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Ductile Shear zones are the widespread structure that localized strain and accommodate deformation in the lithosphere (Ramsay and Graham, 1970; Ramsay, 1980; Simpson, 1981; Shan, 2008; Fossen and Cavalcante, 2017). These zones provide valuable data about the progressive deformation history, such as mineral assemblage and microstructures, P-T conditions and shear sense indicator. These data critically needed to improve understanding of regional tectonics (Watts and Williams, 1983; Zhang et al., 2010; Toy et al., 2012; Mookerjee and Fortescue, 2016).

Primitive research in ductile shear zone proposed that the heterogeneous simple shear is a dominate deformation regime (Ramsay, 1980; Ramsay and Allison, 1979; Ramsay and Graham, 1970; Simpson, 1981; Watts and Williams, 1983) but recent research indicates that in most of the shear zones involve three-dimensional combinations of simple and pure shear strain, such as transpression and transtension zone (Bhattacharyya and Hudleston, 2001; Montesi, 2013; Liang et al., 2015; Fossen and Cavalcante, 2017; Behyari and Moghadam, 2018).

Mylonitic rocks formed as a result of heterogeneous shear strain in the ductile shear zone, and their fabric directly related to the strain rate, finite strain ellipsoid shape and orientation (Watts and Williams, 1983; Zhang et al., 2010; Mulchrone and Talbot, 2014; Fossen and Cavalcante, 2017). Strain ellipsoid shape is an effective tool to the deciphering rheological characteristic and reconstructing progressive ductile deformation history. Therefore in the recent decades, many efforts have been conducted to gain three-dimensional strain ellipsoid shape (Flinn, 1962; Shan, 2008; Mookerjee and Nickleach, 2011; Mulchrone and Talbot, 2014; Soares and Dias, 2015; Mookerjee and Fortescue, 2016).

In the west of Iran recognized shear zone, accommodated mylonite zone that related to the closure of Neo-Tethys and Zagros orogeny contractional event (Mohajjel and Fergusson, 2000; Agard et al., 2005; Allen and Amstrong, 2008; Agard et al., 2011; Mouthereau et al., 2012). Deformation in these shear zones is associated with chemical and mineralogical changes (Aliyari et al., 2009). The circulations of the magmatic fluids convert this zone to be a host for mineralization and ore deposit. The orogenic gold deposit is the typical manifestation in the Zagros collisional Orogenic belt (Aliyari et al., 2009; Alizadeh-Dinabad et al., 2013; Almasi et al., 2015; Almasi et al., 2017). The crustal shear zone has been playing an important role in the localization strain and providing the proper area for mineral deposition (Heidari et al., 2006; Alizadeh-Dinabad et al., 2013). Nevertheless, the kinematic evolution of deformation in the ductile shear zone, as well as P-T conditions, is not well known. For this reason, spatial and temporal relationship between the episode of ductile deformation and geodynamic evolution, it is not limpid.

In this research, we focused on the kinematic evolution of the ore-bearing ductile shear zone in the Zagros Orogeny and applied strain ellipsoid shape as a vector to the interpretation of structure and intensity of deformation. The main aim is graphically represent the relative principle strain ellipticity ratio on the Flinn diagram and interpret structure developed underwent this deformation conditions. Finally, these data used for the reorganization of structural pattern that controls the strain ellipticity ratio and shape on the orogenic shear zone.


The study area is a part of Zagros Orogeny and located in the Sanandaj-Sirjan metamorphic Zone (SSMZ). The Zagros Mountains in Iran is a part of the Alpine--Himalayan Orogenic belt with the northwest-southeast trending (Fig. 1a). As a result of a northeast-dipping subduction process of the Neo-Tethys oceanic crust, beneath the Iranian micro-continent active margin, formed two major parallel domains with the Zagros suture zone, first is Sanandaj--Sirjan metamorphic Zone (SSMZ) and second is Urumieh--Dokhtar Magmatic Arc (UDMA) (Berberian and King, 1981; Mohajjel and Fergusson, 2000; Agard et al., 2005; Allen and Armstrong, 2008). The subduction process initiated in the late Jurassic (Alavi, 1994; Mohajjel et al., 2003; Ghasemi and Talbot, 2006; Agard et al., 2011) and was terminated by the continental collision between the Arabian and Iranian micro-continent. The timing of the final Neo-Tethys ocean closure and the structure is formed in this tectonic regime is one of the most controversial subjects. Early contribution emphasized final colliding occurred in the late Cretaceous (Sengor and Kidd, 1979; Berberian and King, 1981; Alavi, 1991; Alavi, 1994), but most of the recent researches assumed closure of Neo-Tethys happened in the Cenozoic (Mohajjel et al., 2003, Agard et al., 2005; Agard et al., 2011; Mouthereau et al., 2012; Francois et al., 2014). After collision subsequent convergent accommodated along the major thrust and strike-slip fault system and propagated transpression and transtension basin (Jackson, 1992; Mohajjel and Fergusson, 2000; Talebian and Jackson, 2002; Mohajjel and Rasouli, 2014).

