The effects of quartz recrystallization and reaction on weak phase interconnection,strain localization and evolution of microstructure

We conducted axial compression and general shear experiments, at T = 900 °C and P = 1.5 GPa, on samples of banded iron formation (BIF) and synthetic aggregates of quartz, hematite and magnetite to investigate how dynamic recrystallization of quartz promotes strain localization, and the role of weak second phases (oxides) on the rheology and microstructural evolution of the aggregates. Experiments showed strain localization into oxide rich layers, and that the oxide content and oxide distribution are key factors for the strength of the aggregate. Only 2–10 wt.% hematite leads to pronounced weakening and increasing hematite content above ∼10% has only a minor additional effect. Where oxide grains are dispersed, the initial strength contrast with quartz induces stress concentrations at their tips, promoting high stress recrystallization-accommodated dislocation creep of quartz. Fine recrystallized quartz reacts with oxide, forming trails of fine reaction product (ferrosilite/fayalite) leading to the interconnection/percolation of a weaker matrix. The strength contrast between the quartz framework and these fine-grained trails promotes strain localization into micro-shear zones, inducing drastic strain weakening. Thus dynamic recrystallization of quartz promotes syn-deformational reactions leading to a microstructurally-controlled evolution of phase strength contrast. It results in a rheologic transition from load-bearing framework to a matrix-controlled rheology, with transition from S–C′ to S–C fabric with increasing strain.     

Microstructural analysis and P–T conditions of the Azul megashear zone, Tandilia, Buenos Aires province, Argentina

A microstructural analysis was carried out on mylonitic rocks of the Azul megashear zone (AMSZ), Tandilia, which were formed in a range of metamorphic conditions from lower greenschist to amphibolite facies. Tailed porphyroclasts are common and mostly symmetric. Scarce asymmetric rotated porphyroclasts show both sinistral and dextral senses of shear. In sections parallel to the mylonitic foliation, porphyroclasts are round. The AMSZ is probably related to the late Transamazonian orogenic cycle and may be due to NNE–SSW-directed convergence. In weakly deformed protolith and protomylonites, quartz deforms by dynamic recrystallization, mainly subgrain rotation in dislocation creep Regime 2. K-feldspar porphyroclasts and plagioclase show scarce fracturation and deform by dynamic recrystallization along grain boundaries. Quartz microstructures in mylonites indicate predominantly Regime 3 grain boundary migration recrystallization. Feldspar structures indicate recrystallization through the nucleation and growth of new grains at grain boundaries. The temperatures of deformation from mineral assemblages in the CNKFMASH system in four bulk compositions are in the range of 400–450 °C, and the pressures are more than 6 kb.     

On the inter-relationship between grain size sensitive creep and dynamic recrystallization of olivine

The recent theoretical results of Twiss (1976) and Goetze (1978) suggest that at high temperatures and sufficiently high stresses the creep behavior of dry olivine should be dominated by either nonlinear diffusion accommodated grain-boundary sliding or nonlinear Coble creep mechanisms. This would result following the production of a fine grain-size by dynamic recrystallization. For the high-temperature experimental work performed by Karato et al. (1982) dry single crystals of olivine were almost totally recrystallized during creep, and temperature changing experiments were performed on the resulting dynamically recrystallizing polycrystalline aggregates. However, the activation energy for creep determined by Karato et al. (1982) was far higher than that predicted by the models of Twiss (1976) or Goetze (1978), although the conditions required for operation of at least the model of Twiss (1976) apparently were satisfied. The data for the highly recrystallized specimens from the higher stress, lower temperature experiments of Zeuch and Green (1979) and Zeuch (1980) are in good agreement with the results of Karato et al. (1982). These latter experiments were conducted under conditions where either the model of Twiss (1976) or Goetze (1978) should have been applicable. I tentatively conclude that although fine grain sizes were produced in the experiments of Karato et al. (1982), Zeuch and Green (1979) and Zeuch (1980) by dynamic recrystallization, there is no evidence for a transition to grain-boundary diffusional creep mechanisms at high or low stresses despite the predictions of Twiss (1976) or Goetze (1978). Instead, deformation is dominated by dislocation movement with recovery by dynamic recrystallization.     

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.     

