Calcite e-twins as a tectonic indicator, paleo stress pattern and structural evolution of the Zagros hinterland, SE Iran

Through the examination of calcite twins, this research outlines the tectonic development and paleo stress patterns of the Paleozoic Routshon complex situated in the southeastern segment of the Sanandaj–Sirjan zone, a hinterland region of the Zagros orogeny in southeastern Iran. The study of orogenic phase indicates that the deformation event affecting the southern sector of the Sanandaj-Sirjan zone aligns with the Cimmerian orogenic phase of the Late Triassic period. A variety of structural features at both map and outcrop scales highlight the importance of slip partitioning in the structural evolution of this region, driven by inclined transpression. Observations suggest that the deformation related to contractional components includes steeply to moderately plunging folds, dip-slip domain deformation primarily involving thrusts, and ongoing deformation by strike-slip component motion, which results in thrust-related ductile shear zones. The analysis of calcite c-axis fabrics from mylonite samples obtained from these shear zones indicates a low-temperature monoclinic pattern of non-coaxial deformation. This deformation type underscores the impact of the strike-slip component in the development of progressive simple shear within thrust-related shear zones in this segment of the Sanandaj-Sirjan zone. Dynamic analysis of c-axis fabric data reveals a NE-SW orientation for the principal compressive axes (σ1) in this area. This direction, corroborated by additional data such as fault surface, GPS, and earthquake focal mechanism data, confirms that the orientation of the compressive axes (σ1) has remained consistent from the Late Triassic to the present.

1 Introduction

Mechanical e-twinning of calcite represents the primary mechanism of crystal-plastic deformation in coarse-grained limestones and marbles subjected to temperatures below approximately 400°C (Turner, 1953). Twins have been extensively utilized as markers of deformation history, providing valuable insights into the stress regimes that shaped geological formations (Turner and Weiss, 1963), who developed a dynamic technique to deduce stress axes from a population of e-lamellae in deformed calcite rocks (Figure 1). This technique has been subsequently modified and refined to ascertain the principal direction and/or magnitudes of paleo-stress and numerous studies have refined techniques to deduce principal stress directions and magnitudes from calcite twin data (Lacombe and Laurent, 1992; 1996; Burkhard, 1993; Parsons and Thompson, 1993; Shelley, 1993; Lacombe, 2001; 2007; Jaya and Nishikawa, 2013; Yamaji, 2015; Qiu et al., 2016; 2022; Shan et al., 2019; Zheng and Shan, 2020).

Figure 1

Figure 1. (A) Schematic of twin structures in a calcite grain caused by simple shear (Evans and Groshong, 1994) showing the f-axis aligned perpendicular to the ge plane. Dashed lines represent the boundary of the original untwinned calcite grain, with shaded regions indicating the twinned lamellae and unshaded regions showing the host calcite. The angle of rotation (α) from the host portion to the twinned lamellae is measured at 38°17 (Handin and Griggs, 1951) H represents the total thickness of the host and twins combined, and t specifies the thickness of the twin (Jamison and Spang, 1976). (B) Diagram illustrating the geometry of an e-twin in calcite, highlighting the orientations of compression and tension axes (Shelley, 1993).

The scientific issue in the study area revolves around the complex tectonic evolution and paleo stress patterns within the Routshon complex, part of the Sanandaj–Sirjan zone in southeastern Iran. This region, a hinterland of the Zagros orogeny, exhibits a multifaceted geological history influenced by the prolonged convergence between Eurasia and Gondwanaland fragments, as evidenced by ophiolite belts and current GPS vectors (Agard et al., 2011). Despite its significance, the tectonic processes and stress regimes that have shaped this area remain poorly understood, particularly regarding the role of slip partitioning and inclined transpression in its structural evolution.

To address these issues, our research focuses on a detailed analysis of calcite twinning and c-axis fabrics from mylonite samples within thrust-related shear zones in the Routshon complex. Previous studies have identified key structural features and deformation phases in the Zagros region (Berberian and King, 1981; Alavi, 2004), but comprehensive paleo stress analyses integrating these findings with modern techniques are lacking.

