Quartz Microstructures Research Articles

Metamorphic tourmaline and its petrogenetic significance from the Maramureș Mountains (East Carpathians, Romania)

Abstract This study describes mineralogical and crystallochemical characteristics of metamorphic tourmalines from an Alpine shear zone in a Variscan metamorphic rock sequence from the Maramures region in the northern part of the East Carpathians. We use this mineral to unravel aspects of the evolution of the tourmaline bearing host rocks and compare the crystallo-chemical characteristics to other tourmalines from Alps. Petrographic and microstructural observations, as well as electron microprobe analyses on several zoned tourmalines and associated minerals (mica, feldspar) from mylonitic schist of the Rebra terrane (Maramureș Mountains), indicate that the pre-kinematic tourmalines belong to the alkali group (Na dominant), hydroxyl dominated on the crystallographic W-site and can be assigned to the species dravite and schorl. The tourmaline-bearing rocks have a metasedimentary protolith. The analysed porphyroblasts, rotated by simple shear, show corroded rim that are interpreted to have formed due to pressure release. Three main compositional zones were evidenced on a tourmaline porphyroblast: a core zone and two asymmetrically arranged inclusion-poor/free rims, all formed in pre-alpine prograde metamorphic conditions. Based on mineral microstructural relations and geothermobarometry (tourmaline–muscovite, tourmaline–plagioclase geothermometry and phengite geobarometry), the metamorphic peak conditions of the investigated Rebra terrane were evaluated to have been at a temperature of ca. 590 to 620 ± 22 °C and Pmin = 5.5 - 6.0 ± 0.5 kbar. By observing dynamically recrystallized microstructures in quartz and feldspar in the shear zone a temperature of 350 - 400 °C was estimated and the quartz paleopiezometry outlined a differential stress of about 1.5 kbar that implied only minor chemical change in tourmaline outer zone.

Deformation Mechanisms in Orogenic Gold Systems During Aseismic Periods: Microstructural Evidence from the Central Victorian Gold Deposits, Southeast Australia

Abstract In many orogenic gold deposits, gold is located in quartz veins. Understanding vein development at the microstructural scale may therefore provide insights into processes influencing the distribution of gold, its morphology, and its relationship to faulting. We present evidence that deformation processes during aseismic periods produce characteristic quartz microstructures and crystallographic preferred orientations, which are observed across multiple deposits and orogenic events. Quartz veins comprise a matrix of coarse, subidiomorphic, and columnar grains overprinted by finer-grained quartz seams subparallel to the fault trace, which suggests an initial stage of cataclastic deformation. The fine-grained quartz domains are characterized by well-oriented quartz c-axis clusters and girdles oriented parallel to the maximum extension direction, which reveals that fluid-enhanced pressure solution occurred subsequent to grain refinement. Coarser anhedral gold is associated with primary quartz, whereas fine-grained, “dusty” gold trails are found within the fine-grained quartz seams, revealing a link between aseismic deformation and gold morphology. These distinct quartz and gold morphologies, observed at both micro- and macroscale, suggest that both seismic fault-valving and aseismic deformation processes are both important controls on gold distribution.

Stress sensitivity of high-temperature microstructures in ice, with potential applications to quartz

Quartz and ice both exhibit distinctive microstructures when deformed at low stress and high homologous temperature, known as grain-boundary migration (GBM) microstructures. These are difficult to reproduce experimentally in silicate minerals, and no correlation has been established between quantifiable aspects of the microstructure and deformational conditions. We carried out direct shear experiments to investigate the effect of stress and temperature (T) on GBM microstructures in ice, which is crystallographically analogous to quartz. Differential stress was 0.7–6.0 MPa, confining pressure 4–9 MPa, and T −3° to −25 °C. There is a clear transition at ∼ −12 °C from granular microstructures produced by rotation recrystallization at high stress and low T, to GBM microstructures at low stress and high T, accompanied by an increase in strength of the crystallographic preferred orientation (CPO). Samples with GBM microstructure show lobate grain-boundaries and “island grains” where several distinct and separate areas have the same crystallographic orientation, representing lobes of a single grain isolated on the cut surfaces. The size of lobes and island grains define an array with respect to stress with a slope of 0.9, similar to but offset from the slope of published stress/grain-size data. This suggests the possibility of determining paleostress from GBM microstructures in both ice and quartz.

