Quartz and sheet-silicate preferred orientations of low symmetry, Pikikiruna Schist, New Zealand

The Pikikiruna Schist of Nelson, New Zealand, displays a fabric in which the patterns of quartz c-axes, the poles to planes of inequidimensional quartz grains, and the statistical maxima of poles to sheet-silicate cleavages are oblique to each other. The quartz c-axes patterns consist of type-1 and type-2 crossed-girdles. The triclinic fabric can be explained in terms of one complex rotational deformation of an essentially plane strain nature. Rotation of approximately 90° about the intermediate strain-axis was combined at a late stage with subsidiary rotations about the extension axis. The quartz c-axes patterns can be related to the kinematic framework rather than the finite strain-axes. On the other hand, the dimensional quartz preferred orientation may be closely related to the finite strain-axes, though the quantity of strain can not be measured because of recrystallisation.     

Deformation partitioning during folding and transposition of quartz layers

Banded iron formation (BIF) from the Quadrilátero Ferrı́fero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 °C). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (“wet”) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90° to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal 〈a〉 glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating c-axis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.     

Atypical textures in quartz veins from the Simplon Fault Zone

Samples of monomineralic quartz veins from the Simplon Fault Zone in southwest Switzerland and north Italy generally have asymmetric, single girdle c-axis patterns similar to textures measured from many other regions. Several samples have characteristically different textures, however, with a strong single c-axis maximum near the intermediate specimen axis Y (the direction within the foliation perpendicular to the lineation X) and a tendency for the other crystal directions to be weakly constrained in their orientation about this dominant c-axis maximum. This results in ‘streaked’ pole figure patterns, with an axis of rotation parallel to the c-axis maximum. These atypical samples also have a distinctive optical microstructure, with advanced recrystallization and grain growth resulting in a strong shape fabric (S_B) oblique to the dominant regional foliation (S_A), whereas typical samples have a strong S_A fabric outlined by very elongate, only partially recrystallized, ribbon grains. The recrystallized grains of the atypical samples are themselves deformed and show strong undulose extinction and a core-mantle recrystallization structure. The streaked texture is likely to be a direct consequence of lattice bending and kinking during heterogeneous slip on the favoured first-order prism (10 0) (a) system, the heterogeneity itself being due to problems in maintaining coherence across grain boundaries when insufficient independent easy-slip systems are available for homogeneous strain by dislocation glide. Such bending would be particularly prevalent in very elongate, thin ribbon grains, resulting in high internal strain energy and promoting recrystallization. Thus both the texture and the microstructure could be significantly modified by later strain increments affecting quartz grains with an already developed, nearly single-crystal texture.     

Piezoelectric effects in quartz-rich rocks

Piezoelectricity, a polarization of charge produced by an applied stress, occurs in many minerals. It is particularly strong in quartz. Aggregates of piezoelectric grains are themselves piezoelectric if the grains are suitably aligned. Such aggregates may be said to have a piezoelectric fabric. Thus quartz-rich rocks may possess a piezoelectric fabric and this paper discusses the various possible fabrics.To test whether a piezoelectric fabric might be detected in a quartz-rich rock, apparatus was built that hydraulically applied a sinusoidal stress to cubic specimens. The three resulting orthogonal polarizations of charge were measured via a charge amplifier. A specimen of pure quartz was used to verify the experimental method and to ensure that absolute piezoelectric moduli were being measured. Rocks with and without preferred orientation were tested. Of the latter types, those containing little or no free quartz (marble, basalt) did not exhibit measurable piezoelectric effects. However, all quartz-rich rocks (quartzites, granites, gneisses, mylonites) did show piezoelectric effects when stressed. These effects were in two categories

1. (1) effects due to piezoelectric fabrics, called true piezoelectric effects

2. (2) effects due to random distributions of the piezoelectric vectors, called statistical effects.

To distinguish between these two effects, three criteria were used. Firstly, the measured effects were compared with the expected statistical effect for a rock of that grain size and composition. Secondly, where possible, multiple specimens were cut from the one rock sample, all specimens with the same orientation. Specimens from a rock with a piezoelectric fabric should show similar results. Thirdly, the optically observed c-axis distribution and orientation was compared with the piezoelectrically predicted fabric and orientation.This paper shows that while most rocks gave results consistent with statistical effects from a non-polar or random distribution, some rocks exhibited a true piezoelectric effect due to fabric. This effect may be used, with some imprecision, to locate the a-axes and c-axes of quartz in the aggregate. The polarities of the a-axes are also obtained.     

