Recrystallized Quartz Grains Research Articles

Deformation and paleopiezometry of auriferous quartz veins in Archean orogenic gold deposits of the Abitibi greenstone belt

Most Archean orogenic gold deposits are associated with major faults such as the Larder Lake-Cadillac fault zone (LLCFZ) in the Superior craton and are hosted in or adjacent to second-order structures as quartz vein systems. Microstructures induced by quartz deformation in these faults or shear zones, such as typical textures of recovery and dynamic recrystallization, give indication of the P-T conditions during deformation. Electron backscatter diffraction (EBSD) analysis of grain lattice orientation and grain size determination, for flow stress calculation, are commonly performed on mylonite or quartzite samples from shear zones. This study tests the validity of such methods on hydrothermal auriferous quartz veins to better constrain the conditions of quartz vein deformation in the vicinity of the LLCFZ and evaluate the effect of depth and local variations on the microstructures and flow stress. Samples were collected in 24 gold deposits and showings along the LLCFZ. The microstructural observations and EBSD analysis allowed the distinction of six microstructural types within the studied samples. Samples analyzed with EBSD display a log-normal distribution of recrystallized grain sizes that is consistent with data for natural samples. The calculated flow stress is highly variable between samples and ranges from 38.2 + 5.1/-6.1 to 74.5 + 2.2/-3.0 MPa in surface samples only. Quartz recrystallization overprints primary textures in all but one samples, which indicates that recrystallization and its associated deformation occurred after the complete crystallization of the veins. Samples with bimodal distribution of recrystallized quartz grain sizes highlight the potential of quartz veins to record several deformation events. Samples are recrystallized at higher temperature and lower stress in the east than in the west of the LLCFZ, which could indicate the exposure of a deeper crust in the east. These data underline the potential of quartz vein microstructures to contribute to the evaluation and evolution of deformation conditions of auriferous quartz vein in an orogenic context.

Lattice preferred orientation of quartz in granitic gneisses from Tso Morari Crystalline Complex, Eastern Ladakh, trans-Himalaya: evaluating effect of Dauphiné twin in dynamic recrystallization during exhumation

Abstract The Tso Morari Crystalline complex (TMCC) of eastern Ladakh, India, is part of the north Indian continental margin and is characterized by eclogitic enclaves embedded within ortho- and paragneisses known as the Puga Gneiss. Two fault zones bound the TMCC: the Karzok fault to the southwest and the Zildat fault to the northeast. In the present study, we carried out Electron Backscatter Diffraction study of quartz of 10 samples collected from the Puga Gneiss. The relict and recrystallized quartz grains were treated separately to understand the deformation conditions of the Puga Gneiss during early and late deformation stages related to UHP metamorphism and final stage of exhumation during retrogression, respectively. Microstructural observations suggest dynamic recrystallization in quartz and plagioclase at different temperature ranges. Misorientation analysis of both relict and recrystallized quartz grains reveals presence of Dauphiné Twins. Lattice preferred Orientation (LPO) of <c> axis of relict quartz grains generally shows more than one point maxima indicating that the relict grains preserve LPO developed during different stages of metamorphism/deformation. On the other hand, LPO of <c> axis of recrystallized grains from Karzok and Zildat fault zones shows asymmetric single girdle either normal or at an angle to the foliation plane, which suggests simple shear. We conclude that grain size reduction and recrystallization of the Puga Gneiss was greatly influenced by Dauphiné Twin and the final exhumation of the TMCC took place in a simple shear environment aided by activity along its two binding fault zones.

