C-axis Fabrics Research Articles

Inferring the Orientations of the Foliation and Lineation Defined by Quartz c-Axis Fabrics: Methods and Applications

Inferring the Orientations of the Foliation and Lineation Defined by Quartz c-Axis Fabrics: Methods and Applications

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.

Transpression in the Eastern Jiangnan Orogen and its implications for ductile deformation process and regional Tectonics of the South China block

The transpressive deformation in the eastern Jiangnan Orogen during the Early Paleozoic is the structural response to the oblique convergence of the Yangtze-Cathaysia blocks, and this relationship is critical for understanding the tectonics of South China. The Jiangwan–Huangshan shear zone (JHSZ) and Baiji–Sanyang shear zone (BSSZ) exhibit NEE–SSW-trending oblique dextral and sinistral shearing, respectively, and their evolution resulted in local E–W dextral strike-slip shearing. Most kinematic vorticity values range from 0.4 to 0.81, which implies that the JHSZ and BSSZ deformed under the contribution of both pure-shear and simple-shear components, suggesting complicated deformation histories for those orogenic shear zones. The quartz and feldspar deformation mechanisms, opening angles and lattice preferred orientation (LPO) patterns of the quartz c-axis fabrics suggest shear deformation temperatures between 400 and 550 °C in the eastern Jiangnan Orogen. Locally deformation occurred under high-temperature (>600 °C) conditions. Combined with previous data and regional geology, these findings indicate that the shearing deformation in the eastern Jiangnan Orogen accommodated the oblique convergence between the Yangtze and Cathaysia blocks during the Early Paleozoic, which was possibly caused by the final assembly of Gondwana.

Multimaxima crystallographic fabrics (CPO) in warm, coarse-grained ice: New insights

Multimaxima crystallographic c-axis fabrics (CPOs) commonly observed in coarse-grained ice have been interpreted in several different ways but remain enigmatic. We review previous interpretations of these fabrics and present new data from Storglaciären, northern Sweden, for comparative purposes using both U-stage methods (c-axes only) and electron backscatter diffraction (EBSD) (both c-axis and a-axis orientations). Consistent with most previous studies, microstructures in our study indicate that all grains have been dynamically recrystallized with grain boundary migration as the dominant process. The CPO is characterized by grains oriented favorably for easy glide on the basal plane, given the local kinematics. When a representative sample is obtained, areas on the glacier margin undergoing non-coaxial deformation have a CPO generally consistent with simple shear. Multimaxima CPOs developed in the center of the glacier, which is associated with more coaxial deformation, likely represent a small circle girdle or partial girdle fabric associated with flow-parallel compression. In both locations, sampling bias, resulting from repeat measurements of single parent grains appearing several times as “island grains” in a 2D thin section, is likely responsible for the observed multiple maxima that, with bias removed, simplify to patterns expected for simple shear or coaxial deformation. An earlier study suggesting that multimaxima CPOs may result from twinning is not borne out from our data combining c-axes and a-axes that do not show the symmetry required of twinning.

PT conditions of deformation of the Gol-e-Gohar shear Zone, SW Iran: Insights from analysis of quartz c-axis fabrics, recrystallization mechanisms and syndeformational fluid inclusions

An investigation into the correlation between microstructural characteristics and deformation temperatures of quartz in the middle crust is of utmost importance in yielding substantial knowledge of the nature of deformation zones. Mylonitic rocks from the Gol-e-Gohar Shear Zone were investigated to infer innovative insights into the ductile deformation pressure–temperature (PT) conditions of the Sanandaj–Sirjan metamorphic rocks within the Zagros, southwest Iran. This approach used a multi-method strategy including quartz c-axis fabrics, recrystallization mechanisms and fluid inclusions (Fls) data. Geothermobarometric analysis based on these data highlight deformational PT conditions ranging from ∼400 to ∼500 °C and P < 5 kbar that correspond to ductile deformation with greenschist to lower-amphibolite conditions at depth of ∼ 12–19 km. Measured opening angles of quartz c-axis fabric and dominance of basal < a > slip system indicate crystal plastic deformation occurred in T > 400 °C. Microstructures highlighting the occurrence of Grain Boundary Migration (GBM) confirm temperature of 500–600 °C. The deduced P–T–salinity Fls history of deformation-related fluid inclusions is highlighted by Fls (B-type) with H2O-CO2–rich metamorphic fluids. These fluids were trapped during ductile deformation in temperatures ranging from ∼ 355 to ∼ 535 °C and pressure of P < 5 kbar. The petrofabric analyses show that the mylonitic rock in the Gol-e-Gohar shear zone have undergone metamorphism at the amphibolite facies condition, and then followed by a retrograde greenschist facies metamorphism. These results emphasize the utility of microstructural and fluid inclusion techniques to constrain the history of mylonitic rocks exhumation in the deformation zones.

