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Heterogeneous sub-simple deformation in the Gol-e-Gohar shear zone (Zagros, SW Iran): insights from microstructural and crystal fabric analyses
Abstract
Petrofabrics and microstructural characteristics of the quartz-rich mylonites were investigated in the NW–SE-trending Gol-e-Gohar shear zone with length of ca. 25 km within the Zagros Transpression Zone, SW Iran. The semi-quantitative vorticity analysis using quartz c-axis-fabric (Rxz/β method) and rigid porphyroclast geometry (Rigid Grain Net method) reveals that mean kinematic vorticity number (Wm) increases towards the Baghat Thrust Fault displaying the occurrence of spatial and temporal variations of flow during progressive deformation. The Wm values show pure shear-dominated flow progressively was changed to a simple shear-dominated flow toward to the late stages of ductile deformation, implying that the accelerating flow path was accompanied with progressive cooling and strain localization at the lower parts of the zone. The measured vertical thinning/dip-parallel elongation suggests that thrust emplacement and metamorphic rock extrusion likely occurred by a combination of simple shear with a pure shear component of deformation (i.e., general shear). The estimated deformation temperatures based on the quartz c-axis fabric patterns and their opening angles highlight greenschist-to-amphibolite-facies conditions.
... The location of the study area and some of shear zones in the Sanandaj-Sirjan zone presented by the black and with rectangle, respectively. (1) the Dehbid shear zone ; (2) Neyriz shear zone (Faghih and Sarkarinejad, 2011); (3) Heneshk metamorphic rocks (Samani et al., 2020); (4) Gol-e-Gohar metamorphic complex (Keshavarz and Faghih, 2020). (5) the Darizhun shear zone (Derikvand, 2021). ...
... These results, therefore, demonstrate that at the scale of the entire shear zone, pure and simple shear components of deformation were spatially distributed at different structural levels. Our results on the spatial variations of the shear components in the Kahdan mylonitic rocks in the SE part of the Sanandaj-Sirjan zone are in agreement with that reported from the other high strain shear zones from the central and northwest part of this belt, such as the Dehbid shear zone ; Neyriz shear zone (Faghih and Sarkarinejad, 2011); Heneshk metamorphic rocks (Samani et al., 2020); Gol-e-Gohar metamorphic complex (Keshavarz and Faghih, 2020) the Darizhun shear zone (Derikvand, 2021). This strain partitioning is common in regions of oblique convergence and defined in terms of inclined transpression Jones and Tanner, 1995;Teyssier et al., 1995). ...
Microstructural, vorticity and strain analyses, combined with geological field observations are fundamental tools for retrieving information about kinematics of deformation in crustal-scale shear zones. The Kahdan shear zone is located in the SE part of the Sanandaj-Sirjan metamorphic zone within the Zagros orogen, SW Iran. The Zagros Orogen that formed due to the oblique convergence between the Afro-Arabian plate and the central Iranian plateau. Three-dimension strain analyses show that finite strain ellipsoids shape is oblate (K = 0.38–0.89). Vorticity analysis on mylonites yielded arithmetic mean maximum (Wm max) and mean minimum values (Wm min) of 0.83 and 0.72, respectively (63–47% simple shear). Higher values of Wm and Rxz estimated adjacent to the shear zone boundaries and gradually decrease toward the central part of the zone. Our yielded results show a non-unified spatial variations of vorticity and strain values in the deformed metasandstone (Late Jurassic to Early Cretaceous) of the Kahdan shear zone. Vorticity analyses and noncoaxial indicators indicate that this zone experienced general shear deformation. The horizontal shortening of ∼13–27% measured perpendicular to the shear zone boundaries. Based on yielded results, this area provides a good example of strain partitioning in a ductile shear zone.
... The different works are described in Table 3. Different types of rocks with various origins are exposed in the hinterland of Zagros orogenic belt including slate, phyllite, quarzitic and micaceous schist, deformed Precambrian rocks, volcanic rocks, granite mylonite, quartz mylonite, gneiss, and amphibolite (Mohajjel and Fergusson, 2000;Shakerardakani et al., 2015;Sarkarinejad and Derikvand, 2017). The degree of metamorphism varies widely from low-to high-grade metamorphic conditions and occurs under greenschist to amphibolite facies (Behyari and Kanabi, 2019;Keshavarz and Faghih, 2020;Derikvand and Almasi, 2022). ...
... Since previous strain analyses from Zagros used different strain markers within different rock types, it is difficult to compare data amongst different locations. One can at least comment that a general-shear deformation (sub-simple shear; Mulchrone and Mukherjee, 2019) by increasing the simple-shear component towards the basal thrust sheet (Table 3; Sarkarinejad et al., 2015;Sarkarinejad and Heibati, 2017;Keshavarz and Faghih, 2020;Samani et al., 2020;Derikvand, 2021;Derikvand and Almasi, 2022) prevailed. The northern, southern and central parts of the hinterland of the Zagros orogenic belt lack vorticity analyses and can be the target of future studies. ...
New structural data on radiolarite and quartz mylonite rocks from the well-exposed Kamandan area are presented with the aim to shed light on the quantitative finite strain and vorticity analyses and strain partitioning along the Zagros transpression zone. Duplex structures, imbricate fans, thrust sheets and shear zones are well developed in the Kamandan region, located in the Sanandaj-Sirjan Metamorphic Belt, western Iran. Kinematic shear sense indicators associated with Kamandan shear zone show top-to-the-NE sense of ductile shearing deformation. Integrated quantitative finite strain and kinematic vorticity analyses of deformed rocks indicate deformation pattern and geometry. The strain parameter measurements vis. strain ratios and strain ellipsoid shape (k) show that radiolarite and quartz mylonite rocks have deformed in the constrictional to approximately plane-strain conditions. The quartz mylonite rocks highlight general-shear (sub-simple shear) deformation (Wm = 0.68–0.81) associated with the spatial increase in simple-shear component (47%–60%) from NE to SW. Our results of structural, strain, and vorticity analyses confirm a dextral transpression with strain partitioning into the pure-shear and simple-shear components in the Kamandan shear zone. Estimated Wm-values display the θ angle between the transpressional boundary and the maximum instantaneous strain axis (ISA) is the range from 63 to 69°. Accordingly, the oblique convergence angle (α) between the flow apophyses was estimated at about 35–48° that causes a horizontal shortening of ∼32–42% perpendicular to the boundaries of the Kmandan shear zone.
... Most studies (e.g. Authemayou et al., 2009;Sarkarinejad et al., 2010;Babaahmadi et al., 2012;Shafiei and Mohajjel, 2014;Behyari and Shahbazi, 2019;Keshavarz and Faghih, 2020;Partabian and Faghih, 2021) recorded the structural elements and kinematics of flow in the Zagros orogenic belt. In fact, these studies Partabian and Faghih (2021) highlighted the occurrence of transpressional deformation of Harland (1971) in various parts of the Zagros orogen which resulted due to oblique subduction and subsequent collision of the Afro-Arabian plates with the Central-Iranian microcontinent. ...
... All these works confirm that the Zagros is a transpressive orogenic belt consisting from northeast to southwest of a volcanic arc (Urumieh-Dokhtar), a hinterland with high-pressure metamorphic rocks (Sanandaj-Sirjan), and a foreland fold and thrust belt (Zagros Fold-Thrust Belt). The geometry and style of synmetamorphic deformation structures in the Sanandaj-Sirjan metamorphic belt are compatible with their formation in a dextral oblique collision between the Afro-Arabian and Central Iranian continents, in which displacement partitioning has occurred during Late Cretaceous to Recent (Mohajjel and Fergusson, 2000;Sarkarinejad et al., 2009 and for more details see Fig. 11, in Keshavarz and Faghih 2020). However, similar structural style and deformation history were reported from shear zones in the lower crustal rocks at different parts of the other convergence plate boundaries in the world such as Aravalli-Delhi mobile belt, NW India (Tiwari et al., 2019), Eastern Ghats Belt (Saha and Karmakar, 2015), Attico-Cycladic Massif from Internal Hellenides, Greece (Xypolias et al., 2010), Everest massif, Tibet (Law et al., 2004) and Ferriere-Mollières shear zone of the Southern European Variscan Belt (Simonetti et al., 2018). ...
The Tanbour Metamorphic Complex (TMC) is located within the Sanandaj-Sirjan Metamorphic Belt, SW Iran, along the northeastern margin of the Afro-Arabian plate. Detailed kinematic and strain analyses were combined with vorticity estimates on rocks from the TMC to document patterns and geometry of deformation. The strain parameter measurements including strain ratio in the XZ principal plane of strain ellipsoid (RXZ), strain ellipsoid shape (K) and strain intensity (D), highlight the deformed gneissic rocks in the TMC exhibit constrictional to approximately plane-strain shape of the strain ellipsoids. Several kinematic shear sense indicators including S/C fabrics, mantled porphyroclasts, oblique grain shape, micro-shears with bookshelf geometry and mineral fish show top-to-the-SE sense of shear. The quantitative kinematic analyses highlight that Wk varies between 0.64 and 0.78, implying a general shear flow with 44% < simple shear <58% and 42% < pure shear <57%. The exhumation of TMC is attributed to the exhumation-controlled transpressive tectonic process during oblique convergence between the Afro-Arabian continent and the Central-Iranian microcontinent which started since Upper Cretaceous.
... The finite strain data (R XZ ) obtained by R f /ϕ method (Ramsay and Huber, 1983;Lisle, 1985) on elliptical strain markers such as plastically deformed elongate, slightly recrystallized quartz grains lying in the plane of the main foliation (e.g. Xypolias, 2009;Samani, 2013;Keshavarz and Faghih, 2020). About 70 grain orientation (defined by the angle, ϕ, relative to the trace of foliation) and aspect ratio, R f , were measured on a microphotograph of XZ-plane thin sections, then are operated on a Rf/ϕ software of Chew (2003) in excel Platform to obtain Fig. 7. Graph of quartz c-axis fabric opening angles versus estimated deformation temperatures of naturally deformed quartz (Kruhl, 1998). ...