In this study, we performed a dynamic analysis of c-axis fabric data to map out the principal compressive axes (σ1) and their orientations. By examining calcite twins and their deformation patterns, we aimed to reconstruct the paleo stress fields and elucidate the tectonic history of the region. Our approach combines field observations, microstructural analysis, and stereographic projections to provide a nuanced understanding of the stress regimes that have influenced the geological evolution of the Routshon complex.

Our findings indicate that the deformation events in the southern Sanandaj–Sirjan zone align with the Cimmerian orogenic phase of the Late Triassic period, characterized by contractional components and strike-slip motions resulting in ductile shear zones. The NE-SW orientation of the principal compressive axes (σ1), corroborated by fault surface GPS and earthquake focal mechanism data, highlights the consistency of the stress regime from the Late Triassic to the present.

This paper aims to utilize the properties of calcite twins to investigate the paleo-stress patterns and structural evolution of the Paleozoic Routshon complex, which is exposed in the Bazar area (Figure 2) within the Sanandaj-Sirjan Zone (SSZ), southeastern Iran. The study seeks to provide insights into the tectonic evolution of the Zagros orogenic belt’s hinterland.

Figure 2

Figure 2. (A) Simplified tectonic map of Iran, showing the Sanandaj-Sirjan Zone (SSZ) between the Urumieh-Dokhtar Magmatic Assemblage (UDMA) and the Zagros Fold-Thrust Belt. (B) Satellite image of the Bazar area, highlighting major geological features. (C) Key structures within the region in map view and (D) in cross section view, providing a detailed view of fault lines and thrust systems critical to the area’s tectonic framework.

2 Geological setting

The Zagros orogeny is part of the Alpine–Himalayan belt system, formed due to the prolonged convergence between Eurasia and fragments derived from Gondwanaland, as evidenced by ophiolite belts and current GPS vectors (Agard et al., 2011). This orogenic belt comprises southwest-verging mountains, which developed from the persistent subduction of the Neo-Tethyan oceanic lithosphere from the Jurassic to the Cenozoic era, followed by the collision of the Arabian plate with the Central Iranian microplate in the Cenozoic (Berberian and King, 1981; Alavi, 2004; Kamali et al., 2023). This process is the primary cause of the region’s activity, as evidenced by seismic events and morphotectonic features (Derakhshani and Eslami, 2011; Kermani et al., 2017; Rahbar et al., 2017). The formation of this orogeny is attributed to the closure of the Neotethys, following the complete subduction of oceanic crust at a northeast-dipping subduction zone beneath central Iran, and the subsequent Cretaceous continental collision between the Afro-Arabian and Iranian continental fragments (Shafieibafti et al., 2011). The orogenic structure includes: (1) the Mesopotamian–Persian Gulf foreland basin, (2) the Zagros Fold–Thrust belt extending from southwest to northeast (Falcon, 1961; Ghanbarian and Derakhshani, 2022b), (3) the SSZ (Stocklin, 1968), and (4) the Urumieh–Dokhtar Magmatic Assemblage, (UDMA). In contrast to the more extensively studied active tectonics and foreland deformation, there is less knowledge about the hinterland subduction-related arcs parallel to the Zagros, specifically SSZ and UDMA (Agard et al., 2011; Ghanbarian and Derakhshani, 2022a) (Figure 2A). The SSZ and UDMA, located northeast of the Main Zagros Thrust, are believed to result from a northeast-dipping subduction process of the Neo-Tethyan oceanic crust beneath the Iranian continental active margin (Berberian and King, 1981; Amirihanza et al., 2018).

The SSZ stretches approximately 1,500 km from the northwest (Sanandaj) to the southeast (Sirjan) with a width of 150–200 km, running parallel to the Zagros Fold Thrust belt. This zone contains the most intensely deformed rocks, exhibiting a NW-SE structural trend. The southern boundary of the SSZ is defined by the Zagros Main Thrust, which separates it from the Zagros region. Additionally, Central Iran is divided from the SSZ by a series of steep and straight faults, including the Tabriz and Nain-Baft Faults (Şengör and Kidd, 1979; MohammadiNia et al., 2024). The SSZ is characterized by a scarcity of Tertiary volcanic rocks, significant volumes of Mesozoic (and some Tertiary) intrusions, a relatively high abundance of Paleozoic volcanic rocks (Silurian, Devonian, and Permian), and metamorphism due to Cimmerian movements (Aghanabati, 2004; Ghasemi and Talbot, 2006). These features distinguish the SSZ from other geological and structural subdivisions in Iran (Ghazi and Moazzen, 2015). The geodynamic evolution of this zone was driven by the opening and subsequent closure of the Neotethys Ocean along the northeastern margin of Gondwana (Alavi, 1994).