Amalgamation of the Ryoke and Sanbagawa metamorphic belts at the subduction interface: New insights from the Kashio mylonite along the Median Tectonic Line, Nagano, Japan

AbstractWe present a detailed petrological, structural and geochronological study of the mylonitic Ryoke metamorphic rocks within the granitic mylonite (Kashio mylonite) and Sanbagawa metamorphic rocks along the Median Tectonic Line (MTL), Japan. Located in the Oshika area of the Chubu District, the Kashio mylonite is one of the few geologic units that can be used to determine detailed pressure–temperature–time–deformation (P–T–t–D) paths during mylonitization because it occurs as many small tectonic blocks of mylonitic metasediment. Detailed petrological analysis coupled with conventional thermobarometry and P–T pseudosection modelling give estimated peak P–T conditions (M1a) of 650–790°C at 4.6–5.6 kbar for the Ryoke metamorphic rocks. The gneissose Ryoke granitoids were emplaced subhorizontally at around 685–710°C and 4.6–5.8 kbar, after peak metamorphism. The Kashio shear zone developed immediately after the last igneous activity at ca. 71 Ma, and two stages of mylonitization (stages D1 and D2) can be identified from microstructural observations. The retrograde P–T conditions (M1b) recorded in the Kashio mylonite exhibit a systematic change in temperature from 710°C to 450°C at 5.2–2.6 kbar with decreasing distance from the MTL. By contrast, highly deformed mylonites with zoned garnets demonstrate a striking increase in pressure from 4.0 to 8.3 kbar with decreasing temperature from 590°C to 450°C after low‐P/T‐type metamorphism. Such a temperature range indicating isothermal compression is consistent with deformation temperatures of stage D1 determined from quartz microstructures and quartz c‐axis fabric opening‐angle deformation thermometer. Moreover, the timing of the two mylonitization episodes during retrograde metamorphism are estimated to be 69–67 and 66–64 Ma, respectively, with a high cooling rate of ~34°C/Ma using the revised time–temperature relationship of the host Ryoke granitoids. The rapid change in tectonic setting with strain localization occurred during the brief period between 69 and 64 Ma. Our field and petrological observations imply that a thick D1 mylonite zone was formed by rapid subsidence (≥10 km) with cooling of the hangingwall rocks from the middle crust to the subduction interface. It is considered that the underplating of exhumed high‐P/T‐type metamorphic rocks led to further cooling between hangingwall and footwall rocks and the formation of a narrow D2 mylonite zone, which served as an old plate boundary. Thus, low and high‐P/T‐type metamorphic belts had already been amalgamated as paired metamorphic belts beneath the brittle–ductile transition of the subduction zone before exhumation. The rapid cooling of hangingwall rocks at the subduction interface is proposed to play an essential role in the thermal overprinting of exhumed high‐P/T‐type metamorphic rocks.

Grain-scale petrographic evidence for distinguishing detrital and authigenic quartz in shale: How much of a role do they play for reservoir property and mechanical characteristic?

Quartz grains and their microstructural characteristics are essential both to the formation of interparticle porosity and to the influencing reservoir brittleness of siliceous shale. Here we analyze quartz forms, origins, and textures and evaluate their impacts on reservoir property and mechanical behavior's response to hydraulic fracturing. Scanning/transmission electron microscopy imaging reveals the presence of both detrital and authigenic quartz with the content from 13.6 to 88.3%. Detrital quartz occurs as randomly distributed asperities that are characterized by an irregular ellipsoidal shape. At least four distinct authigenic forms are distinguished: (1) euhedral microquartz with well crystallization and similar size, which are derived from smectite-to-illite transformation and fracture fill, (2) nanospheres, which are product of biogenic silica and intraparticle cement, (3) grain-rimming syntaxial overgrowths, which come from pressure dissolution and cementation, and (4) replacement of skeletal grain, which may be of in situ authigenic origins. Reservoir property is controlled by quartz microstructures. The authigenic quartz grains have significant implications for mechanical fracture property. Two mechanisms of grain cushioning and grain rolling effect have been invoked to explain the common perception that authigenic quartz is positively correlated with reservoir brittleness while detrital quartz has little or no effect.

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.

