1. Introduction
Quartz is a common crustal mineral that undergoes crystal plastic deformation under a wide range of conditions, and there are many descriptions of its deformation microstructures and lattice preferred orientation (LPO) from a wide variety of metamorphic grades, strain geometries and finite strains. The analysis of these fabrics has been found to be useful in throwing light on understanding the active slip mechanism, the shape of the finite strain ellipsoid and the strain path (Law, Casey & Knipe, Reference Law, Casey and Knipe1986; Platt & Behrmann, Reference Platt and Behrmann1986; Law, Reference Law, Knipe and Rutter1990; Xypolias & Doutsos, Reference Xypolias and Doutsos2000). As such, well-developed c-axis fabrics in natural tectonites can be used to characterize the kinematics of flow (Schmid & Casey, Reference Schmid, Casey, Hobbs and Heard1986; Sullivan & Law, Reference Sullivan and Law2007), the sense of shear (Law, Reference Law, Knipe and Rutter1990), the vorticity number associated with flow (Xypolias, Reference Xypolias2009), the dominant deformation mechanisms during deformation (Lister & Dornsiepen, Reference Lister and Dornsiepen1982) and the deformation temperature (Kruhl, Reference Kruhl1996, Reference Kruhl1998; Law, Searle & Simpson, Reference Law, Searle and Simpson2004; Morgan & Law, Reference Morgan and Law2004).
This paper presents quantitative data on strain, deformation temperature and vorticity of flow in the Sanandaj–Sirjan Metamorphic Belt of the Zagros Mountains of Iran and concludes with a discussion of the structural implication of the integrated strain, temperature and vorticity data. Our aim is the systematic collection of samples throughout the Sanandaj–Sirjan Metamorphic Belt in order to obtain estimates of the strain ratio and vorticity of flow at varying distances above the Zagros basal thrust. These data allow the identification of detailed geometric and kinematic constraints on the ductile transpression zone in which extrusion of medium- to high-grade metamorphic rocks has occurred. This belt is outlined by ductile transpressional deformation (Mohajjel & Fergusson, Reference Mohajjel and Fergusson2000; Sarkarinejad, Faghih & Grasemann, Reference Sarkarinejad, Faghih and Grasemann2008; Sarkarinejad & Azizi, Reference Sarkarinejad and Azizi2008; Sarkarinejad, Godin & Faghih, Reference Sarkarinejad, Godin and Faghih2009; Sarkarinejad et al. Reference Sarkarinejad, Samani, Faghih, Grasemann and Moradipoor2010), which represents a crustal-scale shear zone accommodating deformation and displacement of metamorphic rocks during oblique collision between the Afro-Arabian continent and the Iranian microcontinents (Alavi, Reference Alavi1994).
2. Geological setting
The Zagros orogenic belt is part of the Alpine–Himalayan Mountain Range and extends for more than 1500 km in a NW–SE direction from eastern Turkey to the Minab Fault system in Southern Iran (Haynes & McQuillan, Reference Haynes and McQuillan1974; Stöcklin, Reference Stöcklin, Burk and Drake1974). The belt is considered to be a complex product of an early Mesozoic separation of the Iranian continental block from the rest of the Gondwana landmass followed by a NE-dipping subduction of the newly generated Neo-Tethyan oceanic crust below the Iranian microcontinents and subsequent collision between the Afro-Arabian and Iranian microcontinents (Alavi, Reference Alavi1994). The Late Cretaceous to Tertiary convergence between the Afro-Arabian continent and Iranian microcontinents accounts for thrusting and large-scale strike-slip faulting associated with crustal shortening in the Zagros orogeny (Alavi, Reference Alavi1994; Mohajjel & Fergusson, Reference Mohajjel and Fergusson2000; Sepehr & Cosgrove, Reference Sepehr and Cosgrove2005; Lacombe et al. Reference Lacombe, Mouthereau, Kargar and Meyer2006). Collision is still an ongoing orogenic process (Talebian & Jackson, Reference Talebian and Jackson2002; Tatar, Hatzfeld & Ghafory-Ashtiyani, Reference Tatar, Hatzfeld and Ghafory-Ashtiyani2004; Allen, Jackson & Walker, Reference Allen, Jackson and Walker2004; Regard et al. Reference Regard, Bellier, Thomas, Abbassi, Mercier, Shabanian, Feghhi and Soleymani2004; Vernant et al. Reference Vernant, Nilforoushan, Hatzfeld, Abbasi, Vigny, Masson, Nankali, Martinod, Ashtiani, Bayer, Tavakoli and Chery2004; Authemayou et al. Reference Authemayou, Bellier, Chardon, Malekzade and Abassi2005) with a N–S-directed convergence rate of approximately 20 ± 2 mm yr−1 (Vernant et al. Reference Vernant, Nilforoushan, Hatzfeld, Abbasi, Vigny, Masson, Nankali, Martinod, Ashtiani, Bayer, Tavakoli and Chery2004).