... However, this is unlikely to be true in most cases, and the NCDS shear zone is of no exception. The kinematic vorticity analysis is an effective way to test the state of deformation and that has been widely applied to better reconstruct the tectonic processes of shear zones, wordlwide (Fossen and Tikoff, 1997;Xypolias and Doutsos, 2000;Bailey and Eyster, 2003;Iacopini et al., 2008;Sullivan, 2008;Frassi et al., 2009;Langille et al., 2010;Xypolias et al., 2010;Law, 2010;Law et al., 2013;Ring et al., 2015Sarkarinejad et al., 2017Keshavarz and Faghih, 2020). According to the applied vorticity methods, mean kinematic vorticity estimates from mylonite within the study area range between 0.50 and 0.72 (49-66% pure shear) using the rigid-grain method and 0.62-0.99 ...
Integrated microstructural, quartz petrofabric and flow vorticity analyses on mylonitic rocks in the lower plate of the Neybaz Chatak Detachment Shear Zone provide significant information regarding the kinematic characteristics coeval with the Chapedony metamorphic core complex development within the Central East Iranian Microcontinent. Several kinematic shear sense indicators including quartz c-axis fabrics, S–C fabrics, rotated feldspar and mica fish reveal that the detachment shear zone has experienced a consistent top-to-the NE-directed progressive shear deformation. Detailed investigations on recrystallization types of quartz and feldspar in the mylonites demonstrate that the deformation occurred under the amphibolite facies condition corresponding to mid-crustal levels. A combination of investigations on mineral assemblages, microstructures of quartz and feldspar, and quartz lattice preferred orientations (LPOs) show that deformation temperatures ranged from 400 to 645 °C that were varied across a distance of ~21 km in the extension direction (NE-SW). The western and eastern parts of the study area experienced lower temperatures compared to the central parts. The gneissic mylonite record sub simple shear deformation (Wm = 0.56–0.85, 64–36%) associated with an increasing of pure shear component. These data highlight that the metamorphic rocks of the study underwent a non-coaxial shear with ductile low-angle normal faulting, resulting in subvertical thinning in the extensional deformation regime responsible for formation of the core complex.
... Oblique convergence, which is commonly recognized in ancient orogens and modern convergent plate margins, results in the formation of transpressional shear zones characterized by combined pure and simple shear components (e.g., Sanderson and Marchini, 1984;Fossen and Tikoff, 1993;Tikoff and Teyssier, 1994;Jiang et al., 2001;Fernández and Díaz-Azpiroz, 2009;Acocella, 2014;Massey et al., 2017). Oblique convergence and the associated strikeslip deformation are crucial factors in orogenic construction, including the accretion and collision of arcs and continental blocks (fragments), and the lateral extrusion and terrane translation parallel to orogen (Tapponnier et al., 1982;Leloup et al., 1995;Umhoefer and Schiarizza, 1996;Koons et al., 2003;Sarkarinejad et al., 2008;Toy et al., 2012;Massey and Moecher, 2013;Acocella, 2014;Keshavarz and Faghih, 2020;Wang et al., 2023;Allen et al., 2023), and they are key factors to understand in the effort to characterize the geodynamic mechanics of convergent plate boundaries (e.g., Little et al., 2002;Philippon and Corti, 2016;Blatchford et al., 2020). Plate-boundary strike-slip systems, which are commonly localized near or within magmatic arcs in the upper plates of subduction systems, play a fundamental role in accommodating margin-parallel shear and have considerable implications for seismic hazard (e.g., Sumatra Island-McCaffrey, 2009; the northern Andes- Alvarado et al., 2016). ...
Transpressional shear zones commonly occur in ancient and modern convergent plate boundaries to accommodate oblique plate convergence. The early Paleozoic Qilian orogen in northeastern Tibet records the subduction of Proto-Tethyan Ocean lithosphere and the accretion-collision of various magmatic arcs and continental terranes. This study focused on the Datong ductile shear zone, which represents the central part of the WNW-ESE−striking ductile shear zone along the northern margin of the Qilian block in the Qilian orogen. This structure bears key information about the evolution of oblique convergence during the early Paleozoic orogeny. The kinematics and timing of the Datong ductile shear zone were investigated via field-based, microstructural, and mica 40Ar/39Ar dating analyses. Mesostructural and microstructural data showed predominantly dextral strike-slip shearing within the Datong ductile shear zone. Microstructural features and quartz c-axis crystallographic preferred orientation patterns indicated that dextral ductile shearing occurred under lower-amphibolite-facies conditions (∼500−550 °C and ∼5.6 kbar) within the shear zone. Microstructures of quartz showed subgrain rotation (SGR) and grain boundary migration (GBM), suggesting dislocation creep−dominated deformation. A strain rate of 10−12 s−1 and a differential stress of 25−39 MPa were estimated by the rheological flow law and quartz paleopiezometry. Finite strain measurements indicated that all deformed rocks of the Datong ductile shear zone exhibit a weakly oblate ellipsoid near the plane strain. Kinematic vorticity (ranging 0.47−0.83) analysis suggested the coexistence of simple shear and pure shear strains within the Datong ductile shear zone, indicating a transpressional setting. Biotite and muscovite 40Ar/39Ar data showed that transpressional shearing deformation started in the Ordovician (before 453 Ma) and lasted to the Silurian (ca. 430 Ma). Our new data combined with regional geological data show that the deformation type, kinematics, and dynamics of the Datong ductile shear zone were controlled by the southward oblique subduction of the Paleo-Qilian Ocean (Proto-Tethyan Ocean) and the following oblique collision between the Qilian block and the Alxa block. The intensive transpressional deformation along the northern Qilian block may reflect strong coupling between the subducting Paleo-Qilian oceanic slab and the overriding Qilian block as well as a high degree of convergence obliquity during the ongoing early Paleozoic convergence.
... Their results were used to draw inferences about temperature of deformation, sense of shear, strain geometry, and kinematics of flow [12,18,30,31,37,40,82,86,88,91,92]. This contribution is aimed to describe and document the microstructures on the quartz rich-mylonitic rocks from the Tanbour metamorphic complex (TMC) in the Sanandaj-Sirjan metamorphic belt, Southwestern Iran. ...
... According to the applied vorticity method, measured kinematic vorticity number (W m =0.55 -0.87) confirms a general shear deformation that significantly deviated from both simple (35%-65%) and pure (45%-65%) shear (Fig. 10). The study of deformation based on quantitative vorticity analysis shows that in most shear zones of the Zagros orogeny, the general shear component is dominant (e. g., Derikvand, 2021;Mansouri et al., 2021;Keshavarz and Faghih, 2020;Samani et al., 2020;. In detail view, The higher values of kinematic vorticity numbers (W m >0.85) and finite strain are observed in stations (T93, T94, and T98) located close to the thrust faults (Fig. 12b) which systematically decreases towards the middle parts of the overlying thrust sheet so W m -value at station T92 reaches less than 0.70. ...
Deformation in the Zagros suture zone is a result of the oblique collision of the Afro-Arabian continent with the Central Iranian microcontinents. Various types of folding and faulting are characteristic features of the study area and indicate the performance of a high strain tectonic regime in this region. To distinguish deformation geometry during the collisional events, strain measurements have been carried out, using the Rf/φ method on deformed radiolarian microfossils, in the Zagros suture zone. Based on the results, the strain ellipsoid shape is in the range of general flattening to plane strain (K=0.16–1.12). Measured mean kinematic vorticity number (Wm) in the deformed radiolarian rocks ranges between 0.50 and 0.87, which implies that exhumation of the Abade-Tashk area was facilitated by a general shear flow (35%
... Mylonitic rocks contain deformed minerals such as quartz, feldspar, and amphibole in a relatively weaker matrix that preserves significant information of the complex deformation history (Mukherjee, 2011(Mukherjee, , 2013. They use for the estimation of finite strain, sense of shear, kinematic vorticity number (KVN), and deformation temperature (Passchier and Trouw, 2005;Iacopini et al., 2008;Mukherjee and Biswas, 2015;Keshavarz andFaghih, 2020, Derikvand, 2021). ...
Quantitative microstructural investigations involving finite strain and vorticity analyses of quartz-rich mylonites provide significant information regarding the kinematic characteristics of the Samen shear zone (SSZ) within the Sanandaj–Sirjan Metamorphic Belt, Zagros Mountain of Iran. A dominant top-to-the-SE sense shear in the study area is recognized by several kinematic indicators. The results of the finite strain analyses indicate approximately plane strain deformation conditions (Rxz = 2.46–3.53). The mylonitic granitoid rocks record general-shear deformation (Wm = 0.37–0.94) associated with a spatial increase in simple-shear component (23%–78%) from W to E. In the study area, kinematic analyses indicate spatial variations of finite strain, deformation temperature, and the kinematic vorticity numbers resulted in partitioned progressive deformation. These spatial variations reveal an increase of all values towards the Samen thrust sheet. Ductile SSZ developed under greenschist-amphibolite facies metamorphic conditions (464–619 °C), as indicated by the presence of quartz, feldspar, biotite, muscovite, hornblende mineral assemblages, opening angles (OAs) of quartz c-axis fabrics and recrystallization regimes. The dynamic and kinematic characteristics of the study area were generally controlled by ongoing oblique convergence of the Afro-Arabian plate and the Iranian plateau with estimation of a horizontal shortening of ∼20–38% measured perpendicular to the boundaries of the SSZ.
... Our study focused on the kinematic analysis, strain geometry and deformation that accompanied by the ophiolite obduction. The deformation distribution is non-uniform in the Zagros collision zone that this variation of the strain geometry imply to the changes in the deformation conditions during the closure of the Neo-Tethys and thrusting of the oceanic crust on the continental edge (Keshavarz and Faghih 2020). The shape of strain ellipsoid or ellipse is the proper technique for the evaluation of strain geometry in the structural geology. ...
The closure of the Neo-Tethys ocean associated with the ophiolite obduction and the Oshnavieh ophiolite is the unknown part of the Neo-Tethys suture zone. Three well-known band ratio combinations applied to ASTER satellite image the result shows the ((2+4) /3, (5+7) /6, (7+9) /8) band ratio is the proper combination for the reorganization of rock units in the ophiolite regions. Principal component analysis of the (PC2, PC4 and PC5) is well discriminated against to the rock unit contacts. The general trend of thrust faults is the NW-SE and dip direction is toward the NE. The rake of slickenline on the fault plane is 80°-90° and the mechanism of movement is the pure thrust. The shear sense indicator such as Z-type parasitic folds or mica-fish and S-C fabrics confirm right-lateral shearing sense in the shear zone. Strain geometry on the obducted slab evaluated by the shape of the mineral grains. In the shear zone strain ellipsoid shape is the prolate type and formed under constrictional regime, the Flinn K-value of these samples changes between 2.71 to 11.67 and lode ratio between-0.42 to-0.63. Most of the samples taken from the thrust fault zone located in the flattening zone and strain ellipsoid are pancake-shaped and formed under contractional regime the k-value varied between 0.44 to 0.80 and Lode ratio range is 0.32 to 0.5. The displacement in the thrust zone and shearing by the shear zone disrupted the ophiolite sequence and created an ophiolite mélange.
