High-temperature creep in a Ni2GeO4: a contribution to creep systematics in spinel

 High-temperature creep behavior in Ni₂GeO₄ spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4 microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to 1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following flow laws: where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion (Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less than 2 μm. Received: 18 June 2000 / Accepted: 3 April 2001     

The mechanical and microstructural behaviour of calcite-dolomite composites: An experimental investigation

The styles and mechanisms of deformation associated with many variably dolomitized limestone shear systems are strongly controlled by strain partitioning between dolomite and calcite. Here, we present experimental results from the deformation of four composite materials designed to address the role of dolomite on the strength of limestone. Composites were synthesized by hot isostatic pressing mixtures of dolomite (Dm) and calcite powders (% Dm: 25%-Dm, 35%-Dm, 51%-Dm, and 75%-Dm). In all composites, calcite is finer grained than dolomite. The synthesized materials were deformed in torsion at constant strain rate (3 × 10⁻⁴ and 1 × 10⁻⁴ s⁻¹), high effective pressure (262 MPa), and high temperature (750 °C) to variable finite shear strains. Mechanical data show an increase in yield strength with increasing dolomite content. Composites with <75% dolomite (the remaining being calcite), accommodate significant shear strain at much lower shear stresses than pure dolomite but have significantly higher yield strengths than anticipated for 100% calcite. The microstructure of the fine-grained calcite suggests grain boundary sliding, accommodated by diffusion creep and dislocation glide. At low dolomite concentrations (i.e. 25%), the presence of coarse-grained dolomite in a micritic calcite matrix has a profound effect on the strength of composite materials as dolomite grains inhibit the superplastic flow of calcite aggregates. In high (>50%) dolomite content samples, the addition of 25% fine-grained calcite significantly weakens dolomite, such that strain can be partially localized along narrow ribbons of fine-grained calcite. Deformation of dolomite grains by shear fracture is observed; there is no intracrystalline deformation in dolomite irrespective of its relative abundance and finite shear strain.     

Dynamic recrystallization of wet synthetic polycrystalline halite: dependence of grain size distribution on flow stress, temperature and strain

It is often observed that dynamic recrystallization results in a recrystallized grain size distribution with a mean grain size that is inversely related to the flow stress. However, it is still open to discussion if theoretical models that underpin recrystallized grain size–stress relations offer a satisfactorily microphysical basis. The temperature dependence of recrystallized grain size, predicted by most of these models, is rarely observed, possibly because it is usually not systematically investigated. In this study, samples of wet halite containing >10 ppm water (by weight) were deformed in axial compression at 50 MPa confining pressure. The evolution of the recrystallized grain size distribution with strain was investigated using experiments achieving natural strains of 0.07, 0.12 and 0.25 at a strain rate of 5×10⁻⁷ s⁻¹ and a temperature of 125 °C. The stress and temperature dependence of recrystallized grain size was systematically investigated using experiments achieving fixed strains of 0.29–0.46 (and one to a strain of 0.68) at constant strain rates of 5×10⁻⁷–1×10⁻⁴ s⁻¹ and temperatures of 75–240 °C, yielding stresses of 7–22 MPa. The microstructures and full grain size distributions of all samples were analyzed. The results showed that deformation occurred by a combination of dislocation creep and solution-precipitation creep. Dynamic recrystallization occurred in all samples and was dominated by fluid assisted grain boundary migration. During deformation, grain boundary migration results in a competition between grain growth due to the removal of grains with high internal strain energy and grain size reduction due to grain dissection (i.e. moving boundaries that crosscut or consume parts of neighbouring grains). At steady state, grain growth and grain size reduction processes balance, yielding constant flow stress and recrystallized grain size that is inversely related to stress and temperature. Evaluation of the recrystallized grain size data against the different models for the development of mean steady state recrystallized grain size revealed that the data are best described by a model based on the hypothesis that recrystallized grain size organizes itself in the boundary between the (grain size sensitive) solution-precipitation and (grain size insensitive) dislocation creep fields. Application of a piezometer, calibrated using the recrystallized grain size data, to natural halite rock revealed that paleostresses can vary significantly with temperature (up to a factor of 2.5 for T=50–200 °C) and that the existing temperature independent recrystallized grain size–stress piezometer may significantly underestimate flow stresses in natural halite rock.     