Quartz [0001]-axes preferred orientation, Bluff, New Zealand: Origin elucidated by grain-size measurements

Permian volcanic sediments at Bluff have been strained and thermally metamorphosed by Permian intrusives to metasediments of hornblende—hornfels facies. Quartz, which crystallised as a secondary mineral during metamorphism, has an unusual preferred orientation with c-axes either forming paired maxima in the plane containing the lineation (=maximum principal strain axis = direction of extension) and the perpendicular to schistosity (=minimum principal strain axis = shortening direction) or a broad maximum parallel to the lineation; the paired maxima are approximately 30° either side of the lineation. Some quartz grains are markedly elongate parallel to the lineation, and according to hypotheses of preferred orientation involving crystal plasticity, there should be some correlation between the shape of such grains and their c-axis orientations. Grain-size and shape analysis of Bluff quartz demonstrate that no such correlation exists; the analyses show that the preferred orientation results from oriented nucleation in the residual stress field immediately following the bulk straining of the rocks, with the distribution of c-axes as predicted by Kamb's hypothesis (1959). The time relationships of rock deformation, thermal metamorphism, and nucleation and growth of quartz are discussed.     

High Temperature Mylonitization of Quartzofeldspathic Gneisses: Example from the Schirmacher Hills, East Antarctica

In the Schirmacher Hills, most of the ductile shearing took place under high to medium grade amphibolite facies metamorphism. The microstructure of the mylonites shows characteristic features of high temperature deformation and thus gives us an idea of deformation mechanisms of the constituent minerals at great crustal depth. The variation in microstructure of the sheared rock is partly due to heterogeneity of the intensity of strain from domain to domain, producing protomylonites, orthomylonites and ultramylonites. However, a large part of the microstructural variation has resulted from syn- to post-tectonic recrystallization and grain growth of constituent minerals. Both quartz and feldspar have deformed by crystal plastic processes with dominant grain boundary migration. The present aspect ratio of the feldspar grains is a result of various degrees of dynamic recrystallization along the grain boundary. The ratio varies between 1.5 and 2. Presence of exsolution lamellae in perthites and formation of myrmekite at the strained grains of K-feldspar suggest diffusion assisted dislocation creep. These mylonites are characterized by the presence of weakly strained or unstrained long quartz ribbons. The development of quartz ribbons with the absence of significant strain suggests grain recovery and grain growth during high temperature mylonitization. The growth of quartz ribbons took place by coalescing neighbouring grains both along and across the ribbon length. At the ultramylonite stage the fine-grained matrix of quartz and feldspar mostly accommodates the bulk strain.     

Simulation of fabric development in recrystallizing aggregates—II. Example model runs

This study investigates the role of the coupling of dynamic recrystallization and lattice rotations in fabric development. A new two-dimensional computer simulation of polycrystalline deformation is used, which combines homogeneous straining, internal lattice rotations and dynamic recrystallization processes. Five example runs of the simulation are described here, which compare the progressive development of fabrics resulting from different recrystallization regimes and different straining geometries. It is found that these fabrics can evolve significantly with progressive strain from one strong fabric pattern to another. The effect of recrystallization is not only to create point maxima concentrations of c-axes, but also to modify and create girdle distributions. Correlations are made between the fabrics generated by this model and previously reported quartz and ice fabrics.     

电子背散射衍射技术(EBSD)在组构分析中的应用和相关问题

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。     

秦岭商丹带沙沟糜棱岩带的显微构造及其(p)-T-t演化路径的再认识

对沙沟糜棱岩带的78个样品进行了显微构造与组构分析。石英以动态重结晶Ⅱ型条带为主,其C-轴组构型式为极密Ⅰ型,同时可见Ⅲ型石英条带残存。长石均显脆性碎裂变形,仅钾长石略具韧性变形。糜棱岩面理普遍绕过石榴石斑晶分布。存在多次后期脆性变形构造。这些显微构造与组构特征表明,该带糜棱岩化阶段处于中─高绿片岩相条件、并大致发生在晚白垩世以后。糜棱岩化阶段之前该带可能存在一个角闪岩相左行韧性剪切变形阶段。糜棱岩化阶段之后,该带直接进入脆性变形阶段。据此,笔者对前人有关沙沟糜棱岩带(p)-T-t演化路径提出修正意见。     

Garnet crystal plasticity in the continental crust,new example from south Madagascar

Garnet (10 vol.%; pyrope contents 34–44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain‐size reduction (from a few cm to ～300 μm). Electron back‐scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain‐size evolution. The garnet grain‐size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm‐sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and/or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grain‐size reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion‐assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (10¹⁰ m^?3 or lower). The rounded Type III garnet experiences rigid body rotation in fine‐grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain‐size‐ and shape‐dependent: dislocation creep is dominant for the few large grains left (>1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 μm or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P–T conditions of garnet plasticity in the continental crust (≥950 °C; 11 kbar) have been identified using two‐feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain‐size decrease in a fine‐grained steady‐state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone.     