... 230 Evidence of dextral transpression is not limited to our study area. Indeed, dextral ductile 231 transpressional zones were reported in other parts of the SSZ (Mohajjel and Fergusson, 2000; 232 Sarkarinejad et al., 2008; Faghih and Sarkarinejad, 2011; Bafti and Mohajjel, 2015; Samani, 2017; 233Keshavarz and Faghih, 2020). In particular, the deformation style in the study area is comparable to 234 that in the June structural complex in the northwestern SSZ(Mohajjel and Fergusson, 2000), where a 235 major D2 with similar geometry and kinematics affected metamorphic rocks. ...
The deformation style associated with the early stage of subduction of the Neo-Tethys under Iranian microcontinent is poorly understood. Utilizing field observations, microscopic thin-section interpretation, and recently published 40Ar-39Ar ages, we conducted a structural analysis of the North Shahrekord Shear Zone. The study area is located in the central Sanandaj-Sirjan Zone in western Iran, in an area that includes high-grade eclogite-bearing and low-grade metamorphic units of the North Shahrekord Metamorphic Complex. Two generations of deformation were observed in the study area, including D1 and D2. D1 structures are rare and involve folds and axial-plane foliations (S1), which have been almost obliterated by D2. D2 is characterized by a dextral transpressional zone, where strain is partitioned into distinct simple shear and pure shear domains. The pure shear domain is observed mainly in low-grade zone and some high-grade metamorphic rocks, and is characterized by NW-striking close to isoclinal folds (F2) and axial-plane foliations (S2). The vergence of inclined F2 folds is towards the southwest and northeast in the northern and southern parts of the study area, respectively. In the central part of the area, F2 folds are approximately upright. These mesoscopic folds are associated with map-scale folds. The simple shear domain is restricted to the high-grade metamorphic rocks of the North Shahrekord Shear Zone, defined by NW-striking steeply dipping mylonitic foliation with low-angle stretching lineation. Kinematic indicators, such as quartz sigmoids, feldspar porphyroclasts, mica fish, and C- and C’-type shear bands, reveal a dextral strike-slip movement with a minor reverse component along the North Shahrekord Shear Zone. Recrystallised phengite from mylonitic eclogite and metagranite have been recently dated (40Ar-39Ar) at 178–170 Ma and ∼110 Ma, respectively, indicating that the Sanandaj-Sirjan Zone was dominated by dextral transpression phases in the Early-Middle Jurassic and Early Cretaceous. The transpressional phases are interpreted to have occurred in response to the oblique subduction of the Neo-Tethys Ocean under the Iranian microcontinent.
... In Iran, transpression deformation zones have been identified in different parts of the Zagros simply folded belt , high Zagros (Agard et al., 2011;, Sanandaj-Sirjan zone and Urumieh-Dokhtar Magmatic Belt (Amirihanza et al., 2018;Mirzaie et al., 2015;Mohajjel and Fergusson 2014;Mohajjel et al., 2003;Mohajjel and Fergusson 2000;Sarkarinejad 2007;Sarkarinejad and Azizi 2008;Shafiei Bafti and Mohajjel 2015;Sarkarinejad et al., 2015;Keshavarz and Faghih, 2020), Alborz (Nabavi et al., 2017), Binalud (Sheikholeslami et al., 2019). In the east of central Iran N-S to NE-SW dextral transpression have been recognized along the boundary between the Lut and Tabas blocks (Cifelli et al., 2013). ...
... In Iran, transpression deformation zones have been identified in different parts of the Zagros simply folded belt , high Zagros (Agard et al., 2011;, Sanandaj-Sirjan zone and Urumieh-Dokhtar Magmatic Belt (Amirihanza et al., 2018;Mirzaie et al., 2015;Mohajjel and Fergusson 2014;Mohajjel et al., 2003;Mohajjel and Fergusson 2000;Sarkarinejad 2007;Sarkarinejad and Azizi 2008;Shafiei Bafti and Mohajjel 2015;Sarkarinejad et al., 2015;Keshavarz and Faghih, 2020), Alborz (Nabavi et al., 2017), Binalud (Sheikholeslami et al., 2019). In the east of central Iran N-S to NE-SW dextral transpression have been recognized along the boundary between the Lut and Tabas blocks (Cifelli et al., 2013). ...
Inclined transpression can be defined in terms of simultaneous contraction and strike-slip and dip-slip shearing. An example of an upper-crustal inclined transpression zone is exposed at Bazargan area, SE Iran. As with most natural transpression zones, deformation at Bazargan area is heterogeneous, and widespread kinematic strain partitioning has given rise to an irregular distribution of strain components and resulting structures. Based on structural and strati graphical evidence, deformational events are divided into two stages: (1) a Late Triassic event and (2) a Late Cretaceous to Miocene event. A wide range of known structures at the map and outcrop scales show the effect of slip partitioning on the structural evolution of this area during this period. The dynamic analysis based on folds, fault striations and calcite twin data show a NNE-SSW trend for the compressive axes in this area. The results of dynamic analysis reveal that the orientation of the compressive axes did not change during late Triassic to present. Moreover, they show that the structural evolution of the Bazargan area occurred under the effect of continues deformation phases which is related to contraction, dip slip and strike slip component. Furthermore, the results indicate that the evolution of Bazargan area in the south of Tabas block is compatible with an inclined transpression model.
... Oblique collision between lithospheric plates leads to exhumation/extrusion of low to metamorphic rocks involving transpressional deformation within large-scale shear zones (e.g., Mukherjee and Mulchrone 2012;Sarkarinejad et al., 2017;Keshavarz and Faghih 2020;Samani et al. 2020;Dutta and Mukherjee 2021;Mahmoudi Sivand et al. 2021). These large-scale shear zones can weaken the crust (Davis et al. 1986;Leloup et al. 1995;Yin 2004;Rosenberg and Handy 2005;Searle 2013;Mukherjee and Mulchrone 2015) and usually have crucial role in the rapid exhumation and extrusion of elliptical-shaped gneissic domes with long axes parallel or perpendicular to the axial trend of the orogenic belt. ...
Detailed structural and kinematic analyses of deformed structures in the Sanandaj-Sirjan Metamorphic Belt in the Ghouri area revealed the occurrence of three gneissic sub-domes with semi-elliptical shapes with their major axes parallel to the Zagros Transpression Zone boundary (NW-SE direction). These sub-domes are cored by sheared quartzo-feldspathic gneisses which surrounded by sequences consisting of garnet amphibolites, phyllites, phyllonites, muscovite schists, and deformed conglomerates. The Kuh-e-Zard, Chah-Sabz, and Chah-Goni sub-domes resulted from the interaction of the lower to middle continental crust and hot subducting Tethyan oceanic crust during subduction/continental collision. These sub-domes were squeezed between thrusts in the Zagros dextral transpression zone. The variations in the kinematic vorticity and strain estimates revealed that sub-simple shear transpressional deformation is highly heterogeneous in each sub-domes.
... The quartz c-axis measurements are displayed on equal-area, lower-hemisphere stereographic projections. In addition, the sense of shear in mylonitic rocks can be revealed through the quartz c-axis orientation patterns(Xypolias 2010;Muto et al. 2011;Law 2014;Faghih and Soleimani 2015;Sarkarinejad et al. 2015Sarkarinejad et al. , 2017Fazio et al. 2016;Yin et al. 2017;Mondal 2018;Xypolias et al. 2018;Keshavarz and Faghih 2020;Samani et al. 2020). Spreadsheet software ...
The Boneh Shurow metamorphic core complex (BSMCC) in the Central East Iranian Microcontinent (CEIM) provides a good example of the Mesozoic succession of nonsynchronous compressional and extensional deformation events attributed to the transitional Cimmerian events. The D1 compression developed subvertical dextral ductile shear zones and corresponds to continental accretion and crustal thickening producing kyanite- and sillimanite-grade rocks and migmatites in the Early Cimmerian orogeny in the CEIM. The D2 deformation event is marked by extension during the mid-Cimmerian orogeny. It is characterized by top-to-the-NE normal sense of shear along a low angle detachment surface. Field evidence for cross cutting relationships of D1- by D2-related structures reveal that the occurrence of Barrovian facies metamorphism and associated partial melting in the core of BSMCC formed during compressional tectonic events. These structures formed before the initiation of extension and the formation of the low-angle detachment shear zone. Finally, during the Late Cimmerian D3 event, the east and west Boneh Shurow reverse faults ruptured on both sides of the MCC. Recognition of the complicated origin and exhumation mechanisms of the BSMCC provide crucial constraints on the prolonged evolution of Paleo- and Neo-Tethys ocean basins and collisional and post-collisional events in this region.
We investigate kinematic evolutionary paths of seventeen major shear zones associated with internal thrust faults from the Himalayan, Appalachian, Caledonian, Zagros, Alpine, and Sevier fold-thrust belt (FTB), by assessing published minimum strain, kinematic vorticity number, and minimum translation. We estimate the pure shear component from the recorded vorticity and reconstruct a first-order kinematic path based on multiple strain markers, wherever possible. The studied shear zones follow decelerating strain paths. In general, sub- simple shear more effectively accumulates translation than pure/simple shear deformation. Shear zones with relatively high pure shear component record higher strain. Incremental strain markers from these shear zones record a progressive evolution from an earlier simple shear dominated to a pure shear dominated flow. These results are in agreement with earlier theoretical studies.
