these earthquakes are consistent with the strikes of these faults southwest of Isfahan, and indicate a normal and strike-slip fault with an N-NW strike, although the 26° dip to NE (Mohajer Ashjaee, 1981) is unexpected (due to low dip angle of fault plane). 4. Earlier stratigraphic, petrologic, and structural studies (e.g., Ghasemi and Talbot, 2006) emphasized the fundamental differences between the northern and southern segments of the SSZ. However, none of these studies defined the precise boundary between these two segments, but rather considered it to be somewhere in the western parts of Isfahan city. In this study, the Isfahan fault system, with its higher elevations to the west and low elevations to the east, is proposed as the precise boundary between the northern and southern segments of the SSZ. 5. The KFS and the Isfahan fault systems are both approximately north-trending dextral strike-slip fault systems. The Isfahan fault system can therefore be considered as the continuation of the KFS across the SSZ. Because of dextral strikeslip of the Main Zagros thrust and other parallel faults these two faults are currently offset by 52 km. 6. Additional research is required in this area, particularly to establish whether the Isfahan fault continues into or through the Urumieh–Dokhtar zone.
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7. Although the KFS has been recognized for many years, this is the first suggestion that this fault system continues to the north as the Isfahan fault system. The clarification here of the Isfahan fault system as the boundary between the northern and southern segments of the SSZ is a step forward in understanding the relationships between this zone and the Zagros orogen. 8. The relationship between the Kazerun fault and the Isfahan fault can be explained with two possible scenarios. a. One scenario is that the Isfahan fault system post-dates the suturing of the Iranian and Arabian plates in the Zagros. The Isfahan fault probably formed in the Late Cretaceous–Paleocene soon after suturing as a result of renewed activity along the Kazerun fault zone. Because of dextral strike-slip of the Main Zagros thrust these two faults are currently offset by 52 km. This model assumes that the MZT represents the suture between the two plates. b. The second scenario considers that the Kazerun fault and Isfahan fault systems are N-S trending Panafrican basement faults. These faults were formally part of the same fault but have subsequently separated by 52 km of dextral strike-slip movement along the MZT. If this scenario is correct then the suture would not coincide with the MZT as both sides of this fault would have originally formed part of the same (i.e. Arabian) plate. This scenario agrees with models considering that the suturing of the Iranian and Arabian plates occurred along the Sanandaj– Sirjan and Urumieh–Dokhtar zones (e.g., Alavi, 1994). For confirmation of the mechanism, however, each scenario requires further work.
Acknowledgements I would like to express my appreciation to all those whose cooperation and support allowed my studies in the Isfahan area. Particular thanks are due to Prof. Christopher Talbot and Dr. Frédéric Mouthereau for critically reading and helping with the text and for good advice. I have benefited greatly from discussions with them. I thank Mansor Shishehfrosh of the Isfahan Head of Province Office for his valuable support. This study was financed by the Isfahan University Research Council (under Project Nos. 800311 and 7-3486). I appreciate their fundamental support and patience. References Alavi, M., 1994. Tectonics of the Zagros Orogenic belt of Iran, new data and interpretations. Tectonophysics 229, 211–238. Ambraseys, N.N., Melville, C.P., 1982. A History of Persian Earthquakes. Cambridge University Press, New York. 219pp. Authemayou, C., Bellier, O., Chardon, D., Malekzade, Z., Abbassi, M., 2005. Role of the Kazerun fault system in active deformation of the Zagros fold-and-thrust belt (Iran). C.R. Geoscience 337, 539–545. Bahroudi, A., Talbot, C.J., 2003. The configuration of the basement beneath the Zagros basin. J. Petrol. Geol. 26 (3), 257–282. Bahroudi, A., Koyi, H.A., 2004. Tectono-sedimentary framework of the Gachsaran Formation in the Zagros foreland basin. Mar. Petrol. Geol. 21, 1295–1310. Baker, C., Jackson, J., Priestley, K., 1993. Earthquakes on the Kazerun line in the zagros Mountains of Iran: strike-slip faulting within a fold and thrust belt. Geophys. J. Int. 115, 41–61. Bayer, R., Chery, J., Tatar, M., Vernant, Ph., Abbassi, M., Masson, F., Nilforoushan, F., Doerflinger, E., Regard, V., Bellier, O., 2006. Active deformation in Zagros– Makran transition zone inferred from GPS measurements. Geophys. J. Int. 165, 373. doi:10.1111/j.1365-246X.2006.02879.x. Berberian, M., 1981. Active faulting and tectonics of Iran. In: Gupta, H.K., Delany, F.M. (Eds.), Zagros-Hindu Kush-Himalaya Geodynamic Evolution, Geodynamics Series, vol. 3. American Geophysical Union, pp. 33–69. Berberian, M., 1995. Master blind thrust faults hidden under the Zagros folds: active basement tectonics and surface morphotectonics. Tectonophysics 241, 193– 224.
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