Kinematic analyses of the asymmetric boudins in the Tutak area within the Zagros hinterland fold-and-thrust belt, Iran

Exhumation of the Tutak gneiss dome occurred in a transpressional regime between the Arabian plate and the Central Iranian microcontinent. The Tutak gneiss dome is located in the eastern part of the basement-involved Zagros hinterland fold-and-thrust belt. This dome consists of exhumed middle crust including the Tutak and Sourian metamorphic complexes, which are lied on its core and edges, respectively. Integrated structural, microstructural, and flow vorticity analyses in quartzo-feldspathic mylonites and well-developed asymmetrical boudins provide significant information regarding the kinematic characteristics coeval with the Tutak gneiss dome development. Asymmetrical boudins have been used as potential shear sense markers throughout the mantle of the Tutak gneiss dome. The deformed quarzitic layers within the slate belt host domino and shear-band boudins, demonstrating both sinistral and dextral shear senses. The mean vorticity number (Wm) was estimated from the quartzo-feldspathic mylonites and asymmetric boudins. Using the hyperbolic distribution (HD) and the RXZ strain ratio/quartz c-axis fabric (RXZ/β) methods. The estimated Wms from the HD method and quartz c-axis patterns are 0.69 to 0.82 and 0.6 to 0.92, respectively, suggesting a combination of 41% to 72% simple- and 28% to 59% pure-shear components. The close match confirms that asymmetric boudins can be a good strain estimator. The estimated deformation temperature (528.33 ± 50 °C) from the opening angles (OAs) of quartz c-axes suggests amphibolite facies conditions. Our results show that the Tutak gneiss dome developed under sub-simple shear conditions, which stretched and shortened the dome parallel and orthogonal to the transpressional boundaries, respectively.

Buoyancy-driven exhumation deformation: Evidence from the Sulu orogen, eastern China

Whether deformation of exhumed crust in a collisional orogen is driven by buoyancy or tectonic stress remains uncertain. The Sulu orogen in eastern China contains slowly subducted and exhumed high-pressure (HP)−ultrahigh-pressure (UHP) terranes, which provide a good opportunity to understand whether the exhumation deformation was driven by buoyancy. We used field and microscopic observations as well as quartz c-axis fabrics to determine the deformation kinematics, temperatures, and evolution of the exhumed crustal slices. Deformed and undeformed dikes were dated by using the zircon U-Pb laser ablation−inductively coupled plasma−mass spectrometry method to constrain the timing of deformation. Our data demonstrate that each crustal slice was involved in pervasive top-to-the-NW (hinterland) or top-to-the-SE (foreland) ductile deformation during Late Triassic exhumation, and that the base of each slice records deformation that was superimposed during the subsequent exhumation of the underlying slice. The crustal exhumation of the southern Sulu orogen is consistent with the multistage ductile extrusion model. The kinematics of exhumation deformation within each crustal slice conform to an upward asymmetric flow. The flow velocity and corresponding shear sense were affected by temperature variations in each HP slice, whereas they were sensitive to migmatization within the UHP slices. The southern Sulu orogen examples show that crustal flow deformation during exhumation was driven by buoyancy and controlled by viscosity. Unlike the consistent kinematics of tectonics-driven deformation, the kinematics of buoyancy-driven deformation are characterized by variations in the senses of shear within a single crustal slice, and this can therefore be used to distinguish the two types of deformation.

Timing of the Kaiyuan-Jiapigou shear zone in the northern margin of the North China Craton: Implications for closure of the Mongol-Okhotsk Ocean

