Stages Of Ductile Deformation Research Articles

Deformation and kerogen macromolecular structure transformations of the Wufeng–Longmaxi shale under the control of the detachment layer

Tectonism caused strong deformation of the organic-rich shale in the detachment layer of the Wufeng–Longmaxi Formation. In this study, the deformation of the samples collected from the detachment layers were observed using an optical microscope and field emission scanning electron microscopy. Fourier transform infrared spectroscopy (FTIR), 13C nuclear magnetic resonance, X-ray diffraction, and high-resolution transmission electron microscopy were employed to characterize the macromolecular structure of extracted kerogen samples. The research shows that tectonically deformed shale (TDS) is commonly developed in the detachment layer of the Wufeng–Longmaxi Formation, dominated by brittle–ductile and ductile deformation. In the brittle–ductile deformation stage, the aliphatic carbon (fal) decreases, manifested as the decrease of –CH or –CH2 (falH), from 6.77% to 3.12%. The Al/–OX (defined by FTIR A3000–2800/A1800–1520) gradually decreases, indicating that the brittle–ductile deformation has a stronger effect on the aliphatic structure. The aromaticity I and aromatic carbon (fa) show an increasing trend due to aromatization. In the ductile deformation stage, the fal and aliphatic carbon bonded to oxygen increase first and then decrease, falH decreases gradually, and the aliphatic side chain becomes shorter. The Al/–OX shows an increasing trend, and carbonyl decreases from 7.82% to 5.12%, indicating that the effect of ductile deformation on oxygen-containing functional groups is stronger. The fa shows an overall increasing trend, and the aromatic ring carbon and protonated aromatic carbon increase more significantly due to aromatic condensation. The change trend of the interlayer spacing is opposite in these two stages. However, both brittle–ductile and ductile deformation can improve the size and orientation of aromatic fringes to varying degrees, and the latter is more significant. The rigid mineral intrusion and organic matter (OM) chemical structure evolution are the two main reasons for the low development of OM pores in TDS.

Batholith-hosted polyphase gold mineralization at the Brothers project, eastern Suriname

The gold mineralization at the Brothers Project, located in eastern Suriname, is hosted by a large tonalitic batholith, emplaced during the Transamazonian orogenic cycle (2260–1950 Ma) and spatially associated with the Marowijne greenstone belt of the Guiana Shield. A multiscale, field-based approach from district to drill core scale was used to study the deformation history, the geometry and spatial distribution of intrusion-hosted, ENE-striking shear zones and associated gold-bearing quartz veins. At the drill core scale, this orogenic gold deposit recorded a polyphase deformation history dominated by a progressive ductile deformation stage D1BR, associated with both shearing and folding that is locally overprinted by a late brittle deformation stage D2BR. Within this structural framework a complex vein system is preserved with syn-shearing tension-vein arrays, massive-laminated fault-fill veins, and post-shearing extensional veins. Two main gold events are related to syn- and post-shearing pyritization (Py1a and Py2). At the camp scale, multiple ENE-striking steeply dipping strike-slip to reverse oblique-slip shear zones occur within the rigid batholith. These gold-bearing structures contain oblique to subvertical ore shoots. At the district scale, identified gold trend distribution and LiDAR-aeromagnetic-based structural patterns highlight that auriferous target zones are located at the intersection between ENE-striking shear zones and NNW-striking brittle structures, that correspond to zones of enhanced permeability and thus intense hydrothermal activity recorded by different vein systems. Analogies with other Paleoproterozoic intrusion-hosted gold deposits of the Guiana Shield and the West African craton show that the central parts of large intrusive complexes can host significant gold mineralization related to orogenic events.

Linked microstructural and geochemical evolution of mylonitic quartzite during exhumation of a core complex

A novel method for combining microstructural analysis and Ti-in-quartz measurements is used to characterize interactions between quartz deformation and Ti equilibration during progressive deformation and cooling in a mylonitic shear zone. Mylonitic quartzite from the Wildhorse detachment, the major bounding structure of the Pioneer metamorphic core complex (Idaho, USA), contains a continuum of microstructures: minimally deformed relict grains, quartz ribbons, and a matrix dominated by subgrain rotation recrystallization. EBSD and semi-quantitative Ti mapping via EPMA and cathodoluminescence imaging are used to correlate quartz microstructure and Ti concentration [Ti]. The Ti-in-quartz thermobarometer is used to estimate P-T conditions during and prior to mylonitization. Relict and ribbon quartz preserve high [Ti] (∼60 ppm) indicative of temperatures >650 °C, whereas recrystallized quartz contains low [Ti] (<5 ppm), reflecting equilibration to temperatures <450 °C. The high-T record may reflect either relict conditions prior to detachment deformation or conditions that prevailed in the early stages of strain localization in the detachment, whereas the low-T record likely reflects the final stages of ductile deformation. The preservation of multiple microstructures with distinct [Ti] facilitates construction of a multi-phase history of linked ductile deformation and Ti equilibration during cooling.