In the study area, rock units consist of metamorphosed sequence from the Precambrian to the Tertiary (comprises: Genesis, schist, Mica-schist, and meta-rhyolite). Mesozoic rocks in the study area deposited after a long interval with older units; this contains a complex of volcanic-sedimentary. These units have widespread exposure, high thickness and metamorphosed to greenschist facies (Aliyari et al., 2009; Alizadeh-Dinabad et al., 2013; Almasi et al., 2015; Almasi et al., 2017) (Fig. 1b).

The Eocene volcanic unit is the youngest non-metamorphic volcanic activity in the study area. The transpression tectonics regime is dominated in the Cenozoic and particularly in the Eocene (Mohajjel and Fergusson, 2000), and could be facilitated volcanic activity (Fig. 1b).

Consideration to the regional map (Fig. 1b) revealed the study area located in the transition zone between Zagros sedimentary basin and SSMZ, indeed this area coincides with the suture zone between the Arabian plate and Iran micro-continent. Ophiolite melange in the Baneh area with NW-SE trend continued to the west over the border of Iraq. This unit is tectonically melange and is impossible to be recognized sedimentary units from ultramafic segments. The sedimentary part includes 20 percent of the whole of ophiolite melange volume. This part includes two different facieses: pelagic limestone with the Paleocene age and grey limestone of the Eocene age (GSI, 1976). The boundary sedimentary units with ultra-mafic units is a thrust fault, the dip of the fault plane is towards N-NE (Agard et al., 2005).

In the southern part of the study area, Eocene deposits were affected by several faults so that in some parts, they mixed with the ophiolite.


Deformation and metamorphism in the convergent plate boundary caused to superimposition of the different phases of deformation and propagated the complex structural pattern, In the study area collisional event of the Neo-Tethys followed by the magmatic body intrusion (Azizi et al., 2018; Zhang et al., 2018; Behyari et al., 2018) and development of complex structures, consists of: foliation, ductile shear zone, and brittle structures such as conjugated fault, fractures and veins. The deformation history of study area can be classified into three stage. The first stage is a co-axial penetrative deformation that affected Paleozoic units and developed pervasive foliation. The second events are non-coaxial and non-penetrative deformation concentrated in the shear zone. In the some parts of the study area this deformation phase overprint on the older deformation phase. The main structures propagated in this phase are mylonitic and S-C fabric, Minerals fish and lensoid structures, and shear folds. The third deformation phase is the semi-brittle phase and fractures and brittle faults are the results of this phase. In this section, the structures describe according to the precedence of evolution and investigated variation of strain ellipticity ratio and shape in the collisional zone:


This phase of deformation occurred contemporaneously with the regional metamorphism in the SSMZ. The foliation is the most important structure developed in this stage (Sarkarinejad et al., 2010; Sheikholeslami, 2015) The deformed Precambrian unit consists of schist, phyllite, and slate, exposed extensively in the east and south-east of the study area. The ribbon of meta-Rhyolite, metamorphosed acidic tuff and ignimbrite lie parallel to this Precambrian unit (Fig. 2).

The foliation general spatial orientation is parallel to the main thrust fault in the study area and the Zagros faults strike NW-SE in this region (Agard et al., 2011; Mohajjel and Fergusson, 2000). In the K03 station near the Barika Au-index foliation planes, attitude is N40[degrees]W/50[degrees]NE, these foliated planes superimposed by subsequent deformation path. Around this area in K04 and K05 foliation, attitude is approximately same to the K01 station (Fig. 2).

