the development of shear zones in polycrystalline camphor urai j l and humphreys f j 1981 the development of shear zones in polycrystalline camphor in g s lister h j behr k weber and h j zwart editors the effect of deformation on rocks tectonophysics 78 677 685 thin polycrystalline specimens of rhombohedral camphor have been deformed in pure shear in the temperature range 283 343k the deformation processes dynamic recrystallization and the development of microstructure were followed by transmission polarized light microscopy when the stress distribution in the specimens was favourable shear zones were initiated the development of microstructure changed drastically above 310320k the shape of the dynamically recrystallized grains which was equiaxed below this temperature became strongly elongated this change probably due to the development of a marked anisotropy in grain boundary mobilities and the activation of new slip systems made the development of shear zones a more frequently occurring phenomenon above 310 320k

convective generation of cirrus near the tropopause 1 during the july 2002 crystal face field program based from key west florida aircraft measurements showed that long lived thin tropopause cirrus ttc layers were often present above thunderstorm anvils this paper describes these clouds and explores their possible origins measurements showed that the horizontal dimensions of ttc layers were nearly identical to convectively formed anvils beneath but that ttc did not appear to have originated from convective detrainment rather they appeared to have formed in stably stratified air derived from high altitudes near the tropopause ttc were separated vertically from the top of the anvil by 1 km they lacked precipitation particles and they were strongly depleted in hdo all of which indicate in situ formation however compared to surrounding clear air near the tropopause ttc were enriched in moisture and trace gases the degree of enrichment was consistent with mixing of air near the tropopause with the same convective air mass that produced the anvil unlike surrounding air ttc had embedded a monochromatic gravity wave with a wavelength of 2 km and an amplitude of several hundred meters this evidence supported by a photograph from crystal face leads to the conjecture that ttc originate as stratiform pileus clouds that form near the tropopause ahead of vigorous convective uplift we hypothesize that the pileus are penetrated by the convection moistened through turbulent mixing and once the convection subsides they are sustained by radiative cooling due to the presence of the anvil layer beneath

water enhanced dynamic recrystallization and solution transfer in experimentally deformed carnallite cylindrical samples of polycrystalline carnallite kmgcl3 6h2o were deformed in a triaxial apparatus at 60 c at confining pressures between 0 1 and 31 mpa and at strain rates between 10 4 and 10 8 s 1 in a number of cases small amounts of saturated carnallite brine were added samples without added brine deform by intracrystalline slip mechanical twinning cracking and by frictional sliding on crack surfaces stress strain curves of these samples are strongly dependent on confining pressure addition of brine has a dramatic effect on both microstructural development and mechanical properties grain boundary migration is strongly enhanced at lower strain rates additional intracrystalline effects start to appear together with the onset of solution transfer rapid compaction in samples deformed with added brine causes high fluid pressures to develop at higher strain rates addition of brine results in a decrease of the flow stress by a factor of two this weakening will increase even further at strain rates below about 10 9 s 1 when solution transfer becomes rate controlling it is argued that deformation of carnallite in nature is adequately described by the flow law found for samples deformed with added brine

syndeformational grain growth microstructures and kinetics abstract grain growth can be an important high temperature process in fine grained polycrystalline materials we studied its effect on texture during deformation using a two dimensional computer model and a see through experiment with octachloropropane syndeformational grain growth in geological materials can slow down the development of a grain shape preferred orientation or even obliterate it important processes are preferentially oriented deformation induced neighbour switches the presence of a grain shape preferred orientation slows down the increase in grain size due to grain growth

dynamic recrystallization of wet synthetic polycrystalline halite dependence of grain size distribution on flow stress temperature and strain abstract 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

seven types of subgrain boundaries in octachloropropane abstract subgrain boundaries in thin sheets of octachloropropane deformed at 0 7 0 8 t m on the stage of a microscope are seen to appear in the material in seven different ways type i boundaries show the classical evolution by polygonization of bent crystals type ii are essentially kink boundaries which migrate sideways during deformation to reach their present positions in the crystals type iii develop at the sites of former grain boundaries by reduction of misorientation of adjacent grains type iv and v originate by impingement of migrating grain boundaries or subgrain boundaries respectively type vi propagate in their own planes behind migrating grain boundaries to which they are attached type vii develop statically from optically strain free grains by a process probably otherwise similar to the type i process two thirds of the boundaries are types i or ii in view of the variety of subgrain boundary histories in ocp interpretation of similar features in minerals ought to be undertaken cautiously criteria are needed for telling the different types of subgrain boundaries apart in situations where only a final view of the structure is available as in optical and electron micrographs of rocks