Construction of crossed girdles by superposing four subfabrics,each with a single maximum

Commonly, basal glide is the predominant deformation mechanism of quartz in tectonites. Therefore, local deformation is probably mostly progressive simple shear rotating the sheared domains as well as deforming them. If a tectonite body is constrained to be deformed irrotationally and approximately homogeneously throughout, it is necessarily traversed by closely spaced material surfaces that are approximately plane and orthogonal originally, and stay so through time. These surfaces act as internal boundaries and enforce cancellation of the rigid-body rotations of, in the general case, four distinct families of domains, with slip planes and directions mutually mirror-symmetric. The overall symmetry of the fabric is orthorhombic, with the mirror planes coinciding with the principal planes of strain. Certain grains with basal planes in favorable orientation for one of the four ideal simple shears could initiate the deformation, and because of the need for compatibility, entrain neighboring grains into a similar strain, making the surroundings of an initiating grain a shear zone. Compatibility also requires thec-axes of grains in a domain to be rotated progressively toward the direction of maximum shortening. If the original orientation of crystallographic axes was random, domains of one family thus acquire a fabric with a single maximum, and the four resulting fabrics with single maxima combine to form crossed-girdle patterns. Depending on the orientation of the average shear planes and slip directions in the four families, the crossed girdles can be of different types; most fabric types that have been observed in quartz tectonites can be obtained by superposition. Crossed-girdle fabrics with low symmetry result from non-coaxial strain histories.     

Microstructures of deformed grains in the augen gneisses of southern Menderes Massif (western Turkey) and their tectonic significance

A detailed fabric and microstructural analysis of the granitic mylonites was carried out on the southern side of Bes,parmak Mountain north of Selimiye (Milas). The mylonitic augen gneisses have?a blastomylonitic texture characterized by large retort-shape porphyroclasts or augen of feldspars, around which a more ductile, medium to fine-grained matrix of muscovite, biotite, quartz and feldspar is deflected. Feldspars behave in both plastic and brittle fashion, because size reduction occurs through grain boundary migration and/or subgrain rotation, and also through fracturing. Typical “core-and-mantle” structure, characterized by a large feldspar core surrounded by a mantle of fine recrystallized grains, is very characteristic. The majority of plagioclase twins obey the albite-twin law; however, the association with pericline-law twinning suggests that many of the twins are mechanical. Evidence of strain, such as deformation twins, bent or curved twins, undulatory extinction, deformation bands and kink bands occur characteristically in plagioclase. Myrmekite is ubiquitous at K-feldspar grain boundaries, most notably on the long sides of inequant grains parallel to the S-foliation direction, which invariably face the maximum finite shortening direction. Deformation of quartz in mylonitic augen gneisses commonly results in the development of core-and-mantle structure and “type-4” quartz ribbons of elongated, preferably oriented, newly recrystallized quartz aggregates suggesting a primary dynamic recrystallization. Undulatory extinction, deformation bands and lamellae are the strain-related features associated with quartz porphyroclasts. Micas, especially biotite, undergo internal deformation by bend gliding and kinking. Most of the micas are completely attenuated and aligned such that their (001) planes are subparallel or parallel to the margins of quartz ribbons and define the foliation in the rock. These microstructures of feldspars, quartz and mica in the mylonitic augen gneisses in this part of the southern Menderes Massif are broadly consistent with fabric development under upper-greenschist- to lower-amphibolite-facies conditions, rather than almandine–amphibolite facies, as was previously believed. This supports the previous contention of the authors that the protoliths of augen gneisses are younger granitoids and do not represent an exposed Precambrian Pan-African basement in the Menderes Massif.     

Development of quartz ribbons in quartzofeldspathic granulites

In high-grade (granulite facies) quartzofeldspathic rocks the progressive development of a fabric records contrasting deformation behaviour of quartz and feldspar. Feldspar has undergone deformation mainly by recrystallization-accommodated dislocation creep and produced smaller recrystallized grains progressively in the course of deformation. Quartz has not deformed solely by dislocation creep but also by a diffusion-controlled mechanism. Dislocation climb is important in the dislocation creep of quartz. In contrast to feldspar, quartz grains have not recrystallized into smaller grains at any stage of deformation. Rather, they have transformed initially to short monocrystalline ribbons and ultimately to long polycrystalline ribbons. This textural change of quartz is a continuous process and has taken place in the course of bulk textural change of the rocks during the deformation.     