The Sanandaj–Sirjan zone’s polyphase deformation structures align with transpressional forces due to the angled collision between the African–Arabian continent and the Iranian microcontinent (Mohajjel and Fergusson, 2000; Sarkarinejad and Azizi, 2008; Sarkarinejad et al., 2008; Shafieibafti and Mohajjel, 2015; Ghanbarian et al., 2021; Mansouri et al., 2021). This transcurrent component arises from an oblique factor in the subduction zone (Shafieibafti, 2007; Sheikholeslami et al., 2008; Shafieibafti and Mohajjel, 2015). This study aims to determine the structural characteristics of the Zagros orogen’s backcountry by using calcite twinning analyses to map out the ancient stress fields and the depth of deformation within the Routshon complex, located in the southeastern part of the SSZ.

In the southern part of the Sanandaj-Sirjan Zone (SSZ), Paleozoic strata are divided into six syntectonic regional metamorphic complexes, each with varying metamorphic grades and ages. These complexes are overlain by non-metamorphosed Early Jurassic basal conglomerates, which contain metamorphic clasts, as well as volcanic-detrital rocks. The continuity of these complexes has been traced from Hajiabad in the southeast to Neyriz in the northwest (Berberian, 1976). Each complex is further divided into three metamorphic units, listed from top to bottom: (1) the upper complex (low grade), (2) the middle complex (low to medium grade), and (3) the lower complex (high grade). A single sample of Kyanite-Sillimanite schist from the Neyriz area has been dated using the conventional K/Ar radiometric method, yielding an age of 404 ± 8 Ma, corresponding to the Silurian-Devonian boundary (Watters and Sabzehei, 1970). The middle complex contains poorly preserved fragments of crinoid stems, several corals, and a single Bryozoan, with Devonian pollen and spores reported from carbonaceous black schist (Berberian, 1976). The stratigraphic units in the study area, which extends from longitude 56°36′15″E to 56°40′30″E and latitude 28°59′15″N to 29°0′45″N (Figures 2B, C), consist of the Paleozoic-Mesozoic Routshon complex (GSI, 1997b; 1997a; 2007).

Some researchers have suggested that these metamorphic complexes represent Precambrian basement rocks (Stocklin, 1968), while others attribute their syntectonic regional metamorphism to Early Cimmerian tectonics (Berberian, 1972; 1973; Hushmandzadeh et al., 1972; Majidi, 1972; 1974; Sabzehei and Berberian, 1972; Berberian and Nogol, 1974; Sabzehei, 1974). The Routshon complex includes a variety of rock types such as dolomitic and marble formations, green schists, black schists, chert, mica schists, calcite and dolomite marbles, and white marble (Figure 3).

Figure 3

Figure 3. Outcrop view of rock units in the study area. Paleozoic units are identified as Dolomites (P_zd), Schists (P_zs), and White Marbles (P_zm). The transitional paleozoic-Triassic unit (P_Tdm) is characterized by Dolomitic marbles, illustrating the lithological diversity and stratigraphic relationships crucial to understanding the region’s geological history.

The Bazar area in the southeastern SSZ exhibits evidence of three distinct deformation events (D1, D2, and D3). Calcite twinning plays a significant role in understanding the deformation mechanisms and stress regimes associated with each phase.