The ductile Kaiyuan–Jiapigou Shear Zone (KJSZ) is located within the eastern part of the northern margin of the North China Craton (NCC). The foliation within the shear zone generally dips at 55° towards N20° and contains a lineation that plunges at 40° towards N30°. Field and thin section observations and quartz c-axis fabrics of the KJSZ indicate that this zone records top-to-the-SSW shearing associated with a NNE–SSW regional compression event. Muscovite, biotite, and hornblende 40Ar/39Ar step-heating ages constrain the timing of shearing and the tectonothermal history of the KJSZ. Quartz c-axis fabrics and mineral deformation features indicate the deformation within the KJSZ occurred at temperatures of 400°C–450°C, which are lower than the Ar–Ar closure temperature of hornblende but higher than the closure temperatures of muscovite and biotite. Samples from the KJSZ yield 40Ar/39Ar step-heating ages from 164.6 ± 0.5 to 165.2 ± 0.6 Ma for hornblende samples and from 161.4 ± 0.4 to 163.1 ± 0.4 Ma for mica samples. Combining these ages with the temperature of deformation within the KJSZ indicates that this shear zone most likely formed between 165.2 ± 0.6 and 161.4 ± 0.4 Ma. Linking these new data with the tectonics and geochronology of the eastern part of the northern margin of the NCC suggests that the deformation within this region was most likely controlled by a nearly N–S compressional event associated with the far field effect of the Mongol–Okhotsk Ocean closure during the Middle–Late Jurassic.

Correlation among the Ailaoshan–Song Ma–Song Chay orogenic belts and implications for the evolution of the eastern Paleo-Tethys Ocean

The timing and mechanism of the combination between the South China Block (SCB) and the Indochina Block (IB) are controversial. Three ophiolitic mélange zones (Ailaoshan, Song Ma, and Song Chay) have been proposed as suture zones within this collisional orogen. However, the relationships among the three corresponding tectonic belts are unclear. In this study, we present detailed structural data for the three tectonic belts. The bulk architectures of the Ailaoshan, Song Ma, and Song Chay belts correlate well with one another. This similarity is also revealed by our new zircon UPb geochronological results from the Song Ma and Song Chay ophiolites. The regional deformation age is constrained to between 250 and 240 Ma by our new muscovite 40Ar/39Ar ages, and the medium-low temperature conditions are revealed by the quartz c-axis fabric. Considering the transformation effect of the Cenozoic large-scale sinistral strike-slip of the RRF and DBF, the Early Mesozoic Ailaoshan, Song Ma, and Song Chay suture zones should represent different segments of the same belt. Based on this hypothesis, we compiled the ages of the magmatism in this region, which allows us to propose an evolutional model as follows: i) ∼380–310 Ma continental rifting and subsequent Ailaoshan–Song Ma–Song Chay ocean spreading as a branch of the Paleo-Tethys, ii) ∼310–250 Ma oceanic subduction coeval with continental-arc magmatism, iii) ∼250–240 Ma continental collision, iv) ∼240–220 Ma post-collisional extension.

Late early Paleozoic continental collision on the northern margin of the Central Qilian Block, NE Tibetan Plateau: Evidence from a two-stage tectono–metamorphic event

The central part of the Qilian orogenic belt (QOB) on the NE Tibetan Plateau is known as the Central Qilian Block (CQB), and research on deformation and metamorphism of CQB improves our understanding of the tectonic evolution of the QOB, which makes up an important part of the continental crust in eastern Asia. Here we present the results of thermobarometric calculation and geochronological analyses for deformed metamorphic rocks on the northern margin of the CQB, and these new datasets provide a record of the continental collision of the CQB. Macrostructures and microstructures indicate that the CQB underwent dextral ductile deformation. C-axis fabrics and dynamic recrystallisation of quartz indicate that deformation took place at temperatures of 400–500 °C. In contrast, phase equilibrium modelling suggests that the region underwent HT–MP metamorphism along a clockwise P–T path, with peak conditions of ∼ 740 °C and ∼ 7.2 kbar. Zircon U–Pb ages obtained from the zircon rims indicate the HT–MP metamorphism took place at 452–449 Ma. The results of petrographic analyses indicate changes in the rims of the garnet porphyroblasts due to late ductile deformation, which suggests that the ductile shearing took place after the HT–MP metamorphism. The records of different temperatures of metamorphism and deformation are consistent with multi-stage metamorphism–deformation. Together with the results of previous research on the metamorphism, magmatism, and deformation in the CQB, our new datasets confirm that the HT–MP metamorphism and ductile deformation of the CQB were developed during the collision of the CQB and the Alxa Block.