In the Gandoman Au-index (K06, K07), two foliation planes propagated, and the dominant foliation is parallel to the previous stations, but the dip of the plane increased respect to the Barika index (K01, K04). On average these set of the foliation plane's attitude are N30[degrees]W/70[degrees]NE. The weaker foliation set trend is NE-SW and has dipped to the NW, the measurement of this set of foliation indicated general orientation is N20[degrees]E/50[degrees]NW. In the other Au-indexes such as Zavakoh (K13), Merganagshina (K14), Shooy (K15) foliation general trend is NW-SE and dip direction is toward NE and dip of foliation plane change in the wide range between 30[degrees] to 70[degrees].

In the thin section used XZ and YZ planes data for the explanation of foliation morphology. In the K03 propagated spaced foliation. Cleavage domain contains opaque mineral and microlithons are consist of very fine grain of plagioclases. A weak crenulation developed in the cleavage domain, but it is not penetrative in the section. This crenulation in somewhere caused to the propagation of new foliated plane. The shape of the cleavage domain is smooth and spatial relation between cleavage domains is anastomosing (Fig. 3a).

The foliation type in the Phyllite, meta-sandstone unit (K05) is continues and very fine grain, fabric elements are homogeneously distributed in the rock. The volume percentage of cleavage domain is above 90 % and quartz filled veins cut the foliation plane (Fig. 3b). In the Meta-volcanic rocks (K10) weak spaced foliation propagated. The cleavage domain contains fine grain sericite and microlithones deformed quartz (Fig. 3c). The foliation type in Precambrian Schist, phyllite and slate units (K12) is fine grain and continuous. These foliation planes penetrated in the whole of the rock volume (Fig. 3d). In the other samples from this unit (K13, K14, K15) the strong superimposition of subsequent deformation phase removed evidence of the D1 foliation. Generally this stage of deformation affected Paleozoic and Mesozoic rock units also with attention to the pervasive effect of D1 event in the SSMZ, assumed this phase occurred contemporaneously with the collision of Arbain plate with the Iranian microcontinent.


The dynamo-thermal regional deformation followed by severe ductile deformation. This phase is the most important event in the study area and associated with the development of the high strain, Au-bearing ductile shear zone (Aliyari et al., 2009; Almasi et al., 2017). Event D2 is related to the post-collisional contraction in the Zagros suture zone. Deformation evidence of this phase mostly has been studied in the ductile shear zone. The mylonitization is one of the significant effect of ductile deformation in this period and propagated mylonitic foliation with the NW-SE general trend.

The shear zone spatial orientation measured in the different station, in average N60W/50NE, is the shear zone orientation, this trend is approximately parallel with the Zagros Orogeny (Fig. 4a). In the ductile shear zone outcrop, the frequent deformed structures are the folded mylonitic foliation and well developed S-C structure. S-plane and C-plane combined with each other and created lensoid shape structures in the rock units (Fig. 4b). Also folded and sheared silica vein is the common structure in the shear zone. These veins are in two sets, first set cut the foliation and second are parallel to the foliation plane (Fig. 4c).