deformation of crystalline materials in thin section a method applicable to weak low melting point model materials permits observation through a petrographic microscope of large simple shearing deformation of samples as thin as ordinary thin sections examples of ductile brittle and intermediate behavior can be studied the method lends itself to teaching and may also prove valuable in research

the evolution of high temperature mylonitic microfabrics evidence from simple shearing of a quartz analogue norcamphor plane strain simple shearing of norcamphor c7h10o in a see through deformation rig to a shear strain of 10 5 at a homologous temperature of th 0 81 yields a microfabric similar to that of quartz in amphibolite facies mylonite synkinematic analysis of the norcamphor microfabric reveals that the development of a steady state texture is linked to changes in the relative activities of several grain scale mechanisms three stages of textural and microstructural evolution are distinguished 1 rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears a symmetrical c axis girdle reflects localized pure shear extension along the main microshear set sa oblique to the bulk shear zone boundary abbreviated as szb 2 progressive rotation of the microshears into parallelism with the szb increases the component of simple shear on the sa microshears grain boundary migration recrystallization favours the survival of grains with slip systems oriented for easy glide this is associated with a textural transition towards two stable c axis point maxima whose skeletal outline is oblique with respect to the sa microshears and the szb and 3 at high shear strains 8 the microstructure texture and mechanism assemblage are strain invariant but strain continues to partition into rotating sets of microshears steady state is therefore a dynamic heterogeneous condition involving the cyclic nucleation growth and consumption of grains

in situ annealing experiments of octachloropropane as a rock analogue kinetics and energetics of grain growth abstract two dimensional in situ observation of the change of size of individual grains of octachloropropane ocp during post deformation annealing revealed that grain boundary migration occurred to reduce the grain boundary energy within the polycrystalline aggregate studied some grains show a cyclic change in grain size others have a stable grain size while most grains were consumed and disappeared in order that the mean grain size could increase during grain coarsening some grains were dissected then coalesced amalgamated with others and their centers migrated across the aggregate the average grain size d can be expressed by d k 0 exp q rt t n where k0 is a constant q is the activation energy r is the gas constant t is the temperature t is the annealing time and n is a constant we obtained n 0 1 0 2 and q 6 2 kcal mol the driving force p is the surface energy stored at grain boundaries the driving force can be expressed as p tm where m is the constant and hence the grain growth rate d can be expressed as d p n 1 m the value of n 1 m ranges from 4 to 10

effect of confining pressure on dilatation recrystallization and flow of rock salt at 150 c microstructural evidence for fluid assisted dynamic recrystallization fadrx is widespread in naturally deformed rock salt however the principal experimental evidence for fadrx in salt has been obtained from stress relaxation experiments and it is unclear whether the process occurs during steady state dislocation creep and what its effect might be here we report deformation experiments performed on natural rock salt at a constant strain rate of 3 5 10 7 s 1 150 c and confining pressures pc of 3 30 mpa samples deformed at pc 3 mpa showed continuous work hardening and minor dilatation microstructurally they exhibited intergranular cracking plus slip band and subgrain structures indicative of dislocation glide creep electron backscatter diffraction analysis revealed a high frequency of boundaries with low angle misorientations in the range 5 10 in contrast samples deformed at pc 6 5 mpa showed work hardening followed by steady state flow at strains 6 7 these samples compacted slightly and crystal plastic deformation was accompanied by extensive fadrx with a predominance of high angle boundaries 30 50 over low angle boundaries we infer that fadrx is suppressed by dilatation at low pressures as a result of grain boundary disruption at pressures high enough to prevent dilatation however fadrx acts as a recovery mechanism counteracting work hardening the results offer a possible explanation for rheological variability seen in previous experiments conducted at pressures up to 30 mpa a simple rate model for diffusion and interface controlled fadrx indicates that fadrx should become increasingly important toward natural halokinetic conditions although the effect on flow stresses is likely to be small