Internal shear zones that act as roof thrusts of duplexes or have stacked imbricate structures in their immediate footwall, generally record relatively higher strain, greater translation and greater pure shear component toward the later stage than similar shear zones without such footwall structures. We interpret that slip-transfer and structural culmination formed during growth of immediate footwall structures contribute to the kinematic evolution of internal shear zones. The same shear zone records along-strike variation in its kinematic path as a result of its varying immediate footwall geometry. Thus, deciphering comprehensive kinematic evolutionary paths of internal shear zones also requires understanding of immediate footwall structures. Additionally, studying kinematic paths of internal shear zones may provide insights into the geometry of immediate footwall structures when they are not exposed.
The main central thrust (MCT) is one of the major thrusts in Himalayas. In central Himalaya, MCT was defined as a contact between underlying Lesser Himalayan Sequence (LHS) and overlying higher Himalayan crystallines (HHC). However, in the Kashmir Himalayas, the main central thrust zone (MCTZ), shear zone associated with MCT, is overlain by Kashmir Tethyan Sequence suggesting that the MCTZ has been deformed through a mechanism different than the mechanism responsible for MCTZ evolution in other parts of the Himalayas. In the present study we used structural, microfabric and kinematic analyses to investigate the deformation kinematics of MCTZ. Microstructural investigation revealed that the quartz in orthogneiss mylonites of MCTZ was dynamically recrystallized by grain boundary migration (GBM) and sub-grain rotation recrystallisation (SGR) with top-to-SW sense of shear. The mean kinematic vorticity number (Wm) just above the thrust ranges from 0.72 to 0.84 (40%–52% pure shear component) decreasing upwards to 0.65–0.71 (35%–50% pure shear component). Deformation in the MCTZ is characterized by Rxz strain ratio varying from 2.7 to 8. The present study suggested that the MCTZ suffered 3%–40% vertical shortening and 3%–66% transport-parallel elongation. The results suggested that the HHC’s were not completely exhumed to the topographic surfaces in the Kashmir Himalayas. Along the basal decollement, i. e., the main Himalayan thrust (MHT), the deformation continued until MCTZ reached the brittle-ductile transition where deformation mechanism changed to the brittle and the MCTZ rocks were transported to the surface through slip on brittle MCT.
The Yeba ductile shear zone is located on the southern margin of the middle Gangdese magmatic belt and southeast of Lhasa, China. In this paper, we systematically investigate the structural deformation history in the Yeba Group and provide the kinematics, strain pattern, mean vorticity number and temperature environment of the Yeba ductile shear zone. Three tectonic events are identified in the Yeba Group of the study area. The Yeba shear zone represents the late stage of the northward subduction of the Neo-Tethys Ocean (D1 stage, 94–85 Ma). Since the collision of the Indian and Eurasian plates, principal compressive stress (σ1) in the north–south direction was generated in the Yeba Group and formed composite folds (D2 stage, ~ 50 Ma). The D3-stage structural deformation event of the Yeba Group in the study area is represented by the Woka ductile shear zone (22.38–14.6 Ma). Electron backscattering diffraction and chlorite geothermometry demonstrate that upper greenschist facies (213–295 °C) and lower greenschist facies (400–550 °C) conditions developed in the Yeba shear zone. Finite strain measurements of the Yeba ductile shear zone indicate that almost all deformed rocks exhibit an oblate ellipsoid consistent with near-flattening strain. Kinematic vorticity analysis of the Yeba shear zone yielded values (Wm) of 0.49–0.78, indicating a bulk general shear deformation regime with a combination of 58% pure shear and 42% simple shear. The kinematic vorticity number (58% pure shear), flattening strain pattern, and presence of opposite indicators in the same mylonitic foliation support the occurrence of a transpressive structure.
The basal decollement Darizhan shear zone in the hinterland of the Zagros orogenic belt exposes the amphibolite-mylonite in NW Iran. In this study, I discuss the deformation partitioning in terms of oblique convergence between the Afro-Arabian continent and the Iranian microcontinent. Rf/\(\phi \)-based finite strain analysis, Rxz varies ranging from 2.25 to 3.04 for the XZ-plane of the strain ellipsoid. The flow vorticity (Wm) along the Darizhun shear zone is estimated employing PHD, PAR, RGN and PD techniques. The Wm estimates vary between 0.6 and 0.85 and demonstrate general shear deformation (pure-shear: 35–58% and simple-shear: 42–65%). The Wm values indicate a gradual increase in the simple-shear component towards the Darizhan thrust sheet. Considering both the results of strain and vorticity analyses, we propose a horizontal shortening of (~ 21–36%) normal to the walls of the shear zone.
Dehbid mylonite nappes associated with the Tutak gneiss dome as the Zagros hinterland involved-basement window in the Zagros Orogenic Belt has significant components of dextral sense of shear along the mylonite nappes and are related to the high proportion of simple shear relative to pure shear deformation. Kinematic vorticity analysis (W k) was made in order to determination of relative amounts of simple shear and pure shear components. The kinematic vorticity analysis has taken as evidence of porphyroclasts rotation relative to internal reference frames during non-coaxial deformation within the Tutak gneiss dome in the Zagros Orogenic Belt. The quartz c-axis preferred orientation evidence used to drive the degree of non-coaxiality during deformation. The mean kinematic vorticity (W m) value in the Tutak gneiss dome revealed contribution of 48% pure shear and 52% simple shear components. Detailed kinematic vorticity studies may allow this interpretation that the convergence between the Afro-Arabian and central Iran continental crusts is characterized by basement involvement during ongoing inclined dextral transpressional regime.
Microstructures inside exhumed crustal shear zones provide a window into the mechanics of Earth’s interior. However, difficulties remain in quantifying their physical properties due to the numerous macro-, meso-, and grain-scale factors controlling their development and modification. The complex interplay between these factors has meant that inconsistencies remain between the knowledge derived from controlled laboratory deformation experiments and that from empirical observations in nature.
We present a quantitative toolbox for identifying variations in the deformation characteristics of shear zones, using the microstructure and crystallographic preferred orientation (CPO) of quartz. The Main Central Thrust in the Alaknanda Valley (NW
Himalaya) is an archetypal major ductile thrust zone that contains abundant quartzites
and quartz–mica schists that permit microstructural characterization across gradients
in strain and temperature. Using a combination of detailed microstructural analysis and
neutron diffraction, we show that quartzites across the Main Central Thrust sequence are
characterized by: (1) systematic increases in grain lobateness and coarseness, indicating
the transition from subgrain rotation (SGR) to dominant grain boundary migration
(GBM) dynamic recrystallization; (2) a concomitant shift in CPO from oblique
(top-to-the-SSW) girdles to a strong single y-maximum, indicating increasing activity of
prismatic slip; and (3) high CPO intensities coinciding with the switch to GBM and
prismatic slip activity. Our analyses suggest the Main Central Thrust may be characterized as an ~3-km-wide zone of dominantly non-coaxial deformation coinciding with a sharp change in peak metamorphic temperatures, from ~450 °C in the footwall to 750–800 °C in the hanging wall. We suggest that the observed systematic microstructural and CPO transitions may typify the deformational behavior of quartz in the Main Central Thrust across the wider Himalaya.
The Zagros hinterland fold-and-thrust belt is located in the central portion of the Zagros Thrust System and consists of the exhumed basement windows associated with NW-striking and NE-dipping flexural duplex structures that contain in-sequence thrusting and related folds. Mylonitic nappes of the basement were exhumed along deep-seated sole thrusts of the Zagros Thrust System. Lattice preferred orientation (LPO) c-axes of quartz show asymmetric type-1 crossed girdles that demonstrate a non-coaxial deformation under plane strain conditions. Based on the opening angles of quartz c-axis fabric skeletons, deformation temperatures vary from 425±50°C to 540±50°C, indicating amphibolite facies conditions. The estimated mean kinematic vorticity evaluated from quartz c-axis of the quartzo-feldspathic mylonites (Wm = 0.55±0.06) indicates the degree of non-coaxiality during mylonite exhumation. The estimated angle θ between the maximum instantaneous strain axis (ISA1) and the transpressional zone boundary is 17°, and the angle of oblique convergence is 57° in the M2 nappe of the basement involved. This indicates that the mylonitic nappe was formed by a combination of 62% pure shear and 38% simple shear during oblique convergence.
The Sirjan mylonite nappe is part of the HP–LT Sanandaj–Sirjan Metamorphic Belt (Zagros Orogenic Belt, Iran). This nappe was emplaced in a transpressional regime during oblique convergence between the Afro–Arabian Continent and Central Iranian Microcontinent. In order to investigate the type of flow, fabrics from deformed quartz and K-feldspar porphyroclasts were analyzed. Quantitative finite strain (RXZ = 1.9–3.8) and mean kinematic vorticity number (Wm = 0.60–0.85) analyses revealed that both the strain ratio and the simple shear component of deformation increase towards SE along a 2 km long NW–SE-oriented transect. Analysis of quartz and K-feldspar textures suggests shearing at c. 400–650 °C. We also conclude that deformation in the Sirjan mylonite nappe began with nearly pure shear flow but became a more simple-shear flow in character as strain accelerated.
Microstructural, finite strain and vorticity analyses of quartz-rich mylonites were used in order to investigate kinematics of rock flow and deformation temperature in the Sirjan thrust sheet exposed in a structural window within the Sanandaj–Sirjan High Pressure – Low Temperature (HP–LT) metamorphic belt that forms part of the hinterland of the Zagros orogenic belt of Iran. A dominant top-to-the-SW sense of shear in the study area is indicated by several shear sense indicators such as asymmetric boudins, rotated porphyroclasts, mica fish and S/C fabrics. Quantitative analyses reveal approximately plane strain deformation conditions with R
xz
values ranging from 2.5 to 4.3 and increasing towards the Sirjan thrust. Opening angles of quartz c-axis fabrics and recrystallization regimes suggest deformation temperatures vary from 430 to 625 ± 50°C in the hanging wall rocks. Oblique grain shape and quartz c-axis fabrics were used to estimate the degree of non-coaxiality during deformation. The obtained vorticity profile indicates a down-section increase in kinematic vorticity number (W
m
) from 0.6 to 0.89. This range of vorticity numbers confirms contributions of both simple (41–68 %) and pure shear (32–59 %) deformation components. The structural characteristics of the study area ultimately were controlled by oblique motion of the Afro-Arabian plate relative to the Iranian plate.