Water release and homogenization by dynamic recrystallization of quartz

Abstract. To evaluate changes in water distribution generated by dynamic recrystallization of quartz, we performed infrared (IR) spectroscopy mapping of quartz in deformed granite from the Wariyama uplift zone in NE Japan. We analyzed three granite samples with different degrees of deformation: almost undeformed, weakly deformed, and strongly deformed. Dynamically recrystallized quartz grains with a grain size of ∼10 µm are found in these three samples, but the percentages of recrystallized grains and the recrystallization processes are different. Quartz in the almost-undeformed sample shows wavy grain boundaries, with a few bulged quartz grains. In the weakly deformed sample, bulging of quartz, which consumed adjacent host quartz grains, forms regions of a few hundred micrometers. In the strongly deformed sample, almost all quartz grains are recrystallized by subgrain rotation. IR spectra of quartz in the three samples commonly show a broad water band owing to H2O fluid at 2800–3750 cm−1, with no structural OH bands. Water contents in host quartz grains in the almost-undeformed sample are in the range of 40–1750 wt ppm, with a mean of 500±280 wt ppm H2O. On the other hand, water contents in regions of recrystallized grains, regardless of the recrystallization processes involved, are in the range of 100–510 wt ppm, with a mean of 220±70 wt ppm; these values are low and homogeneous compared with the contents in host quartz grains. These low water contents in recrystallized regions also contrast with those of up to 1540 wt ppm in adjacent host grains in the weakly deformed sample. Water contents in regions of subgrains are intermediate between those in host and recrystallized grains. These results for water distribution in quartz imply that water was released by dynamic recrystallization.

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.

Quartz Microstructures from the Sambagawa Metamorphic Rocks, Southwest Japan: Indicators of Deformation Conditions during Exhumation

The Sambagawa metamorphic rocks in central Shikoku, southwest Japan consist of an inverted metamorphic sequence from the upper chlorite to oligoclase-biotite zones at the lower structural level (LSL), which is overlain by a normal metamorphic sequence consisting of the albite-biotite and garnet zones at the upper structural level (USL). These sequences form a large-scale recumbent fold called the Besshi nappe. To unravel the mechanism of recrystallization and physical conditions in quartz, and their relation to exhumation tectonics, microstructures of recrystallized quartz grains in quartz schist from the Asemi-Saruta-Dozan River traverse were analyzed. The recrystallized quartz grain size increases with increasing structural level from 40 µm in the upper chlorite zone to 160 µm in the garnet zone of the USL. Further, the mechanism of dynamic recrystallization of quartz changes from subgrain rotation to grain boundary migration with increasing structural level across the uppermost garnet zone of the LSL. From these data, the deformation temperatures in quartz schist are calculated to increase with increasing structural level within the range between 300 and 450 °C using paleopiezometers and experimental flow laws. It could be interpreted that a rapid cooling of the Besshi nappe from above is responsible for the deformation temperatures recorded in quartz schist.

Quartz dissolution associated with magnesium silicate hydrate cement precipitation

Abstract. Quartz has been replaced by magnesium silicate hydrate cement at the Feragen ultramafic body in south-east Norway. This occurs in deformed and recrystallized quartz grains deposited as glacial till covering part of the ultramafic body. Where the ultramafic body is exposed, weathering leads to high-pH (∼ 10), Mg-rich fluids. The dissolution rate of the quartz is about 3 orders of magnitude higher than experimentally derived rate equations suggest under the prevailing conditions. Quartz dissolution and cement precipitation start at intergranular grain boundaries that act as fluid pathways through the recrystallized quartz. Etch pits are also extensively present at the quartz surfaces as a result of preferential dissolution at dislocation sites. Transmission electron microscopy revealed an amorphous silica layer with a thickness of 100–200 nm around weathered quartz grains. We suggest that the amorphous silica is a product of interface-coupled dissolution–precipitation and that the amorphous silica subsequently reacts with the Mg-rich, high-pH bulk fluid to precipitate magnesium silicate hydrate cement, allowing for further quartz dissolution and locally a complete replacement of quartz by cement. The cement is the natural equivalent of magnesium silicate hydrate cement (M-S-H), which is currently of interest for nuclear waste encapsulation and for environmentally friendly building cement, but it has not yet been developed for commercial use. This study provides new insights that could potentially contribute to the further development of M-S-H cement.