Structural Control of Gold Deposits in the Northern Margin of the North China Platform

The North China Platform is one of the oldest continental crusts in China. The northern margin of the North China platform is one of the top gold mining concentration areas in China. Studying its ore-controlling factors is of great practical significance for finding similar gold deposits in areas with similar geological backgrounds. The authors of this paper have concluded that the gold deposits in the northern margin of the North China platform are mainly controlled by various faults. In the long geological history, these fault structures have undergone many stages of activation and transformation almost without exception. Generally, it has experienced the following three stages of development and evolution: 1. Beginning in the early stage of ductile deformation at the end of the Archean period, it is characterized by mylonite. The rock is transformed from the original high amphibolite facies-granulite facies into greenschist facies through ductile shearing. The typical mineral combination is sericite + chlorite + quartz ± feldspar. 2. The brittle-ductile deformation stage occurred in the Hercynian period, that is, the middle activation stage. Due to the different lithology of activation in different sections, the uneven strain of the mining area is caused. Some of the rocks are of continuous inhomogeneous strain, while others are of discontinuous inhomogeneous strain. As a result, hardening in plastic deformation occurs in some areas, and two different types of ore would appear in some mine. 3. The late transformation stage occurred in the early Yanshanian orogeny. It is mainly manifested in the strong fragmentation and transformation of the existing ore bodies, resulting in the dislocation of the ore bodies and complicating their occurrence. This late transformation stage has created favorable conditions for the later oxidative secondary enrichment of ore.

Recognition of broad thermal anomaly around the median tectonic line in central Kii peninsula, southwest Japan: Possible heat sources

AbstractApplication of Raman spectroscopy of carbonaceous material thermometry to samples of metasedimentary rock from the low‐grade Sanbagawa belt in the central Kii peninsula reveals a progressive decrease in temperature from ~390 °C close to the northern boundary, a major continental shear zone—the median tectonic line (MTL)—to ~270 °C and ~7 km to the south and roughly constant temperature distribution thereafter. Within the Sanbagawa belt, the thermal structure is not significantly modified by slip‐on fault boundaries between different geological units or folding. Meso‐ and microstructural observations combined with strain analysis using detrital grains in meta‐mudstone indicate a similar deformation history throughout the area and no correlation between ductile strain and temperature gradients. These observations suggest the observed thermal structure was developed after the main stages of ductile deformation of the Sanbagawa belt were complete and are not due to localized preferential exhumation along with the MTL. The observations also require a heat source along with the MTL. Order of magnitude estimates suggest the influx of warm fluid along the MTL are viable causes of the observed thermal anomaly. Although shear heating would be another possible explanation, thermal calculations require anomalous fast slip rates along the MTL and much greater frictional strength than generally considered reasonable. For these reasons, fluid infiltration is our preferred model.

Syntectonic metamorphism of a collisional zone in the Tuva-Mongolian massif, Central Asian Orogenic belt: P-T conditions, U-Pb ages and tectonic setting

Accretionary-collisional events occurred at the western margin of the Tuva-Mongolian microcontinent are used to explain the largely contradictory Cambrian geodynamic history of the western Central Asian Orogenic Belt (CAOB). This study presents new data for the Erzin metamorphic complex in order to constrain the petrogenesis and tectonic implications of the metamorphic rocks in this complex. The Erzin complex in the tectonic Erzin zone is composed of high-grade metamorphic rocks that have formed under variable metamorphic conditions (T = 730–835 °C, P = 5.3–7.5 kbar). In the Erzin complex, metamorphic rocks have undergone multiple stages of ductile deformation with subvertical mineral lineation and were then superimposed by sub-horizontal ductile deformation. The stages of tectonic deformation are close in time and mark collisional stages characterized by a sequential change from the compression regime to the extension regime in the period of 495 ± 5 Ma. The determination of the time interval is based on structural and petrological data, previously published materials, as well as the dating of granite dikes (U-Pb, zircon) sealing the Erzin complex. P-T conditions and structural-petrological studies analysed in this paper suggest that the formation of the Erzin metamorphic complex occurred as a result of a collisional event between the Tannuola island arc and Tuva-Mongolian microcontinent. We suggest that the Erzin metamorphic complex is important for study of transitional regime between the collisional event and initial orogenic collapse in the ancient fold belts. Metamorphic record of the Tuva-Mongolian microcontinent provides important information about the processes that occurred at the Early Paleozoic geodynamic evolution in the western CAOB.