ABSTRACT: Asymmetric boudins that are widespread in the Seh-
Ghalatoun area in the Neyriz, southwestern Iran are good indicators
to estimate the kinematic vorticity number (Wk) and sense
of shear. This area is part of the Sanandaj-Sirjan HP-LT metamorphic
belt located within the Zagros orogenic belt. Boudins have
been analyzed on the meso- and micro-scales. Domino and shearband
boudins are developed in deformed quartzitic layers associated
with quartzo-feldspathic gneiss, amphibolite, mica-schist and
marble. Shearband boudins with backward rotation and domino
boudins with forward rotation, mantled porphyroclasts, drag folds,
and asymmetric folds which all display a dextral sense of shear.
The mean kinematic vorticity number was estimated by using the
asymmetric boudin hyperbolic distribution method and quartz caxis
patterns. The estimated mean kinematic vorticity number is
Wk = Wm = 0.85 ± 0.04, which indicates 34% pure shear and 66%
simple shear components of deformation. The estimated mean
deformation temperature based on the opening angle of the quartz
c-axis fabrics is 530 ± 50 °C. These quantitative analyses suggest a
sub-simple shear deformation developed at the epidote-amphibolite
to amphibolite facies conditions at depth along the Sanandaj-
Sirjan HP-LT metamorphic belt, which is associated with a transpressional
flow regime with shortening perpendicular to the shear
zone boundary
This paper presents quantitative data on strain, deformation temperatures and vorticity of flow at
the top of the Greater Himalayan Slab. The data were collected from the Tibetan side of the Everest Massif
where two low-angle normal faults bound the upper surface of the Greater Himalayan Slab, the earlier and
structurally lower Lhotse Detachment and the later and structurally higher Qomolangma Detachment.
Greenschist- to sillimanite-grade quartz-rich metasedimentary rocks exposed in the Rongbuk to North Col
region of the Everest Massif are characterized by cross-girdle quartz c-axis fabrics indicating approximate
plane strain conditions. Fabric opening angles progressively increase with depth beneath the overlying Lhotse
Detachment, and indicate progressively rising deformation temperatures of 525–625 � 50 8C at depths of
300–600 m beneath the detachment. Deformation temperatures of c. 450 8C are indicated by fabric opening
angles in epidote amphibolite-facies mylonites located closer to the overlying detachment. A top down-to-thenorth
(normal) shear sense is indicated by the asymmetry of microstructures and c-axis fabrics, but the degree
of asymmetry is low at distances greater than 400 m beneath the detachment, and sillimanite grains are drawn
into adjacent conjugate shear bands but still appear pristine, indicating that deformation occurred at close to
peak metamorphic temperatures. These ‘quenched’ fabrics and microstructures indicate rapid exhumation in
agreement with previous isotopic dating studies. Mean kinematic vorticity numbers (Wm) were independently
calculated by three analytical methods. Calculated Wm values range between 0.67 and 0.98, and indicate that
although a simple shear component is generally dominant, particularly in greenschist-facies mylonites located
between the Lhotse and overlying Qomolangma detachments, there is also a major component of pure shear in
samples located at 400–600 m beneath the Lhotse Detachment (pure and simple shear make equal
contributions at Wk ¼ 0:71). Our integrated strain and vorticity data indicate a shortening of 10–30%
perpendicular to the upper surface of the Greater Himalayan Slab and confirm that the upper surface of the
slab is a ‘stretching fault’ with estimated down-dip stretches of 10–40% (assuming plane strain deformation)
measured parallel to the flow plane–transport direction.
Kinematic analysis of mélange fabrics provides critical information concerning tectonic processes and evaluation of the kinematics of ancient relative plate motion. Systematic kinematic analysis of deformed structures within a tectonic mélange exposed along the Zagros Suture Zone elucidates that this zone is an ancient transpressional boundary. The mélange is composed of a greywacke and mudstone matrix surrounding various lenses, blocks and ribbons of radiolarian chert, limestone, sandstone, pillow lava, tuff, serpentinite, shale and marl. The deformation fabrics of the mélange suggest that the mélange units were tectonically accreted at shallow levels within a subduction complex, resulting in layer-parallel extension and shearing along a NW–SE-trending suture that juxtaposes the Afro-Arabian continent to the south and the Central Iranian microcontinents to the north. The tectonic mélange is characterized by subhorizontal layer-parallel extension and subsequent heterogeneous non-coaxial shear resulting in alternating asymmetric and layer-parallel extensional fabrics such as P–Y fabrics and boudinaged layers. Kinematic data suggest that the mélange formed during oblique subduction of the Neo-Tethys oceanic lithosphere in Late Cretaceous time. Kinematic shear sense indicators reveal that the slip direction (N9°E to N14°E) during accretion-related deformations reflects the relative plate motion between the Afro-Arabian continent and Central Iranian microcontinents during Late Cretaceous to Miocene times.
In metamorphic core complexes it is commonly unclear whether lower plate
mylonites formed as the down-dip continuation of a detachment fault, or
whether they represent a subhorizontal shear zone that was captured by a
more steeply dipping detachment fault. Detailed microstructural, fabric,
and strain data from mylonites in the Buckskin-Rawhide metamorphic core
complex, west-central Arizona, constrain the structural development of
the lower plate shear zone. Widespread exposures of ˜22-21 Ma
granitoids of the Swansea Plutonic Suite enable us to separate Miocene
strain coeval with core complex extension from older deformation.
Mylonites across the lower plate consistently record
top-to-the-NE-directed shear. Miocene quartz and feldspar
deformation/recrystallization mechanisms indicate
˜450-500 °C mylonitization temperatures that were
relatively uniform across a distance of ˜35 km in the
extension direction. Quartz dynamically recrystallized grain sizes do
not systematically vary in the extension direction. Strain recorded in
the Swansea Plutonic Suite is also relatively uniform in the extension
direction, which is incompatible with models in which lower plate
mylonites form as the ductile root of a major detachment fault.
Altogether these data suggest the mylonitic shear zone initiated with a
≤4° dip and was unroofed by a more steeply dipping detachment
fault system. Lower plate mylonites in the Buckskin-Rawhide metamorphic
core complex thus represent a captured subhorizontal shear zone rather
than the down-dip continuation of a detachment fault.
The Zagros Mountains are the result of the Arabia/Eurasia collision initiated at ~ 35 Ma as the rifted Arabian lithosphere was underthrusted beneath the Iranian plate due to its negative buoyancy. The onset of crustal thickening started at ~ 25 Ma, as recorded by the hinterland exhumation and foreland clastic deposition. Deformation throughout the Arabia/Eurasia collision zone and the uplift of the Iranian plateau occurred after 15–12 Ma, as a result of shortening/thickening of the thin Iranian crust. We emphasize that only 42% of the post-35 Ma convergence is partitioned by shortening within central Iran. Tomographic constraints show ongoing slab steepening or breakoff in the NW Zagros, whereas underthrusting of the Arabian plate is observed beneath central Zagros. The current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in NW Zagros and underthrusting in central Zagros. Critical wedge approach applied to the Zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust. In contrast, the weak sedimentary cover deforms by buckling of a thick multilayered cover. Thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes. There is evidence that the role of the slab pull force in driving the Arabian plate motion was reduced after ~ 12 Ma. Large-scale mantle flow induced by mantle upwelling at the Afar plume appears to be the main driver of the Arabia plate motion. We stress that the main kinematic change in the Zagros region occurred at 15–12 Ma as the Zagros uplifted, before the Arabian slab detached. The Zagros appears key to investigate coupling between continental rheology, plate driving forces and mountain building, in which the role of rift inheritance appears to be central.
The Greater Himalayan Slab (GHS) is composed of a north-dipping anatectic core, bounded above by the South Tibetan detachment system (STDS) and below by the Main Central thrust zone (MCTZ). Assuming simultaneous movement on the MCTZ and STDS, the GHS can be modelled as a southward-extruding wedge or channel. New insights into extrusion-related flow within the GHS emerge from detailed kinematic and vorticity analyses in the Everest region. At the highest structural levels, mean kinematic vorticity number (Wm) estimates of 0.74-0.91 (c. 45-28% pure shear) were obtained from sheared Tethyan limestone and marble from the Yellow Band on Mount Everest. Underlying amphibolite-facies schists and gneisses, exposed in Rongbuk valley, yield Wm estimates of 0.57-0.85 (c. 62-35% pure shear) and associated microstructures indicate that flow occurred at close to peak metamorphic conditions. Vorticity analysis becomes progressively more problematic as deformation temperatures increase towards the anatectic core. Within the MCTZ, rigid elongate garnet grains yield Wm estimates of 0.63-0.77 (c. 58-44% pure shear). We attribute flow partitioning in the GHS to spatial and temporal variations that resulted in the juxtaposition of amphibolite-facies rocks, which record early stages of extrusion, with greenschist to unmetamorphosed samples that record later stages of exhumation.
In this brief introductory review the potential geological use of crystallographic fabrics is illustrated by considering selected geological problems which, given appropriate conditions, may be investigated in plastically deformed rocks using fabric analysis. Quartz and calcite are taken as the main illustrative examples. Numerical fabric simulations indicate that the imposed strain path (strain symmetry, vorticity etc.) is reflected in the relationship between the fabric pattern, kinematic framework and finite strain axes. Although these fabric patterns are sensitive to the numerical model and combination of crystallographic slip systems chosen, many of the major fabric types have been observed in experimentally and naturally deformed rocks. The fabric pattern itself may contain important information on strain symmetry, orientation of fields of extension and contraction and operative slip systems. Similarly, the angular relationship between the fabric pattern and finite strain features (foliation and lineation) may provide information on shear sense and vorticity of deformation. Spatial transitions with decreasing grain size in naturally deformed rocks, from strongly defined fabrics to a complete lack of crystallographic preferred orientation, have been interpreted as indicating a switch to deformation mechanisms involving grain boundary sliding. Potential problems associated with using the absence of a fabric as an indication of grain boundary sliding (and by inference superplastic flow) are discussed. Experimental studies indicate that geometrical relationships between intracrystalline strain features and the crystal lattice of individual grains may be used to deduce palaeo-stress directions. Results of palaeo-stress analysis techniques based on such relationships are compared.