Grain growth kinetics of dolomite,magnesite and calcite: a comparative study

The rates of grain growth of stoichiometric dolomite [CaMg(CO₃)₂] and magnesite (MgCO₃) have been measured at temperatures T of 700–800°C at a confining pressure P _c of 300 MPa, and compared with growth rates of calcite (CaCO₃). Dry, fine-grained aggregates of the three carbonates were synthesized from high purity powders by hot isostatic pressing (HIP); initial mean grain sizes of HIP-synthesized carbonates were 1.4, 1.1, and 17 μm, respectively, for CaMg(CO₃)₂, MgCO₃, and CaCO₃, with porosities of 2, 28, and 0.04% by volume. Grain sizes of all carbonates coarsened during subsequent isostatic annealing, with mean values reaching 3.9, 5.1, and 27 μm for CaMg(CO₃)₂, MgCO₃, and CaCO₃, respectively, in 1 week. Grain growth of dolomite is much slower than the growth rates of magnesite or calcite; assuming normal grain growth and n = 3 for all three carbonates, the rate constant K for dolomite (≃5 × 10⁻⁵ μm³/s) at T = 800°C is less than that for magnesite by a factor of ~30 and less than that for calcite by three orders of magnitude. Variations in carbonate grain growth may be affected by differences in cation composition and densities of pores at grain boundaries that decrease grain boundary mobility. However, rates of coarsening correlate best with the extent of solid solution; K is the largest for calcite with extensive Mg substitution for Ca, while K is the smallest for dolomite with negligible solid solution. Secondary phases may nucleate at advancing dolomite grain boundaries, with implications for deformation processes, rheology, and reaction kinetics of carbonates.     

Effect of impurities on grain growth in synthetic calcite aggregates

 We investigated grain growth of calcite aggregates fabricated from crushed natural single crystals with different impurity content. The total trace-element concentration of the starting powders varied from about 170 ppm to more than 930 ppm with Mn as the major component. Samples were produced by hot-isostatic pressing of the different powders at 300 MPa confining pressure at 600 °C for 2 h. The starting material for the anneals was dry and had a uniform microstructure with an average grain size of about 5 μm and a porosity of <2.1%. Three disks with Mn concentrations of 10, 350, and 670 ppm, respectively, were annealed in the same run at a confining pressure of 300 MPa, and temperatures between 700 and 900 °C for up to 20 h. Grain growth was fastest in samples with the highest Mn concentrations. A multivariable fit to the data yields grain-growth exponents of 2.0 ± 0.3 for samples with 10 ppm Mn and 2.3 ± 0.2 for those with 670 ppm Mn. The activation energies for grain growth vary from 99 ± 12 kJ mol⁻¹ to 147 ± 14 kJ mol⁻¹ for the respective calcite compositions. Received: 22 August 2000 / Accepted: 12 March 2001     

Post-deformational annealing of calcite rocks

The evolution of microstructure and crystallographic preferred orientation (CPO) during post-deformational annealing was studied on three calcite rock types differing in purity and grain size: Carrara marble (98% calcite, mean grain size of 115 μm), Solnhofen limestone (96%, 5 μm) and synthetic calcite aggregates (99%, 7 μm). Samples were first deformed in torsion at 727 °C at a shear strain rate of 3 × 10^{− 4} s^{− 1} to a shear strain of 5 and subsequently heat-treated at 727 °C for various durations between 0 and 24 h. Microstructures and CPOs were analysed by optical microscopy, image analysis and electron backscatter diffraction (EBSD).All rock types deformed in the dislocation creep field at the same applied conditions, but their microstructures and CPOs after deformation and after annealing differed depending on starting grain size and material composition. In Carrara marble and in the synthetic calcite aggregate, a strong CPO developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size. During annealing, widespread grain growth and subtle changes of CPO occurred, and equilibrated foam microstructures were approached after long annealing times. The CPO is the only feature in annealed samples indicating an earlier deformation phase, although it is not always identical to the CPO formed during deformation. In the more impure Solnhofen limestone, secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation CPO during deformation and grain size modification during deformation and annealing.     