The microstructure of polar ice. Part II: State of the art

An important feature of natural ice, in addition to the obvious relevance of glaciers and ice sheets for climate-related issues, is its ability to creep on geological time scales and low deviatoric stresses at temperatures very close to its melting point, without losing its polycrystalline character. This fact, together with its strong mechanical anisotropy and other notable properties, makes natural ice an interesting model material for studying the high-temperature creep and recrystallization of rocks in Earth's interior. After having reviewed the major contributions of deep ice coring to the research on natural ice microstructures in Part I of this work (Faria et al., 2014), here in Part II we present an up-to-date view of the modern understanding of natural ice microstructures and the deformation processes that may produce them. In particular, we analyze a large body of evidence that reveals fundamental flaws in the widely accepted tripartite paradigm of polar ice microstructure (also known as the “three-stage model,” cf. Part I). These results prove that grain growth in ice sheets is dynamic, in the sense that it occurs during deformation and is markedly affected by the stored strain energy, as well as by air inclusions and other impurities. The strong plastic anisotropy of the ice lattice gives rise to high internal stresses and concentrated strain heterogeneities in the polycrystal, which demand large amounts of strain accommodation. From the microstructural analyses of ice cores, we conclude that the formation of many and diverse subgrain boundaries and the splitting of grains by rotation recrystallization are the most fundamental mechanisms of dynamic recovery and strain accommodation in polar ice. Additionally, in fine-grained, high-impurity ice layers (e.g. cloudy bands), strain may sometimes be accommodated by diffusional flow (at low temperatures and stresses) or microscopic grain boundary sliding via microshear (in anisotropic ice sheared at high temperatures). Grain boundaries bulged by migration recrystallization and subgrain boundaries are endemic and very frequent at almost all depths in ice sheets. Evidence of nucleation of new grains is also observed at various depths, provided that the local concentration of strain energy is high enough (which is not seldom the case). As a substitute for the tripartite paradigm, we propose a novel dynamic recrystallization diagram in the three-dimensional state space of strain rate, temperature, and mean grain size, which summarizes the various competing recrystallization processes that contribute to the evolution of the polar ice microstructure.     

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.     

Evidence for dominant grain-boundary sliding deformation in greenschist- and amphibolite-grade polymineralic ultramylonites from the Redbank Deformed Zone, Central Australia

Microstructural and textural investigations by scanning (SEM) and transmission electron microscopy (TEM) techniques have been performed on samples taken across two quartzo-feldspathic mylonite zones from the Redbank Deformed Zone, Central Australia. One has been deformed at greenschist-facies (GS), the second at amphibolite-facies (Am), conditions. With increasing strain the rock type changes from protomylonite to mylonite to ultramylonite. The protomylonites and mylonites consist of alternating quartz and polymineralic quartz-feldspar bands. At the highest strains a homogeneous, fine-grained polymineralic ultramylonite occurs. Shear-zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites. TEM and SEM studies of the ultramylonites reveal a rectangular to square grain shape, a continuous alignment of grain and interphase boundaries across several grain diameters, a grain size (GS 0.5 μm; Am 5–11 μm) less than the equilibrium subgrain size, and open and void-containing grain and interphase boundaries. Analysis of local textures by electron back-scatter diffraction (EBSD) in the SEM showed a very weak crystallographic preferred orientation (CPO) for the quartz. The grain misorientation relationships are not consistent, with dislocation creep being the dominant deformation mechanism. All structures are of the type expected if grain-boundary sliding processes had contributed significantly to the deformation. Consequently, the deformation of such quartzo-feldspathic rocks, and by implication the rheology of the Redbank Deformed Zone, must have been controlled by the mechanical properties of these fine-grained polymineralic ultramylonites, deforming by grain-boundary sliding processes. This is in contrast to the pure quartz bands which deformed by dislocation-creep mechanisms and were less important in the rheology of the Redbank Deformed Zone.     

阿尔泰造山带内韧性剪切带的显微、超显微构造和变形机制

阿尔泰造山带的剪切带中发育大量S-C糜棱岩,其内云母产生云母鱼和扭折构造并发生边界重结晶。长石发生不同性质的系列变形,并以出熔成核为主的动态重结晶为主要变形机制,同时伴有强烈的扩散作用,属Naborrow-Herring蠕变。石英的变形呈条带状并发生完全的旋转和边界迁移动态重结晶。石英晶内变形以位错运动和动态恢复为主,形成亚颗粒构造及动态重结晶,以致产生应变弱化和超塑性变形,定量分析确定的石英变形机制主要为低温幂指数蠕变。     

Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress

The microfabrics of folded quartz veins in fine‐grained high pressure–low temperature metamorphic greywackes of the Franciscan Subduction Complex at Pacheco Pass, California, were investigated by optical microscopy, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. The foliated host metagreywacke is deformed by dissolution–precipitation creep, as indicated by the shape preferred orientation of mica and clastic quartz without any signs of crystal‐plastic deformation. The absence of crystal‐plastic deformation of clastic quartz suggests that the flow stress in the host metagreywacke remained below a few tens of MPa at temperatures of 250–300 °C. In contrast, the microfabric of the folded quartz veins indicates deformation by dislocation creep accompanied by subgrain rotation recrystallization. For the small recrystallized grain size of ～8 ± 6 μm, paleopiezometers indicate differential stresses of a few hundred MPa. The stress concentration in the single phase quartz vein is interpreted to be due to its higher effective viscosity compared to the fine‐grained host metagreywacke deforming by dissolution–precipitation creep. The fold shape suggests a viscosity contrast of one to two orders of magnitude. Deformation by dissolution–precipitation creep is expected to be a continuous process. The same must hold for folding of the vein and deformation of the vein quartz by dislocation creep. The microfabric suggests dynamic recrystallization predominantly by subgrain rotation and only minor strain‐induced grain boundary migration, which requires low contrasts in dislocation density across high‐angle grain boundaries to be maintained during climb‐controlled creep at high differential stress. The record of quartz in these continuously deformed veins is characteristic and different from the record in metamorphic rocks exhumed in seismically active regions, where high‐stress deformation at similar temperatures is episodic and related to the seismic cycle.     

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.     

苏鲁榴辉岩在俯冲峰期-折返早期的递进塑性变形过程及对超高压变质岩石折返动力学的启示

榴辉岩的变形特征严重影响俯冲板片的流变学行为,而石榴石的变形机制又直接制约着榴辉岩的变形属性。本文在结合前人研究的基础上,通过对中国大陆科学钻探（CCSD）主孔榴辉岩中石榴子石的典型变形特征的详细对比和分析,进一步证明了榴辉岩在俯冲峰期条件下可以发生强烈的塑性变形。且俯冲峰期-折返早期经历了从以恢复作用主导的位错蠕变,到动态重结晶、扩散蠕变,再到静态重结晶和矿物生长的连续递进变形过程。其中,从动态重结晶到高温静态重结晶的转换,结合短时增温、局部熔融等证据,共同证明了在苏鲁超高压变质岩石折返过程中存在一个短暂的低应力“热”区间,在此前后,折返机制和驱动力发生了转变,由浮力作用下的快速折返转为以挤压透入性剪应力作用下的缓慢抬升阶段。     

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.     

Crystal plasticity and superplasticity in quartzite; A natural example

Plastically deformed quartzites from the Betic Movement Zone (Betic Cordilleras, Spain) exhibit microstructures indicative of crystal plasticity on a mineral grain scale. Quartzites with dynamically recrystallized grain sizes larger than 10 μm have strong crystallographic preferred orientations, narrow grain boundaries, little creep damage, and an inverse proportionality of dislocation density and grain size. Mylonites with grain sizes smaller than 10 μm have low crystallographic preferred orientations, wide grain boundaries (up to 1000 Å), abundant creep damage, and decreasing dislocation density with diminishing grain size. This is thought to reflect a clear-cut shift in deformational regimes from dislocation creep to superplastic flow at 10 μm grain size. Superplasticity can be acquired by quartzites which suffer dynamic recrystallization to grain sizes smaller than 10 μm during an initial dislocation creep stage. Dislocation motion is the major accomodating mechanism for strain incompatibilities that arise during grain-boundary sliding in the mylonites.It seems reasonable to estimate flow stresses from unbound dislocation densities and dynamically recrystallized grain sizes in the tectonite specimens. In the mylonites, dynamically recrystallized grain size probably reflects the stress magnitude before the shift in deformational mechanisms, and an estimate for late stage stresses is provided by unbound dislocation densities. In both deformational regimes the flow strength appears to depend on the extent of dynamic recrystallization.     