Structural and microstructural characteristics, deformation temperatures and flow vorticities of the northern Ailao Shan (ALS) high-grade metamorphic belt provide significant information regarding the nature and tectonic evolution of the Ailao Shan-Red River (ASRR) shear zone. Mineral deformation mechanisms, quartz lattice-preferred orientation (LPO) patterns and the opening angles of quartz c-axis fabrics of samples from the Gasa section indicate that the northern ALS high-grade metamorphic belt has experienced progressive shear deformation. The early stage shearing is characterized by a gradual decrease of deformation temperatures from >650 °C at the northeastern unit to ca. 300 °C at the southwestern unit, that results in the formation of migmatites, mylonitic gneisses, thin bedded mylonites, mylonitic schists and phyllonites from the NE to SW across the strike of the shear zone. The late stage low-temperature (300–400 °C) shearing is superimposed on the early deformation throughout the belt with the formation of discrete, small-scale shear zones, especially in the thin-banded mylonitic rocks along both margins. The kinematic vorticity values estimated by rotated rigid porphyroclast method and oblique grain-shaped/quartz c-axis-fabric method imply that the general shear-dominated flow (0.49–0.77) progressively changed to a simple shear-dominated flow (0.77–1) toward the late stage of ductile deformation. The two stages of shearing are consistent with early shortening-dominated and late extrusion-controlled regional tectonic processes. The transition between them occurred at ca. 27 Ma in the ALS high-grade metamorphic belt along the ASRR shear zone. The large amount of strike-slip displacement along the ASRR shear zone is predominantly attributed to accelerated flow along the shear zone during the late extrusion-controlled tectonic process.
The Zagros Orogen, marking the closure of the Neo-Tethyan Ocean, formed by continental collision beginning in the late Eocene to early Miocene. Collision was preceded by a complicated tectonic history involving Pan-African orogenesis, Late Palaeozoic rifting forming Neo-Tethys, followed by Mesozoic convergence on the ocean's northern margin and ophiolite obduction on its southern margin. The Sanandaj–Sirjan Zone is a metamorphic belt in the Zagros Orogen of Gondwanan provenance. Zircon ages have established Pan-African basement igneous and metamorphic complexes in addition to uncommon late Palaeozoic plutons and abundant Jurassic plutonic rocks. We have determined zircon ages from units in the northwestern Sanandaj–Sirjan Zone (Golpaygan region). A sample of quartzite from the June Complex has detrital zircons with U–Pb ages mainly in 800–1050 Ma with a maximum depositional age of 547 ± 32 Ma (latest Neoproterozoic–earliest Cambrian). A SHRIMP U–Pb zircon age of 336 ± 9 Ma from gabbro in the June Complex indicates a Carboniferous plutonic event that is also recorded in the far northwestern Sanandaj–Sirjan Zone. Together with the Permian Hasanrobat Granite near Golpaygan, they all are considered related to rifting marking formation of Neo-Tethys. Scarce detrital zircons from an extensive package of metasedimentary rocks (Hamadan Phyllite) have ages consistent with the Triassic to Early Jurassic age previously determined from fossils. These ages confirm that an orogenic episode affected the Sanandaj–Sirjan Zone in the Early to Middle Jurassic (Cimmerian Orogeny). Although the Cimmerian Orogeny in northern Iran reflects late Triassic to Jurassic collision of the Turan platform (southern Eurasia) and the Cimmerian microcontinent, we consider that in the Sanandaj–Sirjan Zone a tectonothermal event coeval with the Cimmerian Orogeny resulted from initiation of subduction and closure of rift basins along the northern margin of Neo-Tethys.
Symmetric structures in ductile shear zones range widely in shapes and geneses. Matrix rheology, its
flow pattern, its competency contrast with the clast, degree of slip of the clast, shear intensity and its variation across shear zone and deformation temperature, and degree of confinement of clast in shear zones affects (independently) the degree of symmetry of objects. Kinematic vorticity number is one of the parameters that govern tail geometry across clasts. For example, symmetric and nearly straight tails develop if the clast–matrix system underwent dominantly a pure shear/compression. Prolonged deformation and concomitant recrystallization can significantly change the degree of symmetry of clasts. Angular relation between
two shear zones or between a shear zone and anisotropy determines fundamentally the degree of symmetry of lozenges. Symmetry of boudinaged clasts too depends on competency contrast between the matrix and clast in some cases, and on the degrees of slip of inter-boudin surfaces and pure shear. Parasitic folds and post-tectonic veins are usually symmetric.
The opening-angle of quartz c-axis fabrics (OA) is strongly temperature-dependent and has proven to be a powerful deformation thermometer for natural metamorphic rocks. Previous considerations of empirical data have identified a linear correlation between OA and temperature between 250 and 650 °C, and no correlation above 650 °C. However, possible effects of pressure have not been investigated. We expanded the data set of OA versus temperature, including data from rocks deformed over 300–1050 °C and 2.5–15 kbar. Disregarding possible effects of pressure, the OA-temperature relationship can be described by two linear correlations for the intervals ~ 250–650 °C and ~ 650–1050 °C:
T (°C) = 6.9 OA (degrees) + 48 (250 °C ≤ T ≤ 650 °C and OA ≤ 87°).
T (°C) = 4.6 OA (degrees) + 258 (650 °C ≤ T ≤ 1050 °C and OA ≥ 87°).
The change on the curve slope of the OA-temperature relationship correlates approximately to the low-high quartz transition and to changes in the dynamic recrystallization mechanism from subgrain rotation to grain boundary migration. The available data suggest that pressure has a secondary effect accompanying the major temperature dependence of OA, which is particularly important for temperatures above 650 °C, where the correlation between OA and temperature is less pronounced. For fixed pressures, the OA has logarithmic relationships with temperature over the range 250–1050 °C. The following thermometer equation is formulated from a multiple regression:
T (°C) = 410.44 ln OA (degrees) + 14.22 P (kbar) – 1272.
An uncertainty of ± 50 °C is inherited from the petrological temperature estimates of the natural samples. The data suggest the gradual increasing importance of prism [c] slip relative to < a > slip in quartz with rising temperature. Under conditions of ‘average’ geological strain rate and water weakening, prism [c] slip dominates for deformation above ~ 700 °C.
I conducted new vorticity and deformation temperatures studies to test competing models of the exhumation of the mid-crustal rocks exposed in the Dolpo region (West Nepal). My results indicate that
the Main Central Thrust is located ~5 km structurally below the previous mapped locations. Deformation
temperature increasing up structural section from ~450 C to ~650 C and overlap with peak metamorphic
temperature indicating that penetrative shearing was responsible for the exhumation of the GHS occurred at “close” to peak metamorphic conditions. I interpreted the telescoping and the inversion
of the paleo-isotherms at the base of the GHS as produced mainly by a sub-simple shearing (Wm ¼ 0.88 e1) pervasively distributed through the lower portion of the GHS. My results are consistent with hybrid channel flow-type models where the boundary between lower and upper portions of the GHS, broadly corresponding to the tectonometamorphic discontinuity recently documented in west Nepal, represents the limit between buried material, affected by dominant simple shearing, and exhumed material affected by a general flow dominates by pure shearing. This interpretation is consistent with the recent models suggesting the simultaneous operation of channel flow- and critical wedge-type processes at different structural depth.
The treatment of accretionary complexes on convergent tectonic margins by continuum mechanics as weak wedge-shaped bodies above a decollement surface allows two testable predictions: 1) deformation within such wedges may include large amounts of horizontal extension as well as shortening, and 2) this extension may take place while plate convergence is continuing before continental collision. Extension may be recognized by the presence of abrupt downward increases in metamorphic grade and, at higher structural levels, the formation of extensional sedimentary basins above the orogenic wedge. Extension may also be recognized by using flow path modelling in broad zones of ductile shear. Two former active margins, the Calabrian Arc and the Eastern Alps, are characterized by high-strain regional deformation that caused substantial subvertical thinning prior to continental collision. This deformation is consistent with the predictions of the model and can explain the exhumation of high P/T metamorphic material in both areas. -from Authors
Lattice preferred orientations (LPOs) of quartz were used to establish differences in deformation geometry, finite strain and temperature across an extensional detachment shear zone within the Chapedony Metamorphic Core Complex in the Central-East Iranian Microcontinent along the northern flank of Gondwana. Quartz c-axis data show a continuous evolution across the core complex from asymmetric Type I crossed girdles at the southwest margin, to broken, asymmetric Type I crossed girdle and single girdle with a large concentration of axes plotted in the center of the stereoplot at the central parts of the core complex and small circle girdle pattern at the northeast margin. These variations in quartz c-axis patterns imply change in strain geometry during deformation from plane strain to general flattening and pure flattening. Integrating analyses of quartz c-axis opening angles, quartz c-axis patterns and recrystalization regimes of quartz and feldspar suggests deformation temperatures range between less than 400 °C and 650 °C, which yield greenschist to amphibolite facies conditions. Mean kinematic vorticity number (Wm) measured in the mylonite samples ranges between 0.67 and 0.71, which indicates that exhumation of the metamorphic rocks of the CMCC was facilitated by a significant component of pure shear strain within a general shear regime.
3D finite strain analyses and kinematic vorticity measurements were carried out on the Loghon Anticline within the HP-LT Sanandaj–Sirjan metamorphic belt (Neyriz area, SW Iran). Rƒ/φ and Fry methods were used on the strain markers (e.g. deformed fossils) to interpret geometric relationships between the fold axis, strain ellipsoid axes and shear zone boundaries. The results indicate the predominance of prolate strain in the anticline. Quantitative kinematic analyses show that the Wk parameter is 0. 67 ± 0. 06 (i.e. pure-shear dominated non-coaxial flow). This study quantitatively supports the establishment of a dextral transpressive system, which is responsible for the development of the large-scale right-lateral shear zones that strike sub-parallel to the major folds. Flexural shear combined with regional dextral-shear is suggested to be the most common mechanism of folding in this area. Copyright © 2011 John Wiley & Sons, Ltd.
For quartz-rich tectonites two types of deformation thermometer are currently commonly employed: 1) The quartz c-axis fabric opening-angle thermometer that provides an estimate of deformation temperatures when fabrics were 'locked in' during dislocation creep and dynamic recrystallization. 2) The quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization. A critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development. However, fabric opening-angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening. In this paper the development of these thermometers is reviewed, and their potential sensitivities to competing factors such as temperature, strain rate, water weakening and (in the case of opening-angles) 3D strain type are discussed. Examples of the application of these potential thermometers to naturally deformed quartz-rich rocks are given, and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology-based thermobarometers are highlighted. In the review, attention is focused on problems associated with applying these thermometers to natural deformation, and examples of such problems are discussed.