A number of structural subdivisions are classically recognized in the Zagros Mountains. Starting from the southwest (Fig. 1), the Persian Gulf and Mesopotamian Plain represent the foredeep developed at the front of the orogen and thinning out progressively towards the southwest in the Arabian Platform. To the northeast, the Zagros fold–thrust belt consists of large parallel concentric folds related to Neogene inversion of the former Arabian passive margin (Alavi, Reference Alavi1994, Reference Alavi2004). The range is typically 250–350 km wide, from a southwestern margin along the edge of the Mesopotamian foreland and the Persian Gulf to the edge of the Turkish–Iranian Plateau in the northeast. The Zagros fold–thrust belt is bounded to the northeast by the Sanandaj–Sirjan Metamorphic Belt, consisting mainly of Precambrian metamorphic rocks, and the Urumieh–Dokhtar Magmatic Belt, an Andean-type volcanic magmatic arc including rocks ranging in age from Late Jurassic to Quaternary (Molinaro et al. Reference Molinaro, Leturmy, Guezou, Frizon de Lamotte and Eshraghi2005; Molinaro, Zeyen & Laurencin, Reference Molinaro, Zeyen and Laurencin2005). The Sanandaj–Sirjan Metamorphic Belt is a zone of polyphase deformation, the latest reflecting the collision of the Afro-Arabian continent and Iranian microcontinents and subsequent southward propagation of the fold–thrust belt (Alavi, Reference Alavi1994). This part of the Zagros orogenic belt is 150–200 km wide and more than 1500 km long from the northwest (Sanandaj) to southeast (Sirjan) in the western part of Iran, which runs parallel to and is part of the NW–SE structural grain of the Zagros Mountains. Crustal shortening associated with subduction and collision metamorphosed and deformed late Palaeozoic and Mesozoic rocks in this zone. The Sanandaj–Sirjan Metamorphic Belt is a zone of thrust faults that have transported numerous slices of variously metamorphosed Phanerozoic stratigraphic units. Stratigraphic evidence and synorogenic conglomerates indicate that thrusting initiated in Late Cretaceous time (Alavi, Reference Alavi1994). The study area is located in the Neyriz area, 300 km southeast from Shiraz in southern Iran. Metamorphosed and deformed rocks of this area are part of the Sanandaj–Sirjan Metamorphic Belt (Fig. 1).
Figure 1. (Colour online) (a) Geological map of the study area. The inset map shows a tectonic map of Iran (modified after Alavi, Reference Alavi1994) with the Sanandaj–Sirjan Metamorphic Belt and the location of the study area. (b) The NE–SW transverse cross-section (A–A′) which shows spatial variation of lower hemisphere, equal-area projection of the quartz c-axis fabrics in the study area. The location of the quartzite samples and NE–SW transverse line are displayed in (a). In all these projections, the foliation is vertical and stretching lineation within the foliation is sub-horizontal.