Deformation mechanisms, mineral chemistry and zircon U-Pb geochronological constraints in the south Patos shear zone: implications for the crustal evolution of the Borborema Province, NE Brazil

The southern boundary of the Patos Shear Zone (PSZ) is characterized by mylonites resulting from the deformation of Paleoproterozoic basement gneisses, Neoproterozoic metavolcanic and metasedimentary rocks, granitic and granodioritic intrusions. Among the latter, the Santa Terezinha and Catingueira plutons show stretched shapes in agreement with the the shear zone’s regional trend. Recrystallized quartz grains in these plutons accommodate deformation by dislocation creep. The host banded gneisses show similar deformation structures to the plutons. Quartz crystallographic fabrics mainly record the activity of basal and prism slip planes with local contribution of rhomb planes, whilst feldspar clasts show evidence of intracrystalline deformation in the larger porphyroclasts. These characteristics suggest that shearing occurred at lower-amphibolite facies conditions. U-Pb zircon data for the Santa Terezinha pluton yields an age of 625 ± 7 Ma, which, coupled with the pluton elongated shape and microstructure, suggests that its emplacement was pre-kinematic in relation to the Patos shear zone. Post-emplacement elongation of the granite body occurred along the strike-slip trend. Such structural and geochronological constraints highlight the close association between magma emplacement and shear zone deformation during distinct stages of the Brasiliano-Pan-African orogeny in the Borborema Province.

The application and associated problems of EBSD technique in fabric analysis

在过去的二十年里，EBSD（Electron Backscattered Diffraction），即电子背散射衍射测试技术，已广泛应用于韧性组构分析，成为变形运动学、流变学分析的常规手段。该方法主要应用于流变条件下矿物晶轴组构定向性分析，以判定流变剪切指向、对比应变强度、估算变形温度。理论上讲，EBSD法适用于所有矿物的全部晶轴定向的分析测试。然而鉴于天然变形的复杂性，笔者建议EBSD分析应以石英，特别是经历了动态重结晶的石英条带为组构分析的主要对象。长期以来，石英晶轴组构的不对称性被视作独立的剪切指向标志。然而，近年来基于天然变形和一般剪切实验的研究结果表明，塑性流变的剪切指向含义应为多重流变剪切指向标志综合判别比对的结果。尽管在提出之初，石英的轴组构开角被视作独立可靠的变形温度计（Kruhl，1998）。然而限于天然变形的复杂性，特别是对变质与变形阶段的对应、耦合的认识；尽管石英变形滑移系及石英晶轴组构开角可为动力变质温度提供重要的参考，但是石英晶轴组构开角并非独立的变形温度计。;In the last two decades, EBSD (Electron Backscattered Diffraction) has been widely used as a routine approach in ductile fabric analyses, focusing on flow kinematics and rheology of ductile deformation. It is predominantly applied to determining the shear-sense of the bulk flow, evaluating strain strength, and assessing deformation temperature via analyzing Crystallographic Preferred Orientation (CPO) and deformation mechanisms of recrystallized minerals. Theoretically, EBDS retains the potential to recover the crystal-axis orientation of all minerals. However, dynamically recrystallized quartz grains, particularly, quartz ribbons are recommended for EBDS analysis due to the complexities of natural deformation. Conventionally, quartz c-axis obliquity is used as a reliable and independent shear-sense indicator, while the c-axis opening angle and recrystallization mechanisms of quartz (i.e., bulging, subgrain rotation, and grain boundary migration) have been treated as deformation thermometers. However, recent studies on naturally deformed rocks and general-shear experiments indicate that cautions must be exercised when using EBSD results to extract flow kinematics and multi-criteria are suggested for a proper interpretation of flow kinematics in ductile shear zones. Similarly, limited to our understanding of the process of natural deformation, particularly the coupling between metamorphism and ductile deformation, crystal fabric-based deformation thermometers have their own caveats and limitations that EBSD-derived thermal data, the quartz c-axis opening-angles, may also be subject to multiple interpretations although it was originally proposed as an independent deformation thermometer (Kruhl, 1998).