an in situ optical investigation of the deformation behaviour of sodium nitrate an analogue for calcite abstract in situ observations have been made under the optical microscope of the deformation of thin polycrystalline specimens of sodium nitrate calcite structure at temperatures between 20 c 0 5 tm and 300 c 0 98 tm at low temperatures or high strain rates deformation twinning is observed between 20 c and 180 c inhomogeneous deformation together with dynamic recrystallisation by subgrain rotation results in a core and mantle microstructure at temperatures above 180 c rapid dynamic recrystallisation by grain boundary migration also occurs producing a coarse grained microstructure the results are interpreted in terms of impurity controlled grain boundary motion as proposed for halite and a strong similarity between the deformation behaviour of sodium nitrate and marble is noted

microstructural evolution during initial stages of static recovery and recrystallization new insights from in situ heating experiments combined with electron backscatter diffraction analysis we present in situ observations performed using electron backscatter diffraction of static recovery and recrystallization processes occurring in a plastically deformed geological material namely rocksalt static heating experiments carried out in a scanning electron microscope at temperatures of up to 450 c allowed direct detailed observations of grain boundary migration between substructured grains in deformed polycrystalline samples contrary to expectations crystallographic orientation maps reveal that behind slowly migrating grain boundaries new subgrain boundaries form while pre existing subgrain boundaries are sometimes inherited moreover the crystallographic character especially the preferred orientation of misorientation axes of the new substructures reflects the character of the previous deformation history these results imply that substructural features such as subgrain misorientation distributions may be relatively robust indicators of deformation mechanisms and conditions even in tectonites that have undergone late static recrystallization in addition our observations suggest that the process of static grain boundary migration does not necessarily reset earlier deformation microstructures as commonly assumed the kinematic observations of boundary migrations are inconsistent with simple models in which migration is achieved by single atoms jumping across the boundary

variability of circulation induced by the separation of gaspe current in baie des chaleurs canada observational studies the observed variability of summer circulation in the baie des chaleurs bdc canada is found to be largely controlled by the intrusion separation processes of a coastal jet gaspe current gc at the bay entrance an analysis of hydrographic and current meter data showed that the mean counter wind cyclonic circulation in the bdc from spring to the beginning of the cooling season in 1991 was ascribed to the westward intrusion of the gc the forcing mechanism of the gc on the bdc was determined by the characteristics of the coastal jet separation under the intrusion regime the gc entered into the bdc along the north coast of the bay suppressed the local wind driven eastward currents and formed a cyclonic circulation in the bay under the separation regime gc mainly passed across the entrance of the bdc with the formation of an anticyclonic recirculation at the lee of the jet near the bay entrance and a weaker cyclonic circulation in the bay dynamics of the boundary current separation is utilized to reason the observed variability of flow field in the bdc

the petrological significance of misorientations between grains misorientation analysis quantifies microstructural features in tectonites metamorphic and igneous rocks and allows hypotheses on their formation to be tested the misorientation between two lattices can be expressed by a rotation axis and rotation angle for lattices with symmetry it is conventional to take the minimum angle that enables one lattice to be rotated into the other for a group of lattice measurements two types of misorientation distribution can be calculated selecting random pairs of grains gives the random pair misorientation distribution selecting neighbouring pairs gives the neighbour pair misorientation distribution the forms of both distributions are visualised using histograms or cumulative frequency diagrams they are strongly influenced by any overall crystallographic preferred orientation and by intrinsic crystal symmetry in many rocks the random pair misorientation distribution and neighbour pair misorientation distribution are statistically significantly different quantified using the kolmogorov smirnov test differences between the random pair misorientation distribution and neighbour pair misorientation distribution imply that adjacent grains have physically interacted or are inherited from a precursor microstructure interactions include 1 reduction in surface energy by lattice alignment we show this may have occurred in garnet clusters in schist and olivine in a cumulate it is well known in metals and may be a common geological process 2 nucleation where those nuclei have influenced the orientation of adjacent nuclei 3 mechanical rotations of facetted grains in compacting crystal mushes so that faces become parallel 4 growth twinning inheritance includes 1 subgrain rotation recrystallisation in tectonites deforming by crystal plastic processes 2 mechanical and transformation related twinning 3 domainal microstructures e g where grains have formed from a few large original grains may give rise to spurious correlations when the orientation data cover more than one domain with this proviso misorientation analysis can be used to investigate many important microstructural processes