Crystallographic preferred orientation of quartz deformed at granulite conditions: the Kalinjala Shear Zone, Port Neill, South Australia

Crystallographic preferred orientations of quartz aggregates in mylonitic orthogneisses from the highest strained central region of the Kalinjala Shear Zone, Port Neill South Australia, are products of a combination of constrictional and flattening strains. These rocks were deformed under granulite facies peak metamorphic conditions, reaching pressures of ∼1.0 GPa and temperatures of ∼800 ± 50 °C. Neutron diffraction textural and fabric analysis methods reveal a dominant single c-axis maximum parallel to the Y-strain axis. This is accompanied by a symmetric orthorhombic pattern, where the c-axis fabrics and corresponding pole figure densities of {a}, {m}, {r} and {z} planes are arranged around the XY foliation plane. These fabrics from coexisting adjacent layers suggest that the activation of prism-<a> slip is characteristic for these high-temperature deformation conditions. A small asymmetry in the [c], {a} and {m} pole figures suggests a minor component of dextral shearing. The c-axis pattern is weakened where the foliation is dominated by secondary phases, such as concentrations of biotite mica. The opening angles obtained from cross-girdle c-axis patterns produce temperature estimates that are ∼100 °C lower than the peak metamorphic temperature of ∼800 ± 50 °C, but closer to temperatures related to a component of flattening strain. These c-axis patterns are consistent with other structural characteristics and can be related to the late-stage flattening strains during the Kimban Orogeny. KEY POINTS Mylonitic orthogneisses are deformed under granulite facies conditions. Quartz fabrics were measured by bulk neutron diffraction and optical methods. Mylonite c-axis fabrics can be related to the high-temperature deformation and activation of the prism-< a > slip system. Crystal preferred orientations indicate a large component of flattening strain superimposed on the dextral shearing observed in the Kalinjala Shear Zone.

Structures and chronology of the Yabrai shear zone in the Alxa, NW China: Constraints on the late Paleozoic shear system in central segment of the Central Asian Orogenic Belt

Large-scale strike-slip faults often develop in the collisional and accretionary orogenic belts around the world. The Central Asian Orogenic Belt (CAOB) contains numerous strike-slip faults and provides opportunities to decipher the relative movements among different components. Here, we report new data from the ENE-trending Yabrai dextral ductile shear zone in the Alxa region (NW China) of the southernmost CAOB. We performed field mapping, structural analysis, and muscovite 40Ar/39Ar dating to constrain the geometries, kinematics and timing of the shear zone. The Yabrai shear zone crops out for more than 150 km, and the kinematic indicators from the outcrops, microstructures, and quartz c-axis fabrics suggest a consistent dextral shear motion. The microstructures and EBSD results indicate intermediate-to low-temperature deformation (∼300–530 °C). Muscovite 40Ar/39Ar dating from mylonitic samples yield ages of 254–251 Ma, which imply that the shear zone was active in the Late Permian. The displacement of Yabrai shear zone is estimated to be ca. 30 km. An integration of spatiotemporal patterns of shear zones and regional aeromagnetic data suggests that a giant “S–C”-type shear system occurred in the Alxa region in the Late Permian. Based on this work and previous studies, a crustal-scale strike-slip system of late Paleozoic is developed between the CAOB and the southern cratons. It formed in a transcurrent regime after the closure of the Paleo-Asian Ocean, which was a response to the deformation of the whole CAOB in the end of late Paleozoic.

Shear zones in the southern Eyre Peninsula, South Australia: Quartz c-axis fabrics in granulite facies mylonitic orthogneisses and relationship to mafic dykes

The south eastern portion of the Eyre Peninsula, South Australia, is dominated by a suite of granites and orthogneisses deformed in steep-dipping shear zones during the Kimban Orogeny (1730–1690 Ma). The largest is the transpressional Kalinjala Shear Zone with shortening strains of ≥90% and shear strains of γ = ≥20, with other near-parallel secondary shear zones displaying similar kinematics. These ductile shear zones are dominated by mylonitic orthogneisses, with shearing commonly nucleating on, or adjacent to, precursor mafic dykes. The mafic dykes are highly sheared, folded and boudinaged with a newly formed matrix assemblage (orthopyroxene + clinopyroxene + garnet + magnetite + quartz + K-feldspar) that indicates the peak P/T metamorphic conditions of 0.7–1.0 GPa and 750–850 °C. Partial melting, developed as leucosomes, is associated with this metamorphism. Microstructural relationships between quartz crystallographic preferred orientation (CPO) and shape preferred orientations (SPO) analyses of these polymineralic rocks are best interpreted in terms of a granulite facies isothermal temperature regime of ~750 °C. The quartz c-axis fabrics, obtained using an optical fabric analyser, reveal a range of CPOs from weak girdles normal to the shear plane, to maxima parallel to the Y-strain axis. There is an overall symmetric orthorhombic pattern, with c-axis fabrics oriented with respect to the XY plane of flattening. There is a minor component of non-coaxial deformation in some shear zones. These fabrics indicate the increased activation of predominantly the prism-<a > slip system in quartz within the shear zones, which is characteristic for these high-temperature and high-strain deformation conditions.