Intracrystalline vorticity record of flow kinematics during shear zone reactivation

The vorticity axis of deformation of a shear zone is a rotational axis controlled by the relative movement of its boundaries. When characterized, it is a reliable tool to determine the ductile flow geometry (monoclinic general, monoclinic transpressive, or triclinic) and an asset to identify the kinematics of complex oblique collisions. Advancements in analysis of electron backscattered diffraction (EBSD) maps now permit inference of vorticity axes – known as crystallographic vorticity axes (CVAs) – from the intragranular lattice orientation dispersion of strain-bearing minerals. However, natural shear zones may preserve a record of multiple shear events with contrasting kinematics and it is not understood to which extent the quartz lattice may preserve a kinematic record through multiple deformation events. Herein, we characterize the finite flow kinematic record produced during reactivation of the Eastern Highlands shear zone (EHSZ), in an oblique collision within the Canadian Appalachians. We integrate field and thin section observations, quartz lattice preferred orientations, EBSD maps, and grain classification, leading to a new method to determine and compare CVA within relict and recrystallized quartz grains. Using this method, we were able to conclude that the EHSZ was reactivated in an heterogenous monoclinic flow environment.

Quartz textural analysis from an anastomosing shear zone system: Implications for the tectonic evolution of the Ribeira belt, Brazil

Strain localization and influence of grain-size reduction processes were investigated from field and microstructural observations, and from quartz textural analysis using EBSD, in rocks deformed along an anastomosing network of shear zones in the Ribeira belt in Brazil. Rocks deformed along the Lancinha shear zone (LSZ) display high percentage of recrystallized grains, in which quartz recrystallized grain size from rocks in the southern LSZ reaches a maximum of 50 μm, and in the northern LSZ a maximum of 200 μm. The dominant serrated grain boundaries and the widespread (50–85%) presence of very small (>10–50 μm) recrystallized quartz grains, suggest that BLG was the dominant recrystallization mechanism responsible for grain-size reduction in the south portion of the LSZ. In the northern LSZ the presence of large recrystallized grains with sizes close to subgrains, suggests that grain-size reduction occurred by SGR, and the presence of lobate grain boundaries point to GBM recrystallization, which led to an increase in recrystallized grain size. This suggests grain-size reduction by dynamic recrystallization as a major factor leading to weakening and strain localization. Evidence of intracrystalline deformation and the well-developed quartz crystallographic fabrics suggest that strain localization in the LSZ occurred by dislocation creep deformation mechanisms. Dislocation creep is recorded by dominant activation of basal <a> slip and secondary contribution of rhomb <a> slip, in the southeast of the LSZ, and by activation of rhomb <a> + basal <a> and prism <a> in the northeast. These slip systems together with microstructural observations suggest that plastic deformation occurred at around 400 °C in the southern region, and at around at >400–500 °C in the northern region. S–C foliation, quartz <c> axis distribution and several kinematic indicators suggest sinistral sense of shear along the LSZ, and strain partitioning during progressive deformation. The dominance of sinistral indicators observed in the LSZ are also found in parallel shear zones to the east, such as the Putunã and Serra do Azeite. The sinistral kinematics in this overall dextral transpressional orogenic system may be explained by dual sense of shear on shear zones bounding extruding blocks, likely formed during progressive orogenic evolution instead of the polyphase evolution previously suggested for this southern part of the Ribeira belt.