The effect of ductile deformation on the kinetics and mechanisms of the aragonite-calcite transformation

Abstract The effect of ductile deformation (dislocation creep) on the kinetics of the aragonite-calcite transformation has been studied at 1 atm (330° C and 360° C) and 900-1500 MPa (500° C) using undeformed and either previously or simultaneously deformed samples (500° C and a strain rate of 10^-6 s). Deformation enhances the rate of the transformation of calcite to aragonite, but decreases the rate of transformation of aragonite to calcite. The difference results from a dependence of transformation rate on grain size, coupled with a difference in the accommodation mechanisms, climb versus recry-stallization, of these minerals during dislocation creep. Dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction. The rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries. In aragonite, on the other hand, dislocation climb is difficult and thus plastic strain produces extensive dynamic recry-stallization resulting in a substantial grain size reduction. The transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and/or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries. Thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodation mechanism.     

Fractal analysis of magnetite grains — implications for interpreting deformation mechanism

In the present study, the grain size (d) and shape of 225 magnetite grains, that crystallized at T>600°C in a syntectonic granite (Godhra Granite, India) are evaluated and implications of data to decipher deformation mechanism of magnetite are discussed. Fractal (ruler) dimension (D) analysis of magnetite grains is performed and it is demonstrated that they show fractal behaviour. Smaller magnetite grains tend to be more serrated than the larger ones, which is manifested in the higher fractal (ruler) dimension (D) of the former. Assuming a natural strain rate ranging between 10⁻¹⁰ s⁻¹ and 10⁻¹⁴ s⁻¹, the grain size data fall dominantly in the dislocation creep field of the existing deformation mechanism map of magnetite for 630°C. However, SEM-EBSD studies reveal that subgrains are absent in the magnetite grains and they did not undergo dislocation creep. Thus it is inferred that the shape of magnetite grains was not controlled by dislocation creep. It is concluded that the higher serration and increased fractal dimension of finer magnetite grains implies the importance of diffusion creep as an important deformation mechanism at high-T for magnetite in polymineralic rocks.     

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.     

Dolomite microstructures between 390° and 700 °C: Indications for deformation mechanisms and grain size evolution

Dolomitic marble on the island of Naxos was deformed at variable temperatures ranging from 390 °C to >700 °C. Microstructural investigations indicate two end-member of deformation mechanisms: (1) Diffusion creep processes associated with small grain sizes and weak or no CPO (crystallographic preferred orientation), whereas (2) dislocation creep processes are related with larger grain sizes and strong CPO. The change between these mechanisms depends on grain size and temperature. Therefore, sample with dislocation and diffusion creep microstructures and CPO occur at intermediate temperatures in relative pure dolomite samples. The measured dolomite grain size ranges from 3 to 940 μm. Grain sizes at T_max >450 °C show an Arrhenius type evolution reflecting the stabilized grain size in deformed and relative pure dolomite. The stabilized grain size is five times smaller than that of calcite at the same temperature and shows the same Arrhenius-type evolution. In addition, the effect of second phase particle influences the grain size evolution, comparable with calcite. Calcite/dolomite mixtures are also characterized by the same difference in grain size, but recrystallization mechanism including chemical recrystallization induced by deformation may contribute to apparent non-temperature equilibrated Mg-content in calcite.     