2.1 D1 deformation

The D₁ deformation phase is characterized by folding, manifested in a range from gently inclined to recumbent and spans mesoscopic to macroscopic scales, including axial planar schistosity (Figure 4A). These formations are prominently observed in the calcite and dolomite marbles (PT^d) and white marble (P_m) units of the Routshon complex, as evidenced by detailed field observations and microstructural analysis. Specifically, axial planar schistosity is consistently observed to run parallel to the bedding planes in these units. This parallel alignment is indicative of intense compressive deformation, which is further supported by the presence of well-developed axial planar schistosity, confirmed through microscopic examination of thin sections from multiple samples. These samples exhibit clear evidence of recrystallization and deformation textures consistent with low-grade metamorphic conditions, corroborating the interpretation of the observed fabrics. Our analysis of the F1 folds in the Routshon complex reveals that their axial planes dip at angles of 10°–30° toward the NE, while the fold axes plunge gently to the W. These observations are consistent across multiple outcrops and are supported by detailed field measurements and structural mapping. The gentle plunge of the fold axes and the moderate dip of the axial planes suggest that these folds formed through a process of flexural flow, (Twiss and Moores, 1992; Hatcher, 1995), reflecting the compressive stress regime prevalent during the deformation. In flexural-flow folds, rock material in incompetent layers flows from fold limbs toward fold hinges, and therefore appreciable thickness changes occur in the rock layer. Flexural-flow folds are mostly similar-like folds, but may also include some parallel folds. Some layers in single flexural-flow folds maintain constant thickness; others are thickened into axial zones and thinned into limbs as folding proceeds, indicating a higher contrast in internal ductility. In this area flexural flow evidences are visible in schist in an interbedded marble (Figures 4B, C). At the mesoscopic level, an interference pattern is evident due to the coaxial refolding of F1 folds. While Type I and Type II interference patterns are typically more common, our detailed structural analysis indicates that the overlapping of F1 and F2 folds in the Routshon complex predominantly results in Type III interference patterns. This observation, as outlined by Ramsay and Huber (Ramsay and Huber, 1987), is somewhat unexpected but is supported by consistent field evidence and structural mapping (Figure 4D). The presence of Type III patterns suggests complex deformational histories involving multiple phases of folding and refolding, which we have documented through extensive mesoscopic and microscopic analyses of the fold structures.

Figure 4

Figure 4. (A,B) View of an outcrop featuring an isoclinal fold, highlighting the characteristic features of the F1 folds in the calcite and dolomite marbles (PTd) and white marble (Pm) units of the Routshon complex. The F1 folds are marked by their gently inclined to recumbent geometry with axial planes dipping at angles of 10°–30° toward the NE and axes plunging gently to the W. These folds are indicative of flexural flow deformation. The symbols so and s1 correspond to bedding and schistosity, respectively. (C) Under the flexural flow folding mechanism some layers are thickened into axial zones (yellow lines) and thinned into limbs (blue lines) (D) Type III interference pattern in folding identified as a Hook type pattern, resulting from the coaxial refolding of F1 and F2 folds.

2.2 D2 deformation

This deformation event led to the formation of thrust faults (Figure 2). In the hinterlands of orogenic belts, thrusts typically result from differential flow within a ductile mass, which creates F1-folds. Subsequent shearing between the antiform and synform limbs produces these thrusts (Hatcher and Hooper, 1992). In the study area, five thrust sheets (T1–T4), known as the Bazar thrust system, have been identified. These thrust faults influence the complex outcrop patterns and generally trend NW–SE (T1-T4, dipping northeast) or N–S (dipping east) (Figure 2C). Various geological features arise from these faults. The T1 thrust developed along the southern limbs of gently inclined to recumbent F1-folds (Figures 4A, 5A). The overlying of dolomitic marbles (PTdm) atop green schist, black schist, and mica schist (PZs) indicates the influence of the T2 thrust (Figures 5B–D). The T3 thrust involves the thrusting of dolomites (Pzd) over Schists (Pzs) and White Marbles (Pzm), with associated duplex structures in the shear zones (Figures 5E, F). T4 follows an NNE-SSW trend with an easterly dip. The interposition and increasing thickness of the PTdm unit above the Pzd unit due to T4 thrusting and associated shear zones are evident. In the northern region of the T4 thrust, characteristic klippe outcrops are visible (Figures 5G, H).

Figure 5

Figure 5. (A) Outcrop view of the T₁ thrust, illustrating the truncation and duplication of Dolomites (P_zd). (B) Outcrop view of T₂ thrust, where Dolomitic Marbles (Ptdm) were truncated and thrusted over schists (P_zs). (C) Dolomites (P_zd) overlying the white Marbles (Pzm) and Schists (P_zs) by T₃ thrust. (F) Flexural-slip duplex structures in the Pzm (white Marbles) and Pzs (Schists) units associated with the T3 thrust, characterized by a sigmoidal configuration and suggesting a southwest thrust direction. (D,E) slickenside and striation of T₂ thrust with its stereographic projection (equal area), indicating the direction of movement. (G,H) Typical outcrop of klippes, showing Dolomites (P_zd) over Schists (P_zs) by T₄ thrust.