Structural Networks Constraints on Alteration and Mineralization Processes in the Jiaojia Gold Deposit, Jiaodong Peninsula, China

Structural network studies could give appropriate opportunities to understanding structural/hydrothermal events, transportation of ore-forming fluids and water/rock interaction process. Four structural deformation/hydrothermal events have been identified in the Jiaojia fault zone according to microtexture and deformation of quartz and feldspars. Plagioclase experienced ductile deformation period with bended polysynthetic twin stripes (>450 °C) in the early stage, followed by K-feldspar alteration period with ductile-brittle deformation and subgrain rotation recrystallization of quartz (380–450 °C). Then, sericitization period occurred extensive ductile-brittle deformation (350–420 °C) and extensive subgrain rotation recrystallization with a little bulging recrystallization in quartz. In the last, gold precipitation-related pyrite-sericite-quartz alteration was dominated by brittle deformation (300–380 °C) and total bulging recrystallization of quartz. From the K-feldspar alteration zone and sericitization zone to pyrite-sericitequartz alteration zone, fractal dimension values of dynamically recrystallized quartz grains increase from 1.07 and 1.24 to 1.32, the calculated paleo strain rate values of dynamically recrystallized quartz range from 10−10.7 (380 °C)−10−96 (450 °C) and 10−9.3 (350 °C)−10−82 (420 °C) to 10−9.5 (300 °C)−10−80 (380 °C), and the paleo differential stress values increase from 36.9 and 39.3, to 121.3 MPa. The increase of fractal dimension values and decrease of grain size from pyrite-sericite-quartz alteration zone and sericitization zone to K-feldspar alteration zone decreased average water/rock ratio values, which could lead to different acidity and redox conditions of ore-forming fluids and mineralization differences. Two kinds of orecontrolling fractures have been distinguished which include the gentle dip types (18°–50°) with NW (315°–355°) and SW (180°–235°) dip hosting No. I orebodies and the steep dip types (74°–90°) with NE (45°–85°) and SE (95°–165°) dip hosting No. III orebodies. These faults/fractures crosscut altered Linglong granite of footwall of the Jiaojia fault zone as rhombohedrons that promoted the connection between fractures in the K-feldspar alteration zone and fluid flow passages near the main fault face. Research results indicate No. I and No. III orebodies should be derived from the same mineralization event and belong to different orebody types in different mineralization sites under the same structural networks.

Distribution of ductile deformation around the Main Central Thrust zone at the frontal part of nappe in southeastern Nepal Himalaya

AbstractA geological survey and morphological analysis of quartz grains were performed to investigate the distribution of ductile deformation caused by the Himalayan Main Central Thrust (MCT) around Dhankuta, southeastern Nepal. The MCT was mapped as the lithological boundary between the gneiss of the Higher Himalayan Crystalline (HHC) as the hanging wall and the inverted metamorphic sequence of the Lesser Himalayan Sediments (LHS) as the footwall. The MCT was found to truncate various stratigraphic levels of LHS and cuts a map‐scale gentle fold developed in the LHS. Ductile deformation was quantified by fractal dimension between size and perimeter of dynamically recrystallized quartz grains in bedded metaquartzite intercalated in both HHC and LHS. Serrate and polygonal shapes of quartz indicate large and small strain rates, respectively, when the temperature during ductile deformation was assumed to be uniform. A peak of strain rate was found at the lithological boundary with the peak width of ca. 500 m. Such a thin shear zone is favorable for producing frictional heat to promote the inverted metamorphism in LHS.

Recrystallization and Provenance History of the Upper Kaimur Group Siliciclastics, Son Valley, India: Coupled Petrographic and Fluid inclusion Proxy

Abstract The Mesoproterozoic Kaimur Group belonging to upper part of Vindhyan Supergroup, conformably overlies the carbonate sequence of Semri Group (Lower Vindhyan) in the Son Valley, central India. The Upper Kaimur Group consists of Dhandraul sandstone, Scarp sandstone and Bijaigarh shale. The detrital contents of the Dhandraul and Scarp sandstones are mainly composed of several varieties of quartz followed by feldspar, rock fragments, micas and heavy minerals. Fluid inclusion studies are carried out on the detrital and recrystallized quartz grains of the Dhandraul and Scarp sandstone to know about the fluid phases already present in the source rock and / or introduced in the recrystallisation process. Fluid micro-thermometry reveals the presence of two types of fluids: (i) bi-phase low saline aqueous inclusions, (ii) bi-phase high saline aqueous inclusion. These fluids were trapped during the development of grain and recrystallization processes. The salinity of these inclusions in the quartz grain is in the range of 5.7 to 13.4% suggests that initially there was good proportion of marine water during the initiation of sedimentation. The provenance of these rocks may be granite/metamorphic rocks of Mahakoshal Group and Chhotanagpur granite-gneisses and minor input from Bundelkhand granite complex.