Volume and grain boundary diffusion of calcium in natural and hot-pressed calcite aggregates

 Calcium self-diffusion rates in natural calcite single crystals were experimentally determined at 700 to 900° C and 0.1 MPa in a stream of CO₂. Diffusion coefficients (D) were determined from ⁴²Ca concentration profiles measured with an ion microprobe. The Arrhenius parameters yield an activation energy (Q)=382±37 kJ/mol and pre-exponential factor (D₀)=0.13 m²/s, and there is no measurable anisotropy. Calcium grain boundary diffusion rates were experimentally determined in natural (Solnhofen) limestone and hot-pressed calcite aggregates at 650° to 850° C and 0.1 to 100 MPa pressure. The Solnhofen limestone was first pre-annealed for 24 h at 700° C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments. Values for the product of the grain boundary diffusion coefficient (D′) and the effective grain boundary diffusion width (δ) were determined from ⁴²Ca concentration profiles measured with an ion microprobe. The results show that there is no measurable difference between D′δ values obtained for pre-annealed Solnhofen samples at 0.1 and 100 MPa or between hot-pressed calcite aggregates and pre-annealed Solnhofen samples. The temperature dependence for calcium grain boundary diffusion in Solnhofen samples annealed at 0.1 MPa is described by the Arrhenius parameters D^′ ₀δ=1.5×10⁻⁹ m³/s and Q=267±47 kJ/mol. Comparison of the results of this study with previously published data show that calcium is the slowest volume diffusing species in calcite. The calcium diffusivities measured in this study place constraints on several geological processes that involve diffusive mass transfer including diffusion-accommodated mechanisms in the deformation of calcite rocks. Received: 19 December 1994/Accepted: 30 June 1995     

Rheological evolution of a Mediterranean subduction complex

We use field and microstructural observations, coupled to previously published P-T-time histories, to track the rheological evolution of an intracontinental subduction complex exposed in the Betic Cordillera in the western Mediterranean region. The body of rock we focus on, known as the Nevado-Filabride Complex (NFC), was originally part of the upper crust of the Iberian margin. It was subducted into hot asthenospheric mantle, then exhumed back toward the surface in two stages: an early stage of fast exhumation along the top of the subducting slab in a subduction channel, and a late stage of slower exhumation resulting from capture by a low-angle detachment fault rooted at the brittle-ductile transition. Each stage of deformation in the NFC was punctuated by changes in the dominant deformation mechanism. Deformation during initial subduction of the complex was accommodated by pressure-solution creep in the presence of a fluid phase – the grain sizes, stress magnitudes, and estimated strain rates for this stage are most consistent with a thin-film model for pressure solution in which the diffusion length scale is controlled by the grain size. During the early stages of exhumation within the subduction channel, deformation transitioned from pressure solution to dislocation creep due to increases in temperature, which resulted in increases in both water fugacity and grain size, each of which favor the dislocation creep mechanism. Differential stress magnitudes for this stage were ∼10 MPa, and are consistent with simple models of buoyancy-driven channel flow. With continuing subduction-channel exhumation, deformation remained within the dislocation creep field because sequestration of free water into hydrous, retrogressive minerals suppressed the pressure-solution mechanism. Differential stresses progressively increased to ∼100 MPa near the mouth of the channel during cooling as the rocks moved into mid-crustal levels. During the final, core-complex stage of exhumation, deformation was progressively concentrated into a narrow zone of highly localized strain beneath a mid-crustal detachment fault. Localization was promoted by a transition from dislocation creep to dislocation-creep-accommodated grain boundary sliding at temperatures of ∼350–380 °C, grain sizes of ∼4 μm and differential stress magnitudes of ∼200 MPa. Peak differential stress magnitudes of ∼200 MPa recorded just below the brittle-ductile transition are consistent with Byerlee's law in the upper crust assuming a vertical maximum principal stress and near-hydrostatic pore fluid pressures. Overall, the distribution of stress with temperature, coupled to independent constraints on strain rate from field observations and geochronology, indicate that the naturally calibrated Hirth et al. (2001) flow law for wet quartzite accurately predicts the rheological behavior of mid-crustal rocks deforming by dislocation creep.     