Shear zones, formed in the thrust sheets, are composed of calcite mylonites. The foliation patterns exhibit east-west orientations and dip northward. The elongation lineation within the calcite mylonites is delineated by the elongated axes of the calcite grains, which are ellipsoidal in shape. The mylonites present multiple shear sense indicators across various scales, all suggesting a right-lateral shear direction, as illustrated in Figures 6A–D.

Figure 6

Figure 6. (A) Outcrop view of mylonitic foliation within the T₁ thrust shear zone. (B) C-type shear band featuring horizontal C-planes and S-planes trending from upper right to lower left. (C) δ (delta)- type mantled porphyroclasts. (D) σ(Sigma)-type of asymmetric mantled porphyroclats with backward rotation (Zhang and Fossen, 2020) in dolomite and calcite. Also, fractured porohyroclats with and antithetic microfaults evident in these shear zones.

2.3 D3 deformation

The last deformation phase was characterized by an extensional regime, leading to the formation of normal faults. The bookshelf or domino structures associated with a normal fault can be clearly seen in competence units such as dolomites and marbles (Figure 7). The normal faults show a NNE trend which has a high correlation with dyke orientations. The study area reveals the presence of diacritic dykes within the fault planes, as depicted in Figure 6. The emergence of normal faults is hypothesized to be associated with magmatic activity, a theory that requires more in-depth analysis for verification.

Figure 7

Figure 7. (A) Intrusion of dioritic dykes into Dolomites (Pzd unit), showing the interaction between magmatic activity and existing rock formations. (B) Intrusion of a dioritic dyke into Dolomitic Marbles (P_Tdm unit) demonstrating the penetration of magmatic material into carbonate units. (C,D) Rose diagrams illustrating a high correlation between the trends of normal faults and the orientation of dykes, highlighting the structural relationship between faulting and magmatism.

Calcite twinning plays a crucial role in deciphering the deformation history of the Routshon complex across the D1 to D3 phases. During the D1 deformation phase, calcite twins indicate low-temperature compressive conditions, with consistent NE-SW oriented stress axes corresponding to the flexural flow folds. In the D2 phase, the twinning patterns within the mylonitic shear zones of the thrust faults reveal progressive simple shear deformation, providing kinematic indicators of dextral shear. Finally, in the D3 extensional phase, the reorientation of calcite twins reflects the transition to an extensional stress regime, marking the shift from compressional to extensional tectonics. These twinning patterns serve as microstructural markers, offering valuable insights into the paleostress orientations and the dynamic tectonic evolution of the study area.

3 Sampling strategy and measurement

To delineate the stress and kinematic chronology of the Sanandaj–Sirjan zone, a sampling strategy was employed that involved collecting 15 oriented samples of calcite mylonites from thrust-related shear zones. These samples were aligned normal to the mylonitic foliation and parallel to its strike. Utilizing a U-stage, measurements were conducted to determine the c-axis and orientation of twin lamellae. Approximately 50 grains were examined per sample. The predominant form of calcite twin lamellae observed was type II twins; however, other twin types were also identified in these samples (as shown in Figure 8). The analysis indicated that 70% of the calcite grains exhibited two sets of twins, while the remaining 30% presented a single set of twins.

Figure 8

Figure 8. Microphotograph (plane-polarized light, PPL) of type I, II and III twins. Type II twins are the predominant twin type observed in mylonitic calcites from the Bazar area.