3. Microstructures and quartz c-axis fabrics
In order to constrain the kinematics of the shearing event, quartz c-axis fabric measurements were carried out on eight deformed quartzite samples selected from different structural positions and located along a NE–SW transverse line across the Sanandaj–Sirjan Metamorphic Belt. Quartzite samples have been used for the measurements and the evaluation of lattice preferred orientation. C-axis orientations of 300 or more quartz grains from each specimen from the study area were measured on sections cut parallel to the lineation and perpendicular to the foliation. The analyses of quartz c-axis fabrics were made using an optical microscope equipped with a universal stage located in Shiraz University, Department of Earth Sciences. The c-axis measurements are displayed on equal-area, lower hemisphere stereographic projections. The mesoscopic foliation plane, its normal and the stretching lineation lying on foliation define a reference coordinate frame.
Most of the collected samples record a well-preserved shape and lattice preferred orientation (Fig. 2). The fabric of the dynamically recrystallized quartz was formed by both subgrain rotation and grain boundary migration. Generally, the presence of a lattice preferred orientation is interpreted as evidence for deformation by dislocation glide and creep (Vernooij, Brok & Kunze, Reference Vernooij, Brok and Kunze2006). From the density distribution of quartz c-axes, the fabrics of the study area can be described as single-girdle and Type-I cross-girdle fabrics, which are oblique to the foliation trend indicating the operation of non-coaxial shearing during deformation (Lister, Reference Lister1977; Lister & Hobbs, Reference Lister and Hobbs1980; Schmid & Casey, Reference Schmid, Casey, Hobbs and Heard1986; Law, Reference Law, Knipe and Rutter1990). Deformation geometry and shear sense are deduced from methods described by Simpson & Schmid (Reference Simpson and Schmid1983), Ramsay & Huber (Reference Ramsay and Huber1987), Law (Reference Law, Knipe and Rutter1990) and Bell & Johnson (Reference Bell and Johnson1992). The top-to-the-SE sense of shear is confirmed by microstructures and microtextures. These c-axis fabrics show an obliquity of the central girdle segment (ψ) with respect to the main foliation and lineation, indicating a non-coaxial top-to-the-SE sense of shear (Fig. 3; Table 1). The obliquity of the central girdle segment (external asymmetry parameter) varies between 71° and 82°. Almost all the c-axis fabrics exhibit monoclinic peripheral point-maxima asymmetry with respect to the foliation, which is consistent with the pattern commonly reported for non-coaxial deformation (Wang et al. Reference Wang, Zhang, Fan and Peng2005). Fabric skeletons were prepared from the contoured fabric diagrams by connecting the crests and ridges (Lister & Williams, Reference Lister and Williams1979; Lister & Hobbs, Reference Lister and Hobbs1980; Vissers, Reference Vissers1989; Joy & Saha, Reference Joy and Saha2000). External fabric asymmetry may also be expressed by the relative magnitudes of the parameters c 1 and c 2 within individual samples (Platt & Behrmann, Reference Platt and Behrmann1986; Law, Reference Law1987, Reference Law, Knipe and Rutter1990; Law, Knipe & Dayan, Reference Law, Knipe and Dayan1984). Within the quartzite samples, c 1 is smaller than c 2 indicating a top-to-the-SE sense of shear. The same sense of shear may be expressed by the relative magnitudes of ω 1 and ω 2 (internal asymmetry), where ω 2 is consistently greater than ω 1 in each sample (Fig. 3; Table 1).
Figure 2. Photomicrograph of quartz mylonites from the Sanandaj–Sanandaj Metamorphic Belt in the study area. Elongate and plastically deformed quartz grains form a well-developed shape-preferred orientation. The quartz grains record evidence of dynamic recrystallization by subgrain rotation and grain boundary migration. (a) Relicts of large old quartz grains with undulose extinction and elongate subgrains pass laterally into domains of small, new grains. (b) Polycrystalline quartz with irregular grain boundaries formed in response to grain boundary migration recrystallization. For example, the dark grain is bulging into the light-grey grain in the central part of the image.
Figure 3. The fabric skeletons and angular relationships between quartz c-axis external and internal fabric asymmetry that is used in this paper. The parameters used to define external and internal fabric asymmetry are illustrated. The observed fabric asymmetry indicates a significant component of non-coaxial, top-to-the-SE sense of shear.