Stresses during pseudotachylyte formation - Evidence from deformed amphibole and quartz in fault rocks from the Silvretta basal thrust (Austria)

Pseudotachylyte-bearing, amphibole-rich gneisses with quartz-rich layers concordant to the foliation from the base of the Silvretta nappe were analyzed by polarized light microscopy and scanning electron microscopy to constrain the deformation history. Amphibole shows microfractures, kink bands and mechanical twins associated with pseudotachylytes. (−101)[101] twins document high differential stresses of ≥ 400 MPa. Quartz grains in contact with twinned amphibole in gneisses show subbasal deformation lamellae, short-wavelength undulatory extinction and new grains along cracks with diameters of < 15 μm and crystallographic preferred orientation (CPO) controlled by the orientation of host grains. This quartz microstructure indicates dislocation glide-controlled deformation coeval with high-stress amphibole deformation and pseudotachylyte formation.Quartz-rich layers concordant to the gneiss' foliation commonly show evidence of dislocation creep by homogenous recrystallized grain aggregates with CPO indicating <a> dislocation glide. Systematic differences in distribution of the recrystallized grains and grain sizes, indicate deformation at differential stresses from a few tens to hundred MPa. Yet, no evidence of increasing strain towards the contact to amphibole-rich gneisses is observed. In contrast, a decreasing number of recrystallized grains towards the contact occurs locally. In relation to pseudotachylytes, the recrystallized quartz is affected by shear fractures offsetting the recrystallized microstructure. This, and the missing or decreasing strain gradient of dislocation creep towards the contact to amphibole-rich gneisses, indicate that 1) dislocation creep in quartz-rich layers is preceding and 2) not directly related to the high-stress and high strain-rate deformation, ruling out a major influence of thermal runaway for the formation of the pseudotachylytes. The differences in the deformation behavior is interpreted to be controlled by the distance to the propagating fault tip. This study demonstrates the potential of microstructures to constrain the deformation history controlling seismic faulting and creep at hypocentral depth not directly accessible for in-situ stress measurements.

Shearing and fluid evolution of the Porto Nacional orogenic gold district, western Brazil: microstructural, fluid inclusion, and C-O isotopic evidence

The Porto Nacional Gold District, in the northern Tocantins Province, Brazil, is one of the few crustal-scale shear systems that contain both hypozonal and mesozonal orogenic gold deposits. It is situated within a Paleoproterozoic terrane, hosted by the crustal-scale Neoproterozoic Transbrasiliano-Kandi Shear System. The Cachimbo Shear Zone (CSZ) hypozonal deposits are hosted by phyllonites characterized by staurolite, garnet, and median to high-crystallinity graphite, consistent with amphibolite facies conditions. The mesozonal deposits, hosted in the Mutum Shear Zone (MSZ) and the Conceição Shear Zone (CoSZ), show a paragenesis that reflects greenschist-facies conditions. All zones host quartz fault-fill veins and adjacent extensional quartz-carbonate veinlet swarms. Two mineralizing stages are recognized. Stage 1 occurs in old quartz porphyroclasts in the fault-fill veins of the CSZ and MSZ deposits, and comprises pyrite, Type-2 graphite, gold, and other sulphides. In the CSZ stage 1, pressure-temperature data derived from mineral thermometry, quartz microstructure, and fluid inclusion isochores are compatible with a ductile deformation regime (433° to 580 °C and 3.5 to 6.3 kb), equivalent to 16 to 24 km depths under lithostatic conditions. Conditions for MSZ stage 1 are 350° to 415 °C and 2.0 to 3.1 kb, where the calculated depth (9 to 12 km) suggests the MSZ developed at shallower levels in the crust than the CSZ. In both shear zones the ore fluids are represented by H2O-CO2 fluid inclusions that are CH4-bearing and low salinity (0.3 to 6 wt% NaCl), and have low ƒO2. Reactivation along the shear zones caused folding and boudinaging of the extensional veinlets and recrystallization by subgrain rotation in the vein quartz, suggesting temperatures of ~ 400 °C. Ore stage 2 (gold-galena) occurs in vug-fill quartz in the veins and in extensional veinlets of the MSZ and CoSZ. It is estimated to have formed at lower temperatures (340° and 390℃) and pressures (1.8 to 4.5 kb) than ore stage 1, at depths of 6 to 18 km. The fluids were H2O-CO2 rich, with salinities of 3 to 8 wt% NaCl. The high and fluctuating pressures, above lithostatic conditions, suggest supralithostatic conditions. The wide fluid pressure variation is consistent with the fault-valve model for both ore stages. Later, higher salinity aqueous fluids (up to 15 wt% NaCl) with no CO2 or CH4 circulated at 300℃ and at 1.0 to 1.7 kb, which is more compatible with hydrostatic conditions at depths of 4 to 7 km. The stable isotopic composition of the fluids (δ18OH2O = 2 to 12‰, δ13CCO2 = −4 to −8‰) and the local geological constraints indicate that the mineralizing fluids were metamorphic. The P-T characteristics of the system, which indicate a decrease in T and P from the CSZ to the MSZ and CoSZ suggests that the PNSZ developed concurrently in the greenschist-amphibolite window. The further development of footprints for the hypozonal-mesozonal orogenic gold deposits of the PNGD could help identify new target areas for exploration in Brazil and elsewhere in this Neoproterozoic crustal-scale shear system.