4 Discussion

4.1 Structural model of the bazar area

In the southern region of the SSZ, the Paleozoic rock formations underwent deformation and metamorphism as a result of the early Cimmerian orogenic events that occurred in the Late Triassic period. An unconformity between the deformed and metamorphosed Paleozoic complex and the Jurassic units (conglomerate and sandstones) in the Khabr area (east of the study area) and Faryab area (southeast of the study area) reveals the end of the Cimmerian phase (Shafieibafti et al., 2011; Shafieibafti and Mohajjel, 2015). The early Cimmerian deformation along the southern margin of Iran indicates that this region functioned as an active margin during that period (Berberian and King, 1981; Mohajjel et al., 2003; Sheikholeslami et al., 2008). Structures indicative of the early Cimmerian orogenic phase, such as south-southwest verging folds and type C thrusting, are evident (Hatcher and Hooper, 1992), along with thrust-related shear zones under low-grade green-schist metamorphic conditions (Shafieibafti et al., 2011). In various locations within the Sanandaj-Sirjan zone, structural developments consistent with the inclined transpression model have been confirmed (Shafieibafti et al., 2011; Shafieibafti and Mohajjel, 2015; Mansouri et al., 2021). Inclined transpression results from concurrent contraction and oblique-slip shear, which can be decomposed into strike-slip and dip-slip components (Jones, 2004). This model shows that structural evolution, whether through ductile or brittle mechanisms, is closely linked to slip partitioning. The subsequent section will analyze and interpret the structures of the study area using the strain triangle model (Figure 9).

Figure 9

Figure 9. Structural relationships in the Bazar area, illustrating direct correlations with contractional, dip-slip, and strike-slip components as outlined by the strain triangle in the inclined transpression model (Jones, 2004). A clearer structural strain model, similar to that proposed by Xiao et al. (2023), is used to better depict the control of deformation partitioning in the region. This model provides a more comprehensive understanding of the structural evolution and its implications for deformation processes.

4.1.1 Contractional domain

Deformation related to the contractional component is observed in the calcite and dolomite marbles (PTdm) and white marble (Pzm) units, characterized by folds that range from steeply plunging to moderately inclined (Figures 4A, 9).

4.1.2 Dip-slip domain

In the dip-slip domain, deformation is primarily governed by thrusts dipping north with top-to-the-south movement and thrusts dipping east with top-to-the-west movement. These thrust faults are central to the complexity of the outcrop patterns, typically trending NW–SE or N–S with dips ranging from northeast to east (Figures 2D, 5).

4.1.3 Strike-slip domain

The ongoing deformation under strike-slip component motion is evident in the form of thrust-related ductile shear zones. These zones contain calcite mylonites and display multiple indicators of dextral shear sense (Figures 2D, 5E–H, 9).

4.2 Kinematic interpretation of calcite fabrics

Wenk et al. (1987) classified calcite c-axes fabrics into low-temperature (LT) and high-temperature (HT) categories (Figure 10). LT fabrics, demonstrated in experimental settings, predominantly feature twinning with minor intracrystalline slip as the primary deformation mechanism, whereas HT fabrics are dominated by intracrystalline slip, as seen in experiments. LT fabrics display a distinct peak orientation nearly parallel to the principal stress axes and oblique to the shear plane. This orientation can serve as an indicator of shear sense or as a measure of the degree of non-coaxial deformation. LT fabrics are further differentiated into LT pure shear, where the peak orientation is perpendicular to the shear plane, and LT simple shear, which is similar to LT pure shear but rotated counter to the shear direction. In LT fabrics, the c-axes orientations range from sub-horizontal to gently plunging (Lafrance et al., 1994), presenting a girdle that includes one or two asymmetrical maxima, indicative of monoclinic symmetry.

Figure 10

Figure 10. Computer simulations of c-axis fabrics in calcite based on experimental data, classifying these fabrics into low-temperature (LT) and high-temperature (HT) types. HT fabrics feature simple shear, evolving from rotated pure shear fabrics with added monoclinic distortion. LT fabrics, displayed in the lower part of the figure, consist of both pure shear and simple shear components, as oriented with respect to the external frame (Wenk et al., 1987).

Figure 11 presents plots of optic c-axis orientations from samples 1 to 18. These samples exhibit concentrations of c-axes with highly intense point maxima positioned anticlockwise from the normal to the shear plane, indicative of LT simple shear fabrics.

Figure 11

Figure 11. Presentation of calcite twinning data, showing orientations of c-axes, compression, and tension axes. Panels (A–C) illustrate c-axis fabrics displaying a low-temperature (LT) simple shear pattern, indicative of dextral shear sense in mylonitic shear zones. The data are plotted on lower hemisphere, equal-area stereographic projections with contour levels at 1%.