Table 1. Petrofabric, strain and vorticity data for quartzite samples from the study area
4. Finite strain analysis
Finite strain ratio in the XZ plane (RXZ) was estimated using the Rf/ϕ method (Ramsay, Reference Ramsay1967; Lisle, Reference Lisle1985) on the elliptical strain markers such as plastically deformed quartz grains in the deformed quartzites of the study area (Table 1). The Rf/ϕ method can yield unreliable results on specimens in which repeated recrystallization has significantly modified the original shape of the observed quartz grains. Therefore, thin-sections in which quartz grains showed no evidence of recrystallization were chosen for the finite strain analysis. For each thin-section, strain markers were traced on transparent overlays on the enlarged photomicrographs. At least 80 deformed quartz grains were digitized from enlarged photographs of thin-sections in each sample and then Rf/ϕ diagrams were constructed manually. It is emphasized that strain analysis was not attempted on pure quartzites owing to the fact that in these rocks the aspect ratio of quartz grains was significantly modified by dynamic recrystallization.
5. Degree of non-coaxiality
The analysis of kinematic indicators in the study area reveals differences in quartz c-axis asymmetry, reflecting spatial partitioning of the flow between domains that underwent coaxial flow and those that underwent non-coaxial flow. Quantitative determination of the vorticity is extremely valuable when attempting to reconstruct a deformation because it describes the degree of non-coaxiality of flow, or the relative amounts of pure versus simple shear, and helps to define the kinematic reference frame for the flow in ductile shear zones (Grasemann, Fritz & Vannay, Reference Grasemann, Fritz and Vannay1999; Law, Searle & Simpson, Reference Law, Searle and Simpson2004; Carosi et al. Reference Carosi, Montomoli, Rubatto, Visona, Law, Searle and Godin2006; Jessup et al. Reference Jessup, Law, Searle, Hubbard, Law, Searle and Godin2006; Short & Johnson, Reference Short and Johnson2006; Forte & Bailey, Reference Forte and Bailey2007). The kinematic vorticity number (WK) is a dimensionless number that describes this quality of flow, where pure shear is described by WK = 0, and simple shear is described by WK = 1. General shear is the term used for flows intermediate between the pure and simple shear end members, in which 1 > WK > 0 (Passchier & Trouw, Reference Passchier and Trouw2005). Pure and simple shearing components contribute equally to the instantaneous flow at WK = 0.71 (Law, Searle & Simpson, Reference Law, Searle and Simpson2004). In natural systems the vorticity of flow may vary with both position and time (Fossen & Tikoff, Reference Fossen and Tikoff1993, Reference Fossen, Tikoff, Holdsworth, Strachan and Dewey1998). However, to integrate the vorticity of the flow (WK) over time and space it is more appropriate to consider the mean vorticity number Wm (Passchier, Reference Passchier1988).
The approximately plane-strain condition which is suggested from the cross-girdle pattern of the quartz c-axis fabrics (Law, Reference Law, Knipe and Rutter1990) satisfies the application of two-dimensional vorticity analysis (Tikoff & Fossen, Reference Tikoff and Fossen1995). In this work, a method based on quartz fabrics proposed by Wallis (Reference Wallis1992, Reference Wallis1995) is used in order to estimate Wm. This method is based on measuring Type-I (Lister, Reference Lister1977) cross-girdle quartz c-axis fabrics and strain ratios in the XZ plane of finite strain. The angle β between the perpendicular to the central girdle of the quartz c-axis diagram and the foliation is equal to the angle between the flow plane and the principal plane of normal strain in this method. This angle is a function of RXZ and Wm, as described by Wallis (Reference Wallis1995):
![\begin{eqnarray}
W_m &{=}& \sin\! \left\{\! \tan ^{- 1}\! \left[ \frac{{\sin (2\beta)}}{{[ {{{(R_{XZ} \,{+}\, 1)} / {(R_{XZ} \,{-}\, 1)}}}] \,{-}\, \cos (2\beta)}} \!\right] \right\}\nonumber\\
&&\times\, \frac{{(R_{XZ} + 1)}}{{(R_{XZ} - 1)}}\end{eqnarray}](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20180419074129359-0244:S0016756811000276:S0016756811000276_eqn1.gif?pub-status=live)
This method was applied to the deformed samples where asymmetric quartz c-axis fabrics are observed. The analysis of samples from the study area yielded values of Wm in the range 0.6–0.9 (Table 1).