The investigations of the deformation of the Great Counter Thrust (GCT) in Zedang area, South Tibet: A case study of magnetic fabric and EBSD analysis

仁布-泽当逆冲断层是喜马拉雅大反向逆冲断层（GCT）在藏南地区的重要延伸部分，也是喜马拉雅造山带北部边界新生代最为活动的构造单元之一。新生代以来特提斯喜马拉雅的构造变形组构特征的研究对于深入理解碰撞造山带演化与高原隆升具有重要构造意义。本文综合GCT泽当-琼结段断层的宏观与微观变形特征，对断裂带石英脉、围岩中石英和云母矿物的电子背散射（EBSD）组构及断层两侧岩石磁组构（AMS）特征进行对比分析。结果表明对AMS主要贡献来自顺磁性云母、绿泥石等，磁化率各向异性椭球体以压扁状为主，磁面理与构造面理（劈理、断层面）基本重合，显示较强的构造变形磁组构特征；磁线理优选方向近南北向，且与观测北向逆冲断层方向一致，揭示剪切作用在变形过程中的持续作用。研究发现泽当地区GCT附近石英微观结构从围岩至断层区，石英至少呈现3种不同类型的微观变形机制：围岩区溶解蠕变、断裂带石英以膨凸重结晶和亚颗粒旋转重结晶作用为主。断裂带石英的c轴EBSD组构指示变形为低温（300~400℃）环境，其中黑云母的结晶学优选（CPO）与磁组构主轴优选方向存在高度的一致性，进一步证实了顺磁性矿物黑云母对AMS的主要贡献。综合研究表明泽当地区GCT的韧性变形是断层处在中上地壳韧性带的活动阶段变形的结果，也代表了特提斯喜马拉雅在碰撞、高原隆升期的变形主要特征。;Renbu-Zedang thrust fault is one of the most important parts of the Great Counter Thrust (GCT) in South Tibet, as well as one of the most active faults of the Himalaya Orogeny in Cenozoic. The investigation of the deformed fabric has great importance for deep understanding of the orogen evolution and plateau uplift. In this paper, we combined the mesoscopic and micro-structures of the fault rocks, especially, the comparison analysis of the Electron Backscattered Diffraction (EBSD) fabrics of quartz, mica and the anisotropy of magnetic susceptibility (AMS) of samples around the fault zone. The AMS results suggest an oblate magnetic ellipsoid predominant at most sample sites, and magnetic foliations nearly coincide with the tectonic foliations (cleavage or fault plane), both of which suggest strong tectonic fabric dominant of the area. S-N directed magnetic lineation in line with the fault moving direction indicated the continuing thrusting happened in the fault zone. Quartz microstructure suggests that this fault zone at least underwent three different levels of deformation mechanism, namely, the dissolution creep static recrystallization (quartz grains shows polygon seams), the bulging (BLG) recrystallization and the subgrain rotation (SGR) recrystallization, respectively. Low deformation temperature (300~400℃) is suggested from the c-axes of quartz EBSD fabric. The Crystallographic Preferred Orientation (CPO) of biotite shows strong correspondence with the magnetic fabrics which further confirmed the dominance of paramagnetic to the AMS. The ductile deformation of GCT in the Zedang area is the result of the fault in ductile zone of the middle and upper crust uplift movement stage, also represent the plateau uplift and deformation of the Tethyan Himalayan since Cenozoic.