Deformation mechanisms and flow regimes in limestones from the Helvetic zone of the Swiss Alps

This paper is based on a combined field, transmission-electron (TEM) and transmission-optical (TOM) microscope study of limestones from the Helvetic zone (Swiss Alps) and discusses the deformation mechanisms and flow regimes that governed the deformation of these rocks.During pre-metamorphic regional ductile deformations the limestones deformed by power-law dislocation creep with differential stresses probably not exceeding 1 kbar. Dynamic recrystallization with grain-boundary sliding and grain-boundary migration allowed the grains to be less elliptical than the strain ellipse. A characteristic of the structure is the existence of dislocation-free subgrains. In the footwall of and approaching the Lochseiten calc-mylonite along the Glarus overthrust, grain-boundary sliding becomes more important (shift to diffusional creep or superplastic flow).During a syn- and post-metamorphic deformation, dynamic recovery seems to have become less competitive (no dislocation-free sub-grains), and along thrust faults twinning indicates a shift to higher differential stresses at the close of the deformation.It was not possible to separate these deformation phases on the basis of the dislocation debris. Sub-grain sizes as observed in TEM and TOM were identical.In limestones that underwent cataclastic deformation the rocks seem to have started breaking up along the grain boundaries. The new grain fragments are very small (0.1–0.3 μm) and are heavily twinned. In TEM the old large grains show very long straight glide dislocations, cleavage and, when shattered, ring patterns in diffraction.     

The transition between deformation regimes dominated by intercrystalline diffusion and intracrystalline creep evaluated by oxygen isotope thermometry

The dominant flow mechanism in tectonic processes depends on the rheological properties of geological materials and the physical conditions prevailing during deformation. We have evaluated the relative importance of intercrystalline diffusion and intracrystalline creep in crustal deformation in terms of temperature and grain size.Oxygen isotope thermometry has been used to elucidate the thermal environment obtaining during deformation and contemporaneous metamorphism of Dalradian rocks from Southwest Scotland. The temperature and grain size data, applied in conjunction with microstructural criteria for evaluating independent mechanisms of steady-state flow, allow recognition of a low-temperature deformation regime dominated by intercrystalline diffusion, and a high-temperature regime dominated by dislocation processes.The transition between the fields of intercrystalline diffusion and dislocation creep for quartz and calcite of 100 Mm grain size occurs at about 450° C and about 300° C, respectively. These empirically derived results are consistent with the temperature intervals over which intercrystalline diffusion and dislocation creep, respectively, are predicted to be dominant at geologically reasonable strain rates, as derived from theoretically formulated deformation mechanism maps for quartz and calcite.Grain growth may play an important role in delimiting the higher-temperature boundary of the intercrystalline diffusion field. Intercrystalline diffusion is the only deformation mechanism that involves mass transfer over distances that are large in relation to the grain size. This result has important consequences for geochemical transport phenomena.     

Ductile deformation of garnet in mylonitic gneisses from the Münchberg Massif (Germany)

Mylonitic gneisses from the Münchberg Massif contain single grains (type I) and polycrystalline aggregates (type II) of garnet displaying a distinct elongation parallel to a macroscopic lineation which is interpreted as the result of ductile deformation. Lattice-preferred orientations of quartz (textures) symmetrical to the macroscopic foliation and lineation and the lack of rotational microfabrics indicate that the bulk deformation was pure shear at least during the latest strain increments. Garnet textures measured by EBSD together with microprobe analyses demonstrate that these two structural types of garnet can be related to two different processes of ductile deformation: (1) For the single grains stretching can be attributed to diffusion creep along grain boundary zones (Coble creep). The related mass transfer is indicated by the fact that primary growth zones are cut off at the long faces of the grains while the related strain shadow domains do not show comparable chemical zoning. Pressure solution and precipitation suitable to produce similar structures can be largely ruled out because retrogressive reactions pointing to the presence of free hydrous fluids are missing. (2) For the polycrystalline garnet aggregates consisting of cores grading into fine-grained mantles, dislocation creep and associated rotation recrystallization can be assumed. Continuous lattice rotation from the core to the outer polycrystalline rim allow a determination of the related dominant slip systems which are {100}<010> and equivalent systems according to the cubic lattice symmetry. The same holds for garnets which appear to be completely recrystallized. For this type of fine-grained aggregates an alternative nucleation model is discussed. Due to penetrative dislocation glide in connection with short range diffusion and the resulting lattice rotation, primary growth zones are strongly disturbed.Since for the considered rock unit of the Münchberg Massif peak metamorphic temperatures between 630 and 670 °C can be assumed, this study clearly demonstrates that the inferred processes of ductile garnet deformation can occur not only in HT regimes as often suggested in the literature even if embedded within a matrix of “low-strength” minerals like quartz, feldspars and micas.     