Preferred orientation in experimentally deformed limestone

Over sixty syntectonic deformation experiments in uniaxial compression have been done on fine-grained limestones in the stability fields of calcite I, calcite II and aragonite. X-ray techniques and spherical harmonic analysis of the data were used to determine preferred orientation quantitatively, and inverse pole-figures were derived for these axially symmetric specimens. They display in most cases strong preferred orientation which varies as a function of the experimental conditions, mainly temperature and pressure. At temperatures below 350° C recrystallization is lacking and flattened grains indicate that translation, twin gliding and kinking have been the dominant deformation mechanisms. The inverse pole-figure shows a maximum at c with a shoulder towards or a second maximum at e. This is in agreement with preferred orientation observed in experimentally deformed Yule marble and can be explained as the product of dominant twin gliding on e and translation gliding on r (Turner et al., 1956). At high temperatures (900–1000° C) strong grain growth (from 4 to 50 microns) indicates that the fabric recrystallized. Grains are equidimensional and clear with a marble-like texture. The inverse pole-figure shows a single maximum at r, and c-axes are oriented in a small circle around the axis of compression, ₁. Such a pattern of preferred orientation would be expected on thermodynamic grounds assuming that recrystallized grains will be oriented in such a way that the strain energy is a maximum (e.g. MacDonald, 1960). Decrease in confining pressure caused a decrease of the maximum at c and the formation of a secondary maximum at highangle positive rhombs in the inverse pole-figure. This can be interpreted as r translation dominating over e twinning. In all deformation experiments an equilibrium in preferred orientation was reached after 20 percent shortening. The strength of preferred orientation decreased with increasing temperature. Aragonite was produced within its hydrostatic stability field at temperatures above 500° C. Close to the phase boundary, coarse-grained textures showed preferred orientation with poles to (010) parallel to ₁. At higher pressures the fabric is fine-grained and [001] is aligned parallel to ₁. Evidence is given that the phase change from calcite to aragonite in these deformation experiments is a diffusive and not a martensitic transformation.Publication No. 1043, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California.     

Quartz microfabric of the Laxfordian Canisp Shear Zone,NW Scotland

The Canisp Shear Zone transects layered Lewisian gneisses near Lochinver, NW Scotland. It is a vertical ductile shear zone with a dextral shear sense, formed during Laxfordian amphibolite facies metamorphism, transposing the layering to new foliation and linear structures. Minerals in the layered gneisses show little or no shape fabric, while a strong shape fabric defines the foliation. For quartz, this shape fabric is accompanied by development of a preferred crystal orientation with fabric patterns reflecting the geometry of the shear deformation. The quartz fabric shows a pole-free area around the lineation with the c-axes concentrated in an asymmetric cross-girdle or a point maximum perpendicular to the shear plane, and a monoclinic symmetry consistent with the shear sense.     

Grain growth kinetics and the effect of crystallographic anisotropy on normal grain growth of quartz

Annealing experiments on agate were performed to investigate grain growth kinetics and the effect of crystallographic anisotropy on normal grain growth of quartz. The experiments were conducted using a piston-cylinder apparatus at 700–800°C and 0.5 GPa for 0–66 h. The grain growth rate was expressed by D ⁿ −D ₀ ⁿ  = kt with k = k ₀ exp(−H*/RT) where D ₀ is the initial grain size at t = 0, with n = 4.4 ± 0.3, and H* = 191.3 ± 11.0 kJ/mol is the activation enthalpy and logk ₀  = 19.8 ± 1.4. While the grain aspect ratios are nearly constant at ~0.7 (short/long) during grain growth, the longest axis in individual grains tends to be oriented parallel to their c-axis, indicating that a primary crystal-preferred orientation of c-axis of the agate could result in the development of a weak shape-preferred orientation during grain growth.     

A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz

Water content of quartz in and around a greenschist facies mylonitic shear zone located in the western Adirondacks was analyzed by micro-FTIR spectroscopy. The shear zone is within a pegmatitic dike, which cuts across a granitic gneiss. The thickness of the shear zone varies along strike from 15 cm wide and encompassing all of the pegmatite dike at its northern most exposure to 5 cm wide approximately 10 m south, along strike. Microstructures, including quartz ribbons and recrystallized grains, indicate quartz and feldspar within the mylonite underwent dislocation creep. Infrared spectral analysis was carried out using a Nicolet micro-FTIR on mylonitic quartz ribbons, pegmatitic quartz and gneissic quartz. A small aperture size (56 μm by 50 μm) for the IR beam allowed optically clear regions of the quartz grains to be analyzed without any contribution from grain boundaries. The smallest dimension of the quartz ribbons is 0.3 mm, whereas the pegmatitic quartz has a grain size of 3 to 5 cm. Results show mylonitic quartz ribbons contain the most water (320 H:10⁶ Si average, range of 50 to 1120 H:10⁶ Si); pegmatite quartz contains much less water (30 H:10⁶ Si average, range of 20–40 H:10⁶ Si) and the gneissic quartz contained an intermediate amount (200 H:10⁶ Si average, range of 20 to 870 H:10⁶ Si). These data indicate that water was preferentially incorporated into the deformed quartz ribbons.     