The thin section feature has been used for determination of sense of shear, the intensity of deformation, P-T condition in the shear zone and deciphering strain ellipsoid shape. Thirty thin sections from ten samples covering all the deformation event in this phase. At the K03 station in the XZ plane dominant structures are distinct light and dark bands that are typical of in the ultra-mylonite. The striped mylonite structure consists of continuous bands of mostly recrystallized grains, this plane attitude is N40[degrees]W/60[degrees]NE approximately parallel to the foliation and connected with the shear plane. These structures proposed sinistral shear sense for this zone (Fig. 5a). In the K06 recrystallized fabric with frequent porphyroclasts revealed earlier mylonitization stage. Relict quartz porphyroclasts underwent ductile deformation indicate sweeping and undulatory extinction. Also fabric elements in the result of shear strain showing preferred orientation. Cleavage domain composed of dark biotite, opaque minerals, and a few hornblende and microlithon contains light long and lozenge-shaped relict quartz grains. The width of dark and light zone increased in this sample (Fig. 5b). In the K07 station grains indicated more evidence of strain than the K06 station, proposed that the deformation was accommodated by recrystallization and grain size reduction. In the cleavage domain increased the concentration of dark grains and in the microlithone decreased prophyroclast size and frequency (Fig. 5c). At the other stations and samples, the structure elements are similar to the described stations, mylonitic foliation and shear structure are dominated and attitude of structures is approximately identical. The deformed basement in the study area comprises of Precambrian schist, phyllite, slate, and cut by Au-bearing ductile shear zone samples K12, K13, K14 are from these shear zones. The main change in K12 respect to the other station is the propagation of continuous foliation accompanied by notable grain size reduction. The veins cut the foliation plane and showed sinistral displacement along the shear plane (Fig. 5d). One of the Au-bearing shear zones is the Zavekouh shear zone in the east of the study area. The sample K13 is quartzite mylonite (medium grade mylonite) and taken from this shear zone. The main difference with respect to other shear zones in the study area is well-developed quartz porphyroclast texture accompanied by dextral sheared microstructure and preferred orientation. The quartz mineral fish widely propagated and indicated dextral shear component also long and lozenge shape quartz microstructures are immersed in a very fine-grained matrix (Fig. 5f). Mylonitic foliation is a penetrative structure in the shear zone. Deformation mechanism in the K14 (Merganagshina) station is similar to the K13. The quartz porphyroclasts showing dextral shear and mostly have preferred orientation. With attention to the commonly used classification of mylonites is based on the percentage of matrix respect to the porphyroclast) this shear zone is medium grade mylonite (Passchier and Trouw, 2005). In the hand sample folding of the foliated plane is emerged (Fig. 5g). Quartz grains, mostly have sigmoid shape. The striped structure propagated by the sequence of dark and light grains arranged in the approximately mono-minerals layers.


The latest deformation path in the study region is a low-temperature brittle deformation phase acted after ductile deformation and mylonitization processes. This retrograde deformation phase characterized by the faults, micro-fracture which cut through the foliation propagated during ductile events. Micro-fracture, mostly filled with quartz grains and caused to the propagation of silicification and sulfide alteration. Geochemical analysis revealed these zones coincident with the concentration of Au in the ore-bearing shear zone (Aliyari et al., 2009). Several types of faults with the various attitude developed during this brittle events. Fault zone characterized by the concentration of fracture and fault breccia. Conjugated faults also are one of the common structures in this brittle zone, the general trends of faults are NE-SW and NW-SE (Fig. 6a). Brecciated zone width with tens of centimeters is most common (Fig. 6b). Precambrian recrystallized deformed basements exhumed along the thrust faults in some outcrops thrust fault plane preserved, field data revealed these type faults, general attitude is N45[degrees]E/40[degrees]SE and slickenlines spatial orientation are 185[degrees]/80[degrees] and have dextral strike-slip component (Fig. 6c). In the fault zone under compressional conditions propagated folded layers (Fig. 6d). Fracture in the fault zone, mostly filled with the quartz and calcite, these veins can be divided into two categories based on the orientation of the vein with respect to the foliation plane. The first set is cut through the foliation plane, these veins are more abundant and have a less aperture than other veins (Fig. 6e). The second set of veins inject along the foliation plane and are parallel to these surfaces. These veins are mostly thick and filled with calcite (Fig. 6f).


For calculating the strain ellipsoid shape in the ore-bearing shear zone in the study area, applied a suite of the geological program developed by (Mookerjee and Nickleach, 2011) and written in the Mathematica. The advantage of this program is utilized of arbitrary sectional ellipses to determining a general ellipsoid. The sample preparation process conducted according to the (Mookerjee and Fortescue, 2016), for this purpose 10 samples collected along the ore-bearing shear zone in the study area. Thin section for microscopic study prepared in three perpendicular directions. XY section indeed is foliation plane and XZ section are perpendicular to XY section and foliation plane strike, finally YZ section are perpendicular to the both of XY and XZ section. In the next step with attention to the lithology units of the study area selected quartz grain as strain index. Photomicrographs were taken and merged with each other to gain complete thin section image (Fig. 7a). Graphical program effects applied to the edge enhancement of quartz grain, then selected grain boundary to drown on the photomicrographs (Fig. 7b), in each thin section between 50 to 100 grains used to refine strain estimation. This drawing figures exported as an individual layer to the ImageJ program (Fig. 7c). This program used to trace the best fit ellipse for imported quartz grains (Fig. 7d). The resulted dataset exported as Microsoft office Excel datasheet. This dataset imported to the Mathematica script, "Best-Fit Ellipsoid with Statistics for ImageJ with Vort-v3_0.nb" (Mookerjee and Nickleach, 2011). This process conducted for elliptical quartz marker in the three XY, XZ and XY section (Fig. 7e). Finally best fit ellipsoid extracted with each of sectional planes (Fig. 7f).