Characterizing syn-convergent extension along the Neybaz-Chatak detachment shear zone, Central Iran: Insights from microstructures, quartz petrofabrics and flow vorticity analysis

Integrated microstructural, quartz petrofabric and flow vorticity analyses on mylonitic rocks in the lower plate of the Neybaz Chatak Detachment Shear Zone provide significant information regarding the kinematic characteristics coeval with the Chapedony metamorphic core complex development within the Central East Iranian Microcontinent. Several kinematic shear sense indicators including quartz c-axis fabrics, S–C fabrics, rotated feldspar and mica fish reveal that the detachment shear zone has experienced a consistent top-to-the NE-directed progressive shear deformation. Detailed investigations on recrystallization types of quartz and feldspar in the mylonites demonstrate that the deformation occurred under the amphibolite facies condition corresponding to mid-crustal levels. A combination of investigations on mineral assemblages, microstructures of quartz and feldspar, and quartz lattice preferred orientations (LPOs) show that deformation temperatures ranged from 400 to 645 °C that were varied across a distance of ~21 km in the extension direction (NE-SW). The western and eastern parts of the study area experienced lower temperatures compared to the central parts. The gneissic mylonite record sub simple shear deformation (Wm = 0.56–0.85, 64–36%) associated with an increasing of pure shear component. These data highlight that the metamorphic rocks of the study underwent a non-coaxial shear with ductile low-angle normal faulting, resulting in subvertical thinning in the extensional deformation regime responsible for formation of the core complex.

Subduction accretion, thermal overprinting, and exhumation of high-pressure/low-temperature metasedimentary rocks of the south-central brooks range

ABSTRACT Lithological, petrological and structural observations and measurements of graphite thermometry, Si-in-phengite barometry, and 40Ar/39Ar phengite thermochronology were performed along two transects across the south-central Brooks Range of northern Alaska. The integrated data set reveals a history of accretion, protracted thermal overprinting of high-pressure/low-temperature (HP/LT) metamorphic assemblages, and subsequent exhumation. Tectonic domains characterized by upper greenschist facies metamorphic rocks display a dominant E-W axial planar cleavage/foliation fabric that overprints bedding and earlier metamorphic foliations. Raman maturation thermometry and Si-in-phengite barometry document a southward increase from greenschist facies conditions (ca. 360–400°C; 3–7 kbar) in the Doonerak window and Endicott Mountains allochthon to epidote-blueschist facies conditions (~515°C and 10–13 kbar) within the Schist Belt. Within the Central and Schist belts albite porphyroblasts overgrow the earlier deformation fabric and quartz microstructures exhibit evidence for diffusive recovery. This indicates thermal overprinting of deeper portions of the accretionary complex caused protracted (ca. 25 m.y.) greenschist facies overprinting of HP/LT assemblages within the Ar partial retention zone. The Schist Belt – Central Belt boundary is defined by a singular, penetrative foliation that displays a weak shear-band fabric. 40Ar/39Ar results along this boundary indicate that the fabric records deformation along the lower boundary of a nappe that began to move up the subduction channel as subduction ceased by ~135-130 Ma. Subsequent deformation along the Schist Belt – Phyllite Belt boundary reflects the exhumation of HP/LT rocks along a south-dipping normal-sense shear zone. Phengite 40Ar/39Ar age gradients coupled with the first arrival of coarse-grained Brooks Ranges detritus within the adjacent Yukon Koyukuk Basin indicate that tectonic denudation along this shear zone partly exhumed the Schist and Phyllite belts to mid-crustal depths by 110–105 Ma. Final exhumation of the HP/LT rocks was delayed until the Cenozoic which resulted in < 15 km of additional denudation.

Fracture Analysis of α-Quartz Crystals Subjected to Shear Stress

This study assesses the correlations between the intensity of stress undergone by crystals and the morphological characteristics of particles and fracturing products. The effects of the fractures on the microstructure of quartz are also studied. Alpha quartz, subjected to shear stress, is quickly crushed according to a fracturing sequence, with a total fracture length that is correlated to the stress rate. The shear stress generates a sequence of macro and microstructural events, in particular localized melting phenomena, never highlighted before on quartz and the formation of different polymorphs, such as cristobalite and tridymite together with amorphous silica.