Fabric and Deformation of Omphacite in Dabie Ultra-high-pressure Ecologites

The rheological characters of omphacites in Dabie ultra-high-pressure eclogite have been studied interms of fabric, dislocation and micro-structures. 1. The eclogite has undergone high-temperature deformation, thus forming omphacite lattice preferred orientation. In addition to creep dislocation, the omphacite ductile deformation may have other mechanisms, such as diffusion creep and grain boundary migration. 2. The main-phase deformation of eclogite is coaxial, but asymmetry strain also exists due to strain partitioning in the Dabie erogenic belt. 3.The twin measured by the universal stage is (100), indicating that omphacite high-T deformation was superimposed by low-T deformation. 4. Subgrain structure is common in omphacite, but the deformation features of the omphacites in the Shuanghe area and Bixiling area are different, the latter being dominated by dynamic recrystallization. 5. The Flinn plots show that the strain of omphacite belongs to the constriction ellipsoid and stretching strain, which is     

Mechanisms and orientation of melt segregation paths during pure shearing of a partially molten rock analog (norcamphor–benzamide)

In-situ deformation experiments were performed on partially molten analog materials (norcamphor in the presence of a benzamide–norcamphor melt) undergoing pure shearing at a constant melt fraction of 0.13. Melt in the samples induces a strain-dependent transition from purely dislocation creep to dislocation creep associated with minor intergranular fracturing and grain boundary sliding (GBS). Intergranular fractures drain the melt from initially isotropic melt pockets to grain boundaries. Along such boundaries, grain-boundary migration recrystallization is inhibited, while GBS occurs. Intergranular melt pockets occur along grain boundaries oriented subparallel to the shortening direction, but melt must have migrated parallel to the elongation direction of the samples, as indicated by melt accumulations at both extruding ends of the sample. Intergranular melt pockets parallel to the elongation direction were only rarely observed, because melt was rapidly expelled from these sites. Nevertheless, these grain boundaries are the pathways of melt segregation in the samples.     

Variscan deformation,microstructural zonation and extensional exhumation of the Moravian Cadomian basement

Abstract

The Cadomian Dyje Batholith, in the foot–wall of the Variscan Moravian nappe pile, has been involved in Variscan ductile deformation. The Cadomian Brunovistulian rocks were obliquely underthrusted during Carboniferous dextral transpression.

Strain intensity is inversely proportional to the distance from the contact of the Variscan thrust front. The microstructures of deformed granodiorites and quartz–diorites show a characteristic zonality marked by relatively high temperature flow in the west (550–580 °C) characterized by dynamic recrystallization of feldspars and grain boundary migration recrystallization of quartz. The size of quartz grains decreases with decreasing strain towards the east. At the easternmost part of the autochthonous Dyje massif, fracturing of feldspar and subgrain rotation recrystallization of quartz predominate. Flow stress estimates calculated from recrystallized quartz grain size show a regional increase of stress intensity from the highly strained margin towards the less deformed core of the Dyje massif. This microstructural zonation is oblique with respect to the major thrust boundary and corresponds roughly to metamorphic isogrades. The microstructural zonation reflects underthrusting of the Brunovistulian domain below the Moldanubian nappe.

The main ductile tectonic event D₁ is followed by a retrogressive brittle–ductile and brittle deformation D₂. D₂ results in the development of shear zones and faults superimposed on the D₁ mylonite fabric. D₂ is related to extension oblique to the D₁ fabric, associated with detachment and the westward movement of the Moravian nappes. © Elsevier, Paris     

Ductile faulting, dynamic recrystallization and grain-size-sensitive flow of olivine

Twiss (1976) has suggested that the “ductile faulting” events observed by Post (1973) during high temperature creep of dunite are due to a transition from creep by dislocation movement to a diffusion accommodated, grain-boundary sliding mechanism following a reduction in grain size by dynamic recrystallization. Similarly, Goetze (1978) has explained both ductile faulting and water weakening of dunite by transition to a “nonlinear Coble” creep mechanism. However, the fundamental assumption made by Twiss (1976) that the stress exponent, n, reduces to unity during ductile faulting events is questionable. If the stress exponent remains high, (n≥3), then a diffusion-accomodated grain-boundary sliding mechanism is excluded. “Nonlinear Coble” creep would remain a viable alternative; however, this model fails to adequately explain the water weakening phenomenon, and the available data do not constrain us to this model. Assuming that the water-weakening phenomenon can be explained by other models (e.g., Blacic, 1972), it will be shown (by analogy with the behavior of metals) that a third model, also consistent with the available data, also qualitatively explains the observations associated with ductile faulting without appeal to a transition in creep mechanisms. The model is similar to one for metals undergoing deformation by dislocation movement and recovery by dynamic recrystallization, which commonly exhibit behavior virtually identical to that observed in dunite during ductile faulting events without transition to grain-size-sensitive creep mechanisms.