We report new deformation temperature and flow vorticity data from the
base of the Greater Himalayan Series (GHS) exposed in the Sutlej Valley
and Shimla Klippe of NW India. We focus on three groups of transects
across the hanging wall of the Main Central Thrust (MCT). In order of
relative foreland - hinterland positions, they are the Shimla Klippe,
Western and Eastern Sutlej transects. Deformation temperatures indicated
by quartz c-axis fabric opening-angles increase both from foreland to
hinterland at a given structural distance above the MCT and up
structural section from the MCT within individual transects. Deformation
temperatures in the immediate hanging wall to the MCT are estimated at
˜510-535, 535-550 and 610 °C on the Shimla, Western Sutlej and
Eastern Sutlej transects, respectively. The steepest inferred field
gradients in deformation temperatures are recorded adjacent to the MCT
and progressively decrease up structural section following a power law
relationship. Comparison with temperature estimates based on
multi-mineral phase equilibria data suggests that penetrative shearing
occurred at close to peak metamorphic conditions. Vorticity analyses
indicate that shearing along the base of the GHS occurred under
sub-simple shear conditions (Wm values of 0.9-1.0) with a minor
component of pure shear.
This work develops an analytical model of shear senses within an inclined ductile simple shear zone with parallel rigid boundaries and incompressible Newtonian viscous rheology. Taking account of gravity that tends to drive the material downdip and a possible pressure gradient that drives it upward along the shear zone, it is shown that (i) contradictory shear senses develop within two sub-zones even as a result of a single simple shear deformation; (ii) the highest velocity and least shear strain develop along the contact between the two sub-zones of reverse shear; (iii) for a uniform shear sense of the boundaries, a zone of reverse shear may develop within the top of the shear zone if the pressure gradient dominates the gravity component; otherwise it forms near the bottom boundary; (iv-a) a ‘pivot’ defined by the intersection between the velocity profile and the initial marker position distinguishes two sub-zones of opposite movement directions (not shear sense); (iv-b) a pivot inside any non-horizontal shear zone indicates a part of the zone that extrudes while the other subducts simultaneously; (v) the same shear sense develops: (v-a) when under a uniform shear of the boundaries, the shear zone remains horizontal and the pressure gradient vanishes; or alternatively (v-b) if the shear zone is inclined but the gravity component counterbalances the pressure gradient. Zones with shear sense reversal need to be reinterpreted since a pro-sheared sub-zone can retro-shear if the flow parameters change their magnitudes even though the same shear sense along the boundaries is maintained.
This paper presents quantitative data on the finite strain, quartz crystal fabric, geometry of flow and deformation temperatures in deformed quartzite samples to characterize the ductile deformation along the thrust sheets constituting the Sanandaj–Sirjan Metamorphic Belt within the Zagros Mountains of Iran. The results of this study emphasize the heterogeneous nature of deformation in this belt, showing a spatial variation in strain magnitude and in degree of non-coaxiality. A dominant top-to-the-SE sense of shear is indicated by the asymmetry of microstructures and quartz c-axis fabrics. Quartz c-axis opening angles suggest deformation temperatures range between 435° ± 50°C and 510° ± 50°C, which yield greenschist to amphibolite facies conditions during the ductile deformation. Mean kinematic vorticity number (Wm) measured in the quartzite samples ranges between 0.6 and 0.9 with an average of 0.76, which indicates that extrusion of the metamorphic rocks of the region was facilitated by a significant component of pure shear strain. Traced towards the basal thrust of the Zagros Thrust System from northeast to southwest, the quartz grain fabrics change from asymmetric cross-girdle fabrics in the internal part of the deformation zone to an asymmetric single-girdle fabric at distances close to the basal thrust. This variation may depend on the structural depth and on the geometry of the ductile deformation zone. The observed increase in strain and vorticity within the study area is comparable with patterns recorded within metamorphic rock extrusions within other orogens in the world.
The combination of inclined collision and plate boundary shape can
control the nature of deformation and the sense of shear along a
transpression zone. The present study investigated the effects of a
boundary zone with curvilinear shape along a transpression zone on the
kinematics of deformation. The kinematics of the Zagros transpression
zone varies with the orientation of the zone boundary. Detailed
structural and microstructural studies showed sinistral sense of shear
on the southeastern part of the Zagros inclined transpression zone (Fars
Arc), but dextral sense of shear on the northwestern part of the zone.
It is inferred that the both senses of shear were developed coevally
under a bulk general shear, regional-scale deformation along a curved
inclined transpression miming the shape of the Fras Arc of the Zagros
and the reentrant of the Bandar Abbas Syntaxis. The Zagros transpression
zone formed by inclined continental collision between the Afro-Arabian
continent and Iranian microcontinent.
In the Greater Himalayan sequence of far northwestern Nepal, detailed mapping, thermobarometry, and microstructure analysis are used to test competing models of the construction of Himalayan inverted metamorphism. The inverted Greater Himalayan sequence, which is characterized by an increase in peak metamorphic temperatures up structural section from 580 to 720 °C, is divided into two tectonometamorphic domains. The lower domain contains garnet- to kyanite-zone rocks whose peak metamorphic assemblages suggest a metamorphic field pressure gradient that increases up structural section from 8 to 11 kbar, and which developed during top-to-the-south directed shearing. The upper portion of the Greater Himalayan sequence is composed of kyanite- and sillimanite-zone migmatitic gneisses that contain a metamorphic pressure gradient that decreases up structural section from 10 to 5 kbar. The lower and upper portions of the Greater Himalayan sequence are separated by a metamorphic discontinuity that spatially coincides with the base of the lowest migmatite unit. Temperatures inferred from quartz recrystallization mechanisms and the opening angles of quartz c-axis fabrics increase up section through the Greater Himalayan sequence from ∼530 to >700 °C and yield similar results to peak metamorphic temperatures determined by thermometry. The observations from the Greater Himalayan sequence in far northwestern Nepal are consistent with numerical predictions of channel-flow tectonic models, whereby the upper hinterland part evolved as a ductile southward tunneling mid-crustal channel and the lower foreland part ductily accreted in a critical-taper system at the leading edge of the extruding channel. The boundary between the upper and lower portions of the Greater Himalayan sequence is shown to represent a foreland–hinterland transition zone that is used to reconcile the different proposed tectonic styles documented in western Nepal.
The kinematic vorticity number (Wk) can be calculated for three-dimensional as well as two-dimensional geologic deformations. For steady-state deformations, Wk can be correlated to and analyzed in terms of finite strains. The analysis shows that assumptions commonly made for two-dimensional deformations are not applicable to three-dimensional deformations. A single Wk describes an infinite number of three-dimensional deformations. Further, even knowledge of flow apophyses orientation, instantaneous stretching axes orientation, and/or Wk are not sufficient to describe deformation. Three-dimensional deformations also require knowledge of the deformation ‘type’ or boundary conditions of deformation (e.g. transpression). Hence, in addition to being difficult to estimate, the value of knowing Wk for three-dimensional deformations is greatly reduced compared with plane strain. The most useful methods of determining Wk from naturally deformed rocks are presented.
We present the first P-T, deformation time, and kinematic constraints on the only known blueschist facies rocks (BS) present in the Zagros (Hajiabad area). The BS were underplated below the Sanandaj-Sirjan zone and crop out as kilometer-scale bodies within extensive colored melange units marking discontinuously the Neotethyan suture zone. P-T estimates point to high-pressure/low-temperature (HP-LT) conditions around 11 kbar and 520–530°C for the majority of BS, along a ∼15°C km−1 gradient. Some exotic blocks in matrix serpentinite reached 17–18 kbar at ∼500°C. In situ laser probe 40Ar-39Ar radiometric age constraints on phengite cluster between 85 and 95 Ma and suggest that (1) synconvergence exhumation of Zagros BS from 35–50 km to depths
This paper presents quantitative data from the Stack of Glencoul on flow vorticities associated with mylonite generation in the hanging wall and footwall of the Moine Thrust, using samples collected in a vertical traverse from 80 m above the thrust plane to 8.5 m beneath the thrust. Estimated vorticity numbers (Wm) in Moine pelites and psammites above the thrust range from 0.775-0.725 (c. 43-47% pure shear component) increasing downwards to 0.83-0.75 (35-45% pure shear) at 10 cm above the thrust. Wm values in dynamically recrystallized Cambrian quartzites at 0.5-14.5 cm beneath the thrust range from 0.99-0.90 (10-30% pure shear). At 3.0-8.5 m beneath the thrust estimated Wm values are less than 0.75 in the quartzites, although there is some thin section-scale partitioning withWmvalues of 0.75-0.65 (45-55% pure shear) in domains of dynamically recrystallized quartz and Wm values <0.65 >.55% pure shear) in domains of relict detrital quartz grains. Integration of strain and vorticity analyses indicates a vertical shortening of 50-75% in these gently dipping mylonites located at the base of the Moine Nappe. The tectonic implications of vertical shortening (thinning) and transport-parallel stretching at the base of the Moine Nappe are discussed.
In this paper we review microstructural and petrofabric work carried out on the Moine Thrust zone and overlying thrust nappes. Our review is primarily historical, and starts with contributions made by both 'amateur' and 'professional' geologists from the 1880s through to the early 1920s during, and immediately following, the original field-mapping of the Moine Thrust zone by the Geological Survey. From the 1920s to the early 1950s contributions were first dominated by Geological Survey work on the microstructural and metamorphic transition between the thrust zone mylonites and the overlying Moine metasedimentary rocks. Subsequent university-based quartz petrofabric work, primarily focused on the Moines, would ultimately lead to the 'Moine Petrofabric Controversy' that ran from the late 1940s to the early 1960s. The later stages of this controversy overlapped, from the early 1950s-mid 1960s, with a phase of microstructural and quartz petrofabric work that concentrated on the thrust zone mylonites and immediately overlying Moine Schists. Our review concludes with an overview of microstructural, petrofabric and related strain analyses undertaken since the early 1970s, both within the Moine Thrust zone and its immediate foreland and in the overlying higher grade thrust sheets. Throughout our review we emphasize and track the changing tectonic interpretations that have been placed on available microstructural and petrofabric data.