The stress and strain state of Yalahe fault in the Kangding segment of the Xianshuihe fault zone and its seismogenic environment

鲜水河断裂带位于青藏高原东缘，是中国大陆内部地震活动性最强的大型左行走滑断裂之一，揭示其深部应力状态与应变是认识孕震环境和评估断裂带地震危险性的重要依据。本文选择鲜水河断裂带未来强震危险性较高的康定段作为研究对象，聚焦雅拉河断裂带内的糜棱岩，通过光学显微镜和扫描电镜（SEM）的显微构造分析、粒度统计、石英组构和Ti含量测试，确定石英递进变形过程中差应力的变化和不同阶段的变形温度，构建鲜水河断裂带康定段的地壳强度剖面。观测结果显示，糜棱岩在抬升过程中石英发生颗粒边界迁移（GBM）、亚颗粒旋转（SGR）和膨凸重结晶（BLG）三种类型的变形机制。构建的地壳强度剖面显示，雅拉河断裂带应变速率约为6.08×10^-13~1.62×10^-11s^-1，脆韧性转化带发生在~12.5±2.5km深度，该处岩石极限强度~145.5MPa。实际岩石强度（μ=0.383）小于理论破裂摩尔圆（μ=0.85）的岩石强度发育应变弱化。当韧性变形阶段由位错蠕变形成的应变局部化以及细粒化等机制降低岩石的强度至临界条件时，会导致破裂频次的增强，应力释放发生地震。同时，位错蠕变引起的应变弱化导致岩石强度降低，使高应力无法积累。;The Xianshuihe fault zone, located at the eastern margin of the Tibetan Plateau, is a sinistral strike-slip fault zone with current ~10mm/yr slip rate. Calculation of crustal stress state and strain rates of fault zone is important to understand seismogenic environment and assess the seismic risk along the active segments of fault zone. In this contribution we select the Kangding section of the Xianshuihe fault zone, which has a higher risk of strong earthquakes in the future, as the research object, focusing on the mylonite in the Yalahe fault. According to optical microscope and scanning electron microscope (SEM) microstructure analysis, grain size statistics, quartz fabric and Ti content test to determine the change of differential stress and temperature at different stages during the progressive deformation of quartz, and construct the crustal strength profile of the Kangding section of the Xianshuihe fault zone. Three types of quartz deformation mechanisms can be identified, including grain boundary migration (GBM), sub-grain rotation (SGR), and bulging recrystallization (BLG) during the gradual mylonization. The established crustal strength profile shows that the strain rate of the Yalahe fault is about 6.08×10^-13~1.62×10^-11s^-1, and the ultimate strength of the rock in the brittle-ductile transition zone is ~145.5MPa, which is corresponding ~12.5±2.5km depth. The actual rock strength (μ=0.383) is smaller than the theoretical rupture Mohr circle (μ=0.85), which indicated the strength of the rock is weakened within the fault zone. The strain localization and grain size reduction at the ductile deformation stage can reduce the strength of the rock, which may lead to an increase in the frequency of rupture, stress release and earthquakes. However, the weakening of the mylonite caused by the dislocation creep leads to a decrease in the strength of the rock, which makes it impossible to accumulate high stress earthquake.

Structural and Fractal Characterizations of Nanopores in Middle-Rank Tectonically Deformed Coals - Case Study in Panguan Syncline.