Origin of englacial stratigraphy at three deep ice core sites of the Greenland Ice Sheet by synthetic radar modelling

AbstractDuring the past 20 years, multi-channel radar emerged as a key tool for deciphering an ice sheet's internal architecture. To assign ages to radar reflections and connect them over large areas in the ice sheet, the layer genesis has to be understood on a microphysical scale. Synthetic radar trace modelling based on the dielectric profile of ice cores allows for the assignation of observed physical properties’ variations on the decimetre scale to radar reflectors extending from the coring site to a regional or even whole-ice-sheet scale. In this paper we rely on the available dielectric profiling data of the northern Greenland deep ice cores: NGRIP, NEEM and EGRIP. The three records are well suited for assigning an age model to the stratigraphic radar-mapped layers, and linking up the reflector properties to observations in the cores. Our modelling results show that the internal reflections are mainly due to conductivity changes. Furthermore, we deduce fabric characteristics at the EGRIP drill site from two-way-travel-time differences of along and across-flow polarized radarwave reflections of selected horizons (below 980 m). These indicate in deeper parts of the ice column an across-flow concentratedc-axis fabric.

Tectonic transformation from orogen-perpendicular to orogen-parallel extension in the North Himalayan Gneiss Domes: Evidence from a structural, kinematic, and geochronological investigation of the Ramba gneiss dome

The North Himalayan Gneiss Domes consist of a series of isolated domes cored by leucogranite and/or gneiss body. The Ramba dome core comprises gneiss and two-mica granite, mantled mainly by schist sequences, and the degree of deformation decreases outward from pervasive mylonitization to weak mylonitization. Pre-doming deformation is characterized by the formation of regional-scale top-to-the-north shearing (D1) in the Tethyan Himalayan Sequence, locally preserved strong folding (F1), and intrusion of 46.3 Ma-40.6 Ma leucogranite. The D2 deformation consists of map-scale ductile shearing fabrics and top-to-the-north detachment along the South Tibetan Detachment System (STDS) in the middle and lower units of the dome. Intrusion of 26.1–21.0 Ma syn-kinematic leucogranite veins in the schist units occurred during the D2 north-directed extensional episode. The D3 deformation was characterized by top-to-the-east shear during 12.0–10.2 Ma, concurrent with the initiation of activity in the N–S-trending Yadong–Gulu rift. The latest deformation (D4) involved the ascent of two-mica leucogranite through diapirism at about 8.17 Ma. Microstructural analyses, as well as quartz c-axis fabrics measured by EBSD (Electron backscatter diffraction), indicate deformation temperature ranges of D2 extensional shearing from 500 to 600 °C in the middle unit of the Ramba dome to >600 °C in the lower unit. These new data imply that this type of doming in the Ramba region represents a classic type of transformation from orogen-perpendicular to orogen-parallel extensional deformation during Miocene India–Asia collision.

Quartz C-Axis Fabric Characterization of the Strike-Slip Ductile Deformation within the Three Pagodas Shear Zone, Western Thailand

Quartz C-Axis Fabric Characterization of the Strike-Slip Ductile Deformation within the Three Pagodas Shear Zone, Western Thailand

Kinematic vorticity, finite strain, and deformation thermometry analyses of the exhumed mylonites in the Samen ductile shear zone (Sanandaj-Sirjan Metamorphic Belt, Iran)

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.

Structural and rheological features of the northeastern Sanyang shear zone in the eastern Jiangnan orogen: Indications for early Paleozoic orogeny in the South China Block