4.3 Paleostress orientations from calcite twins

Data were organized into distinct files for c-axes, e-twin lamellae, compression directions, and tension directions, formatted for immediate application in stereographic projection software. In instances where a calcite grain features two or three e-twin lamellae, the corresponding c-axis data for that grain is recorded multiple times in the dataset (Shelley, 1989; Shelley, 1993). Figure 11 illustrates the orientations of c-axes alongside the derived compression and tension directions. These orientations of the principal stress axes were determined by analyzing twin plane and c-axis measurements to deduce compression and tension directions for each analyzed grain. The aggregated data from all measured grains in the sample set were then contoured on an equal-area stereographic projection to display the results (Turner, 1953; Weiss, 1954; Pfiffner and Burkhard, 1987).

Figure 10 illustrates a monoclinic stress pattern derived from c-axes and twin planes data, where σ3 aligns with the tension axis maximum, and σ1 is close to the compression axis maximum. The compression axes (σ1) exhibit a NE–SW trend with shallow plunges ranging from 7°–19°, while the tension axes (σ3) display a NW–SE trend with moderate plunges of 12°–54°. Both stress axes, σ1 and σ3, are nearly horizontal, whereas the σ2 axis is predominantly vertical. These mean principal stress axes are mapped on a structural diagram, revealing a geometric alignment between compressional stress axes and thrusts, suggesting dextral movement in shear zones (Figures 5E–H).

The compressional stress indicated by calcite twinning in the Routshon complex is likely linked to the subduction of the Neotethys beneath the southern Sanandaj-Sirjan zone in central Iran during the Middle Triassic. Our findings suggest that a dextral inclined transpression regime was dominant in the metamorphic complexes within the hinterland of the Zagros orogen from the Middle Triassic to the Jurassic, associated with the oblique subduction of the Neotethys beneath central Iran. Notably, the Triassic-Early Jurassic σ1 trends observed in the Bazar area (this study) and the Faryab area (Shafieibafti, 2007) in the hinterland of the Zagros orogen align closely with the σ1 trend in the Fars province (Lacombe, 2007) of the Zagros Simply Folded Belt, located in the foreland of the Zagros orogen (resulting from the Mio-Pliocene Arabia–Eurasia collision). These consistent NE-SW orientations correlate with the current compressional trend in eastern and central Iran (Ebrahimi et al., 2021; Rashidi et al., 2021; 2023; Ezati et al., 2022; Rashidi and Derakhshani, 2022; Abbaspour et al., 2023), as shown in Figures 12A–E. Despite the significant time gap and the complex geodynamic context, this similarity in stress orientations may suggest that the stress regime in the upper plate (Zagros Simply Folded Belt) has remained relatively consistent from the onset of subduction in the Middle Triassic to the Mio-Pliocene Arabia–Eurasia collision (Shafieibafti, 2007).

Figure 12

Figure 12. (A) Topographic map of Iran using SRTM 30 data, showing the orientation of compressive stress (azimuth of σ1). Different arrow styles represent various methodologies used to determine stress orientations. Key to methodologies: 1: this study; 2: (Shafieibafti and Mohajjel, 2015); 3: (Shafieibafti et al., 2011); 4: (Ebrahimi et al., 2021); 5: (Lacombe, 2007); 6: (Jentzer et al., 2017); 7: (Zarifi et al., 2014); 8: (Karagianni et al., 2015); 9: (Fattahpour and Moosavi, 2010); 10: (Zamani et al., 2008); 11: (Shabanian et al., 2010); 12: (Zanchi et al., 2006); 13: (Malekzade et al., 2016); 14: (Navabpour et al., 2008); 15: (Navabpour et al., 2007); 16: (Mobasher and Babaie, 2008); 17: (Authemayou et al., 2005); 18: (Javadi et al., 2013); 19: (Yazdi et al., 2012); 20: (Farbod et al., 2011); 21: (Lacombe et al., 2011); 22: (Authemayou et al., 2006); 23: (Navabpour and Barrier, 2012); 24: (Javidfakhr et al., 2011b; 2011a); 25: (Javadi et al., 2015); 26: (Kargaranbafghi et al., 2011); 27: (Lacombe et al., 2006); 28: (Dolati and Burg, 2013); 29: (Rashidi et al., 2022). (B–E) Schematic models illustrating inclined transpression within the Bazar, Khabr, and Faryab areas of the Sanandaj-Sirjan zone in the Zagros orogen hinterland, and the Bazargan area in the central Iran microcontinent, providing a comparative view of stress patterns and tectonic evolution across these regions.

