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An investigation into the fault patterns in the Chadegan region, west Iran: Evidence for dextral brittle transpressional tectonics in the Sanandaj–Sirjan Zone
Journal of Asian Earth Sciences 43 (2012) 77–88
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An investigation into the fault patterns in the Chadegan region, west Iran: Evidence for dextral brittle transpressional tectonics in the Sanandaj–Sirjan Zone Abbas Babaahmadi a,⇑, Mohammad Mohajjel a, Abbas Eftekhari a, Ali Reza Davoudian b a b
Department of Geology, Tarbiat Modares University, PO Box 14115-175, Tehran, Iran Department of Natural Resources, Shahrekord University, Shahrekord, Iran
a r t i c l e
i n f o
Article history: Received 5 September 2010 Received in revised form 17 July 2011 Accepted 10 August 2011 Available online 16 September 2011 Keywords: Transpressional tectonics Sanandaj–Sirjan Zone Riedel faults Dalan fault Ben fault
a b s t r a c t The NW–SE trending Sanandaj–Sirjan Zone (SSZ) is the internal part of the Zagros continental collision zone, which mainly consists of metamorphic rocks deformed in a dextral transpressional zone. This dextral transpression is attributed to brittle deformation related to late Cenozoic Arabia–Eurasia oblique continental collision. Major NW-trending faults, including the Dalan, Garmdareh, Yasechah, Sheida, and Ben faults, are reverse faults with a dextral strike-slip component. These faults were displaced by NW-trending synthetic and NE-trending antithetic faults. There are also E-trending thrusts and N-trending normal faults developing in directions that are, respectively, almost normal and parallel to the major shortening direction. The NW-trending Ben, Yasechah, and Sheida faults are NE-dipping faults, and the Dalan and Garmdareh faults are SW-dipping faults. These faults indicate the presence of a transpressive flower structure zone that probably led to the exhumation of Jurassic high-grade metamorphic rocks, such as eclogite, in the central part of the study area. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The Zagros continental collision zone is a part of Alpine orogenic system in west Iran. Timing of the convergence and the subsequent collision of the Arabian and Eurasian plates extends back to the Middle Jurassic (Berberian and King, 1981; Alavi, 1994; Mohajjel et al., 2003; Agard et al., 2005). Convergence was oblique, causing deformation partitioning among structures in the Sanandaj–Sirjan Zone (SSZ). Oblique convergence produced strike-slip ductile shear zones and multiple folding as well as thrust and strike-slip faults (Mohajjel and Fergusson, 2000). The Chadegan region, west of Esfahan, is located in the central part of the SSZ, which includes the hinterland crystalline part of the Zagros Orogen. (Fig. 1b). A map-scale anticline contains deformed metamorphic rocks in the study area. The core of this anticline, including high-grade metamorphic rocks, is strongly affected by a strike-slip ductile shear zone (Babaahmadi, 2008). Abundant later faults have displaced the metamorphic rocks. Recent studies on the structure and tectonics of the SSZ have concentrated on the ductile deformation that resulted from Mesozoic–Cenozoic Arabia–Eurasia convergence. Studies by Mohajjel and Fergusson (2000) in the northwest of the SSZ and by Sarkarinejad et al. (2008) in the southeast reveal dextral ductile transpressional tectonics during the Mesozoic-Tertiary convergence of the Arabian and Eurasian plates. Brittle deformation in the SSZ is mainly
⇑ Corresponding author. Tel.: +98 2182883434; fax: +98 2188630482. E-mail address: [email protected] (A. Babaahmadi). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.08.012
attributed to a number of SW-verging thrust faults, truncating folded and foliated metamorphic rocks (Mohajjel and Fergusson, 2000). Studies on these faults in the SSZ are insufficient. Therefore, in this study, we investigated brittle deformation in the SSZ. We attempted to identify fault systems, their generation and the displacement mechanisms affecting the metamorphic rocks of the SSZ in the Chadegan region. We utilized Landsat ETM+, IRS 1-C PAN, and IRS P5 satellite images as well as field investigations to map and interpret fault systems in the study area. 2. Geological and tectonic setting The Zagros continental collision zone in west Iran consists of three main tectonic zones. From northeast to southwest, these zones are as follows: (1) the Urumieh–Dokhtar Magmatic Arc (UDMA), (2) the Sanandaj–Sirjan Zone (SSZ), and (3) the Zagros Fold and Thrust Belt (ZFB) (Alavi, 1994). The most elevated part of the ZFB is named the High Zagros (HZ, Fig. 1). The UDMA is an Andean-type magmatic arc composed mainly of Eocene tholeiitic, calc-alkaline, and K-rich alkaline intrusive and extrusive rocks along the active margin of the Iranian plate (Alavi, 1994; Shahabpour, 2007). The HZ has a width of 10–65 km and includes the highest mountains in the Zagros range. The boundaries of the HZ are the Main Zagros Reverse Fault (MZRF) to the NE and the High Zagros Fault (HZF) to the SW (Berberian, 1995). The ZFB has a width of 150–250 km and consists of ca. 13–14 km of folded shelf deposits of late Paleozoic to early Tertiary age (James and Wynd, 1965; Stocklin, 1968). The SSZ has a NW–SE trend that is 1500 km long and 150–250 km wide, and it is located
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Fig. 1. (a) 90 m SRTM digital elevation model of Iran showing the major faults and zones of western Iran. MRF, Main Recent Fault; MZRF, Main Zagros Reverse Fault; HZF, High Zagros Fault; KFS, Kazerun Fault System; BFS, Ben Fault System; DFS; Dalan Fault System; DF, Dehshir Fault; QZF, Qom-Zefreh Fault; KNF, Kushk-e-Nosrat Fault; IF, Indes Fault; MF, Mendak Fault; AF, Avaj Fault; UDMA, Urumieh-Dokhtar Magmatic Arc; SSZ, Sanandaj–Sirjan Zone; HZ, High Zagros; ZFB, Zagros Fold Belt. The white arrow indicates a velocity vector for the Eurasian plate (Vernant et al., 2004). (b) Simplified tectonic map of southwestern Iran. The Sanandaj–Sirjan Zone is divided into five subzones (radiolarite, Bisotun, ophiolite, marginal, and complexly deformed). The black rectangle indicates the location of the study area west of Esfahan (after Mohajjel et al., 2003). (c) The earthquake distribution in western Iran from 1964 to 1998 (catalogue of Engdahl et al., 1998) showing the low seismicity of the SSZ (see caption of figure (a) for name abbreviations).
to the northeast of the ZFB. Mohajjel et al. (2003) subdivided the SSZ into five sub-zones: (1) the radiolarite sub-zone that consists of Jurassic–Cretaceous shallow marine limestone and deep-marine radiolarite; (2) the Bisotun sub-zone including Late Triassic–Late Cretaceous limestone; (3) the Late Cretaceous ophiolite sub-zone; (4) the marginal sub-zone, including Late Jurassic–Early Cretaceous volcanic rocks and shallow marine strata; and (5) the complexly deformed sub-zone including deformed metamorphic rocks (Fig. 1b). The study area is located in the complexly deformed sub-zone and has three metamorphic units: the high-grade zone, the low-grade zone and the very low-grade zone (Davoudian et al., 2006). Rocks of the high-grade zone include amphibolite, eclogite, and gneiss, while the rocks of the low-grade zone are schist, phyllite, marble, and slate. The very low-grade zone comprises metamorphosed sandstone, limestone, shale and volcanic rocks (Fig. 2). Cretaceous limestone and Eocene conglomerate exist at the northeastern and southwestern boundaries of the study area. Patches of Permian limestone occur in the area (Fig. 2).
Investigations of brittle deformation have mainly been conducted in tectonic units of western Iran, including the UDMA and the Zagros belt. Several researchers have recently studied brittle tectonics in the UDMA (e.g., Safaei et al., 2008; Meyer and Le Dortz, 2007; Morley et al., 2009; Babaahmadi et al., 2010b). There are several right-lateral reverse faults to the west of Central Iran, generating young dextral transpressional tectonics in the UDMA (Meyer and Le Dortz, 2007; Safaei et al., 2008; Babaahmadi et al., 2010b; Fig. 1a). Several research projects on brittle deformation in the Zagros belt indicate the presence of an active dextral transpression in the Zagros (e.g., Authemayou et al. 2005, 2006; Talebian and Jackson, 2002, 2004; Navabpour et al., 2007; Hessami et al., 2001; Axen et al., 2010). Transpression in the Zagros collision event has been produced by oblique collision of the Arabian platform and SW Iran. The sutured area in the HZ has been affected by a strong shear component parallel to the orogen (Mohajjel and Behyari, 2010). Right-lateral transpressional flower structures are present along
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Fig. 2. Geological map of the study area (after Davoudian et al., 2007) showing the location of areas depicted in later figures.
the Zagros suture in the Nahavand area in NW Iran (Mohajjel and Behyari, 2010). Oblique convergence in the Zagros collision is confirmed by active tectonic measurements (Vernant et al., 2004). GPS data indicate that the Arabian and Eurasian plates converge at 21 mm/yr around 50°E. The shortening of the Zagros belt is NNE-trending with a rate of 7 ± 2 mm/yr, implying that the convergence is oblique (Vernant et al., 2004). The MZRF is a major fault separating the Zagros Belt from the SSZ (Stocklin, 1968; Berberian, 1995). At present, the MZRF is an important seismotectonic feature specifying an abrupt rupture between the intense seismicity of the Zagros and the almost aseismic central Iranian plateau (Talebian and Jackson, 2004 Fig. 1c). In the NW Zagros, a major NW–SE right-lateral strike-slip fault system roughly follows the Zagros Belt for almost 800 km and is known as the Main Recent Fault (MRF) (Talebian and Jackson, 2002). The MRF is one of the most important structural features of the active tectonics of the Zagros and has been responsible for several large earthquakes (Berberian, 1995). The total offset of the MRF is probably approximately 50 km, and it may be moving as quickly as 10–17 mm/yr (Talebian and Jackson, 2002). A series of NNW-trending faults, such as the Kazerun fault, cut the Zagros belt transversely toward the SW termination of the MRF and dextrally displace the NW-trending folds and thrusts (Hessami et al., 2001; Authemayou et al., 2006).