The reservoir properties of tectonically deformed coals (TDCs) differ significantly compared with their neighboring primary coals which are also known as unaltered or underformed coals. However, the heterogeneity of nanopores in TDCs under the syncline control has been seldom reported, and also the middle-rank level was minimally investigated to date. Thus, in this paper, the structures and multiscale fractal characteristics of nanopores in middle-rank TDCs under the controlling effect from Panguan Syncline were investigated via high-pressure mercury injection (HPMI), low-pressure CO2/N2 adsorption (LPCO2/N2GA), and fractal theory. The results show that both the pore volume (PV) and specific surface area (SSA) of macropores increase significantly in the stage of cataclastic–schistose coal. For ductile deformed coals, the PV increases, while the SSA remains stable. The SSA of mesopores increases slightly in the brittle deformation stage, but significantly in the ductile deformation stage. For micropores, both the PV and SSA for TDCs are significantly higher than primary coals. Moreover, the ductile deformation has a more significant promotion effect for the microporous PV and SSA than the brittle deformation. The fractal dimension of the adsorption pore (induced from the Sierpinski model) increases; however, that of seepage pores (Sierpinski model) decreases with the enhancement of tectonic deformation. The fractal dimension for mesoporous (induced from the FHH model, Frenkel–Halsey–Hill) at 2–6 nm keeps stable in the stage of cataclastic–schistose coal but significantly increases in the ductile deformation stage. For mesopores of 6–100 nm, their heterogeneities were also enhanced in the ductile deformation stage. The fractal dimension of 0.3–0.6 nm micropores is close to 3 and changes slightly with the enhancement of tectonic deformation, indicating that the heterogeneity of smaller micropores is stronger than that of larger micropores. The results are of broad interest for CBM exploration and gas outburst prediction.

Polyphase deformation along the South Bohemian Batholith-Moldanubian nappes boundary – The Freyenstein Fault System (Bohemian Massif/Austria)

Abstract This work describes the Freyenstein Fault System, which extends over 45 km in the southeastern part of the Bohemian Massif (Lower Austria). It represents a ductile shear zone overprinted by a brittle fault located at the eastern edge of the South Bohemian Batholith towards the Moldanubian nappes. It affects Weinsberg- and a more “fine-grained” granite, interlayered aplitic granite and pegmatite dikes as well as paragneiss of the Ostrong Nappe System. The ductile shear zone is represented by approximately 500 m thick greenschist-facies mylonite dipping about 60° to the southeast. Shear-sense criteria like clast geometries, SCC`-type shear band fabrics as well as abundant microstructures show top to the south/ southsouthwest normal shearing with a dextral strike-slip component. Mineral assemblages in mylonitized granitoid consist of pre- to syntectonic muscovite- and biotite-porphyroclasts as well as dynamically recrystallized potassium feldspar, plagioclase and quartz. Dynamic recrystallization of potassium feldspar and the stability of biotite indicate upper green-schist-facies metamorphic conditions during the early phase of deformation. Fluid infiltration at lower greenschist-facies conditions led to local sericitization of feldspar and synmylonitic chloritisation of biotite during a later stage of ductile deformation. Finally, a brittle overprint by a north-south trending, subvertical, sinistral strike-slip fault that shows a normal component is observed. Ductile normal shearing along the Freyenstein Shear Zone is interpreted to have occurred between 320 Ma and c. 300 Ma. This time interval is indicated by literature data on the emplacement of the hostrock and cooling below c. 300°C inferred from two Rb-Sr biotite ages measured on undeformed granites close to the shear zone yielding 309.6 ± 3 Ma and 290.9 ± 2.9 Ma, respectively. Brittle sinistral strike-slip faulting at less than 300°C presumably took place not earlier than 300 Ma. Early ductile shearing along the Freyenstein Fault System may be genetically, but not kinematically linked to the Strudengau Shear Zone, as both acted in an extensional regime during late Variscan orogenic collapse. A relation to other major northeast-southwest trending faults of this part of the Bohemian Massif (e.g. the Vitis-Pribyslav Fault System) is indicated for the phase of brittle sinistral movement.

Heterogeneous sub-simple deformation in the Gol-e-Gohar shear zone (Zagros, SW Iran): insights from microstructural and crystal fabric analyses

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.