Complicated deformation history in the study area is related to the geodynamic setting of this region and collisional event in the Zagros orogeny (Agard et al., 2011; Mohajjel and Fergusson, 2000; Mouthereau et al., 2012). This collision event coincident with regional deformation in green-schist facies in the SSMZ. After collision contractions in the suture zone accommodate by transpression shear zone (Mohajjel and Rasouli, 2014). These shear zones in the earth crust provide a high permeability fractured zone, and facilitated the circulation of hydrothermal fluid that mostly accompanied with the mineralization. High concentration of Au in the ductile shear zone directly related to the deformation intensity in the study area (Aliyari et al., 2009; Alizadeh-Dinabad et al., 2013; Almasi et al., 2015; Almasi et al., 2017). Strain ellipsoid geometry provides important data about deformation history in the shear zone (Mookerjee and Nickleach, 2011). The perfectly oblate strain ellipsoid is common in the shear zone that deformed underwent flattening strain and perfectly prolate ellipsoid developed under constructional strain regime (Fossen and Cavalcante, 2017; Mookerjee and Peek, 2014). To gain strain ellipsoid shape requested the axial ratio of the long and intermediate (Rxy) and intermediate and short axes (Ryz). These data extracted from the deformed quartz shape on the XY and YZ thin section for each sample also shear strain are proportional with Rxz therefore, this value also calculated for each sample. Strain ellipsoid principal axes ratio increasing, accompanied with increasing of deformation intensity. In the study area high deformed zone characterized by grain size reduction and recrystallization of protolithic grain, relict porphyroclast frequency decreased and finally increased foliation volume percentage respect to the whole of the rock unit. Evaluating of calculating Rxz in the samples indicated the value varied between 2.45 to 4.25 and total strain ellipsoid principal axes ratio range is between 2.04 to 3.12. Strain ellipsoid shape in all stations except K12 is prolate and formed under constructional strain conditions. In the K12 strain ellipsoid shape is oblate and propagated in the flattening regime. This interpretation has the important geodynamic concept. In the collision zone expected oblate strain ellipsoid shape and dominant deformation mechanism be flattening. This data revealed Au-bearing ductile shear zone cannot be coeval with the collisional event (D1 deformation phase). After the closure of Neo-Tethys and continental collision contraction in the suture zone accommodate by propagation of ductile shear zones. In this period Urmia-Dokhtar magmatic arc was active in the study area and magmatic fluid penetrated in the ductile shear zones and deposit the Au mineral. Constructional strain in the shear zones caused to developed sheared and stretched structure. Indeed post-collisional (D2 phase) transtension evidence superimposed on collisional (D1 phase) transpression structure in the Zagros collision zone. The evidence of this superimposition of trenstenssion on transpression in the brittle shear zone well documented by (Mohajjel and Rasouli, 2014). The strain ellipsoid parameters and shear intensity varied in the different samples and stations. In the K03 station (Barika Au-index), Rxy=1.29 and Ryz=2.43 and the total finite strain ratio equal to 2.05, the foliated plane (XY plane) developed on the D1 phase superimposed by stretching lineation in D2. K05 station is located near the high strain ductile shear zone with NW-SE trend. Rxy=1.31 and Ryz=3.82 and total final strain ratio are 3.12. Strain ratio in the XZ plane equal to Rxz=4.22 and imply to high shear strain in this station. This strain condition caused to propagation ultra-mylonite unit and increased foliation percentage to the rock volume intersection of the foliated plane and stretching lineation increased the permeability of the zone. K06 and K07 located near the K05 station on the Phyllite, metasandstone unit the common characterize of these stations is the relatively high value of strain ellipsoid principal axes ratio on the XZ plane. Gandoman Au-bearing ductile shear zone strain ellipsoid studied by samples K10 and K11. In the K10 total strain ellipsoid principle axes ratio is 2.04 and in K11 is 2.21. Rxz also calculated on the XZ plane for K10=2.65 and K11=2.93. The K12 is a spatial sample, because in this station strain ellipsoid shape is different with other station. This sample taken from a thrust fault zone and contraction of fault zone caused developed the oblate shape of strain ellipsoid. Mean principal axes orientations calculated for all stations and projected in the lower hemisphere, equal-area stereonet. This data indicated wide range, diversity for long, intermediate and short axes orientation. The rotation of the principal strain axes is related to high vorticity during the progressive deformation in the shear zone. Approximate estimation imply the general orientation of strain ellipsoid long principal axes is NE-SW and plunge varied between 30[degrees] to 50[degrees] (Table 1).