3. IRS-P5 images with a spatial resolution of 2.5 m. Satellite images with diverse spatial resolutions are useful for identifying fault traces and structural patterns at different scales (Babaahmadi et al., 2010a). Satellite images were rectified using 1:50,000 and 1:25,000 topographic data. In the rectification procedure, a UTM projection was adopted for zone N39 and a WGS84 datum. For better interpretation of structures and rock units, a 7-4-2 band combination was selected. Image processing methods used in this study are Principle Component Analysis (PCA), fusion of multispectral Landsat ETM+ images and high resolution IRS-1C PAN images, edge sharpening and gradient filters, and contrast reverse function. Recognition of fault lineaments was from mapping: (1) Offset and dragging of rock units, geological structures and geomorphic features. (2) Lineaments with pronounced traces, especially in Quaternary alluviums. (3) Lensoid and en-echelon structures indicating the presence of hidden faults under alluviums. In the study area, five fault systems were recognized from differences in strike patterns (Fig. 3):
3. Methods We used remote sensing and field observations to map fault patterns in the study area. A variety of satellite images were used to accomplish structural analysis: 1. Landsat ETM+ images with a spatial resolution of 28.5 m. 2. IRS-1C PAN images with a spatial resolution of 5.8 m.
1. NW-trending major faults with an average strike of AZ 124. 2. NW-trending synthetic faults with an average strike of AZ 155. 3. NE-trending antithetic faults with an average strike of AZ 058. 4. N-trending faults with an average strike of AZ 185. 5. E-trending faults with an average strike of AZ 095.
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Fig. 3. (a) Processed satellite image of the study area. (b) Map of fault lineaments recognized from satellite images. (c) Rose diagram of major fault lineaments. (d–g) Rose diagrams of E-trending, N-trending, NW-trending synthetic and NE-trending antithetic fault lineaments, respectively.
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Fig. 4. (a) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) of the Dalan fault trace, showing the locations of later figures (see Fig. 2 for location). (b) 3D view of the IRS-P5 image (spatial resolution 2.5 m) superimposed on DEMs showing the Dalan fault between Cretaceous limestone and Eocene conglomerate. Cretaceous units have a rougher morphology than Eocene units. (c) 3D view of the IRS-P5 image (spatial resolution 2.5 m) superimposed on DEMs showing the effect of an antithetic fault on the Dalan faults. The crush zone of this fault is obvious in the Eocene conglomerate.
Field trips to measure the brittle crush zones of faults verified the interpretation of the satellite images. Brittle S–C fabrics are prevalent kinematic indicators that are widely developed in these fault zones. C Surfaces are discrete shear surfaces that develop sub-parallel to the fault plane, and S surfaces are foliations transected by C surfaces (e.g., Lin, 2001; Passchier and Trouw, 1996). We measured these S–C fabrics to analyze the kinematics of major faults. 4. Major faults Major NW-trending faults were identified by their significant length and wide crush zones. The major faults in the study area are (from northeast to southwest) the Dalan, Garmdareh, Yasechah, Sheida and Ben faults (Fig. 3). 4.1. The Dalan fault system The Dalan fault system consists of three parallel, dextral reverse faults (DF1, DF2 and DF3) displacing Jurassic shale and limestone, Cretaceous limestone and Eocene conglomerate (Figs. 2 and 4a). These faults have an orientation of 45–60° toward the SW. Dextral shear sense is indicated by S–C fabrics that have developed in crush zones of the Dalan faults (Fig. 5b and c). These faults form a linear and elevated trace separating the Mastan Mountains from the Karvan plain (Fig. 4a), and they are cut by several synthetic and antithetic faults (Figs. 3, 4c, 13a and b). The DF1 fault has a steep
dip toward the SW, placing Jurassic shale over Cretaceous limestone (Figs. 2 and 3). The DF2 fault is a right-lateral reverse fault, dipping 45–60° toward the SW, placing Cretaceous limestone over Eocene conglomerate (Figs. 3, 5a and e). On the IRS-P5 satellite image (spatial resolution 2.5 m), the DF2 fault is a marked linear trend between Cretaceous and Eocene units (Fig. 4b). On the satellite image, Cretaceous units have a rougher morphology than do Eocene units. The DF3 fault is a right-lateral reverse mountain front fault and displaces Eocene conglomerate (Fig. 5d). Both geomorphic and geological evidence imply that the DF3 fault has been active in the Quaternary because young alluvial fans have been tilted by fault activity (Fig. 4a). Because of the tilting of alluvial fans, channel incision has occurred on streams crossing the DF3 fault, causing the younger alluvial fans to form downslope, away from their former locus (Fig. 4a). The existence of some springs along the DF3 fault shows that the DF3 fault has displaced underground water aquifers along the Dalan mountains. The DF3 fault crosses the Golab water-conveyance tunnel path, causing water with a 30 l/s volumetric flow rate to enter the tunnel during its excavation (Eftekhari and Taheri, 2005). 4.2. The Garmdareh fault system The dextral reverse Garmdareh faults pass along the Zayandehrud river, displacing metamorphic rocks including schist, amphibolite and gneiss units (Figs. 2, 3 and 6a). The strikes of these faults
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Fig. 5. (a) The Dalan fault between Cretaceous limestone and Eocene conglomerate (see Fig. 4a for location). (b) Development of S–C fabrics in the crush zone of the Dalan fault (see Fig. 5a for location). (c) S–C fabrics plotted on a lower hemisphere equal-area stereonet showing the dip-slip movement of the Dalan faults approximately toward the N. (d) The DF3 fault displacing Eocene conglomerate (see Fig. 4a for location). (e) The Dalan fault placing Cretaceous limestone over Eocene conglomerate (see Fig. 4a for location). Note that Cretaceous units are gray colored and that Eocene units are red colored. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
are AZ 124, and their dips are 45–55° toward the SW. A stereonet analysis of kinematic indicators shows that the dip-slip movement is toward the NNE (Fig. 7a–c). The Garmdareh fault system comprises two en-echelon faults: the GF1 and GF2 faults (Fig. 6a). The GF1 fault is a dextral reverse fault with 30 m of crush zone, placing amphibolite over gneiss (Fig. 7a). The amphibolite has a darker color than the gneiss on the satellite image (Fig. 6a). On the IRS P5 image (spatial resolution 2.5 m), some acidic intrusion bands are apparent and displaced along the trace of the fault (Fig. 6b). A river has been deflected dextrally along the GF1 by 1.2 km, indicating the Quaternary nature of this fault (Fig. 6c). The GF2 fault is a dextral reverse fault displacing low-grade and high-grade metamorphic rocks (Fig. 6a). 4.3. The Yasechah fault system The Yasechah fault system comprises two parallel faults with a strike of AZ 118 and displace low-grade and very low-grade metamorphic rocks (Figs. 2 and 3). They are high-angle reverse faults dipping toward the NE (Fig. 8a–e). Stereonet analysis of kinematic indicators shows that the dip-slip movement is toward the SE, indicating a dextral oblique slip of the Yasechah faults (Fig. 8e). The Yasechah faults have been intersected by a number of syn-
thetic and antithetic faults (Fig. 3). The 50 m crush zone of the northern Yasechah fault is observed in both low-grade and very low-grade metamorphic units, while the southern Yasechah fault, with a crush zone of 80 m, has displaced very low-grade metamorphic units, including metamorphosed sandstone and limestone (Figs. 2 and 8).
4.4. The Sheida fault The Sheida fault is a dextral reverse fault with a strike of AZ 118, placing Permian limestone over Jurassic shale and sandstone (Figs. 2 and 3). Fig. 9a shows a 3D-perspective satellite image of the Sheida mountain where the Sheida fault passes between darker-colored Permian units and lighter-colored Jurassic units. Based on field observations, the Sheida fault has an orientation of 55°/ 033° and a dextral strike-slip component (Fig. 9b–e). Satellite images show that the Sheida fault is cut by some synthetic and antithetic minor faults (Fig. 3). Field observations indicate that a number of calcite veins have been dextrally displaced in the crush zone of the Sheida fault (Fig. 9d). Kinematic indicators such as S–C fabrics show that the dip-slip movement of the Sheida fault is approximately toward the S (Fig. 9e).