Domainal and fabric heterogeneities in the Cap de Creus quartz mylonites

A characteristic domainal configuration is reported for both micro-structures and c-axis fabrics in the Cap de Creus pure quartz mylonites as displayed in 50 samples from the centres of different shear zones. Three types of domains are found a, b and c. Each domain has a distinct c-axis orientation pattern. These three fabric elements, also labelled a, b and c make up the total fabric. c-axis fabrics are symmetric or asymmetric with respect to the main mylonitic foliation depending on the presence or absence of the b domain and its fabric element. The boundaries of the domains are parallel to the main mylonitic foliation. Two domain types, a and b display an internal foliation defined by preferred grain boundary alignment parallel to the direction of optical orientation within the domain. The internal foliations are oblique to the main mylonitic foliation in two different senses giving the sample a herring-bone appearance. These internal foliations are shown to be related to extensional crenulations. Domains are not produced by host-controlled recrystallization. The fabric elements and corresponding domains are the expression of kinematic heterogeneities on the scale of the thin section.     

Microstructure and fabric development in ice: Lessons learned from in situ experiments and implications for understanding rock evolution

In this contribution we present a review of the evolution of microstructures and fabric in ice. Based on the review we show the potential use of ice as an analogue for rocks by considering selected examples that can be related to quartz-rich rocks. Advances in our understanding of the plasticity of ice have come from experimental investigations that clearly show that plastic deformation of polycrystalline ice is initially produced by basal slip. Interaction of dislocations play an essential role for dynamic recrystallization processes involving grain nucleation and grain-boundary migration during the steady-state flow of ice. To support this review we describe deformation in polycrystalline ‘standard’ water-ice and natural-ice samples, summarize other experiments involving bulk samples and use in situ plane-strain deformation experiments to illustrate the link between microstructure and fabric evolution, rheological response and dominant processes. Most terrestrial ice masses deform at low shear stresses by grain-size-insensitive creep with a stress exponent (n ≤ 3). However, from experimental observations it is shown that the distribution of plastic activity producing the microstructure and fabric is initially dominated by grain-boundary migration during hardening (primary creep), followed by dynamic recrystallization during transient creep (secondary creep) involving new grain nucleation, with further cycles of grain growth and nucleation resulting in near steady-state creep (tertiary creep). The microstructural transitions and inferred mechanism changes are a function of local and bulk variations in strain energy (i.e. dislocation densities) with surface grain-boundary energy being secondary, except in the case of static annealing. As there is a clear correspondence between the rheology of ice and the high-temperature deformation dislocation creep regime of polycrystalline quartz, we suggest that lessons learnt from ice deformation can be used to interpret polycrystalline quartz deformation. Different to quartz, ice allows experimental investigations at close to natural strain rate, and through in-situ experiments offers the opportunity to study the dynamic link between microstructural development, rheology and the identification of the dominant processes.     

Dynamic recrystallization and fabric development during the simple shear deformation of ice

In situ observations of polycrystalline ice deformed in simple shear between −10 and −1°C are presented. This study illustrates the processes responsible for the deformation, the development of a preferred crystallographic orientation and the formation of a preferred dimensional orientation. Intracrystalline glide on the basal plane, accompanying grain rotations and dynamic recrystallization, helps to accommodate the large intragranular strains. These are the most important mechanisms for crystallographic reorientation and produce a stable fabric that favours glide on the basal plane. Localized kinks, developed in grains unfavourably oriented for easy glide, are unstable and are overprinted by dynamic recrystallization. Dynamic recrystallization is a strain softening process with nucleation occurring in the form of equiaxed grains that grow subparallel to pre-existing grain anisotropies and become elongate during deformation. Plots of grain axial ratio against orientation ( ) indicate a weak shape fabric which does not correspond to the theoretical foliation and elongation for the appropriate increment of shear strain. We argue that estimates of the strain magnitude made from orientation of elongate grains are unreliable in high temperature shear zones. These results are applicable to both geological and glacial shear environments.