For the visualized the strain ellipsoid shape and interpretation data all principle axes measurement in the XY and YZ plotted to the Flinn diagram. Strain ellipsoid geometry in the study area, mostly are prolate and only K12 sample set in the oblate zone (Fig. 8a). Strain magnitude generally increases away from the origin, therefore in K10, K13, K05 occurred maximum stretching (Flinn, 1962; Fossen, 2016). In the Hsu diagram data plotted on the 60[degrees] sector of the circle, the lode ratio (V) defined the strain ellipsoid shape and shear strain ([epsilon]s) represent the intensity of strain in the shear zone (Fig. 8b). This diagram indicated shear strain in most of shear zones in the study area is more than 0.9.


Structural analysis in the study area revealed a poly-phase deformation that caused to propagate complicate high strain zone accompanied with Au mineralization. The first deformation phase is dynamo-thermal regional event could be coincides with the closure of Neo-Tethys in the Zagros orogeny and propagated a penetrative spaced foliation under the flattening conditions and nearly oblate strain ellipsoid. The second phase followed by the contraction in the suture zone and accommodated by the transpressional ductile shear zone. Stretching in the ductile shear zone under constructional regime caused to propagation the prolate strain ellipsoid shape. The strain axes ratio in the XY and YZ section at high strain ductile shear zone indicated strain ellipsoid shape in the most of the shear zone is prolate and formed under the constructional regime and only in the one station oblate strain ellipsoid propagated under flattening conditions. With increasing deformation in ductile shear zone ascending the Au deposition and evaluating of strain magnitude showed generally in the shear zones of the study area shear strain are more than 0.9 and maximum shear accommodate in the station K10, K13, K05.

The strain ellipsoid shape also indicated propagation of shear zone could not be coeval with the continental collision because the collision region expected the ellipsoid shape to be oblate. Indeed, transpression in the Zagros Orogeny contemporaneous with the collisional event (D1 phase) and widespread in the SSMZ but transtension event is related to post-collisional deformation phase (D2 phase) and localized in the ductile shear zone. The third deformation phase is a brittle event and caused to the propagation of veins. These veins somewhere cut the foliation and in the other place are parallel to foliation planes.


This project financials support provide by Urmia University. We grateful to research Bureau of Urmia University to the authorities. We also thankful to the Dr. Matty Mookerjee for kindly help to the utilized of Mathematica script.


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Mahdi BEHYARI (*) and Afshin KANABI

Department of Geology, Faculty of Sciences, Urmia University, 5756151818, Urmia, Iran

(*) Corresponding author's e-mail:

Cite this article as: Behyari M, Kanabi A: Constraining of strain ellipsoid shape from sectional data in the Au bearing shear zone west of Iran. Acta Geodyn. Geomater., 15, No. 2 (194), 131-142, 2019. DOI: 10.13168/AGG.2019.0010

DOI: 10.13168/AGG.2019.0010


Article history: Received 31 October 2018

Accepted 11 February 2019

Available online 21 February 2019
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Title Annotation:ORIGINAL PAPER
Author:Behyari, Mahdi; Kanabi, Afshin
Publication:Acta Geodynamica et Geromaterialia
Geographic Code:7IRAN
Date:Apr 1, 2019

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