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Fig. 7. (a and b) Photographs showing S–C fabrics in the Garmdareh fault zone between amphibolite and gneiss (see Fig. 6a for location). S–C fabrics are specified on figure (b). (c) Lower hemisphere equal area stereographic projection of S–C fabrics showing the dip-slip movement of the Garmdareh fault toward the NNE.
Fig. 6. (a) Landsat ETM+ image (spatial resolution 15 m) of the Garmdareh faults, including the GF1 and GF2 faults. (b) IRS-P5 image (spatial resolution 2.5 m) showing a granitic band displaced along the trace of the Garmdareh fault zone (see figure (a) for location). (c) IRS-P5 image (spatial resolution 2.5 m) showing dextral river deflection by 1.2 km and elevated dry valleys along the Garmdareh fault (see figure (a) for location).
4.5. The Ben fault system The Ben fault system, including a series of parallel faults, has displaced Cretaceous limestone and Eocene conglomerate units south of Ben city (Figs. 2, 3, 10a and 11a). The NE-dipping Ben faults have propagated toward the SW, creating a duplex thrust system (Nemati and Yassaghi, 2010). Brittle crush zones of the Ben faults have developed in Cretaceous limestone, resulting in the development of S–C fabrics (Figs. 10c and 11b). The fault slip analysis of S–C fabrics shows that the dip-slip movement is toward the SSE, indicating dextral oblique slip (Fig. 11d). Some synthetic faults have displaced the Ben faults (Fig. 10a and b). According to Nemati and Yassaghi (2010), the Ben faults involved the basement, demonstrating their deep-seated nature.
5. Minor faults 5.1. NW-trending synthetic faults The NW-trending synthetic faults, with an average strike of AZ 155, have dextral offsets as shown by dragged structures and rock units (Figs. 3 and 12). In addition to the prevalent dextral strikeslip component, these faults have a reverse dip-slip component toward either the NE or SW (Fig. 12d). A number of synthetic faults are truncated by other synthetic faults. Fig. 12e shows a younger synthetic fault with an orientation of 45°/060° that cuts another
Fig. 8. (a and b) Crush zone of the northern Yasechah fault showing development of S– C fabrics (see Fig. 2 for location). S–C fabrics are observed in figure (b). (c and d) Field pictures showing S–C fabrics in the crush zone of the southern Yasechah fault in very low-grade metamorphic rocks (see Fig. 2 for location). S–C fabrics are observed in figure (d). (e) Lower hemisphere equal area stereographic projection of S–C fabrics approximately indicating the dip-slip movement of the Yasechah faults toward the SE.
fault with an orientation of 60°/230°. From satellite images, it is clear that some synthetic faults have displaced features in en-echelon patterns (Fig. 12a–c). Some synthetic faults cut the major faults and have displaced them significantly (Figs. 10b and 12a and b).
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Fig. 9. (a) 3D view of Landsat ETM+ superimposed on DEMs showing the Sheida fault between Permian and Jurassic units (see Fig. 2 for location). (b) A field picture of the Sheida fault between Permian and Jurassic units. (c) Crush zone of the Sheida fault showing the development of S–C structures. (d) Calcite veins have been dextrally displaced in the crush zone of the Sheida fault. (e) Lower hemisphere equal area stereographic projection of S–C fabrics indicating the dip-slip movement of the Sheida fault toward the S.
Fig. 10. (a) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) of the trace of the Ben faults showing latter figures. Small white arrows show the trace of synthetic faults displacing the Ben fault. (b) A synthetic NW-trending fault offsetting the trace of the Ben faults. (c-d) S–C structures developed in a crush zone of the Ben faults (see figure (b) for location).
5.2. NE-trending antithetic faults The NE-trending antithetic faults with an average strike of AZ 058 were identified from the sinistral displacement of geological structures and rock units (Figs. 2, 3 and 13). They have a chiefly reverse component to the NW or SE (Figs. 4 and 13d–e). A small number of NE-trending normal faults were also observed. The antithetic faults have mainly extended in en-echelon patterns, displacing and dragging geomorphic features and major fault traces (Fig. 13a–c). Some of the best examples of antithetic faults that offset major faults are the Abpuneh faults. The Abpuneh faults, including some parallel NE-trending faults, sinistrally offset the Dalan fault traces by more than 2 km (Figs. 13a and b). 5.3. N-trending and E-trending faults The N-trending faults have a normal dip-slip component, chiefly deforming metamorphic rock units (Figs. 3 and 14b). The E-trending faults were difficult to identify from satellite images because they are thrust faults with a low dip angle (Fig. 2). The E-trending faults were mapped in the field and have an average orientation of 38°/185°. Duplex structures occur in the crush zone of the E-trending faults, mainly in low- and high-grade metamorphic rocks (Fig. 14a). E-trending deformation is mostly observed as folding in ductile rocks such as Jurassic shale (Fig. 14c). 6. Discussion Riedel faults are usually associated with strike-slip shear zones (Naylor et al., 1986). In a simple strike-slip shear zone, Riedel faults
Fig. 11. (a) A field picture of the Ben fault displacing Cretaceous limestone (see Fig. 10a for location). (b and c) Brittle crush zone in another part of the Ben faults; displacing Cretaceous limestone and development of S–C surfaces in this crush zone (see Fig. 10a for location). (d) Lower hemisphere equal area stereographic projection of S–C structures indicating the dip-slip movement of the Ben faults toward the SSW.
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Fig. 12. (a and b) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) showing dextral dragging of amphibolite rocks, gneiss rocks, and the Garmadareh fault by synthetic faults. Synthetic faults are specified with a green color in figure (b) (see Fig. 2 for location). (c) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) showing a sigmoid shape formed by dextral en-echelon synthetic faults in the metamorphic rocks (see Fig. 2 for location). (d) Lower hemisphere equal area stereographic projection of the synthetic faults showing dips toward both the NE and SW. (e) A field photograph of two synthetic faults. A synthetic NE-dipping fault has displaced another synthetic fault dipping in the opposite direction (see Fig. 2 for location). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
are categorized as synthetic and antithetic faults. From experimental and theoretical points of view, synthetic faults (R) have a low angle of 17° to the shear direction, whereas antithetic faults (R0 ) have a high angle of 70–90° (Naylor et al., 1986; Wilcox et al., 1973). Transpression involves strike-slip shear accompanying horizontal shortening and vertical lengthening in the shear plane (Sanderson and Marchini, 1984; Woodcock and Schubert, 1994; Fossen et al., 1994). In transpression, the Riedel faults are formed at a higher angle to the shear direction. In theoretical models, the average angle between synthetic faults and the shear direction is 37° and has a maximum of 60° (Naylor et al., 1986). The Riedel faults form at a lower angle with respect to the shear direction during increasing shear zone displacement (Naylor et al., 1986). In transpression, folds and thrusts are formed at a low angle to the shear direction, whereas normal faults are formed at higher angle to the shear direction (Woodcock and Schubert, 1994; Sanderson and Marchini, 1984). In the study area, the major faults are dextral reverse faults. The average strike of the major faults is AZ 124, which is the mean trend of the dextral transpressional zone in the area. The Dalan, Garmdareh, Yasechah, Sheida, and Ben faults are the major faults with right-lateral strike-slip movements. The Ben and Dalan faults are the largest fault systems in the study area. According to Nemati and Yassaghi (2010), the Ben faults are basement faults, displacing the basement top by 3–4 km. The Dalan faults have significantly displaced Jurassic, Cretaceous, and Eocene units. The dextral movement along the Ben and Dalan faults has created a dextral transpressional zone in the study area. The NW-trending synthetic faults (R faults) have a