Quartz Flow Law Revisited: The Significance of Pressure Dependence of the Activation Enthalpy

AbstractAn accurate flow law for dislocation creep of quartz aggregates is critical for the understanding of continental rheology, numerical modeling of lithospheric processes, and the interpretation of microstructures of natural quartz‐bearing mylonites. Despite many decades of research, considerable discrepancies still exist among quartz flow laws determined from different experiments. We demonstrate that the key to reconcile these discrepancies is to consider the pressure dependence of the activation enthalpy. From existing high‐quality creep experiments on quartz aggregates, we critically identified test runs having microstructures and stress‐strain curves indicating steady state regimes 2 and 3 dislocation creep and used the data to obtain a set of flow law parameters most consistent among existing experiments. Because estimates of strain rate and stress from natural mylonites are bound with large uncertainties, they cannot be used to construct a flow law better than one determined from well‐controlled creep experiments. A large number of geological studies and modern GPS observations suggest that crustal scale ductile shear zones likely flow at a strain rate range between 10−13 and 10−11 s−1. In this strain rate range, our flow law predicts flow stresses broadly consistent with stress estimates from many natural mylonites based on well‐calibrated piezometers for dynamically recrystallized quartz grains.

Structural characteristics and geochronology of migmatites in the North Dabie Complex unit: Timing of post-collisional deformation

Extensive migmatization and large-scale post-collisional intrusions occurred in the Dabie orogen, east-central China, during the Early Cretaceous, characterized by distinct deformation preserved in migmatites in the North Dabie Complex unit. The North Dabie Complex unit can be subdivided into three areas based on detailed field observations: the north of the Tiantangzhai pluton, the Luotian area and the Yuexi area. Banded migmatites crop out in the north of the Tiantangzhai pluton while anisotropically deformed migmatites occur in the Luotian area, and both types coexist in the Yuexi area. Microscopy reveals similar micro-structures are in migmatites from the north of the Tiantangzhai pluton, the Yuexi area and border of the Luotian area, while static recrystallization appears in migmatites from the core of the Luotian area. The Lattice-Preferred Orientation of dynamically recrystallized quartz grains in the migmatites are measured using electron backscattered diffraction, revealing prism slip or slip in migmatite from the north of the Tiantangzhai pluton and the Yuexi area and in one sample from the Luotian area. A Type I crossed girdle is developed in another sample from the Luotian area, indicating top-to-the-SE shearing that developed under greenschist facies conditions. Zircon U-Pb dates from four migmatites reveal that mainly Early Cretaceous ages are from the north of the Tiantangzhai pluton, only four Early Cretaceous ages are from the Luotian area, and all zircons from the Yuexi area record Indosinian metamorphic ages. Although zircon U-Pb results show multiple migmatization events, all samples record a migmatization age of about 132 Ma, suggesting an orogen-scale event at that time. The subsolidus deformation in migmatites indicates that deformation were soon after migmatization. Overall, this study shows that deformation in migmatites of the North Dabie Complex unit occurred somewhat later than the migmatization (~132 Ma) at about 131 Ma. The most likely mechanism for thinning of the thickened crust in the Dabie orogen involved removal of the upper along the detachment fault and ductile adjustment of lower crust during development of the North Dabie metamorphic core complex.