Microstructure of Vein Quartz Aggregates as an Indicator of Their Deformation History: An Example of Vein Systems from Western Transbaikalia (Russia)

We investigated the microstructural and crystallographic features of quartz from complex vein systems associated with the development of thrust and shear deformations in Western Transbaikalia using electron back scatter diffraction (EBSD) and optical microscopy. Vein quartz systems were studied to obtain insights on the mechanisms and localization of strains in quartz, in plastic and semibrittle conditions close to the brittle–ductile transition, and their relationship to the processes of regional deformations. Five types of microstructures of vein quartz were distinguished. We established that the preferred mechanisms of deformation of the studied quartz were dislocation glide and creep at average deformation rates and temperatures of 300–400 °C with subsequent heating and dynamic and static recrystallization. The formation of special boundaries of the Dauphiné twinning type and multiple boundaries with angles of misorientation of 30° and 90° were noted. The distribution of the selected types in the differently oriented veins was analyzed. The presence of three generations of vein quartz was established. Microstructural and crystallographic features of vein quartz aggregates allow us to mark the territory’s multi-stage development (with the formation of syntectonic and post-deformation quartz).

EBSD‐based criteria for coesite‐quartz transformation

AbstractUltra high pressure (UHP) metamorphism observed in continental terranes implies that continental crust can subduct to ~40 kbar before exhuming to the surface. This process is one of the least understood and widely debated parts of the orogenic cycle. The dominantly felsic composition of UHP continental terranes means that many petrology‐based techniques for determining peak pressures and temperatures are often not possible. In such cases, the detection of UHP conditions depends on the preservation of coesite, a rarely preserved mineral in exhumed UHP terranes as it rapidly transforms to quartz on decompression. Consequently, the qualitative identification of palisade quartz microstructures that form during the retrograde transformation of coesite to quartz is often used to identify UHP terranes. In this study, we conduct electron backscatter diffraction and misorientation analysis of palisade quartz inclusions in the coesite‐bearing pyrope quartzite from the Dora Maira massif in the Alps, and matrix‐scale palisade quartz in the Polokongka La granite from Tso Morari in the Ladakh Himalaya, in order to quantitatively define crystallographic characteristics of quartz after coesite. The repeatability of our observations in two unrelated occurrences of UHP rocks supports our interpretation that the following features provide a systematic and predictable set of criteria to identify the coesite to quartz transition: (1) Quartz crystallographic orientations define spatially and texturally distinct subdomains of palisade quartz grains with ‘single crystal’ orientations defined by distinct c‐axis point maxima. (2) Adjacent subdomains are misorientated with respect to each other by a misorientation angle/axis of 90°/&lt;a&gt;. (3) Within each subdomain, palisade quartz grain boundaries commonly have intra‐ and inter‐granular misorientations of 60°/[0001], consistent with the dauphiné twin law. Our observations imply that the coesite‐quartz transformation is crystallographically controlled by the epitaxial nucleation of palisade quartz on the former coesite grain, specifically on potential coesite twin planes such as (01) and (021).

Epithermal clast coating inside the rock avalanche-debris flow deposits from Mount Meager Volcanic Complex, British Columbia (Canada)

The observational and semi-quantitative sedimentological analyses of the lithofacies assemblage contribute to describe at different scales the breccia matrix along the sheared and transformed contact between rock avalanche deposits and trailing debris flow deposits emplaced by a large landslide at Mt. Meager volcano in British Columbia in 2010. An inverted cataclastic gradient of crushed breccias in a fluidized and mixed matrix implies a structurally controlled flow regime and rapid deposition. The coating matrix changed the initial polymodal distribution of the debris-flow lithofacies. Clast shape evolution helps to characterize the cataclastic sorting during transport and fluidized disaggregation. A plot of matrix percent against matrix/gravels helps to distinguish primary hot fracturing of about 61% from initial dilation and sheared fracturing involving ~22% matrix. Extensional disaggregation between 73 and 79% matrix is related to hydromagmatic fragmentation within an epithermal system. Clayey mineral assemblages identified by XRD patterns are related to colloidal aluminium gel, cataclastic shear bands, and quartz microstructures in epithermal breccia zones (pH = 2–3, 200–350 °C, <10–20 GPa).Microstructural analysis differentiates the inner rim of coated clasts from their border and the surrounding matrix in impact melt breccias. Sequential coating stages are inferred during the propagation of the shock wave with an oscillatory relative speed during the inter-seismic period. We differentiate: 1) shock faulting which contributes to the impacted quartz (10–35 GPa) in a devitrified matrix and pseudotachylite coating related to frictional melting at the margin of a conduit; 2) shock response (85 GPa) in epithermal vein with calcic spheroids, CO2 dissociation, and basaltic melt (70–101 GPa, >1500 °C); and 3) the secondary fracturing with flash heating and pressure pulse during cavitation (ΔP ~ 10 GPa, >1000–1500 °C), which generates pockets of partial melting, quartz spheroids, and a roll-over effect for the inner rim of coated clasts. The formation of impact melt breccias and the debris flow are related to the slowing elastic impact wave with an oscillatory relative speed during the inter-seismic period along a proximal strike-slip fault.This study helps identify how the proximal rock avalanche transformed into a highly mobile debris flow, larger examples of which pose a hazard to the town of Pemberton, at a distance of 65 km from Mt. Meager.