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maximum angle of 49°, a minimum angle of 15°, and an average angle of 30° to the mean trend of the dextral shear zone. The antithetic NE-trending faults (R0 faults) have a maximum angle of 130°, a minimum angle of 87°, and an average angle of 113° to the mean trend of the dextral zone. The N-trending normal faults have extended at the higher angle with respect to the shear direction and are almost parallel to the regional shortening direction. The E-trending thrusts and folds have also developed at a lower angle to the shear direction and approximately normal to the regional shortening direction. Fig. 15 illustrates the creation of the NW- and NE-trending Riedel faults, N-trending normal faults and E-trending thrusts in a dextral brittle transpressional zone in the study area. The study area has characteristics of a positive flower structure. The Ben, Yasechah and Sheida faults are NE-dipping faults, and the Dalan and Garmdareh faults are SW-dipping faults, giving rise to a positive flower structure zone. Exhumation of Jurassic high-grade rocks such as eclogite, amphibolite, and gneiss in the central part of the study area can be related to the activity of this zone. Because the Dalan and Ben faults have deformed Eocene deposits, they seem to have become activated after the Eocene. Gavillot et al. (2010) used (U–Th)/He thermo-chronometry to achieve ages of thrust activity in the HZ. Their results indicate that thrust activity in the HZ and continental suturing along the Zagros suture initiated no later than 23 Ma in the late Oligocene or early Miocene (Gavillot et al., 2010). Because most faults in the study area have a young nature, the timing of faulting is interpreted to be related to this age. Determining the dextral slip rate along most faults is difficult because reliable geomorphic and stratigraphic markers do not exist on both sides of the faults. However, a dextral 1.2 km river deflection along the Garmdareh fault indicates activity of the faults during the Quaternary (Fig. 6c). Our interpretation of the fault patterns in the SSZ is in agreement with the results of studies in the UDMA and the HZ. Detailed studies of the fault patterns of the Qom-Zefreh, Indes, and Kushk-e-Nosrat faults revealed a dextral transpressional zone in the UDMA located west of Central Iran (Nogol-Sadat, 1985; Safaei et al., 2008; Babaahmadi et al., 2010b), Fig. 1a. For example, there are five classes of faults along the Qom-Zefreh fault with trends of AZ 120-125, AZ 140-155, AZ 170-185, AZ 90, and AZ 50, implying a dextral transpressional system (Safaei et al., 2008). The dextral reverse faults in the UDMA have developed in en-echelon patterns, producing Quaternary restraining step-over zones (Babaahmadi et al., 2010b). Deformation partitioning is also observed among the structures in the UDMA (Babaahmadi et al., 2010b). Transpressional patterns in the HZ have mainly been studied along the MZRF, MRF, and HZF (Talebian and Jackson, 2002, 2004; Agard et al., 2005; Authemayou et al., 2006; Axen et al., 2010, Fig. 1a). Dextral movement along major thrusts has generated significant dextral strain in the HZ since the onset of the orogeny in late Oligocene or early Miocene (Axen et al., 2010; Gavillot et al., 2010). A series of minor faults with strikes of NW to NE has been generated between the MZRF and HZF due to a dextral transpressional zone in the HZ (Authemayou et al., 2006). The presence of dextral thrusts with opposite dipping between the MZRF and HZF and exhumation of Paleozoic rocks along the HZF reveal a dextral transpressive flower structure zone in the HZ (Authemayou et al., 2006). Deformation partitioning has occurred at a dominant transpressional zone in the HZ in the late Pliocene, giving rise to dextral strike-slip movement along the MRF (Talebian and Jackson, 2002; Agard et al., 2005; Authemayou et al., 2006). In contrast to the UDMA and the HZ, deformation partitioning has not developed along the structures in the study area. The dextral brittle transpression observed in the SSZ, the HZ, and
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Fig. 13. (a and b) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) of the sinistral Abpuneh faults displacing the trace of the Dalan faults sinistrally by more than 2 km (see Fig. 4a for location). (c) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) showing dragging of the Yasechah fault traces by sinistral en-echelon faults. Dark arrows show the trace of the Yasechah faults (see Fig. 2 for location). (d) Lower hemisphere equal area stereographic projection of the antithetic faults showing dips toward both the NW and SE. (e and f) Field pictures of a NW-dipping antithetic fault with a reverse component (see figure (c) for location).
the UDMA in western Iran is interpreted to be related to the oblique continental collision of the Arabian and Eurasian plates. Although geomorphic and structural evidence shows that the fault systems are young and active during the Quaternary, the study area in the SSZ exhibits low seismic activity, and no large earthquakes have been detected historically or instrumentally (Fig. 1c). The most destructive earthquakes have occurred at the junction zone of the SSZ and the HZ along the MRF (e.g., Silakhor earthquake in 1909 and Nahavand earthquake in 1958, Berberian, 1976). A sudden drop is observed in the concentration of instrumentally recorded earthquakes from the MRF toward the SSZ (Fig. 1b, Engdahl et al., 1998). Our knowledge of historical events in the SSZ, as in other parts of the Iranian plateau, is poor (Ambraseys and Melville, 1982). However, the limited knowledge about historical earthquakes in the SSZ must not be considered to be due to a lack of large earthquakes because the reoccurrence
periods of large earthquakes can exceed intervals of historical and instrumental records.
7. Conclusions Satellite images and field observations were used to investigate fault patterns in the Chadegan region in the SSZ. The Dalan and Ben faults are the largest major faults in the area, and the dextral movement along them has created a dextral transpressional zone in the study area. As a result of the activity of the dextral transpressional zone, minor faults, including NE-trending antithetic faults, NW-trending synthetic faults, E-trending thrusts, and N-trending normal faults, have extended into the area. The brittle transpression observed in the SSZ is thought to be related to late Cenozoic ArabiaEurasia oblique continental collision.
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Fig. 14. (a) Development of large duplex structures in an E-trending fault crush zone. Lower hemisphere equal area stereographic projection of this fault shows its low angle dip toward the S (see Fig. 2 for location). (b) A N-trending normal fault displacing Jurassic gneiss and schist units. Lower hemisphere equal area stereographic projection of this fault shows dipping toward the E (see Fig. 2 for location). (c) An E-trending recumbent fold in the Jurassic shale. Lower hemisphere equal area stereographic projection of this fault shows dipping toward the NNE (see Fig. 2 for location).
Fig. 15. A schematic model indicating the structures generated in the dextral brittle transpression in the SSZ.
Acknowledgments We would like to express our appreciation to C. Fergusson and B. Natalin for their helpful reviews and valuable comments. We wish to thank N. H. Woodcock for sending required papers to the authors. The authors are also thankful to S. Madanipour, S. Sadeghi and H. Vafa for their beneficial help. National Geographical Organization and Iranian Space Agency are gratefully acknowledged for providing IRS satellite images. We wish to thank Global Land Cover Facility (GLCF; http://www.glcf.umd.edu) for Landsat ETM+ data. References Agard, P., Omrani, J., Jolivet, L., Mouthereau, F., 2005. Convergence history across Zagros (Iran): constraints from collision and earlier deformation. International Journal of Earth Sciences 94, 401–419. 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, Cambridge, UK. Authemayou, C., Bellier, O., Chardon, D., Malekzade, Z., Abassi, 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. Authemayou, C., Chardon, D., Bellier, O., Malekzadeh, Z., Shabanian, E., Abassi, M., 2006. Late Cenozoic partitioning of oblique plate convergence in the Zagros fold-and-thrust belt (Iran). Tectonics 25, 1–21. Axen, G.J., Fakhari, M.D., Guest, B., Gavillot, Y., Stockli, D.F., Horton, B.K., Paper No. 39-5. Distributed oblique dextral transpression in the High Zagros Mountains, Iran, Tectonic Crossroads: Evolving Orogens of Eurasia–Africa–Arabia, Middle East Technical University; Ankara, Turkey from 3 to 8 October 2010. Babaahmadi, A., 2008. The structural geology of metamorphic rocks of in the south of Chadegan in the Sanandaj – Sirjan zone, Iran. M.Sc. Thesis, Tarbiat Modares University. Babaahmadi, A., Safaei, H., Yassaghi, A., Vafa, H., Naeimi, A., Madanipour, S., Ahmadi, M., 2010a. A study of quaternary structures in the Qom region, West Central Iran. Journal of Geodynamics 50, 355–367. Babaahmadi, A., Yassaghi, A., Naeimi, A., Dini, G.H.R., Taghipour, S., 2010b. Mapping quaternary faults in the west of Kavir plain, North Central Iran from satellite imageries. International Journal of Remote Sensing 31 (19), 5111– 5125. Berberian, M., 1976. Documented Earthquake faults in Iran, Rep. 39, Geological Survey of Iran. Berberian, M., 1995. Master blind thrust faults hidden under the Zagros folds: active basement tectonics and surface morphotectonics. Tectonophysics 241, 193– 224. Berberian, M., King, G.C.P., 1981. Towards a paleogeography and tectonic evolution of Iran. Canadian Journal of Earth Sciences 18, 210–265. Davoudian, A.R., Khalili, M., Noorbehsht, I., Dachs, E., Genser, J., Shabanian, N., 2006. Geochemistry of metabasites in the north of the Shahrekord, Sanandaj–Sirjan Zone, Iran. Jb Mineral Abh 182, 291–298. Davoudian, A.R., Genser, J., Dachs, E., Shabanian, N., 2007. Petrology of eclogites from north of Shahrekord, Sanandaj–Sirjan Zone, Iran. Mineralogy and Petrology. doi:10.1007/s00710-007-0204-6. Eftekhari, A., Taheri, D., 2005. Analysis of seismicity along the Golab waterconveyance tunnel in the Chadegan region, western Iran. In: 9th conference of Geological Society of Iran, Tarbiat Modares University, Tehran, Iran. Engdahl, E.R., van der Hilst, R., Buland, R., 1998. Global tele-seismic earthquake relocation with improved travel times and procedures for depth determination. Bulletin of the Seismological Society of America 88, 722–743. Fossen, H., Tikoff, B., Teyssier, C., 1994. Strain modeling of transpressional and transtensional deformation. Norsk Geologisk Tidsskrift 74, 134–145. Gavillot, Y., Axen, G.J., Stockli, D.F., Horton, B.K., Fakhari, M.D., 2010. Timing of thrust activity in the high Zagros fold-thrust belt, Iran, from (U–Th)/He thermochronometry. Tectonics 29, TC4025, doi: 10.1029/2009TC002484. Hessami, K., Koyi, H.A., Talbot, C.J., 2001. The significance of strike-slip faulting in the basement of the Zagros fold and thrust belt. Journal of Petroleum Geology 24, 5–28.