Evolution of fabric in Chitradurga granite (south India) - A study based on microstructure, anisotropy of magnetic susceptibility (AMS) and vorticity analysis

In this paper, the fabric in massive granite (~2.6Ga) from the Chitradurga region (Western Dharwar Craton, south India) is analyzed using microstructure, anisotropy of magnetic susceptibility (AMS) study and kinematic vorticity analysis. The microstructural investigation on the granite shows a progressive textural overprint from magmatic, through high-T to low-T solid-state deformation textures. The mean magnetic foliation in the rocks of the region is dominantly NW-SE striking which have developed during regional D1/D2 deformation on account of NE-SW shortening. The plunge of the magnetic lineation varies from NW to vertical to SE, and interpreted to be a consequence of regional D3 deformation on account of NW-SE to E-W shortening. The vorticity analysis from magnetic fabric in the region reveals that the NW-SE oriented fabric formed under pure shear condition during D1/D2 regional deformation. However, some parts of the region particularly close to the adjacent Chitradurga Shear Zone show that the magnetic fabrics are oblique to the foliation as well as shear zone orientation and inferred to be controlled by simple shearing during D3 regional deformation. The shape preferred orientation (SPO) analysis from oriented thin sections suggest that the shape of the recrystallized quartz grains define the magnetic fabric in Chitradurga granite and the degree of the SPO reduces away from the Chitradurga Shear Zone. It is interpreted that the change in magnetic fabrics in some parts of the granite in the region are dominantly controlled by the late stage sinistral shearing which occurred during the development of Chitradurga Shear Zone.

Dextral strike-slip along the Kapıdağ shear zone (NW Turkey): evidence for Eocene westward translation of the Anatolian plate

The northern part of the Kapidag Peninsula (Marmara Sea, NW Turkey) is affected by the E–W trending Kapidag shear zone, which cuts through calc-alkaline granitoids of the Ocaklar pluton resulting in mylonitic orthogneiss. Macroscopic and microscopic shear-sense indicators, such as SC fabrics, shear bands, σ-clasts and mica fish, unequivocally suggest dextral strike-slip for the Kapidag shear zone. Based on petrographic data, deformation microfabrics of quartz and feldspar, and the slip systems in quartz, the dextral shearing should have been active at T = 500–300 °C and P < 5 kbar. Published K–Ar and 39Ar–40Ar cooling ages of hornblende and biotite suggest that cooling below 500–300 °C occurred during the Eocene (ca. 45–ca. 35 Ma), meaning that the Kapidag shear zone should have been active during Middle to Late Eocene times. The differential stress related to the shearing was <50 MPa as is indicated by the size of recrystallized quartz grains. Based on the new and published data, it is concluded that the westward movement of the Anatolian plate might have been active almost continuously from the Middle Eocene until recent times.

Determination of the stress conditions of the ductile-to-brittle regime along the Asuke Shear Zone, SW Japan

The stress conditions of the ductile-to-brittle regime have been assessed along the Asuke Shear Zone (ASZ), which strikes NE–SW in the Cretaceous Ryoke granite terrain in SW Japan. Along the ASZ, pseudotachylyte and mylonitized pseudotachylyte are locally developed together with cataclasite. The simultaneous operation of dislocation creep and grain-size-sensitive creep, as indicated by the coexistence of the Z-maximum and relatively random c-axis lattice preferred orientations as well as the sizes of dynamically recrystallized quartz grains (6.40–7.79 μm) in the mylonitized pseudotachylyte, suggest differential stresses of 110–130 MPa at ∼300 °C. The e-twin morphology, twinning ratio, and distribution of the glide direction on the e-twin plane of the twinned calcite in the amygdules of the pseudotachylyte suggest the stress conditions of the σ1 and σ3 axes trend 228° and 320° and plunge 55° and 1°, respectively, and indicate differential stresses of 40–80 MPa at 150–200 °C. Based on kinematic indicators in the fault rocks, the stress conditions estimated from calcite twins, and the cooling history of the granitic protolith, the ASZ is inferred to have been activated under a stress state that caused sinistral normal movements before and after pseudotachylyte formation at 70–50 Ma.