6. Deformation temperature
During plastic deformation and dynamic crystallization, increased temperature results in an increased opening angle of peripheral legs of quartz c-axis fabrics (Tullis, Christie & Griggs, Reference Tullis, Christie and Griggs1973; Lister & Hobbs Reference Lister and Hobbs1980; Lister & Dornsiepen Reference Lister and Dornsiepen1982). The relationship between the opening angle of quartz c-axis fabrics and deformation temperatures is linear through greenschist to upper amphibolite facies conditions (Kruhl, Reference Kruhl1996, Reference Kruhl1998; Law, Searle & Simpson, Reference Law, Searle and Simpson2004). In the case of plane strain deformation this opening angle is defined as the angle between the two c-axis girdles measured in the plane perpendicular to the foliation and parallel to the lineation. The basic assumption made in using this geothermometer is that the range in strain rates and effects of hydrolytic weakening likely to be encountered in natural deformation is encompassed in the geothermometer. For natural deformation this opening angle thermometer may give deformation temperatures with an uncertainty of ±50°C (Law, Searle & Simpson, Reference Law, Searle and Simpson2004). In order to estimate temperatures during ductile deformation of the analysed deformed quartzites, the geothermometer of Kruhl (Reference Kruhl1996, Reference Kruhl1998) modified by Law, Searle & Simpson (Reference Law, Searle and Simpson2004) and Morgan & Law (Reference Morgan and Law2004) was used. The asymmetric crossed girdles display opening angles ranging from 55° to 75°. Using this modified geothermometer the fabric opening angles indicate that deformation temperature varies from 435° ± 50°C to 510° ± 50°C in the study area (Fig. 4).
Figure 4. Quartz lattice preferred orientations (LPO) and associated temperature indicators. (a) Simplified stereonets showing dependence of quartz LPOs and inferred slip systems on increasing temperature (modified from Passchier & Trouw, Reference Passchier and Trouw2005). (b) Graph of the quartz c-axis fabric opening angles versus estimated temperatures of deformation for eight quartzite samples of naturally deformed rocks in the Neyriz area showing effect of temperature on the opening angle of small-circles of the quartz c-axes in an LPO diagram (after Kruhl, Reference Kruhl1996, Reference Kruhl1998; Law, Searle & Simpson, Reference Law, Searle and Simpson2004; Morgan & Law, Reference Morgan and Law2004). Fabric opening angle is defined in inset diagram. All boxes (except one with dashed lines, which shows data from this research) indicate data taken from the literature (Kruhl, Reference Kruhl1996, Reference Kruhl1998; Law et al. Reference Law, Morgan, Casey, Sylvester and Nyman1992; Okudaira et al. Reference Okudaira, Takeshita, Hara and Ando1995).
7. Thinning and dip-parallel elongation
Deviation from ideal simple shear deformation implies thinning of the Sanandaj–Sirjan Metamorphic Belt perpendicular to the deformation zone boundaries (flow plane) and resultant dip-parallel elongation. For plane strain deformation, the stretch magnitude both normal and parallel to the flow plane can be calculated. In order to study the importance of the deviation from simple shear within the study area, the stretch magnitude both normal and parallel to the flow plane was calculated combining strain and vorticity data by using the mathematical expression suggested by Wallis, Platt & Knott (Reference Wallis, Platt and Knott1993):
![\begin{eqnarray}
S &=& \left\{\!\frac{1}{2}\big(1 \,{-}\, W_K ^2 \big)^{1/2} \!\left[\!\left(R_{XZ} \,{+}\, R_{XZ} ^{- 1} \,{+}\, 2\frac{{\big(1 \,{+}\, W_K ^2 \big)}}{{\big(1 \,{-}\, W_K ^2 \big)}}\right)^{\!\!1/2}\right.\right.\nonumber\\
&&+\left.\left.\!\!