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James, G.A., Wynd, J.G., 1965. Stratigraphic nomenclatures of Iranian Oil Consortium agreement area. American Association of Petroleum Geologists Bulletin 49, 2182–2245. Lin, A., 2001. S–C fabrics developed in cataclastic rocks from the Nojima fault zone, Japan and their implications for tectonic history. Journal of Structural Geology 23, 1167–1178. Meyer, B., and Le Dortz, K., 2007. Strike-slip kinematics in Central and Eastern Iran: Estimating fault slip-rates averaged over the Holocene, Tectonics 26, TC5009, doi: 10.1029/2006TC002073. Mohajjel, M., Behyari, M., 2010. Young transpressive positive flower structure long the Zagros collision zone, Nahavand area, west Iran. In: Proceedings of a conference held at the University of New England, Armidale, New South Wales, Australia, pp. 242–251. Mohajjel, M., Fergusson, C.L., 2000. Dextral transpression in late Cretaceous continental collision, Sanandaj–Sirjan Zone, western Iran. Journal of Structural Geology 22, 1125–1139. Mohajjel, M., Fergusson, C.L., Sahandi, M.R., 2003. Cretaceous-tertiary convergence and continental collision, Sanandaj–Sirjan Zone, western Iran. Journal of Asian Earth Science 21, 397–412. Morley, C.K., Kongwung, P., Julapour, A., Abdolghafourian, M., Hajian, M., Waples, D., Warren, J., Otterdoom, H., Srisuriyon, K., Kazemi, H., 2009. Structural development of a major late Cenozoic basin and transpressional belt in central Iran: the Central Basin in the Qom-Saveh area. Geosphere 5, 1–38. Navabpour, P., Angelier, J., Barrier, E., 2007. Cenozoic post-collisional brittle tectonic history and stress reorientation in the High Zagros Belt (Iran, Fars Province). Tectonophysics 432, 101–131. Naylor, M.A., Mandle, G., Sijpesteijn, C.H.K., 1986. Fault geometries in basementinduced wrench faulting under different initial stress states. Journal of Structural Geology 8, 737–752. Nemati, M., Yassaghi, A., 2010. Structural characteristics of the transitional zone from internal to external parts of the Zagros orogen, Iran. Journal of Asian Earth Sciences 39, 161–172.
Nogol-Sadat, M.A.A., 1985. Transpressional structures in Iran; results of structural analysis of Qom region. Geological Survey of Iran. Report 55. Passchier, C.W., Trouw, R.A.J., 1996. Microtectonics. Springer_Verlog, p. 253. Safaei, H., Taheri, A., Vaziri-Moghadam, H., 2008. Structural analysis and evolution of the Kashan fault (Qom-Zefreh Fault), Centarl Iran. Journal of Applied Science 8, 1426–1434. Sanderson, D.J., Marchini, W.R.D., 1984. Transpression. Journal of Structural Geology 6, 449–454. Sarkarinejad, K., Faghih, A., Grasemann, B., 2008. Transpressional deformations within the Sanandaj–Sirjan metamorphic belt (Zagros Mountains, Iran). Journal of Structural Geology 30, 818–826. Shahabpour, J., 2007. Island-arc affinity of the central Iranian volcanic belt. Journal of Asian Earth Sciences 30, 652–665. Stocklin, J., 1968. Structural history and tectonics of Iran: a review. American Association of Petroleum Geologists Bulletin 52, 1229–1258. Talebian, M., Jackson, J.A., 2002. Offset on the main recent fault of NW Iran and implication for the late Cenozoic tectonics of the Arabia–Eurasia collision zone. Geophysical Journal International 150, 422–439. Talebian, M., Jackson, J.A., 2004. A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran. Geophysical Journal International 156, 506–526. Vernant, P., Nilforoushan, F., Hatzfeld, D., Abbassi, M.R., Vigny, C., Masson, F., Nankali, H., Martinod, J., Ashtiani, A., Bayer, R., Tavakoli, F., Chery, J., 2004. Contemporary crustal deformation and plate kinematics in Middle East constrained by GPS measurements in Iran and northern Oman. Geophysical Journal International 157, 381–398. Wilcox, E., Harding, T.P., Seely, D.R., 1973. Basic wrench tectonics. American Association of Petroleum Geologists Bulletin 57, 74–96. Woodcock, N.H., Schubert, C., 1994. Continental Strike-Slip Tectonics. In: Hancock, P.L. (Ed.), Continental Deformation. Pergamon Press.
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Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
An investigation into the fault patterns in the Chadegan region, west Iran: Evidence for dextral brittle transpressional tectonics in the Sanandaj–Sirjan Zone Abbas Babaahmadi a,⇑, Mohammad Mohajjel a, Abbas Eftekhari a, Ali Reza Davoudian b a b
Department of Geology, Tarbiat Modares University, PO Box 14115-175, Tehran, Iran Department of Natural Resources, Shahrekord University, Shahrekord, Iran
a r t i c l e
i n f o
Article history: Received 5 September 2010 Received in revised form 17 July 2011 Accepted 10 August 2011 Available online 16 September 2011 Keywords: Transpressional tectonics Sanandaj–Sirjan Zone Riedel faults Dalan fault Ben fault
a b s t r a c t The NW–SE trending Sanandaj–Sirjan Zone (SSZ) is the internal part of the Zagros continental collision zone, which mainly consists of metamorphic rocks deformed in a dextral transpressional zone. This dextral transpression is attributed to brittle deformation related to late Cenozoic Arabia–Eurasia oblique continental collision. Major NW-trending faults, including the Dalan, Garmdareh, Yasechah, Sheida, and Ben faults, are reverse faults with a dextral strike-slip component. These faults were displaced by NW-trending synthetic and NE-trending antithetic faults. There are also E-trending thrusts and N-trending normal faults developing in directions that are, respectively, almost normal and parallel to the major shortening direction. The NW-trending Ben, Yasechah, and Sheida faults are NE-dipping faults, and the Dalan and Garmdareh faults are SW-dipping faults. These faults indicate the presence of a transpressive flower structure zone that probably led to the exhumation of Jurassic high-grade metamorphic rocks, such as eclogite, in the central part of the study area. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The Zagros continental collision zone is a part of Alpine orogenic system in west Iran. Timing of the convergence and the subsequent collision of the Arabian and Eurasian plates extends back to the Middle Jurassic (Berberian and King, 1981; Alavi, 1994; Mohajjel et al., 2003; Agard et al., 2005). Convergence was oblique, causing deformation partitioning among structures in the Sanandaj–Sirjan Zone (SSZ). Oblique convergence produced strike-slip ductile shear zones and multiple folding as well as thrust and strike-slip faults (Mohajjel and Fergusson, 2000). The Chadegan region, west of Esfahan, is located in the central part of the SSZ, which includes the hinterland crystalline part of the Zagros Orogen. (Fig. 1b). A map-scale anticline contains deformed metamorphic rocks in the study area. The core of this anticline, including high-grade metamorphic rocks, is strongly affected by a strike-slip ductile shear zone (Babaahmadi, 2008). Abundant later faults have displaced the metamorphic rocks. Recent studies on the structure and tectonics of the SSZ have concentrated on the ductile deformation that resulted from Mesozoic–Cenozoic Arabia–Eurasia convergence. Studies by Mohajjel and Fergusson (2000) in the northwest of the SSZ and by Sarkarinejad et al. (2008) in the southeast reveal dextral ductile transpressional tectonics during the Mesozoic-Tertiary convergence of the Arabian and Eurasian plates. Brittle deformation in the SSZ is mainly
⇑ Corresponding author. Tel.: +98 2182883434; fax: +98 2188630482. E-mail address: [email protected] (A. Babaahmadi). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.08.012
attributed to a number of SW-verging thrust faults, truncating folded and foliated metamorphic rocks (Mohajjel and Fergusson, 2000). Studies on these faults in the SSZ are insufficient. Therefore, in this study, we investigated brittle deformation in the SSZ. We attempted to identify fault systems, their generation and the displacement mechanisms affecting the metamorphic rocks of the SSZ in the Chadegan region. We utilized Landsat ETM+, IRS 1-C PAN, and IRS P5 satellite images as well as field investigations to map and interpret fault systems in the study area. 2. Geological and tectonic setting The Zagros continental collision zone in west Iran consists of three main tectonic zones. From northeast to southwest, these zones are as follows: (1) the Urumieh–Dokhtar Magmatic Arc (UDMA), (2) the Sanandaj–Sirjan Zone (SSZ), and (3) the Zagros Fold and Thrust Belt (ZFB) (Alavi, 1994). The most elevated part of the ZFB is named the High Zagros (HZ, Fig. 1). The UDMA is an Andean-type magmatic arc composed mainly of Eocene tholeiitic, calc-alkaline, and K-rich alkaline intrusive and extrusive rocks along the active margin of the Iranian plate (Alavi, 1994; Shahabpour, 2007). The HZ has a width of 10–65 km and includes the highest mountains in the Zagros range. The boundaries of the HZ are the Main Zagros Reverse Fault (MZRF) to the NE and the High Zagros Fault (HZF) to the SW (Berberian, 1995). The ZFB has a width of 150–250 km and consists of ca. 13–14 km of folded shelf deposits of late Paleozoic to early Tertiary age (James and Wynd, 1965; Stocklin, 1968). The SSZ has a NW–SE trend that is 1500 km long and 150–250 km wide, and it is located
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Fig. 1. (a) 90 m SRTM digital elevation model of Iran showing the major faults and zones of western Iran. MRF, Main Recent Fault; MZRF, Main Zagros Reverse Fault; HZF, High Zagros Fault; KFS, Kazerun Fault System; BFS, Ben Fault System; DFS; Dalan Fault System; DF, Dehshir Fault; QZF, Qom-Zefreh Fault; KNF, Kushk-e-Nosrat Fault; IF, Indes Fault; MF, Mendak Fault; AF, Avaj Fault; UDMA, Urumieh-Dokhtar Magmatic Arc; SSZ, Sanandaj–Sirjan Zone; HZ, High Zagros; ZFB, Zagros Fold Belt. The white arrow indicates a velocity vector for the Eurasian plate (Vernant et al., 2004). (b) Simplified tectonic map of southwestern Iran. The Sanandaj–Sirjan Zone is divided into five subzones (radiolarite, Bisotun, ophiolite, marginal, and complexly deformed). The black rectangle indicates the location of the study area west of Esfahan (after Mohajjel et al., 2003). (c) The earthquake distribution in western Iran from 1964 to 1998 (catalogue of Engdahl et al., 1998) showing the low seismicity of the SSZ (see caption of figure (a) for name abbreviations).