High-stress creep preceding coseismic rupturing in amphibolite-facies ultramylonites

Coeval pseudotachylytes (solidified melts produced during seismic slip) and mylonites are generally regarded as the geological record of transient seismic events during dominant ductile flow. Thermal runaway has been proposed as a model to explain the pseudotachylyte-mylonite association. In the Mont Mary unit (Western Alps), pseudotachylyte fault veins occur along the amphibolite-facies (ca. 550°C; 0.35 GPa) ultramylonitic foliation of paragneisses. These veins formed at the same metamorphic conditions of the ultramylonites, thus potentially recording thermal runaway. We analysed the microstructure of quartz in ultramylonite and of ultramylonite clasts in pseudotachylyte to investigate the possible occurrence of thermal runaway. Quartz aggregates show an evolution under constant temperature to ultrafine-grained recrystallised grain size (2.5 μm), reflecting creep under high differential stresses (> 200 MPa) and high strain rates (10−9 s−1), along very narrow foliation-parallel layers. In the ultrafine aggregates, viscous grain boundary sliding became dominant and promoted cavitation leading to disintegration of quartz aggregates and precipitation, in the pore space, of biotite, oriented parallel to the main ultramylonitic foliation. The strain rate-limiting process was aseismic fluid-assisted precipitation of biotite. The potential occurrence, at the deformation conditions of the Mont Mary ultramylonites, of thermal runaway in pure quartz layers was investigated by numerical modelling. The models predict a switch from stable flow to thermal runaway at background strain rates faster than 10−9 s−1 for critical differential stresses that are comparable to the brittle strength of rocks. Deformation of ultramylonites occurred close to the conditions for thermal runaway to occur, but based on the microstructural record we conclude that the Mont Mary pseudotachylyte-mylonite association is best explained by brittle failure, triggered by transients of high differential stress and strain rate causing a downward deflection of the brittle-ductile transition.

Preferred orientation distribution of shock-induced planar microstructures in quartz and feldspar.

Shocked quartz and feldspar grains commonly exhibit planar microstructures, such as planar fractures, planar deformation features, and possibly microtwins, which are considered to have formed by shock metamorphism. Their orientation and frequency are typically reported to be randomly distributed across a sample. The goal of this study is to investigate whether such microstructures are completely random within a given sample, or whether their orientation might also retain information on the direction of the local shock wave propagation. For this work, we selected samples of shatter cones, which were cut normal to the striated surface and the striation direction, from three impact structures (Keurusselkä, Finland, and Charlevoix and Manicouagan, Canada). These samples show different stages of pre‐impact tectonic deformation. Additionally, we investigated several shocked granite samples, selected at different depths along the drill core recovered during the joint IODP‐ICDP Chicxulub Expedition 364 (Mexico). In this case, thin sections were cut along two orthogonal directions, one parallel and one normal to the drill core axis. All the results refer to optical microscopy and universal‐stage analyses performed on petrographic thin sections. Our results show that such shock‐related microstructures do have a preferred orientation, but also that relating their orientation with the possible shock wave propagation is quite challenging and potentially impossible. This is largely due to the lack of dedicated experiments to provide a key to interpret the observed preferred orientation and to the lack of information on postimpact orientation modifications, especially in the case of the drill core samples.