\,{+}\, R_{XZ} ^{- 1} \,{+}\, 2\frac{{\big(1 \,{+}\, W_K ^2 \big)}}{{\big(1 \,{-}\, W_K ^2 \big)}}\right)^{\!\!1/2}}
\big(R_{XZ} + R_{XZ} ^{- 1} - 2\big)^{1/2}\right] \right\}^{- 1}\end{eqnarray}](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20180419074129359-0244:S0016756811000276:S0016756811000276_eqn2.gif?pub-status=live)
where S is stretch perpendicular to the deformation zone. Integration of strain and vorticity data yields dip-parallel elongation between 25% and 80% and thinning perpendicular to the flow plane between 22% and 44%.
8. Discussion
The microscope-based and detailed skeletal fabric parameter analyses indicate that subtle fabric asymmetries are consistent with a top-to-the-SE sense of shear. Strong c-axis fabrics were observed in all samples and can be divided into two groups: (i) single girdles with an asymmetric distribution with respect to the foliation; (ii) a cross girdle with an asymmetric distribution with respect to the foliation. These different groups may reflect different strain histories, with the asymmetric c-axis recording a non-coaxial deformation history (Lister, Reference Lister1977; Lister & Hobbs, Reference Lister and Hobbs1980). The sense of asymmetry observed in the c-axis fabrics is consistent with other observed microscopic and mesoscopic kinematic indicators (Fig. 5), which show top-to-the-SE sense of shear. Information about strain symmetry can be retrieved from the obtained quartz c-axis fabrics (Schmid & Casey Reference Schmid, Casey, Hobbs and Heard1986; Law, Reference Law, Knipe and Rutter1990). The crossed-girdle pattern of the quartz c-axis fabrics in the samples is interpreted to indicate approximately plane-strain conditions during top-to-the-SE ductile shearing.
Figure 5. Mesoscopic kinematic indicators of the Sanandaj–Sirjan Metamorphic Belt which show top-to-the-SE sense of shear. (a) Boudin trains formed during shear deformation indicating top-to-the-SE sense of shear and a strong stretching component parallel to the shear zone boundary. Diameter of coin is approximately 20 mm. (b) Field photo of a sigmoidal-shaped quartz lens with monoclinic symmetry which shows top-to-the-SE sense of shear. Pen is approximately 14 cm long.
Pattern geometry is primarily controlled by activity of different glide systems, which in turn are temperature sensitive. Both theoretical and experimental studies have indicated that c-axis fabric intensity increases with increasing finite strain (Hongn & Hippertt, Reference Hongn and Hippertt2001). The strain ratio increases systematically from the northeast to the southwest approaching a value of 6 close to the basal thrust of the Zagros Thrust System and a value of 2.8 in the inner parts of the study area. Similar integrated strain ratio changes have recently been reported for other convergent orogens including the Hellenides of Greece (Xypolias & Doutsos, Reference Xypolias and Doutsos2000; Doutsos et al. Reference Doutsos, Koukouvelas, Poulimenos, Kokkalas, Xypolias and Skourlis2000; Xypolias & Koukouvelas, Reference Xypolias and Koukouvelas2001; Kokkalas & Doutsos, Reference Kokkalas and Doutsos2004), the Himalaya (Law, Searle & Simpson, Reference Law, Searle and Simpson2004) and the European Alps (Ring & Kassem, Reference Ring and Kassem2007). The continuous evolution of cross-girdles to single-girdles has been interpreted as resulting from an increasing finite strain (Schmid & Casey, Reference Schmid, Casey, Hobbs and Heard1986). Quartz c-axis data show a continuous evolution from asymmetric cross-girdles to oblique single-girdles from northeast to southwest in the study area. The quartz c-axis fabric and kinematic vorticity number values might reflect a relative increase of the simple shear component towards the basal thrust of the Zagros Thrust System (i.e. from northeast to southwest).