to the northeast of the ZFB. Mohajjel et al. (2003) subdivided the SSZ into five sub-zones: (1) the radiolarite sub-zone that consists of Jurassic–Cretaceous shallow marine limestone and deep-marine radiolarite; (2) the Bisotun sub-zone including Late Triassic–Late Cretaceous limestone; (3) the Late Cretaceous ophiolite sub-zone; (4) the marginal sub-zone, including Late Jurassic–Early Cretaceous volcanic rocks and shallow marine strata; and (5) the complexly deformed sub-zone including deformed metamorphic rocks (Fig. 1b). The study area is located in the complexly deformed sub-zone and has three metamorphic units: the high-grade zone, the low-grade zone and the very low-grade zone (Davoudian et al., 2006). Rocks of the high-grade zone include amphibolite, eclogite, and gneiss, while the rocks of the low-grade zone are schist, phyllite, marble, and slate. The very low-grade zone comprises metamorphosed sandstone, limestone, shale and volcanic rocks (Fig. 2). Cretaceous limestone and Eocene conglomerate exist at the northeastern and southwestern boundaries of the study area. Patches of Permian limestone occur in the area (Fig. 2).
Investigations of brittle deformation have mainly been conducted in tectonic units of western Iran, including the UDMA and the Zagros belt. Several researchers have recently studied brittle tectonics in the UDMA (e.g., Safaei et al., 2008; Meyer and Le Dortz, 2007; Morley et al., 2009; Babaahmadi et al., 2010b). There are several right-lateral reverse faults to the west of Central Iran, generating young dextral transpressional tectonics in the UDMA (Meyer and Le Dortz, 2007; Safaei et al., 2008; Babaahmadi et al., 2010b; Fig. 1a). Several research projects on brittle deformation in the Zagros belt indicate the presence of an active dextral transpression in the Zagros (e.g., Authemayou et al. 2005, 2006; Talebian and Jackson, 2002, 2004; Navabpour et al., 2007; Hessami et al., 2001; Axen et al., 2010). Transpression in the Zagros collision event has been produced by oblique collision of the Arabian platform and SW Iran. The sutured area in the HZ has been affected by a strong shear component parallel to the orogen (Mohajjel and Behyari, 2010). Right-lateral transpressional flower structures are present along
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Fig. 2. Geological map of the study area (after Davoudian et al., 2007) showing the location of areas depicted in later figures.
the Zagros suture in the Nahavand area in NW Iran (Mohajjel and Behyari, 2010). Oblique convergence in the Zagros collision is confirmed by active tectonic measurements (Vernant et al., 2004). GPS data indicate that the Arabian and Eurasian plates converge at 21 mm/yr around 50°E. The shortening of the Zagros belt is NNE-trending with a rate of 7 ± 2 mm/yr, implying that the convergence is oblique (Vernant et al., 2004). The MZRF is a major fault separating the Zagros Belt from the SSZ (Stocklin, 1968; Berberian, 1995). At present, the MZRF is an important seismotectonic feature specifying an abrupt rupture between the intense seismicity of the Zagros and the almost aseismic central Iranian plateau (Talebian and Jackson, 2004 Fig. 1c). In the NW Zagros, a major NW–SE right-lateral strike-slip fault system roughly follows the Zagros Belt for almost 800 km and is known as the Main Recent Fault (MRF) (Talebian and Jackson, 2002). The MRF is one of the most important structural features of the active tectonics of the Zagros and has been responsible for several large earthquakes (Berberian, 1995). The total offset of the MRF is probably approximately 50 km, and it may be moving as quickly as 10–17 mm/yr (Talebian and Jackson, 2002). A series of NNW-trending faults, such as the Kazerun fault, cut the Zagros belt transversely toward the SW termination of the MRF and dextrally displace the NW-trending folds and thrusts (Hessami et al., 2001; Authemayou et al., 2006).
3. IRS-P5 images with a spatial resolution of 2.5 m. Satellite images with diverse spatial resolutions are useful for identifying fault traces and structural patterns at different scales (Babaahmadi et al., 2010a). Satellite images were rectified using 1:50,000 and 1:25,000 topographic data. In the rectification procedure, a UTM projection was adopted for zone N39 and a WGS84 datum. For better interpretation of structures and rock units, a 7-4-2 band combination was selected. Image processing methods used in this study are Principle Component Analysis (PCA), fusion of multispectral Landsat ETM+ images and high resolution IRS-1C PAN images, edge sharpening and gradient filters, and contrast reverse function. Recognition of fault lineaments was from mapping: (1) Offset and dragging of rock units, geological structures and geomorphic features. (2) Lineaments with pronounced traces, especially in Quaternary alluviums. (3) Lensoid and en-echelon structures indicating the presence of hidden faults under alluviums. In the study area, five fault systems were recognized from differences in strike patterns (Fig. 3):
3. Methods We used remote sensing and field observations to map fault patterns in the study area. A variety of satellite images were used to accomplish structural analysis: 1. Landsat ETM+ images with a spatial resolution of 28.5 m. 2. IRS-1C PAN images with a spatial resolution of 5.8 m.
1. NW-trending major faults with an average strike of AZ 124. 2. NW-trending synthetic faults with an average strike of AZ 155. 3. NE-trending antithetic faults with an average strike of AZ 058. 4. N-trending faults with an average strike of AZ 185. 5. E-trending faults with an average strike of AZ 095.
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Fig. 3. (a) Processed satellite image of the study area. (b) Map of fault lineaments recognized from satellite images. (c) Rose diagram of major fault lineaments. (d–g) Rose diagrams of E-trending, N-trending, NW-trending synthetic and NE-trending antithetic fault lineaments, respectively.
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Fig. 4. (a) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) of the Dalan fault trace, showing the locations of later figures (see Fig. 2 for location). (b) 3D view of the IRS-P5 image (spatial resolution 2.5 m) superimposed on DEMs showing the Dalan fault between Cretaceous limestone and Eocene conglomerate. Cretaceous units have a rougher morphology than Eocene units. (c) 3D view of the IRS-P5 image (spatial resolution 2.5 m) superimposed on DEMs showing the effect of an antithetic fault on the Dalan faults. The crush zone of this fault is obvious in the Eocene conglomerate.