The N55°E−trending Sanyang shear zone is located in the eastern Jiangnan orogen, and the northeastern junction between the Cathaysia and Yangtze blocks is the key to understanding the evolution of the South China Block (SCB) for early Paleozoic crustal reworking. Most shear-sense indicators (e.g., asymmetric folds, porphyroclasts, S–C fabrics, and mica fish) indicate that the Sanyang shear zone is characterized mainly by oblique sinistral shear and locally by dextral shear. Mineral deformation behavior, quartz C-axis fabrics, and the Kruhl thermometer demonstrate that shear deformation developed under high greenschist-to low amphibolite facies conditions with temperatures ranging from 450 to 550 °C. Differential stresses deduced from recrystallized quartz grain sizes were approximately 22.4–46.5 MPa, and strain rates were on the order of 10−9 to 10−10 S−1. Three dimensional (3D) strain ellipsoid measurements and the Flinn diagram of the Sanyang shear zone indicate that all samples yield oblate and flattened strain ellipsoids, which imply a contractional regime. The kinematic vorticity (Wk) 0.716–0.999, estimated by using 3D strain ellipsoid-based methods, indicates a high simple shear contribution in the shear zone. All structural, strain, and vorticity analyses suggest that the Sanyang shear zone experienced general shear with transpression. The numerical estimates allow kinematic modeling of the Sanyang shear zone, which provides important information about the structural framework for the SCB. The development of the Sanyang shear zone was a response to the oblique convergence of the Yangtze and Cathaysia blocks during the early Paleozoic orogeny.

The East Variscan Shear Zone: a structural and geochronological review for improving paleogeographic reconstruction of the southern Variscides

The Alpine orogenesis partially overprinted the Southern European Variscan Belt complicating paleogeographic reconstructions. A main debate concerns the extension and the tectonic significance of the regional-scale East Variscan Shear Zone (EVSZ). This work reviews the data obtained thanks to a systematic and multidisciplinary study of the Ferriere-Mollières shear zone in the Argentera Massif, the Emosson-Bèrard shear zone in the Aiguilles Rouge Massif and the Cavalaire Fault in the Maures Massif. A pure shear-dominated transpression with a a variable component of simple shear is recognized in the studied shear zones. Syn-kinematic paragenesis, microstructures and quartz c-axis fabrics revealed syn- shear decreasing temperature starting from amphibolite-facies. Petrochronology constrained the timing of deformation. Ages range between ~340-330 Ma and ~320 Ma. Older ages are interpreted as the beginning of the activity of the EVSZ along its older branches (Ferriere-Mollières shear zone) while ages of ~320 Ma, obtained in all the shear zones, demonstrate that they were active at the same time. The data revealed a similar tectono-metamorphic evolution of the shear zones and highlights that transpressional deformation affected the European Variscan Belt after the metamorphic peak linked to the initial collisional stage. The review constrains the extension, the kinematics and the timing the EVSZ.

Formation of the Cenozoic Ailao Shan mid-crustal tectonic discontinuity: Role of Oligo-Miocene stratified sub-horizontal middle to lower crustal flow in the southeastern Tibetan Plateau

Structural, kinematic and geothermometry analyses are applied to unravel the structural evolution of the Ailao Shan Tectonic belt (ALTB) with a specific goal of understanding how crustal masses in southeastern Tibetan Plateau flowed during the Indian-Eurasian continental collision. A mid-crustal tectonic discontinuity that is characterized by a narrow ultramylonite belt separates the highly sheared high-grade metamorphic unit (HMU) from the low-grade metamorphic unit (LMU) along the ALTB. It is shown that rocks from the HMU and LMU possess deformation associations of identical structural geometries and kinematics, but at contrasting metamorphic conditions. Calculated mean kinematic vorticity numbers indicate that the ultramylonite belt was affected by dominant simple shearing, while the HMU and LMU on both sides of the belt were deformed by general shearing dominated by pure shearing. Geothermometry analyses on sheared two-mica schists from the HMU and quartz veins from the LMU reveal different deformation conditions, i.e., 610–777 °C, 0.4–0.6 GPa and 320–391 °C, respectively. There is also a sharp decrease in deformation temperature inferred from deformation microstructures and quartz c-axis fabrics, from 550–650 °C for the HMU to 350–400 °C for the LMU. These results suggest that there is an apparent temperature break of ca. 220 °C over a total structural distance of approximately 1 km between these two units at the present outcrop level. Our results show that the HMU and LMU are kinematically linked while rheologically decoupled, and were originally deformed by sub-horizontal shearing at different crustal levels in the same strain field. The ultramylonite belt between them potentially inherited a preexisting basement/cover contact along the ALTB and brought the two units into contact. We suggest that progressive stratified flow of middle to lower crustal masses during the extrusion of the Sundaland block at Oligocene to Early Miocene was responsible for the concurring high- and low-temperature fabrics at different crustal levels. Contemporaneous doming during the progressive crustal flow resulted in exhumation of the lower crustal rocks. During the process, the ultramylonite belt occurred as a tectonic discontinuity (TDC) that leads to the loss of a thick pile (ca. ∼6.3–7.8 km) of crustal masses in the middle crust.