Using incremental elongation and shearing to unravel the kinematics of a complex transpressional zone

This study presents in-depth geometric and kinematic analyses of a complex transpressional shear zone (Fellos Shear Zone, FSZ) that integrates structural mapping with microstructural and quartz crystallographic texture data. The FSZ strikes NE-SW and formed in the short limb of a map-scale antiform. The foliation pattern within the zone indicates dextral shearing whereas the macroscopic object lineation is dispersed over a half great-circle girdle along the mean mylonitic foliation. Based on this deformation pattern, the FSZ could be interpreted as a dextral, NE-directed triclinic transpressional zone. However, the integration of field-based with microtectonic data reveal a more complicate kinematic history. We show that the elongation trend is dispersed along an entire great-circle girdle when we take into account the trends of incremental elongations, recorded by fabrics with different strain memories. Mapping of incremental shear directions implies that the FSZ initiated as a NE-directed dextral transpressional shear zone, and progressively evolved into a NW-directed dextral zone. The passage from NE-to NW-directed shearing was accompanied by transpression whilst local transtension likely occurred during the last stages of ductile deformation. Deformation in the FSZ ended up, at semi-ductile conditions, with localized NE-directed dextral shearing. Our study demonstrates that the integration of field observations and fabrics/microstructures that have different strain memories is a powerful tool for unravelling the complex kinematics of high-strain zones.

Brittle reactivation of ductile precursor structures: The role of incomplete structural transposition at a nuclear waste disposal site, Olkiluoto, Finland

Reactivation of discrete deformation zones that are orientated favourably with respect to the stress field is a well-known phenomenon. What is less clear is the role of other structural features and heterogeneities in localizing deformation. In this paper we describe how brittle deformation structures are localized into zones of incomplete structural transposition inherited from earlier ductile deformation phases. In our example, these zones of incomplete structural transposition are characterised by localised high-strain structures of the latest ductile deformation stage, including short limbs of strongly asymmetric folds and anastomosing networks of minor shear fabrics. When such zones are systematically organized, and orientated favourable with respect to the stress field, they can be very efficient in localizing deformation and forming new fault zones. Applied to the site of the planned geological repository of nuclear waste in Olkiluoto, Finland, the recognized structural inheritance provides tools to understand the geometries, networks and kinematics of the brittle fault zones and the related secondary fracturing which together define the rock mechanical and hydrogeological framework for the repository.

Age constraints on the deformation style of the South Tibetan Detachment System in Garhwal Himalaya

In the Garhwal Himalaya, the Malari granite, a small pluton, intrudes the upper portion of the Greater Himalayan Sequence (GHS) and is deformed in the South Tibetan Detachment System (STDS) shearing. In the present work, we undertook a multidisciplinary approach involving microstructural, chemical and geochronological analyses of the samples in order to constrain the timing of shearing along the STDS. Microstructural observations of samples suggest a switch in deformation features with structural position from the structurally upper samples to the structurally lower one. X-ray maps and electron microprobe analyses reveal a quite uniform chemical composition of muscovite in the three selected samples. 40Ar/39Ar muscovite ages constrain the waning stages of ductile deformation: in rocks ductilely deformed by the STDS activity, they record recrystallization at 16.0-16.5 Ma; the age of static muscovite grown after sillimanite at 14.3 Ma post-dates cessation of movement on the STDS and pre-dates the main activity of the Main Central Thrust in the Western Himalaya.

Macromolecular response to tectonic deformation in low-rank tectonically deformed coals (TDCs)

The nano-porous and optical properties of tectonically deformed coals (TDCs) are well characterized while their macromolecular structures have been minimally reported in recent years. In this study, the structural transformations of low-rank TDC macromolecular structures were identified utilizing 13C nuclear magnetic resonance (13C NMR) and Fourier transform infrared spectroscopy (FTIR). Both the NMR aromaticity (fa’) and the ratio of aromatic bridge carbon to aromatic peripheral carbon (xBP) increase with the increasing deformation intensity, indicating the aromatization process induced by the stress condensation. The carbonyl first increases then decreases with the increasing deformation intensity, leading the maximum value in granulitic coal. The decrease of A (CH2)/A (CH3) (defined by FTIR A2940-2900/A3000-2940) indicates the increasing branching degree and shorting aliphatic chains especially for the ductile- and strong brittle deformation processes. The Al/-OX (defined by FTIR A3000-2800/A1800-1520) firstly increases until the granulitic coal and then decreases, indicating that the loss rate of the oxygen functional groups is higher than that of the aliphatic carbons during the brittle deformation process but lower than the latter during the shear- and ductile deformation stages. The ductile- and strong brittle deformations (especially for the deformation process of mortar- to granulitic coal) have a more significant promote effect for aliphatic loss than the weak brittle deformations (i.e. deformation process of primary-, cataclastic-, and mortar coal). The loss of oxygen functional groups is significant during shear- and brittle deformation stages but rather slight during the ductile deformation stage. The strong shear TDCs (i.e. mortar- and granulitic coal) have a relative abnormal high content of ether, hydroxybenzenze, and CH2CO, indicating that the shear deformation process has a significant promote effect for the diversification in the aliphatic carbons and oxygen functional groups. Then, the relative content of ether, hydroxybenzenze, and unsaturated carboxylic acids overall decreases with the increasing deformation intensity and the reduction rate all manifests as high at the shear deformation stage and low at the ductile deformation stage.