Field trips to measure the brittle crush zones of faults verified the interpretation of the satellite images. Brittle S–C fabrics are prevalent kinematic indicators that are widely developed in these fault zones. C Surfaces are discrete shear surfaces that develop sub-parallel to the fault plane, and S surfaces are foliations transected by C surfaces (e.g., Lin, 2001; Passchier and Trouw, 1996). We measured these S–C fabrics to analyze the kinematics of major faults. 4. Major faults Major NW-trending faults were identified by their significant length and wide crush zones. The major faults in the study area are (from northeast to southwest) the Dalan, Garmdareh, Yasechah, Sheida and Ben faults (Fig. 3). 4.1. The Dalan fault system The Dalan fault system consists of three parallel, dextral reverse faults (DF1, DF2 and DF3) displacing Jurassic shale and limestone, Cretaceous limestone and Eocene conglomerate (Figs. 2 and 4a). These faults have an orientation of 45–60° toward the SW. Dextral shear sense is indicated by S–C fabrics that have developed in crush zones of the Dalan faults (Fig. 5b and c). These faults form a linear and elevated trace separating the Mastan Mountains from the Karvan plain (Fig. 4a), and they are cut by several synthetic and antithetic faults (Figs. 3, 4c, 13a and b). The DF1 fault has a steep
dip toward the SW, placing Jurassic shale over Cretaceous limestone (Figs. 2 and 3). The DF2 fault is a right-lateral reverse fault, dipping 45–60° toward the SW, placing Cretaceous limestone over Eocene conglomerate (Figs. 3, 5a and e). On the IRS-P5 satellite image (spatial resolution 2.5 m), the DF2 fault is a marked linear trend between Cretaceous and Eocene units (Fig. 4b). On the satellite image, Cretaceous units have a rougher morphology than do Eocene units. The DF3 fault is a right-lateral reverse mountain front fault and displaces Eocene conglomerate (Fig. 5d). Both geomorphic and geological evidence imply that the DF3 fault has been active in the Quaternary because young alluvial fans have been tilted by fault activity (Fig. 4a). Because of the tilting of alluvial fans, channel incision has occurred on streams crossing the DF3 fault, causing the younger alluvial fans to form downslope, away from their former locus (Fig. 4a). The existence of some springs along the DF3 fault shows that the DF3 fault has displaced underground water aquifers along the Dalan mountains. The DF3 fault crosses the Golab water-conveyance tunnel path, causing water with a 30 l/s volumetric flow rate to enter the tunnel during its excavation (Eftekhari and Taheri, 2005). 4.2. The Garmdareh fault system The dextral reverse Garmdareh faults pass along the Zayandehrud river, displacing metamorphic rocks including schist, amphibolite and gneiss units (Figs. 2, 3 and 6a). The strikes of these faults
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Fig. 5. (a) The Dalan fault between Cretaceous limestone and Eocene conglomerate (see Fig. 4a for location). (b) Development of S–C fabrics in the crush zone of the Dalan fault (see Fig. 5a for location). (c) S–C fabrics plotted on a lower hemisphere equal-area stereonet showing the dip-slip movement of the Dalan faults approximately toward the N. (d) The DF3 fault displacing Eocene conglomerate (see Fig. 4a for location). (e) The Dalan fault placing Cretaceous limestone over Eocene conglomerate (see Fig. 4a for location). Note that Cretaceous units are gray colored and that Eocene units are red colored. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
are AZ 124, and their dips are 45–55° toward the SW. A stereonet analysis of kinematic indicators shows that the dip-slip movement is toward the NNE (Fig. 7a–c). The Garmdareh fault system comprises two en-echelon faults: the GF1 and GF2 faults (Fig. 6a). The GF1 fault is a dextral reverse fault with 30 m of crush zone, placing amphibolite over gneiss (Fig. 7a). The amphibolite has a darker color than the gneiss on the satellite image (Fig. 6a). On the IRS P5 image (spatial resolution 2.5 m), some acidic intrusion bands are apparent and displaced along the trace of the fault (Fig. 6b). A river has been deflected dextrally along the GF1 by 1.2 km, indicating the Quaternary nature of this fault (Fig. 6c). The GF2 fault is a dextral reverse fault displacing low-grade and high-grade metamorphic rocks (Fig. 6a). 4.3. The Yasechah fault system The Yasechah fault system comprises two parallel faults with a strike of AZ 118 and displace low-grade and very low-grade metamorphic rocks (Figs. 2 and 3). They are high-angle reverse faults dipping toward the NE (Fig. 8a–e). Stereonet analysis of kinematic indicators shows that the dip-slip movement is toward the SE, indicating a dextral oblique slip of the Yasechah faults (Fig. 8e). The Yasechah faults have been intersected by a number of syn-
thetic and antithetic faults (Fig. 3). The 50 m crush zone of the northern Yasechah fault is observed in both low-grade and very low-grade metamorphic units, while the southern Yasechah fault, with a crush zone of 80 m, has displaced very low-grade metamorphic units, including metamorphosed sandstone and limestone (Figs. 2 and 8).
4.4. The Sheida fault The Sheida fault is a dextral reverse fault with a strike of AZ 118, placing Permian limestone over Jurassic shale and sandstone (Figs. 2 and 3). Fig. 9a shows a 3D-perspective satellite image of the Sheida mountain where the Sheida fault passes between darker-colored Permian units and lighter-colored Jurassic units. Based on field observations, the Sheida fault has an orientation of 55°/ 033° and a dextral strike-slip component (Fig. 9b–e). Satellite images show that the Sheida fault is cut by some synthetic and antithetic minor faults (Fig. 3). Field observations indicate that a number of calcite veins have been dextrally displaced in the crush zone of the Sheida fault (Fig. 9d). Kinematic indicators such as S–C fabrics show that the dip-slip movement of the Sheida fault is approximately toward the S (Fig. 9e).
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Fig. 7. (a and b) Photographs showing S–C fabrics in the Garmdareh fault zone between amphibolite and gneiss (see Fig. 6a for location). S–C fabrics are specified on figure (b). (c) Lower hemisphere equal area stereographic projection of S–C fabrics showing the dip-slip movement of the Garmdareh fault toward the NNE.
Fig. 6. (a) Landsat ETM+ image (spatial resolution 15 m) of the Garmdareh faults, including the GF1 and GF2 faults. (b) IRS-P5 image (spatial resolution 2.5 m) showing a granitic band displaced along the trace of the Garmdareh fault zone (see figure (a) for location). (c) IRS-P5 image (spatial resolution 2.5 m) showing dextral river deflection by 1.2 km and elevated dry valleys along the Garmdareh fault (see figure (a) for location).
4.5. The Ben fault system The Ben fault system, including a series of parallel faults, has displaced Cretaceous limestone and Eocene conglomerate units south of Ben city (Figs. 2, 3, 10a and 11a). The NE-dipping Ben faults have propagated toward the SW, creating a duplex thrust system (Nemati and Yassaghi, 2010). Brittle crush zones of the Ben faults have developed in Cretaceous limestone, resulting in the development of S–C fabrics (Figs. 10c and 11b). The fault slip analysis of S–C fabrics shows that the dip-slip movement is toward the SSE, indicating dextral oblique slip (Fig. 11d). Some synthetic faults have displaced the Ben faults (Fig. 10a and b). According to Nemati and Yassaghi (2010), the Ben faults involved the basement, demonstrating their deep-seated nature.
5. Minor faults 5.1. NW-trending synthetic faults The NW-trending synthetic faults, with an average strike of AZ 155, have dextral offsets as shown by dragged structures and rock units (Figs. 3 and 12). In addition to the prevalent dextral strikeslip component, these faults have a reverse dip-slip component toward either the NE or SW (Fig. 12d). A number of synthetic faults are truncated by other synthetic faults. Fig. 12e shows a younger synthetic fault with an orientation of 45°/060° that cuts another
Fig. 8. (a and b) Crush zone of the northern Yasechah fault showing development of S– C fabrics (see Fig. 2 for location). S–C fabrics are observed in figure (b). (c and d) Field pictures showing S–C fabrics in the crush zone of the southern Yasechah fault in very low-grade metamorphic rocks (see Fig. 2 for location). S–C fabrics are observed in figure (d). (e) Lower hemisphere equal area stereographic projection of S–C fabrics approximately indicating the dip-slip movement of the Yasechah faults toward the SE.
fault with an orientation of 60°/230°. From satellite images, it is clear that some synthetic faults have displaced features in en-echelon patterns (Fig. 12a–c). Some synthetic faults cut the major faults and have displaced them significantly (Figs. 10b and 12a and b).
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Fig. 9. (a) 3D view of Landsat ETM+ superimposed on DEMs showing the Sheida fault between Permian and Jurassic units (see Fig. 2 for location). (b) A field picture of the Sheida fault between Permian and Jurassic units. (c) Crush zone of the Sheida fault showing the development of S–C structures. (d) Calcite veins have been dextrally displaced in the crush zone of the Sheida fault. (e) Lower hemisphere equal area stereographic projection of S–C fabrics indicating the dip-slip movement of the Sheida fault toward the S.
Fig. 10. (a) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) of the trace of the Ben faults showing latter figures. Small white arrows show the trace of synthetic faults displacing the Ben fault. (b) A synthetic NW-trending fault offsetting the trace of the Ben faults. (c-d) S–C structures developed in a crush zone of the Ben faults (see figure (b) for location).
5.2. NE-trending antithetic faults The NE-trending antithetic faults with an average strike of AZ 058 were identified from the sinistral displacement of geological structures and rock units (Figs. 2, 3 and 13). They have a chiefly reverse component to the NW or SE (Figs. 4 and 13d–e). A small number of NE-trending normal faults were also observed. The antithetic faults have mainly extended in en-echelon patterns, displacing and dragging geomorphic features and major fault traces (Fig. 13a–c). Some of the best examples of antithetic faults that offset major faults are the Abpuneh faults. The Abpuneh faults, including some parallel NE-trending faults, sinistrally offset the Dalan fault traces by more than 2 km (Figs. 13a and b). 5.3. N-trending and E-trending faults The N-trending faults have a normal dip-slip component, chiefly deforming metamorphic rock units (Figs. 3 and 14b). The E-trending faults were difficult to identify from satellite images because they are thrust faults with a low dip angle (Fig. 2). The E-trending faults were mapped in the field and have an average orientation of 38°/185°. Duplex structures occur in the crush zone of the E-trending faults, mainly in low- and high-grade metamorphic rocks (Fig. 14a). E-trending deformation is mostly observed as folding in ductile rocks such as Jurassic shale (Fig. 14c). 6. Discussion Riedel faults are usually associated with strike-slip shear zones (Naylor et al., 1986). In a simple strike-slip shear zone, Riedel faults
Fig. 11. (a) A field picture of the Ben fault displacing Cretaceous limestone (see Fig. 10a for location). (b and c) Brittle crush zone in another part of the Ben faults; displacing Cretaceous limestone and development of S–C surfaces in this crush zone (see Fig. 10a for location). (d) Lower hemisphere equal area stereographic projection of S–C structures indicating the dip-slip movement of the Ben faults toward the SSW.