4.4 Strengths and limitations of the data

Our study leverages a detailed analysis of calcite twinning and c-axis fabrics to interpret the tectonic evolution and paleostress patterns within the Routshon complex of the Sanandaj-Sirjan zone. The following points highlight the strengths and limitations of our data:

4.4.1 Strengths

• Comprehensive Sampling and Analysis: The study is based on a dataset of 15 oriented samples of calcite mylonites from thrust-related shear zones. Each sample has been meticulously analyzed for c-axis orientations and twin lamellae using a U-stage, ensuring high accuracy and reliability of the data.

• Consistency with Experimental Findings: The observed low-temperature (LT) simple shear patterns in the c-axis fabrics align well with experimental studies, providing confidence in the interpretation of the shear sense and deformation mechanisms.

• Structural and Microstructural Correlations: Our interpretations are supported by detailed structural mapping and microstructural analysis, which reveal consistent geometries and kinematic indicators across multiple samples and outcrops.

4.4.2 Limitations

• Potential Rigid Body Behavior of Porphyroclasts: Under low-temperature conditions, porphyroclasts might behave as rigid bodies, potentially complicating the interpretation of ductile deformation. This aspect necessitates caution in interpreting the shear sense purely from calcite fabrics.

• Absence of EBSD Analysis: While Electron Backscatter Diffraction (EBSD) analysis could provide additional insights into the deformation mechanisms and validate our interpretations, it was not performed due to resource constraints. Future studies incorporating EBSD would help confirm our conclusions.

• Localized Data: The data and interpretations are derived from specific locations within the Routshon complex. While these findings provide valuable insights, they may not fully represent the broader regional deformation patterns and tectonic history.

5 Conclusion

The structural evolution observed in the study area is closely tied to slip partitioning, reflecting a comprehensive analysis within the domains of strike-slip, dip-slip, and contraction, as framed by the strain triangle model. The examination of optical c-axis orientations reveals pronounced point maxima, positioned anticlockwise from the normal to the shear plane, which indicates a low-temperature (LT) simple shear fabric. This LT fabric aligns with experimental findings, suggesting that twinning, is the predominant mechanism of deformation within the region. Further analysis identifies a dextral shear sense within mylonitic shear zones, characterized by a monoclinic stress pattern. This pattern is discernible through the orientation of c-axes and twin planes data, where σ₃ aligns with the tension axes’ maximum and σ₁ is proximal to the compression axes’ maximum. Notably, the compression axes (σ₁) exhibit a NE–SW orientation with shallow plunges ranging between 7°–19°, while the tension axes (σ₃) present a NW–SE trend with moderate plunges of 12°–54°. The orientations of σ₁ and σ₃ are predominantly sub-horizontal, while the σ₂ axis being sub-vertical. The plotted orientations of the mean principal stress axes on a structural map reveal a geometric relationship between compressional stress axes and thrusts, indicating a dextral movement across shear zones. Conclusively, the findings of this research confirm the prevalence of a dextral inclined transpression regime during the Middle Triassic-Jurassic period within the metamorphic complexes of the Zagros orogen hinterland.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

HD: Formal Analysis, Investigation, Software, Writing–original draft. SS: Conceptualization, Data curation, Methodology, Project administration, Supervision, Validation, Writing–original draft, Writing–review and editing. SK: Data curation, Methodology, Project administration, Supervision, Writing–review and editing. JO: Formal Analysis, Methodology, Supervision, Writing–review and editing. AR: Software, Validation, Visualization, Writing–review and editing. MN: Conceptualization, Validation, Visualization, Writing–review and editing. RD: Conceptualization, Funding acquisition, Resources, Supervision, Writing–review and editing.

Funding

The author(s) declare that no financial support was received for the research, authorship, and/or publication of this article.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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