Deformation conditions and kinematics of fault rocks in the Hatagawa Fault Zone and the Shajigami Shear Zone, Northeast Japan: Insights from calcite and quartz microstructures

Calcite grains are plastically deformed at lower temperatures than are quartz grains, which means that investigating the microstructures of both minerals should provide useful information regarding the deformation history of faults. We investigated the conditions of deformation and kinematics of fault rocks within the Hatagawa Fault Zone and the Shajigami Shear Zone, Northeast Japan, using deformation microstructures and crystallographic preferred orientations of quartz and calcite. The Hatagawa Fault Zone trends NNW–SSE, and the associated foliated granite cataclasites show sinistral shear. Limestone mylonite samples showing sinistral shearing display girdles of calcite c-axes close to the YZ plane and Z-maximum, suggesting relatively high and low temperatures of deformation, respectively. The Shajigami Shear Zone trends NE–SW, and quartz microstructures in granodiorite mylonites of the shear zone indicate dislocation creep at temperatures of 400–500 °C. The limestone mylonites have Z-maximum patterns of calcite c-axes. These fault rocks were deformed during the Cretaceous (before the Cenozoic 40° counterclockwise rotation of Northeast Japan) in a kinematic framework characterized by a shortening direction that rotated from N–S to NW–SE. The kinematics and deformation ages of the fault rocks correspond to a change in the direction of motion of the Izanagi Plate at c. 100 Ma.

Structural setting of a transpressive shear zone: insights from geological mapping, quartz petrofabric and kinematic vorticity analysis in NE Sardinia (Italy)

AbstractThe Posada–Asinara Line is a crustal-scale transpressive shear zone affecting the Variscan basement in northern Sardinia during Late Carboniferous time. We investigated a structural transect of the Posada–Asinara Line (Baronie) with the aid of geological mapping and structural analysis. N-verging F2 isoclinal folds with associated mylonitic foliation (S2) are the main deformation features developed during the Posada–Asinara Line activity (D2). The mineral assemblages and microstructures suggest that the Posada–Asinara Line was affected by a retrograde metamorphic path. This is also confirmed by quartz microstructures, where subgrain rotation recrystallization superimposes on grain boundary migration recrystallization. Crystallographic preferred orientation data, obtained using electron backscatter diffraction, allowed analysis of quartz slip systems and estimation of the deformation temperature, vorticity of flow and rheological parameters (flow stress and strain rate) during the Posada–Asinara Line activity. Quartz deformation temperatures of 400 ± 50 °C have been estimated along a transect perpendicular to the Posada–Asinara Line, in agreement with the syn-kinematic post-metamorphic peak mineral assemblages and the late microstructures of quartz. The D2 phase can be subdivided in two events: an early D2early phase, related to the metamorphic peak and low kinematic vorticity (pure shear dominated), and a late D2late phase characterized by a lower metamorphic grade and an increased kinematic vorticity (simple shear dominated). Palaeopiezometry and strain rate estimates associated with the D2late deformation event showed an intensity gradient increasing towards the core of the shear zone. The D2early deformation developed under peak temperature conditions, while the D2late event was active at shallower structural levels.

Late Miocene constrictional strain in the northern Apennines: A case study from the Barabarca metaconglomerate (Elba Island, Italy)

Finite strain analyses were performed on a deformed metaconglomerate from the Calamita Unit in the Island of Elba. The Calamita Unit is a synkinematic contact aureole that was shaped by Late Miocene contractional deformation coeval with high‐temperature metamorphism. The metaconglomerate occurs as an L‐tectonite in the footwall of a major thrust, entirely surrounded by S‐tectonites developed in schistose rocks. Object lineations, defined by the preferred orientation of clasts, trend subparallel to the stretching lineations in the associated rocks. Quartz microstructures registered ductile deformation of clasts by grain boundary migration to bulging recrystallization, suggesting temperature decrease during deformation. Rf/ϕ analyses were carried out on three metaconglomerate samples using quartzite clasts as markers. The finite strain data show that the metaconglomerates are strongly deformed in the constrictional field with K values between ~3 and ~7. The constrictional deformation registered by the metaconglomerate with respect to the surrounding metapelites, which likely deformed under plane strain, can be interpreted as the result of flow partitioning in rheologically heterogeneous sequences during deformation. These results suggest the presence of significant strain gradients in the Calamita Unit, strictly associated with heterogeneously distributed ductile shear zones.