Characterizing a middle to upper crustal shear zone: Microstructures, quartz c-axis fabrics, deformation temperatures and flow vorticity analysis of the northern Ailao Shan-Red River shear zone, China

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. 27Ma 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.

Evidence of Variscan and Alpine tectonics in the structural and thermochronological record of the central Serbo-Macedonian Massif (south-eastern Serbia)

The Serbo-Macedonian Massif (SMM) represents a composite crystalline belt within the Eastern European Alpine orogen, outcropping from the Pannonian basin in the north to the Aegean Sea in the south. The central parts of this massif (south-eastern Serbia) consist of the medium- to high-grade Lower Complex and the low-grade Vlasina Unit. Outcrop- and micro-scale ductile structures in this area document three major stages of ductile deformation. The earliest stage D1 is related to isoclinal folding, commonly preserved as up to decimetre-scale quartz–feldspar rootless fold hinges. D2 is associated with general south-eastward tectonic transport and refolding of earlier structures into recumbent metre- to kilometre-scale tight to isoclinal folds. Stages D1 and D2 could not be temporally separated and probably took place in close sequence. The age of these two ductile deformation stages was constrained to the Variscan orogeny based on indirect geological evidence (i.e. ca. 408-ca. 328). During this period, the SMM was involved in a transpressional amalgamation of the western and eastern parts of the Galatian super-terrane and subsequent collision with Laurussia. Outcrop-scale evidence of the final stage D3 is limited to spaced and crenulation cleavage, which are probably related to formation of large-scale open upright folds as reported previously. 40Ar/39Ar thermochronology was applied on hornblende, muscovite, and biotite samples in order to constrain the age of tectonothermal events and activity along major shear zones. These 40Ar/39Ar data reveal three major cooling episodes affecting the central SMM. Cooling below greenschist facies conditions in the western part of the Vlasina Unit took place in a post-orogenic setting (extensional or transtensional) in the early Permian (284 ± 1 Ma). The age of activity along the top-to-the-west shear zone formed within the orthogneiss in the Božica area of the Vlasina Unit was constrained to Middle Triassic (246 ± 1 Ma). This age coincides with widespread extension related to the opening of the Mesozoic Tethys. The greenschist facies retrogression in the Lower Complex probably occurred in the Early Jurassic (195 ± 1 Ma), and it was related to the thermal processes in the overriding plate above the subducting slab of the Mesozoic Tethys Ocean.

Magnetic fabric as a vorticity gauge in syntectonically deformed granitic rocks

A concept is presented to quantify vorticity using magnetic fabric data determined from anisotropy of magnetic susceptibility (AMS) analysis in syntectonic granites, whose emplacement is synchronous with tectonics of adjacent shear zones. The latter is considered to define the direction of extensional flow apophysis (Ae). It is suggested that the magnetic foliation traces the direction of maximum instantaneous stretching axis during the final stage of ductile deformation. Hence, the angle between mean orientation of magnetic foliation and Ae gives the kinematic vorticity number (Wn). This concept is tested in two granites from India — Godhra Granite (western India) and Chakradharpur Granitoid (eastern India). The analysis explains the kinematics of fabric development within the granites and also the evolution of structural elements in the surrounding rocks. It is also suggested that in cases where granite margins get mylonitized synchronously with tectonic activity along adjacent shear zones, the angle between mean magnetic foliation of the granite margin samples and the shear zone can help calculate Wn. The example of Malanjkhand Granite (central India) is discussed to highlight this. Using magnetic fabric, a value of Wn=0.98 is recorded for granite margin samples implying their deformation by dominantly simple shear. Oblique quartz foliation recorded on the microscale in the granite margin samples yields Wn=0.94, which is similar to the value obtained using AMS. It is thus concluded that magnetic fabric provides a possibility to quantify vorticity in syntectonic granites in 2-dimension.