SEM/EBSD-based orientation and misorientation analyses are described for a lower amphibolite facies simple shear zone (Torridon, NW Scotland). It is shown that as well as conventional crystal-slip processes (i.e. basal-a, prism-a, rhomb-a and negative second order rhomb-a slip), dauphine twinning also plays a role in both microstructural and petrofabric evolution. Twinning assists in the initial grain size comminution processes, including dynamic recrystallization, from originally coarse wall rock grains to a typical nivlonitic microstructure in the centre of the shear zone. Subsequently, twinning helps to accommodate high shear strains in the mylonite whilst maintaining a stable microstructure and constant single crystal' petrofabric. The role of dauphine twinning appears to be to allow efficient switching between relatively 'soft' and relatively 'hard' slip directions that possibly exploit a distinction between negative and positive crystal forms. Misorientation analysis emphasizes the relationships between crystal-slip systems and grain boundary network, including dauphine twin planes, and suggests that the mylonitic microstructure contains preferred orientations of both tilt and twist boundaries that help to explain shear zone microstructural evolution and stability.
Strain and vorticity analysis of two Late Palaeozoic high-strain zones from the southern Appalachian Piedmont indicates that these zones experienced general shear transpression with a monoclinic to triclinic symmetry. Granitic rocks in the Brookneal highstrain zone from the southwestern Virginia Piedmont were transformed into mylonites under greenschist facies conditions. Sectional strains, estimated from quartz grain shapes, in mylonites range from three to ten and three-dimensional fabrics record flattening strains. The mean vorticity number (W(m)) estimated with the R(s)/theta method ranges from 0.3 to 0.95. In the central Virginia Piedmont, lower amphibolite facies deformation in the Spotsylvania high-strain zone affected biotite gneisses, amplubolites, and granitic pegmatites. Minimum sectional strains, estimated from folded and boudinaged pegmatite dykes, of 8-20 are common and three-dimensional strains are dominantly constrictional. Porphyroclast hyperbolic distribution analysis of ultramylonites yields Wn values from 0.4 to 0.8. The kinematic significance of these transpressional high-strain zones is threefold: they record tens to hundreds of kilometres of strike-slip offset-, 40 to 70% contraction normal to the zone: and significant orogen-paraltel material elongation.
Feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations. Recrystallizing aggregates have a lower strength because of the cyclic production of small, strain-free grains, and they develop a strong preferred orientation, consistent with that observed in mylonites. Thus, recrystallization-accommodated dislocation creep may be responsible for the grain-size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones.
Finite strain and vorticity of flow analyses were carried out within the Deh Vazir deformed conglomerates in the Sanandaj-Sirjan metamorphic belt (Zagros Mountains, Iran). This belt is in a sequence of tectono-metamorphic complexes made of low- to high-grade metamorphic rocks affected by a polyphase deformation history. Using Rf/φ technique for two-dimensional strain analysis showed that the finite strain of XZ plane is RS=3.7±0.70. The calculation of the harmonic mean from the axial ratios of the extracted pebbles and plotting these data in the Flinn diagram resulted in K=0.7±0.38 (i.e. oblate strain ellipsoid). Kinematic vorticity analysis of deformed pebbles showed that WK parameter is 0.59±0.10 (i.e. pure-shear dominated non-coaxial flow). The results quantitatively support the recognition of a partitioned dextral inclined transpressive regime with bulk triclinic symmetry along the Sanandaj-Sirjan metamorphic belt resulted from the oblique collision between the African–Arabian continent and the Iranian microcontinents.
The Zagros accretionary prism in southwestern Iran is exposed along the NW–SE trending of the Zagros Thrust System and inclined Zagros transpression zone. This accretionary prism consists of two units: the upper sedimentary mélange unit on top and the high-pressure metamorphic mélange unit at the bottom. Both units show characteristics of a tectonic wedge. The upper unit consists of type-I, II and III mélanges which display S-, C- and C′-type shear-band cleavages, quartz ribbons and rectangular or fish-head boudins. The lower unit fabrics display σ-, and δ-type porphyroclasts and quartz ribbon mylonites. These fabrics formed from a combination of 60.5% simple shear and 39.5% pure shear. Both components were involved in a lateral exhumation of the high-pressure/low-temperature metamorphic rocks in an inclined transpression wedge-shaped geometry. The estimated kinematic vorticity number (Wk) was calculated from quartz c-axis patterns, rotation of porphyroclasts and orientation of finite strain with respect to shear zone boundaries. Using the mean estimated Wm value of 0.84, the inclination angle for the thrust wedge on top of the NE-subducting Neo-Tethyan oceanic lithosphere is 18°. The 40Ar/39Ar plateau ages of early generations of biotite from the lower metamorphic mélange are 119.95±0.88Ma and 112.58±0.66Ma. This late Aptian age is related to early thrusting and formation of HP–LT metamorphic rocks. The dating of two amphibole samples from the amphibolite yields a weighted mean age of 91.23±0.89Ma. This Turonian–Cenomanian age suggests a later metamorphic event associated with subduction and obduction of the Neyriz ophiolite and later lateral extrusion of HP–LT metamorphic rocks along the inclined Zagros accretionary prism.
The Sanandaj–Sirjan Zone of western Iran is a metamorphic belt (greenschist–amphibolite) that was uplifted during Late Cretaceous continental collision between the Afro-Arabian continent and the Iranian microcontinent. In the June area, 300 km southwest of Tehran, the Late Palaeozoic–Mesozoic succession was affected by two major episodes of deformation. The first deformation formed tight folds and axial plane schistosity. These are strongly overprinted by second deformation structures that formed during Late Cretaceous continental collision under dextral transpression. The convergence has a low obliquity and has significant deformation partitioning into two domains. (1) A widespread schist and marble domain with intensely folded and foliated rocks that are cut by thrusts and have an overall south-southwest vergence. (2) A domain with wide zones of mylonitic granite, amphibolite and less common calcite mylonite that are affected by a foliation with the same orientation as in rocks of the schist and marble domain. Rocks of this domain also contain an intense sub-horizontal stretching lineation and abundant shear-sense criteria indicating dextral shear. This contrasts with many zones of transpression where strike-slip shearing is taken up along discrete faults. A syn-D2 pluton (the Galeh–Doz pluton) has a major S-shaped bend within it, imparted during the dextral transpression.
A summary discussion is given of attempts to quantify the degree of non-coaxiality of rock flow from observations of rock structure, and of some principles governing the sensitivity of structure to non-coaxiality. This is preceded by an extended review of relevant terminology, since some of the current confusion about rotational features in rocks stems from confusion about basic concepts and terminology from continuum mechanics. The kinematic vorticity number has been estimated for naturally deformed rocks from three localities. These yield vorticity numbers ranging from 0.35 to 0.9, corresponding to flows intermediate in character between pure and simple shearing.
Quartz crystallographic fabric transitions in well-exposed mylonites immediately beneath the Moine Thrust at the Stack of Glencoul (NW Scotland) have been investigated by optical microscopy, X-ray texture goniometry and Orientation Distribution Function analysis. A progressive change is observed from asymmetrical kinked single girdle c-axis fabrics at 0.5 cm beneath the Moine Thrust, through asymmetrical Type I cross-girdle fabrics to symmetrical Type I cross-girdle fabrics at 30 cm beneath the thrust. This c-axis fabric transition is accompanied by a transition from asymmetrical single a-axis maximum fabrics (0.5 cm beneath the thrust) through asymmetrical two maxima fabrics to essentially symmetrical two maxima a-axis fabrics. ODF analysis of these S >L and L - S tectonites indicates that c-axis positions on the ‘leading edge’ of the fabric skeleton are related by a common (a) direction oriented within the XZ plane at a moderate angle to the lineation (X). In contrast, c-axis positions on the peripheral ‘trailing edge’ are related by a positive (r) rhomb pole oriented close to Z; (a) directions lying within this common rhomb plane progressively change through 180° in orientation traced around the c-axis fabric skeleton. Such contrasting ‘single crystal’ rhomb (a) preferred orientations on the ‘leading’ and ‘trailing’ edges of the fabric skeleton are interpreted as indicating localized (grain scale) plane strain and flattening deformation, respectively. They result in tectonites with essentially symmetrical c- and a-axis fabrics which display strongly asymmetrical positive (r) and negative (z) rhomb pole figures. The observed transition in quartz c- and a-axis fabrics is interpreted as indicating an increasing importance of non-coaxial plane-strain deformation as the Moine Thrust is approached. Even immediately (<1 cm) beneath the thrust, however, flow has still significantly departed from bulk simple shear and involved an important (heterogeneous) component of contemporaneous flattening deformation.
Distortions of a homogeneous fabric, which occur around rigid objects in rocks deformed by non-coaxial ductile flow are useful to determine the sense of shear, but also contain information on other aspects of the flow. Rigid objects in a non-coaxial flow fall into two categories: those which are permanently rotating and those which can reach stable positions at high finite strain. The orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow, the ratio of instantaneous stretches and the axial ratio of the object. In naturally deformed rocks, immobilized rigid objects may be recognized from the geometry of the surrounding fabric. Tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow, and the shape of tails reflects the rotational behaviour of the porphyroclast. The axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain. In practice such an analysis is as yet difficult, but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the French Pyrenees.
A field example of strain partitioning has been analysed along the Nurra–Asinara transect of the NW Sardinian Variscan chain (Italy). The section in the Nurra–Asinara area is in a continuous sequence of tectono-metamorphic complexes made of low- to high-grade metamorphic rocks affected by a polyphase tectonic history. The principal fabric of the area is controlled by a D2 progressive deformation phase in which the strain is partitioned into folds and shear zone domains. The D2 stretching lineation and shear sense show a clear change from south to north. The principal meso- and micro-structures, vorticity gauges and a quantitative kinematic analysis of local strain suggest that the D2 kinematic history could be envisaged as an oblique heterogeneous deformation similar to the transpressive systems described in ancient and modern settings elsewhere. Using a simple kinematic model we also propose that both a transpressive system followed by “thrusting” or a partitioned transpressive system could be responsible for the fabric distribution and strain accumulation described in the study transect.
Some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking. Apparently conflicting evidence from rotated pressure-shadow regions around porphyroclasts and porphyroblasts is clarified. The use of quartz-crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations. -Authors