Lattice preferred orientation in the quartz aggregates is an indicator of deformation temperature since the fabric pattern is controlled by the relative contributions of different slip systems, which in turn are temperature sensitive (Fig. 4a). Slip in the quartz occurs primarily in the <a> crystallographic direction, predominantly on the basal, rhomb and prism planes. Basal <a> slip is dominant at lower temperatures and at faster strain rates, causing a c-axis fabric maximum near the Z-axis of the finite strain ellipsoid. With increasing temperature, a rhombohedral <a> slip system becomes activated, causing a fabric maximum at an intermediate orientation between the Y- and Z-axes. Ultimately, at still higher temperature and slower strain rates, the prism <a> slip system operates, causing a fabric maximum near the Z-axis of the finite strain ellipsoid (Bouchez, Reference Bouchez1977; Hobbs, Reference Hobbs and Wenk1985; Lister & Dornsiepen, Reference Lister and Dornsiepen1982; Mainprice et al. Reference Mainprice, Bouchez, Blumenfeld and Tubia1986; Ralser, Hobbs & Ord, Reference Ralser, Hobbs and Ord1991; Schmid & Casey, Reference Schmid, Casey, Hobbs and Heard1986; Tullis, Reference Tullis1977; Tullis, Christie & Griggs, Reference Tullis, Christie and Griggs1973). Using a modified geothermometer and estimates of mean temperature we propose that the quartzites of the northern parts of the study area were deformed under greenschist facies conditions whereas the quartzites close to the basal thrust were deformed under amphibolite facies conditions. The opening angles of the quartz fabrics reveal that deformation temperature varies from 435 ± 50°C within the northern parts of the study area to 510 ± 50°C within the southern parts of the study area (Fig. 4b). The appearance of crossed-girdles in the northern parts of the study area indicates the dominance of basal <a> gliding, with less pronounced rhombohedral <a> and prism <a> gliding, which is common for deformation under low grade metamorphic conditions (Schmid & Casey, Reference Schmid, Casey, Hobbs and Heard1986; Law, Reference Law, Knipe and Rutter1990). According to the dependence of quartz lattice preferred orientations and inferred slip systems on increasing temperature (Passchier & Trouw, Reference Passchier and Trouw2005), it seems that the southern parts of the study area experienced higher temperatures compared to the northern parts (Fig. 4a).
Using numerical relationships originally established by Wallis, Platt & Knott (Reference Wallis, Platt and Knott1993), integration of the strain and vorticity data allows us to estimate the amount of shortening in the mylonitic quartzites within the Sanandaj–Sirjan Metamorphic Belt, measured perpendicular to the flow plane. For plane strain deformation, stretching parallel to the flow plane in the transport direction is simply the reciprocal of the shortening value. However, integrated strain and vorticity data indicate that vertical thinning and transport-parallel stretching were important tectonic processes during thrusting at the base of the Zagros Thrust. This suggests that nappe emplacement occurs by a combination of simple shear with a pure shear component of vertical flattening/shortening. Vertical thinning by penetrative deformation associated with combined pure and simple shear provides at least one mechanism for extrusion of deeper crustal rocks to shallower levels (Feehan & Brandon, Reference Feehan and Brandon1999).
9. Conclusion
Extensive quantitative new structural, textural and quartz petrofabric analyses within the Sanandaj–Sirjan Metamorphic Belt reveal that mesoscopic and microscopic kinematic indicators show unequivocal top-to-the-SE sense of shear. Quantitative vorticity analyses in conjunction with quartz fabric data and microstructural analyses indicate that deformation within this belt involved general non-coaxial flow with contemporaneous contributions of pure and simple shear. The results of this study demonstrate that both strain ratio and the simple shear component of deformation increase towards the basal thrust of the Zagros Thrust System. The observed increase in strain and vorticity within the study area is comparable with patterns recorded within other orogens in the world.
Acknowledgements
The authors would like to thank the editors Mark Allen, Bernhard Grasemann, Guy Simpson and Olivier Lacombe for their editorial authority. We are grateful to Michael Edwards and an anonymous reviewer for helpful and valuable reviews of the manuscript. The Research Council of the Shiraz University (RCSU) has supported this study, which is gratefully acknowledged.