Petrostructural evolution of ultramafic rocks of the Kalninsky chromite-bearing massif, Western Sayan

The Kalininsky ultramafic massif is a fragment of lower structural zone of the Kurtushiba ophiolitic belt in the extreme northeastern part of the Western Sayan. The massif is composed largely of rocks making up the dunite-garzburgite banded complex. The northeastern part of the massif is composed mainly of dunite with linear NW-trending chromite-bearing zones, the localization of which is controlled by banding of the dunite-harzburgite complex. Harzburgite and dunite are characterized by inhomogeneous structures and textures caused by nonuniform ductile deformation, which is expressed as heterogeneous extinction, kink bands, and syntectonic and annealing recrystallization. The petrostructural patterns of olivine in harzburgite and dunite provide evidence for three stages of ductile deformation. At the first stage under deep mantle-crustal conditions, the ductile flow of ultramafic rocks developed mainly in a regime of axial compression, high temperature (>1000°C), and low strain rate (ɛ 1000°C). The translational gliding in olivine proceeded largely along (010)[100] and was accompanied by diffusion creep. As the temperature rose, ductile deformation gave way to secondary recrystallization of annealing, which facilitated the growth of olivine grains free of dislocations owing to absorption of individual grains oriented adversely relative to the compression axis and deformed grains saturated with dislocations. As a result, dunite and harzburgite with a coarse-grained porphyroblastic microstructure have been formed. The third stage of ductile flow was apparently related to their transport along deep-seated thrust faults under settings of intense shear deformations at a high temperature (∼1000°C) and strain rate (ɛ >10−4 s−1). The ductile flow in olivine resulted in heterogeneous translational gliding along (010)[100] and accompanied by intense syntectonic recrystallization with the formation of a porphyroblastic microstructure. Chromite mineralization in dunite is controlled by internal banding. Intense ductile flow facilitated the metamorphic separation of linearbanded Cr-spinel segregations. Thus, the results of a petrostructural study show that ultramafic rocks of the Kalninsky massif, ascending to the upper lithosphere, underwent both axial and shear ductile deformations in the mantle and lower crust, and these deformations controlled chromite mineralization.

Linking temperature estimates and microstructures in deformed polymineralic mantle rocks

To constrain deformation temperatures of mantle shear zones, we studied a strike-slip shear zone (Hilti massif, Semail ophiolite, Oman) and focused on the interaction between microstructural mechanisms and chemical equilibration processes. Quantitative microfabric analysis on harzburgites with different deformation intensity (porphyroclastic tectonite, mylonite, and ultramylonite) was combined with orthopyroxene geothermometry. The average grain size of all phases decreases with decreasing shear zone thickness. Dynamic recrystallization of porphyroclasts in combination with dissolution-precipitation and nucleation result in small-sized, chemically equilibrated pyroxenes. The composition of orthopyroxene was used to calculate deformation temperatures. In the case of the porphyroclastic tectonites, the chemical composition of orthopyroxene has been reset by diffusion yielding temperature estimates of 880–900°C. The mylonites were deformed by dislocation creep of olivine and show a broad range of calculated temperatures, which result from a combination of grain size reduction and inheritance of equilibrium compositions from earlier high-temperature events and diffusion. In mylonites, diffusion profiles combined with geothermometry and grain size analysis indicate a mylonitic deformation temperature of 800–900°C possibly followed by diffusion. In ultramylonites, the smallest grains (<30 μm) reveal equilibration at temperatures of ∼700°C during the last stages of ductile deformation, which was dominated by diffusion creep of olivine. Our results provide a crucial link between temperature and evolution of microstructures from dislocation creep to diffusion creep in mantle shear zones.

Strength and damage of marble in ductile failure

To study the effects of loading paths and stress states on rock strength and deformation, marble specimens were axially compressed to various displacements under a confining pressure (CP) firstly, and then the damaged specimens were recompressed under another CP. The bearing capacity of a marble specimen depends merely on CP at the stage of ductile deformation, and it has no relation with the loading history when CP keeps constant or increases. However, the damaged specimen turns into brittle when it is recompressed uniaxially or at a lower CP, and the Young’s modulus and strength are lower than those of a dense specimen. The increasing ratio of triaxial strength to CP has a close relation with the areas of fissures in the damaged specimens but not the internal friction angle. Material strength and bearing capacity are two different conceptions for rocks. Material strength decreases continually as the plastic deformation increases; however, the bearing capacity is determined by both the stress state and the material strength.