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Fig. 12. (a and b) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) showing dextral dragging of amphibolite rocks, gneiss rocks, and the Garmadareh fault by synthetic faults. Synthetic faults are specified with a green color in figure (b) (see Fig. 2 for location). (c) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) showing a sigmoid shape formed by dextral en-echelon synthetic faults in the metamorphic rocks (see Fig. 2 for location). (d) Lower hemisphere equal area stereographic projection of the synthetic faults showing dips toward both the NE and SW. (e) A field photograph of two synthetic faults. A synthetic NE-dipping fault has displaced another synthetic fault dipping in the opposite direction (see Fig. 2 for location). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
are categorized as synthetic and antithetic faults. From experimental and theoretical points of view, synthetic faults (R) have a low angle of 17° to the shear direction, whereas antithetic faults (R0 ) have a high angle of 70–90° (Naylor et al., 1986; Wilcox et al., 1973). Transpression involves strike-slip shear accompanying horizontal shortening and vertical lengthening in the shear plane (Sanderson and Marchini, 1984; Woodcock and Schubert, 1994; Fossen et al., 1994). In transpression, the Riedel faults are formed at a higher angle to the shear direction. In theoretical models, the average angle between synthetic faults and the shear direction is 37° and has a maximum of 60° (Naylor et al., 1986). The Riedel faults form at a lower angle with respect to the shear direction during increasing shear zone displacement (Naylor et al., 1986). In transpression, folds and thrusts are formed at a low angle to the shear direction, whereas normal faults are formed at higher angle to the shear direction (Woodcock and Schubert, 1994; Sanderson and Marchini, 1984). In the study area, the major faults are dextral reverse faults. The average strike of the major faults is AZ 124, which is the mean trend of the dextral transpressional zone in the area. The Dalan, Garmdareh, Yasechah, Sheida, and Ben faults are the major faults with right-lateral strike-slip movements. The Ben and Dalan faults are the largest fault systems in the study area. According to Nemati and Yassaghi (2010), the Ben faults are basement faults, displacing the basement top by 3–4 km. The Dalan faults have significantly displaced Jurassic, Cretaceous, and Eocene units. The dextral movement along the Ben and Dalan faults has created a dextral transpressional zone in the study area. The NW-trending synthetic faults (R faults) have a
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maximum angle of 49°, a minimum angle of 15°, and an average angle of 30° to the mean trend of the dextral shear zone. The antithetic NE-trending faults (R0 faults) have a maximum angle of 130°, a minimum angle of 87°, and an average angle of 113° to the mean trend of the dextral zone. The N-trending normal faults have extended at the higher angle with respect to the shear direction and are almost parallel to the regional shortening direction. The E-trending thrusts and folds have also developed at a lower angle to the shear direction and approximately normal to the regional shortening direction. Fig. 15 illustrates the creation of the NW- and NE-trending Riedel faults, N-trending normal faults and E-trending thrusts in a dextral brittle transpressional zone in the study area. The study area has characteristics of a positive flower structure. The Ben, Yasechah and Sheida faults are NE-dipping faults, and the Dalan and Garmdareh faults are SW-dipping faults, giving rise to a positive flower structure zone. Exhumation of Jurassic high-grade rocks such as eclogite, amphibolite, and gneiss in the central part of the study area can be related to the activity of this zone. Because the Dalan and Ben faults have deformed Eocene deposits, they seem to have become activated after the Eocene. Gavillot et al. (2010) used (U–Th)/He thermo-chronometry to achieve ages of thrust activity in the HZ. Their results indicate that thrust activity in the HZ and continental suturing along the Zagros suture initiated no later than 23 Ma in the late Oligocene or early Miocene (Gavillot et al., 2010). Because most faults in the study area have a young nature, the timing of faulting is interpreted to be related to this age. Determining the dextral slip rate along most faults is difficult because reliable geomorphic and stratigraphic markers do not exist on both sides of the faults. However, a dextral 1.2 km river deflection along the Garmdareh fault indicates activity of the faults during the Quaternary (Fig. 6c). Our interpretation of the fault patterns in the SSZ is in agreement with the results of studies in the UDMA and the HZ. Detailed studies of the fault patterns of the Qom-Zefreh, Indes, and Kushk-e-Nosrat faults revealed a dextral transpressional zone in the UDMA located west of Central Iran (Nogol-Sadat, 1985; Safaei et al., 2008; Babaahmadi et al., 2010b), Fig. 1a. For example, there are five classes of faults along the Qom-Zefreh fault with trends of AZ 120-125, AZ 140-155, AZ 170-185, AZ 90, and AZ 50, implying a dextral transpressional system (Safaei et al., 2008). The dextral reverse faults in the UDMA have developed in en-echelon patterns, producing Quaternary restraining step-over zones (Babaahmadi et al., 2010b). Deformation partitioning is also observed among the structures in the UDMA (Babaahmadi et al., 2010b). Transpressional patterns in the HZ have mainly been studied along the MZRF, MRF, and HZF (Talebian and Jackson, 2002, 2004; Agard et al., 2005; Authemayou et al., 2006; Axen et al., 2010, Fig. 1a). Dextral movement along major thrusts has generated significant dextral strain in the HZ since the onset of the orogeny in late Oligocene or early Miocene (Axen et al., 2010; Gavillot et al., 2010). A series of minor faults with strikes of NW to NE has been generated between the MZRF and HZF due to a dextral transpressional zone in the HZ (Authemayou et al., 2006). The presence of dextral thrusts with opposite dipping between the MZRF and HZF and exhumation of Paleozoic rocks along the HZF reveal a dextral transpressive flower structure zone in the HZ (Authemayou et al., 2006). Deformation partitioning has occurred at a dominant transpressional zone in the HZ in the late Pliocene, giving rise to dextral strike-slip movement along the MRF (Talebian and Jackson, 2002; Agard et al., 2005; Authemayou et al., 2006). In contrast to the UDMA and the HZ, deformation partitioning has not developed along the structures in the study area. The dextral brittle transpression observed in the SSZ, the HZ, and
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Fig. 13. (a and b) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) of the sinistral Abpuneh faults displacing the trace of the Dalan faults sinistrally by more than 2 km (see Fig. 4a for location). (c) Merged image of Landsat ETM+ and IRS-1C PAN (spatial resolution 5.8 m) showing dragging of the Yasechah fault traces by sinistral en-echelon faults. Dark arrows show the trace of the Yasechah faults (see Fig. 2 for location). (d) Lower hemisphere equal area stereographic projection of the antithetic faults showing dips toward both the NW and SE. (e and f) Field pictures of a NW-dipping antithetic fault with a reverse component (see figure (c) for location).
the UDMA in western Iran is interpreted to be related to the oblique continental collision of the Arabian and Eurasian plates. Although geomorphic and structural evidence shows that the fault systems are young and active during the Quaternary, the study area in the SSZ exhibits low seismic activity, and no large earthquakes have been detected historically or instrumentally (Fig. 1c). The most destructive earthquakes have occurred at the junction zone of the SSZ and the HZ along the MRF (e.g., Silakhor earthquake in 1909 and Nahavand earthquake in 1958, Berberian, 1976). A sudden drop is observed in the concentration of instrumentally recorded earthquakes from the MRF toward the SSZ (Fig. 1b, Engdahl et al., 1998). Our knowledge of historical events in the SSZ, as in other parts of the Iranian plateau, is poor (Ambraseys and Melville, 1982). However, the limited knowledge about historical earthquakes in the SSZ must not be considered to be due to a lack of large earthquakes because the reoccurrence
periods of large earthquakes can exceed intervals of historical and instrumental records.
7. Conclusions Satellite images and field observations were used to investigate fault patterns in the Chadegan region in the SSZ. The Dalan and Ben faults are the largest major faults in the area, and the dextral movement along them has created a dextral transpressional zone in the study area. As a result of the activity of the dextral transpressional zone, minor faults, including NE-trending antithetic faults, NW-trending synthetic faults, E-trending thrusts, and N-trending normal faults, have extended into the area. The brittle transpression observed in the SSZ is thought to be related to late Cenozoic ArabiaEurasia oblique continental collision.
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Fig. 14. (a) Development of large duplex structures in an E-trending fault crush zone. Lower hemisphere equal area stereographic projection of this fault shows its low angle dip toward the S (see Fig. 2 for location). (b) A N-trending normal fault displacing Jurassic gneiss and schist units. Lower hemisphere equal area stereographic projection of this fault shows dipping toward the E (see Fig. 2 for location). (c) An E-trending recumbent fold in the Jurassic shale. Lower hemisphere equal area stereographic projection of this fault shows dipping toward the NNE (see Fig. 2 for location).
Fig. 15. A schematic model indicating the structures generated in the dextral brittle transpression in the SSZ.
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