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

intragranular dynamic recrystallization in naturally deformed calcite marble diffusion accommodated grain boundary sliding as a result of subgrain rotation recrystallization abstract intragranular microshear zones within a greenschist facies calcite marble were studied to try to constrain better the processes of dynamic recrystallization as well as the deformation processes that occur within newly recrystallized grains intragranular recrystallized grains within large twinned calcite porphyroclasts can be related to the host from which they have recrystallized and are the focus of an electron backscatter diffraction study lattice distortions low angle boundaries and some high angle boundaries 15 in the microshears within a porphyroclast have the same misorientation axes suggesting that deformation occurred by climb accommodated dislocation creep involving subgrain rotation recrystallization changes in the ratio of host and twin domain as the deformation zone is entered show that twin boundary migration also occurred recrystallized grains have similar sizes 10 60 m to subgrains suggesting that they formed by subgrain rotation however within the intragranular microshear zones the misorientations between recrystallized grains and porphyroclasts are considerably larger than 15 and misorientation axes are randomly oriented moreover recrystallized grain orientations average around the porphyroclast orientation we suggest that the recrystallized grains once formed are able to deform partly by diffusion accommodated grain boundary sliding which is consistent with predictions made from lab flow laws

temperature dependent grain boundary migration in deformed then annealed material observations from experimentally deformed synthetic rocksalt abstract grain boundary migration between strained substructured grains and newly appearing strain free grains has been observed during static in situ annealing of pre deformed rocksalt in the sem with increasing temperature t the migration velocity increases and the character of grain boundary migration changes as temperature increases there is an increase in the length of individual migrating boundary segments that move at similar rates in addition the frequency of migrating boundaries that form traces of a 100 boundary plane of at least one of the crystals involved increases and moving grain boundaries between new and old grains change from highly irregular to smooth straight boundaries at the same time there is a decrease in the influence of the substructure of pre existing strained grains on the grain boundary movement resultant microstructures reflect these changes at 325 350 c the deformed then annealed microstructure is characterized by very irregular grain boundaries a high abundance of 5 50 m scale remnants of old substructured grains within new grains giving a poikilitic microstructure at 350 400 c grain boundaries often exhibit elongate embayments into the strained grains and most remnants of old strained grains are located at former grain boundaries at 400 c grain boundaries between new and old strained grains are straight to smoothly curved the grain boundary velocity observations are explained by the effect of temperature on mobility coupled with local driving force variations additionally at low annealing temperature impurity solute drag and driving force variations are influential while at high temperature the anisotropy in grain boundary energy with crystallographic orientation becomes more important transferring the knowledge from our experiments to geological samples enables us to recognize and interpret similar microstructures in rocks thereby making it is possible to relate microstructural characteristics to the pre annealing and post deformational annealing history

rheology of rocksalt abstract we review progress in experimental determinations of transient and steady state flow properties and processes of natural rocksalt aggregates focusing primarily on results from avery island louisiana domal salt the steady state flow field established from constant stress and constant strain rate tests at temperatures from 50 to 200 c strain rates from 10 5 to 10 9 s 1 and differential stresses from 20 7 to 2 5 mpa has been separated into two flow regimes each fit by a power law relation at the higher stresses and strain rates this relation is e 1 6 10 4 exp 68 1 rt 10 3 5 3 for which the pre exponential constant is expressed in mpa 5 3 s 1 and the apparent activation energy is in j mol 1 creep rates predicted by equation a do not differ appreciably from those predicted previously the relatively low stress low strain rate data are very well fit by e 8 1 10 5 exp 51 6 rt 10 3 2 4 which is comparable conditions predicts creep rates higher and equivalent viscosities lower than does equation a by two orders of magnitude the change in behavior from a to b is ascribed to a change in rate limiting mechanism from cross slip of screw dislocations to the climb of edge dislocations in this dry material subgrain formation dominates microstructural development during steady state flow of salt and from determinations of average subgrain diameters in crystals from 20 different rocksalt bodies flow stress levels between 0 6 and 1 4 mpa have been estimated these values do not bear directly on arguments concerning the nature of forces initiating salt pillow growth because apparently evidence relating to the early deformational history has been overprinted at that stage fluid assisted grain boundary diffusional processes might dominate dislocation creep leading to a linear stress strain rate mechanical response considering buoyancy forces alone behavior described by equation b rather than a reduces the relief necessary to initiate pillow growth but a 1000 fold amplification is required to produce stress differences near 1 mpa that forces other than buoyancy are important is indicated by the occurrence of these same paleostress levels in bedded salts and in shallow offshore concordant intrusions differential loading generally provides the most plausible initial driving force for the growth of diapiric salt structures