Macro‐and Microstructural,Textural Fabrics and Deformation Mechanism of Calcite Mylonites from Xar Moron‐Changchun Dextral Shear Zone,Northeast China

The calcite mylonites in the Xar Moron-Changchun shear zone show a significance dextral shearing characteristics. The asymmetric(σ-structure) calcite/quartz grains or aggregates, asymmetry of calcite c-axes fabric diagrams and the oblique foliation of recrystallized calcite grains correspond to a top-to-E shearing. Mineral deformation behaviors, twin morphology, C-axis EBSD fabrics, and quartz grain size-frequency diagrams demonstrate that the ductile shear zone was developed under conditions of greenschist facies, with the range of deformation temperatures from 200 to 300°C. These subgrains of host grains and surrounding recrystallized grains, strong undulose extinction, and slightly curved grain boundaries are probably results of intracrystalline deformation and dynamic recrystallization implying that the deformation took place within the dislocation-creep regime at shallow crustal levels. The calculated paleo-strain rates are between 10~(–7.87)s~(–1) and 10~(–11.49)s~(–1) with differential stresses of 32.63–63.94 MPa lying at the higher bound of typical strain rates in shear zones at crustal levels, and may indicate a relatively rapid deformation. The S-L-calcite tectonites have undergone a component of uplift which led to subhorizontal lifting in an already non-coaxial compressional deformation regime with a bulk pure shear-dominated general shear. This E-W large-scale dextral strike-slip movement is a consequence of the eastward extrusion of the Xing'an-Mongolian Orogenic Belt, and results from far-field forces associated with Late Triassic convergence domains after the final closure of the Paleo-Asian Ocean.     

橄榄石集合体的简单剪切形变实验研究

本文利用高精度Paterson气体介质变形装置对富铁橄榄石集合体(Mg0.5,Fe0.5)2SiO4进行了高温简单剪切变形试验。试验在温度1473K和围压300MPa的条件下进行,差应力为64~153MPa, 应变率为10-5~10-3s-1。 一共进行了三组试验,试件的剪切变形量分别为89%, 131%和200%, 通过对变形后试件的反射光学显微结构分析,得到富铁橄榄石集合体动态重结晶的微观机制,由结晶各向异性分析给出晶格最优取向和波速各向异性的分布及随应变的增加而表现出的演化特征。     

Recrystallisation,phase mixing and strain localisation in peridotite during rapid extrusion of sub-arc mantle lithosphere

The Anita Peridotite, in southwestern New Zealand, is a ∼1 × 20 km ultramafic massif that was rapidly extruded from beneath a Cretaceous arc within the 4 km wide mylonitic Anita Shear Zone. The peridotitic body contains a spectacular array of textures that preserve evidence for changing temperature, stress, and deformation mechanisms during the exhumation process. Olivine and orthopyroxene microstructures and lattice-preferred orientations (LPO) record a three-phase deformation history. Dislocation glide on the C- and E-type slip systems is recorded by coarse pre-mylonitised olivine grains, and occurred under hydrous conditions at T ∼650 °C, stress ∼200–700 MPa and strain rate ∼10⁻¹⁵ s⁻¹, probably within hydrated sub-arc mantle lithosphere. Rare protomylonite pods record deformation by dislocation creep in porphyroclasts and dislocation-accommodated grain boundary sliding in the matrix on {0kl}[100] in olivine and (100)[001] in orthopyroxene, under conditions of T ∼730–770 °C, stress ∼52–700 MPa and strain rate ∼10⁻¹⁵ s⁻¹. The massif, however, is dominated by mylonite and ultramylonite that wrap the protomylonite pods, comprising mostly fine-grained olivine neoblasts that lack internal distortions and have uniform LPOs. These textures indicate deformation occurred by grain-size sensitive (GSS) creep at T ∼650 °C, stress ∼69–137 MPa and strain rate ∼10⁻¹⁵ s⁻¹, and thus during conditions of cooling and decreasing stress. GSS creep became more dominant with time, as the proportion of randomly-oriented neoblasts increased and formed interlinked networks that accommodated much of the strain. Grain boundary pinning allowed GSS creep to be maintained in polyphase regions, following mixing of olivine and orthopyroxene, which may have occurred by grain boundary transport in a fluid phase during a “creep cavitation” process. The results indicate that the Anita Peridotite recrystallised and underwent rheological weakening at a constant strain rate, with strain distributed across the entire section. This widespread deformation caused rapid exhumation of the peridotite from the lithospheric mantle into the overlying arc crust. The massif therefore records multiple overprinting phases of deformation under mantle and crustal conditions associated with the rapid exhumation of a large orogenic peridotite.     