Major softening at brittle‐ductile transition due to interplay between chemical and deformation processes: An insight from evolution of shear bands in the South Armorican Shear Zone

AbstractThe formation of S‐C/C′ fabrics in the South Armorican Shear Zone has been evaluated by detailed microstructural study where the focus was given to initiation and early evolution of the C/C′ fabric shear bands. Our observations suggest that the S‐C/C′ fabrics formed at distinct temperature conditions indicating &gt;550°C for the S fabric and 300–350°C at 100–400 MPa for the C/C′ fabric shear bands. The evolving microstructure within shear bands documents switches in deformation mechanisms related to positive feedbacks between deformation and chemical processes and imposes mechanical constraints on the evolution of the brittle‐ductile transition in the continental transform fault domains. Three stages of shear band evolution have been identified. Stage I corresponds to initiation of shear bands via formation of microcracks with possible yielding differential stress of up to 250 MPa. Stage II is associated with subgrain rotation recrystallization and dislocation creep of quartz and coeval dissolution‐precipitation creep of microcline. Recrystallized quartz grains show continual increase in size and decrease in stress and strain rates from 94 MPa to 17–26 MPa and 3.8 × 10−12 s−1–8 × 10−14 s−1 associated with deformation partitioning into weaker microcline layer and shear band widening. The quartz mechanical data allowed us to set some constrains for coeval dissolution‐precipitation of microcline which at our estimated pressure‐temperature conditions suggests creep at 17–26 MPa differential stress and 3.8 × 10−13 s−1 strain rate. Stage III is characterized by localized slip along white mica bands accommodated by dislocation creep at strain rate 3.8 × 10−12 s−1 and stress 9.36 MPa. Our mechanical data point to dynamic evolution of the studied brittle‐ductile transition characterized by major weakening to strengths ~10 MPa. Such nonsteady state evolution may be common in crustal shear zones especially when phase transformations are involved.

Structures, kinematic analysis, rheological parameters and temperature-pressure estimate of the Mesozoic Xingcheng-Taili ductile shear zone in the North China Craton

The NE to ENE trending Mesozoic Xingcheng-Taili ductile shear zone of the northeastern North China Craton was shaped by three phases of deformation. Deformation phase D1 is characterized by a steep, generally E–W striking gneissosity. It was then overprinted by deformation phase D2 with NE-sinistral shear with K-feldspar porphyroclasts forming a subhorizontal low-angle stretching lineation on a steep foliation. During deformation phase D3, lateral motion accommodated by ENE sinistral strike-slip shear zones dominated. Associated fabrics developed at upper greenschist metamorphic facies conditions and show the deformation characteristics of middle- to shallow crustal levels. In some parts, the older structures have been in turn overprinted by late-stage sinistral D3 shearing. Finite strain and kinematic vorticity in all deformed granitic rocks indicate a prolate ellipsoid (L-S tectonites) near plane strain. Simple shear-dominated general shear during D3 deformation is probably of general significance. The quartz c-axis textures indicate prism-gliding with a dominant rhomb <a> slip and basal <a> slip system formed mainly at low-middle temperatures. Mineral deformation behavior, quartz c-axis textures, quartz grain size and the Kruhl thermometer demonstrate that the ductile shear zone developed under greenschist facies metamorphic conditions at deformation temperatures ranging from 400 to 500 °C. Dislocation creep is the main deformation mechanism at a shallow crustal level. Fractal analysis showed that the boundaries of recrystallized quartz grains had statistically self-similarities. Differential stresses deduced from dynamically recrystallized quartz grain size are at around 20–39 MPa, and strain rates in the order of 10−12 to 10−14 s−1. This indicates deformation of granitic rocks in the Xingcheng-Taili ductile shear zone at low strain rates, which is consistent with most other ductile shear zones. Hornblende-plagioclase thermometer and white mica barometer indicate metamorphic conditions of medium pressures at around ca. 3–5 kbar and temperatures of 400–500 °C within greenschist facies conditions. The main D3 deformation of the ENE-trending sinistral strike-slip ductile shearing is related to the roll-back of the subducting Pacific plate beneath the North China Craton.