Experimental deformation of partially melted granitic aggregates

Abstract The effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions (900°C, 1500 MPa, 10^-4 to 10^-6/s) where melt-free samples deform by dislocation creep, with microstructures approximately equivalent to those of upper greenschist facies. Experiments were performed on samples of various grain sizes, including an aplite (150 μm) and sintered aggregates of quartz-albitemicrocline (10–50 and 2–10 μm). Water was added to the samples to obtain various amounts of melt (1–15% in the aplite, 1–5% in the sintered aggregates). Optical and TEM observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels, while the melt in the aplites is inhomogeneously distributed. The effect of partial melt on deformation depends an melt amount and distribution, grain size and strain rate. For samples deformed with ? 1% melt, all grain sizes exhibit microstructures indicative of dislocation creep. For samples deformed with 3–5% melt, the 150 μm and 10–50 μm grain size samples also exhibit dislocation creep microstructures, but the 2–10 μm grain size samples exhibit abundant TEM-scale evidence of dissolution-precipitation and little evidence of dislocation activity, suggesting a switch in deformation mechanism to predominantly melt-enhanced diffusion creep. At natural strain rates melt-enhanced diffusion creep would predominate at larger grain sizes, although probably not for most coarse-grained granites. The effects of melt percentage and strain rate have been studied for the 150 μm aplites. For samples with ? 5 and 10% melt, deformation at 10^–6/s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep. Conversely, deformation at 10^-5/s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results. For samples with 15% melt, deformation at both strain rates produces cataclasis, presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains, which would produce high stress concentrations at point contacts. At natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate. However, if the melt is confined and cannot migrate, a high pore fluid pressure should promote brittle deformation.     

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.     

Water assisted dynamic recrystallization and weakening in polycrystalline bischofite

Artificially prepared specimens of bischofite (MgCl₂-6H₂O) have been experimentally deformed at temperatures between 20 and 100°C, strain rates between 10⁻⁴ and 10⁻⁸⁸ s⁻¹, and confining pressures between 0.1 and 28 MPa. Development of microstructure with strain was studied by in-situ deformation experiments, and results of these were correlated with observations made on thin sections of deformed samples.In a first series of experiments the effect of grain size, impurity content and water content on the flow behaviour was investigated. Addition of about 0.1 wt.% water to dry samples was found to decrease the flow stress by a factor of 5. This effect was found to be associated with the formation of a thin fluid film on grain boundaries, strongly enhancing dynamic recrystallization due to the movement of high-angle grain boundaries, and possibly also to enhanced intracrystalline plasticity due to excess water present in the lattice. In a second series of experiments the strain-rate sensitivity of the flow stress of selected samples was investigated. Two regimes could be distinguished: one with a stress exponent n = 4.5 in the power law creep equation for values of the differential stress above 2.0 MPa, and one with n = 1.5 for stresses below this value.The main deformation mechanisms were intracrystalline slip, twinning, and grain-boundary sliding. Recrystallization occurred by subgrain rotation and high-angle grain-boundary migration. The rates of grain-boundary migration fell into two different regimes, one regime being distinguished by extremely fast migration rates. The applicability of the experimentally found flow law to the behaviour of bischofite rocks in nature is discussed.     

Thermomechanics of an extensional shear zone,Raft River metamorphic core complex,NW Utah

A detailed structural and microstructural analysis of the Miocene Raft River detachment shear zone (NW Utah) provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes. Combined microstructural, electron backscattered diffraction, strain, and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east, along the transport direction, compatible with observed finite strain markers and a model of ``necking'' of the shear zone. Microstructural evidence (quartz microstructures and deformation lamellae) suggests that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where meteoric fluids played an important role on strain hardening, embrittlement, and eventually seismic failure.Empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results, indicate that the shear zone developed under stress ranging from 40 MPa to 60 MPa. Using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10⁻¹² and 10⁻¹⁴ s⁻¹. We suggest that a compressed geothermal gradient across this detachment, which was produced by a combination of ductile shearing, heat advection, and cooling by meteoric fluids, may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone.