deformation enhanced fluid distribution in feldspar aggregates and implications for ductile shear zones experiments at 900 c and 1 4 gpa show that the distribution of aqueous fluid in fine grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation the isolated pore distribution is rapidly regained during annealing following deformation the deformation enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength it also increases the bulk transport rate through the aggregate by more than an order of magnitude a change in fluid distribution is not observed in fine grained quartz aggregates deformed at similar conditions if deformation enhanced fluid distribution occurs in naturally deformed feldspathic rocks it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones

fluid flow and volume loss during mylonitization an origin for phyllonite in an overthrust setting north carolina u s a abstract in the western blue ridge province of the southern appalachians north carolina granite gneisses are converted to phyllitic mylonites phyllonites along the base of the rector branch thrust sheet the mylonites show evidence for fluid infiltration and large volume losses with decreasing distance from the thrust and local shear zones feldspars undergo grain size reduction by cracking followed by chemical breakdown to muscovite sericite and zoisite the following reaction was important during mylonitization 3 k feldspar 2 h muscovite 2 k 6 sio 2 quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300 c and 500 c in the presence of a fluid the mylonites are enriched by a factor of at least 2 5 in elements such as tio 2 p 2 o 5 zr y v la ce and depleted in sio 2 in comparison to their protolith gneisses the immobile element enrichments are attributed to enrichment in residual phases such as ilmenite zircon apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60 mass balance calculations based on major element chemistry indicate the mylonites lost large amounts of sio 2 and smaller amounts of alkalies and were infiltrated by fluid rock weight ratios of between 10 2 and 10 3 phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas reaction enhanced softening was apparently important in strain localization

an observational and modeling study of mesoscale air masses with high theta e abstracttypically the cool side of an airmass boundary is stable to vertical motions due to its associated negative buoyancy however under certain conditions the air on the cool side of the bou

neotectonics of western india evidence from deformed quaternary fluvial sequences mahi river gujarat a quaternary fluvial sequence along the mahi river gujarat western india shows evidence for a discrete episode of normal faulting and related surface slumping four mapped nw striking faults have throws ranging from 1 m to over 3 m and have related growth folds in synchronous sediment units a slump sheet moved tens of metres northeastward down the 0 7 dip slope of one tilt block during displacement on its bounding fault the exposed part of the internally folded sheet is probably one side of a lobate slump mass several hundred metres in width fault activity began after deposition of a lower gravel which regional correlations suggest is about 300 ka old faulting ended just before deposition of an upper gravel sand complex dated by luminescence techniques in correlated sections at no younger than about 60 ka the mahi river section shows that neotectonic activity in peninsular india exemplified by the 1993 killari latur earthquake has a mid to late pleistocene component

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

grain boundary sliding and development of grain boundary openings in experimentally deformed octachloropropane abstract in situ observations of grain boundary sliding and associated accommodation mechanisms in experimentally pure and simple sheared octachloropropane at 75 85 of its absolute melting temperature and at strain rates of 10 5 10 6 s 1 are discussed discontinuities in the strain rotation and or translation components of deformation across the grain boundary induce grain boundary sliding the influence of crystallographic orientation of grains on grain boundary sliding is so strong that grains unfavorably oriented for basal slip contribute to deformation mainly by grain boundary sliding grain boundary diffusion and intragranular plastic deformation are observed to accommodate grain boundary sliding grain boundary diffusion also causes grain boundary migration types ii and iii that is different from conventional grain boundary migration type i in being non conservative and in the details of boundary movement with respect to material points within grains grain boundary openings filled with fluid octachloropropane vapor are strongly associated with grain boundary sliding openings develop preferentially along grain boundaries at low angles to the shortening direction once openings grow they are closed by thrusting of sliding grains and by diffusion in faster strain rate experiments and entirely by diffusion in slower strain rate experiments an approximately steady openings ratio of 0 5 3 of the sample volume persists without the development of any large scale fracture all grain boundary openings disappear during static readjustment of the microstructure after deformation

multiscaling of shear zones and the evolution of the brittle to viscous transition in continental crust 1 a new method is introduced for quantifying the scale and the intensity of strain localization from maps of natural shear zones the method employs autocorrelation functions to determine local areal scales of geometric homogeneity these homogenization scales are used to calculate scale dependent localization fractions of deformed rock the strain localization intensity is quantified from measurements of mean relative to maximum shear strain this approach is used to analyze shear zones on different scales from an exposure cap de creus spain of the fossil brittle to viscous transition bvt changes in the scaling characteristics of shear zones are interpreted to reflect a time sequence of localization during the evolution of the continental bvt we show that shear zone scaling is related to inherited anisotropies older schistosity lithological layering pegmatite bodies and to the predominant mode of deformation brittle viscous the length to width ratio of shear zones increases with their length up to the meter scale and decreases for larger length scales as they evolve from isolated shear fractures to interconnected mylonitic shear zones variations in strain localization intensity calculated along a single shear zone indicate that such shear zones weakened from their brittle tips to their mylonitic centers thus driving their propagation and growth to larger scales our results imply that the bvt evolves by network widening a process whereby strain localizes on progressively larger scales until a dense network of weak mylonitic layers tens to hundreds of meters wide and hundreds to thousands of meters long forms subparallel to the regional shearing plane

mechanics and mechanisms of magmatic underplating inferences from mafic veins in deep crustal mylonite dioritic to gabbro dioritic veins with extreme length to width ratios 1000 1 are localized along an amphibolite facies shear zone the cmb line between exposed segments of originally middle and lower continental crust strona ceneri and ivrea verbano zones northern italy the geometry of these veins and their mutual cross cutting relationships with the mylonitic foliation indicate that veining was coeval with noncoaxial flattening of the lower crust in early permian time the veins formed as closely spaced extensional shear fractures and propagated parallel to the originally gently to moderately dipping 30o mylonitic foliation vein opening at high angles 60 90o to the inferred1 direction and subparallel to the pre existing planar fabric requires that melt pressure slightly exceeded the lithostatic pressure and that differential stress was low 10 20 mpa in the vicinity of the veins the interaction of regions of tensile stress concentration at vein tips caused the concordant veins to curve and link up across the mylonitic foliation once interconnected the veins served as conduits for the rapid movement of mafic melt along the shear zone thermal modelling constrains the mafic melt in the narrowest 1 mm wide veins to have crystallized almost instantaneously such veins extend no more than a meter from host veins into the country rock indicating that the minimum rate of vein tip propagation and melt flow was at least several m s maximum crystallization times of only hundreds to thousands of years for even the thickest mafic veins 10 100 m in the ivz are short compared to the 15 20 ma duration of early permian crustal attenuation and magmatism in the southern alps this suggests that veining in the lower crust occurred episodically during extended periods of mylonitic creep concordant vein networks within deep crustal shear zones that are inclined as the cmb line may have been can also channel overpressurized mafic melt from deeper sources e g lower crustal magma chambers into cooler intermediate crustal rock this locally widens the depth interval of combined viscous and brittle deformation within the crust and can trigger partial melting of the country rock 1999 elsevier science b v all rights reserved

temperature and strain rate dependent microfabric evolution in monomineralic mylonite evidence from in situ deformation of norcamphor norcamphor c7h10o was subjected to plane strain simple shear in a see through deformation rig at four different strain rate and temperature conditions two transient stages in the microfabric evolution to steady state are distinguished the grain scale mechanisms associated with the microstructural and textural evolution vary with the applied temperature strain rate and strain in high temperature low strain rate experiments computer integrated polarization microscopy reveals that the texture evolution is closely related to the crystallographic rotation paths and rotation rates of individual grains high c axis rotation rates at low to intermediate shear strains are related to the development of a symmetrical c axis cross girdle by the end of the first transient stage 1 5 to 2 during the second transient stage 1 5 to 6 the cross girdle yields to an oblique c axis single girdle as c axis rotation rates decrease and the relative activity of grain boundary migration recrystallization increases steady state 8 is characterized by a stable end orientation of the sample texture and the cyclic growth rotation and consumption of individual grains within the aggregate

fluid infiltration and volume change during mid crustal mylonitization of proterozoic granite king island tasmania the development of 10 30 m wide mylonite zones at mid crustal depths in late proterozoic granitoids on king island tasmania was associated with pervasive infiltration of low 18o fluids 5 to 7 on the scale of the shear zones syndeformational fluid rock interaction produced substantial differences in mineral composition and bulk rock chemistry among several adjacent shear zones which are hosted by the same granite in a shear zone at cape wickham with a normal slip component changes in whole rock chemistry between granite and mylonites indicate a gain of ca and losses of k and na during deformation which was nearly isovolumetric notable losses of k rb and si occurred during partial retrograde alteration of mylonites near the western margin of this shear zone the alteration suggests a component of up temperature fluid flow in contrast 3 km to the south east in a strike slip shear zone at disappointment bay complete albitization of plagioclase was associated with na gain and ca loss deformation also involved losses of mg and fe up to 60 volume gain occurred during the formation of closely spaced mesoscopic to microscopic quartz veins during mylonitization the substantial silica gain in this as well as in two mylonite zones further south east is interpreted to have been associated with upward flow of aqueous fluids along these shear zones on the basis of a gradient reaction model minimum time integrated fluid fluxes of 106 m3 m2 are estimated for the disappointment bay west shear zone

microstructures and rheology of hydrous synthetic magmatic suspensions deformed in torsion at high pressure the relationship between magma rheology and characteristic magmatic microstructures was investigated by performing high temperature high pressure deformation experiments on hydrous synthetic magmatic suspensions in the range of 0 to 76 solid fraction alumina grains torsion experiments were conducted at 300 mpa confining pressure temperatures ranging from 475 c to 1000 c and shear strain rates ranging from 2 0 10 5 to 2 1 10 3 s 1 up to total strains of 21 3 flow is newtonian for a solid fraction of s 0 0 0 16 with a log dynamic viscosity 10 3 pa s t 500 c a deviation from newtonian behavior is observed for s 0 16 with an increase in apparent viscosity of about 1 order of magnitude between s 0 16 and 0 54 the shape fabric of the solid phase is characterized by a unimodal orientation that is almost stable and nearly parallel to the shear direction both shape fabric and deviation from newtonian behavior originate from the increase in the number of particle clusters in the suspension the apparent viscosity increases by 1 5 orders of magnitude between s 0 54 and 0 65 and extrapolation of the data suggests a very sharp increase in apparent viscosity for s 0 65 at t 550 c and s 0 54 the solid phase forms an almost entirely connected network composed of two alternating orientation domains at t 550 c and s 0 65 intragranular fracturing and tensile fractures result from high local stresses at contact points between neighboring particles the resulting bulk extensional fabric is almost parallel to the shortening direction

the role of transiently fine grained reaction products in syntectonic metamorphism natural and experimental examples it is proposed that one of the most important mechanisms of interrelationship between deformation and metamorphism is the facilitation of one of several grain size sensitive deformation mechanisms resulting from the formation of fine grained products of a metamorphic reaction during prograde metamorphism such effects are likely to be transient because grain coarsening and textural equilibration are likely in response to rising temperature conditions thus deformation mechanisms are often difficult to infer from such naturally deformed rocks in localized shear zones exhibiting retrogressive metamorphism evidence of enhanced deformability by such mechanisms is most likely to be preserved because cooling conditions inhibit grain growth and both deformed and undeformed rocks are likely to be preserved an experimental study has been made of the effects of deformation on serpentinite under conditions of progressive dehydration but with controlled pore pressure a marked weakening near linear viscous rheol

metamorphic fluids and anomalous porosities in the lower crust abstract because of the plastic behavior of rocks at high temperature and confining pressure the porosity of lower crustal rocks is a variable dependent on the amount of free fluid present on the basis of hydrologic theory it is predicted that the permeability k of such rocks can be expressed by a relationship of the form k n where and n are material specific constants the exponent n is likely to vary between 3 and 6 whereas which is ultimately related to grain size and shape probably varies over several orders of magnitude consequently substantial contrasts between the k relationships of lower crustal rocks are to be expected the case in which thin crustal layers aquitards are characterized by relatively low values of or n with respect to the crust in general is of particular interest such layers would acquire high porosities in the presence of miniscule steady state crustal fluid fluxes and are a possible explanation for anomalous seismic brightspots and high electrical conductivities in the lower crust modelling of collision belt metamorphism suggests that even long after the cessation of active metamorphism and isostatic reequilibration relict metamorphic fluid fluxes could generate aquitard porosities between 0 1 and 0 4 adequate to explain such geophysical anomalies

the analysis of progressive deformation in rock analogues abstract two dimensional deformation experiments using analogue materials such as octachloropropane and camphor have proven to be a powerful tool in the investigation of microstructural development since they make it possible to observe the specimen throughout its deformation history the addition of dispersed marker particles makes it possible to trace the movement of material points during deformation from this information the flow and deformation can be interpolated for any point within the specimen at any time the geometry of the flow and deformation can then be visualized in the form of grids contour plots strain ellipses and other methods despite this apparently ideal setting for studying flow and deformation the need to use marker particles to trace material points introduces unexpected problems in the analysis we present a computer program to analyse the movement of marker particles digitization of the positions of the particles is improved by the use of video input and semi automatic digitization the use of polynomials to describe the complete path of marker particles and least squares best fit solving improves the analysis of the data

deformation and fluid rock interaction in metasomatic dilatant shear bands abstract the microstructure and microchemistry of shear bands from phyllonites along the merens fault in the axial zone of the pyrenees are described deformation was accompanied by the influx of a metasomatic fluid carrying fe and mg which altered muscovite to chlorite and precipitated chlorite and quartz in dilatant sites overall volume increases in shear bands were two to two and a half times if the mobility of al was low al was probably supplied to chlorites growing from solution by isovolumetric replacement of nearby muscovite by chlorite but fe and mg must have been transported on a larger scale chlorite platelets grew with a linear decussate fabric with 001 planes parallel to the local extension direction the most likely geometry for dilation was in pull apart sites on stepped micro faults or shear zones such sites acted as pumps drawing reactive fluid in from outside the rock along high permeability pathways parallel to the shearbands the larger scale pattern of circulation is unknown but field relations suggest fluid movement on a scale of hundreds of metres or more the relatively low concentration of fe and mg in natural hydrothermal brines implies that fluid rock ratios in the shear bands were high this requires a cyclical crack seal mode of infiltration reactions may have enhanced ductility by producing fine grained products and by allowing the opening of dilatant sites but deformation undoubtedly influenced the location of reactions and possibly their rates

see through deformation experiments on brittle viscous norcamphor at controlled temperature strain rate and applied confining pressure abstract an integrated experimental analytical deformational system allows the observation of evolving microstructures in rock analogues under an optical macroscope at controlled confining pressure p c 0 001 0 3 mpa temperature t 10 150 c and strain rate 10 2 10 6 s 1 this system is ideally suited for studying grain scale processes across the complete spectrum of deformational behaviour from brittle cataclastic to fully viscous mylonitic flow the experimental system comprises two main elements 1 a modified urai means deformation rig and 2 a steel pressure vessel that accommodates the deformation rig yet is small enough to fit under an optical microscope first results of controlled confining pressure experiments on fluid bearing polycrystalline norcamphor an organic quartz analogue indicate that deformation involves a combination of dislocation creep and intergranular hydrofracturing leading to the formation of discrete shear surfaces comparable to c planes in mylonite at applied confining pressure the relative activity of brittle and viscous deformation mechanisms varies across the sample this in turn affects the distribution of strain on the sample scale the pressure dependence of strain localization in these experiments is interpreted to reflect a combination of local variations in the effective stresses and strain incompatibilities associated with the experimental simple shear configuration

the origin of shape preferred orientations in mylonite inferences from in situ experiments on polycrystalline norcamphor abstract polycrystalline norcamphor c 7 h 10 o undergoing plane strain simple shear in a see through deformation rig develops intergranular microshears whose activity and orientation is closely related to shape preferred orientation spo of dynamically recrystallized grains and grain aggregates intergranular microshears nucleate at 80 90 to the sample s shear zone boundary szb and rotate synthetically toward this boundary they accommodate increasing amounts of incremental shear strain at angles ranging from 40 to 60 s b orientation with respect to the szb with progressive simple shear both the rotation rate and the amount of strain accommodated by these microshears decrease but strain accommodation increases as the microshears attain a lower angle 10 30 s a orientation to the szb the microshears gradually deactivate as they acquire inclination angles to the szb of less than 10 the deactivation of such microshears is accompanied by the nucleation of fresh high angle microshears this heterogeneous deformation is associated with two shape preferred orientations in the norcamphor mylonite a steep oblique grain spo comprising the long axes of dynamically recrystallized grains 40 60 subparallel to the s b oriented microshears and gently inclined domainal spo 20 subparallel to the s a oriented microshears the domain spo is defined by the length axes of grain aggregates with a uniform crystallographic preferred orientation the angle between domainal and grain spos is a potential measure of the bulk vorticity during mylonitization

brittle to ductile transition due to large strains along the white rock thrust wind river mountains wyoming abstract the deformed foreland of the rocky mountain cordilleras is characterized by major reverse faults that bring precambrian basement rocks to the surface the white rock thrust in the wind river mountains of wyoming is one such fault that developed at fairly shallow levels in the crust detailed study of grain size variations and textures within the fault zone provide a framework for discussion of the mechanics of movement along the fault textural criteria are suggested for distinguishing between dominantly brittle and dominantly ductile deformation a transition from brittle fracturing to ductile deformation is seen with increasing strain and resultant decrease in grain size along the fault zone diffusional creep in the final stages of deformation leads to strain softening and may allow large strains to take place along narrow zones at low deviatoric stresses this may be one way of obtaining large displacements along major faults at shallow levels in the crust

metamorphism transient mid crustal rheology strain localization and the exhumation of high grade metamorphic rocks 1 we present a series of three dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid crustal section in the new england appalachians the natural mid crustal section records evidence for spatially and temporally variable mid crustal strength as a function of metamorphic grade during prograde polymetamorphism our models address changes in strain rate partitioning and topographic uplift as a function of strengthening weakening in the middle crust as well as the resultant changes in deformation kinematics and potential exhumation patterns of high grade metamorphic rock results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates deformation kinematics recorded in the orogen are also affected by strengthening weakening with complete reversals in shear sense occurring as a function of strengthening weakening without changes in plate boundary kinematics

porosity redistribution enhanced by strain localization in crystal rich magmas magma degassing characterized by changes in permeability and porosity distribution has a crucial control on the style of eruption during ascent magma might develop large porosities and crystallize while it is subjected to shear shear in turn enhances complex fabrics that result from the reorganization of the different phases crystals gas melt such fabrics have not yet been evaluated experimentally on a three phase system we performed torsion experiments on a synthetic crystal rich hydrous magma at subsolidus conditions with 11 vol porosity to establish a link between strain partitioning and porosity redistribution crystals induce non newtonian deformation resulting in localization of the shear strain three dimensional microtomography and two dimensional scanning electron microscope imaging show gas accumulation in local microstructures caused by shear induced crystal fabric our data show that strain localization is a mechanism that could enable magma degassing at very low vesicularity

the origin of s c mylonites and a new fault zone model abstract this paper presents a synthesis of experimental data on mechanical behaviour and deformation textures of simulated halite shear zones with special regard to the internal structures of s c mylonites and their mechanical implications halite is the only mineral so far for which a complete transition between brittle shearing deformation to fully ductile shearing flow in the dislocation glide regime has been studied experimentally under large shear strains that may be encountered along faults or plate boundaries moreover deformation textures in the shear zones are similar to those of mylonites and cataclasites thus the experimental results provide an insight into the petrogenesis of fault rocks mylonites in particular and the establishment of a realistic fault model between the brittle regime and the fully ductile regime i e pressure insensitive inelastic regime there exists a wide and distinct regime called semiductile in which deformation textures are nearly identical to those developed in the ductile regime yet the shear resistance is pressure dependent and potentially unstable fault motion can occur in the low pressure part in cooking salt shear zone experiments the characteristic texture in the semiductile and ductile regimes is foliation resembling mylonitic foliation which formed in the direction of maximum elongation in the semiductile regime nearly homogeneous shearing deformation characterized by the formation of uniform and pervasive foliation changes into heterogeneous deformation after a critical shear strain and this strain localization induces potentially unstable fault motion the primary mode of internal slip after the strain localization is slip along y riedel shears parallel to the shear zone boundary interconnected with r 1 and some p riedel shears these shear surfaces of concentrated deformation are very similar to the internal structures of so called s c mylonites hence the semiductile regime is the primary candidate for the formation of s c mylonites and at least some s c mylonites must have formed at seismogenic depths this interpretation is consistent with recently reported interlaced pseudotachylytes and mylonites it is also shown that the size of recrystallized halite grains presumably mostly postdeformational decreases with increasing shear strains but not with the shear stress and this is consistent with the grain size reduction with strain concentration which has been confirmed for many mylonites in the world thus estimation of flow stress from recrystallized grains needs to be done with great care and might often be erroneous existing fault models are critically examined from the viewpoint of the halite experiments and a new fault model is proposed whereby fault zones are divided into brittle semibrittle semiductile and ductile regimes with increasing depth the shear resistance of faults is highest in the semiductile regime and the seismogenic depth is likely to extend down to the upper part of this regime the semiductile regime thus emerges as an area of primary interest with regard to the petrogenesis of many s c mylonites modelling of large and great shallow earthquakes along pre existing faults and plate boundaries and quantitative evaluation of the mechanical interactions of plates across their boundaries

episodic metamorphic reactions during orogenesis the control of deformation partitioning on reaction sites and reaction duration textural unconformities or truncations are common in porphyroblasts with complex inclusion trails they reflect cycles of successive foliations that develop against competent porphyroblasts during orogenesis and are preserved by successive growth increments their truncational character results from shear and dissolution along a particular foliation generating a differentiated crenulation cleavage the increment of porphyroblast growth that follows a textural unconformity may or may not mark a significant compositional change depending on the amount of movement of the rock through p t space between cleavage forming events although historically interpreted to result from a significant metamorphic hiatus most textural unconformities indicate that the reactions involved in the formation of these minerals are episodic during continuous prograde metamorphism starting and stopping as a function of the stage of crenulation of the matrix foliation and the pattern of deformation partitioning such episodic reaction behaviour can only occur for multivariant reactions or successive but different univariant reactions the reason why garnet is the most common porphyroblast to exhibit evidence for episodic reactions is probably the fact that it grows by multivariant reactions over a much wider p t range than most other common porphyroblast phases porphyroblast growth is micrometasomatic it is episodic because a significant reduction of strain occurs within domains of progressive shortening each time continuous progressive shearing domains form on their margins this stops microfracture development across the progressive shortening domains thereby preventing rapid access and interaction of fluid ions and complexes with porphyroblast boundaries shifting patterns of deformation partitioning and resulting small scale juxtaposition of different compositional layers spreads the duration and location of multivariant reactions and causes differential timing of porphyroblast growth along a particular stratigraphic horizon it may also locally preserve metastable metamorphic assemblages in regionally metamorphosing deforming pelites near simultaneous cessation of porphyroblast growth on all rims once continuous differentiated progressive shearing domains have formed nearby precludes fluid recirculation as a significant process for removal of material during cleavage development alternatively diffusion of simple molecules and dissociated ions along actively shearing and micro gaped phyllosilicates with recomplexing in fluid filled microfractures readily explains the control of deformation partitioning on reaction site and reaction duration

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

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

strain localization and fluid pathways in mylonite inferences from in situ deformation of a water bearing quartz analogue norcamphor simple shearing of polycrystalline norcamphor containing 10 15 vol water at near atmospheric pore fluid pressure and a range of constant temperatures 3 35 c and shear strain rates 5 10 5 4 10 4 s 1 induces localization of both strain and fluid flow prior to deformation the water is located at grain triple junctions and pockets along grain boundaries it forms an average dihedral angle of 46 with the surrounding norcamphor grains during initial shearing c1 grain boundaries oriented subparallel to the principal shortening direction dilate and fill with water at 1 15 c the tips of the shear surfaces propagate alternately by intracrystalline plasticity and subcritical fracturing concomitant with dynamic recrystallization in the rest of the sample the episodic interconnection of dilatant shear surfaces is associated with short term increases in displacement rate parallel to the surfaces these surfaces coalesce to form a high strain fluid filled network subparallel to the experimental shear zone however this network never spans the entire length of the shear zone at any given time even after shearing to c 8 5 the deformation is more homogeneous and fewer dilatant shear surfaces develop at higher deformational temperatures and or lower strain rates due to the increased activity of dislocation creep prolonged stress annealing removes most microstructural evidence of the syntectonic fluid pathways dilatant shear surfaces in norcamphor resemble relics of mica filled synmylonitic fractures in dynamically recrystallized quartz from greenschist facies mylonite suggesting that fluid played a similar mechanical role in nature and experiment the coalescence of dilatant fluid filled shear surfaces represents a strain dependent increase in pore connectivity within mylonitic shear zones the experiments indicate that fluid flow along deep crustal mylonitic shear zones is probably limited by the rate at which the tips of the dilatant shear surfaces propagate subparallel to the shearing plane 2000 elsevier science b v all rights reserved

the migration of fluid filled grain boundaries in recrystallizing synthetic bischofite first results of in situ high pressure high temperature deformation experiments in transmitted light the effect of fluids on recrystallization behaviour is well known however the detailed microscale distribution of fluid in grain boundaries and the influence of fluid on grain boundary migration are still unresolved in this study in situ deformation experiments in transmitted light microscopy were undertaken as this allows continuous and direct observation of the whole range of processes involved in fluid assisted grain boundary migration a new see through deformation apparatus was developed to enable the control of fluid pressure bischofite containing small amounts of aqueous fluid was deformed at temperatures between 50 and 90 c over a range of fluid pressure from 0 5 to 1 mpa and strain rates of 5 10 6 to 1 10 4 s 1 the rates of grain boundary migration were measured at different temperatures and strain rates detailed observations during and after the deformation illustrate the evolution of migrating fluid filled grain boundaries and show that the incorporation of fluids from inclusions as well as their pinch off is dependent on the grain boundary velocity the thickness of the grain boundary and the size and shape of the inclusions direct evidence is presented for the contraction of the grain boundary fluids into isolated inclusions after equilibrium conditions are attained

strain hardening and strain softening in fault zones from foreland thrusts the movement of thrust sheets is accomplished in part by deforming the rocks adjacent to thrust faults in foreland fold and thrust belts emplacement generates an array of mesoscopic faults in a thick layer along the base of a sheet ductile deformation of rocks in the layer occurs by displacing fault bounded blocks relative to one another in a kind of mesoscopic grain boundary sliding analyses of data from three southern appalachian thrust sheets indicate that this block sliding is first accompanied by strain hardening then by strain softening and it is accommodated by intra block fractures small faults and veins the density of accommodation features increases near thrusts and where these features become sufficiently numerous a layer of cataclastic rocks forms along the thrust we describe foliated cataclasites from two thrusts the microstructures of which indicate that the cataclasites exhibited extreme ductility interrupted occasionally by brittle processes deformation of the cataclasites was accompanied by strain softening strain softening deformation in fault arrays near thrusts is genetically related to strain softening deformation in cataclasites during the formation of the cataclasite rocks in the sheet are impelled to cross a strain energy hump after which they deform at either higher rates or lower stresses

the interaction of migrating grain boundaries and fluid inclusions in naturally deformed quartz a case study of a folded and partly recrystallized quartz vein from the hunsruck slate germany abstract we studied the microstructure of a folded and partly recrystallized quartz vein from the hunsruck slates in germany focusing on the morphology and distribution of fluid inclusions in the old and new grains and along the different types of grain boundaries blocky vein quartz grains show undulose extinction and develop subgrains new grains form by subgrain rotation and grain boundary migration with a bimodal size distribution the old deformed grains contain numerous complex h2o co2 graphite inclusions with significant differences in fluid inclusion setting along subgrain boundaries the new grains have a lower content of h2o rich inclusions than the old grains and do not contain graphite and there is a significant difference in volume and density of fluid inclusions between the large and small new grains grain boundaries between old and new grains are irregular containing similar fluid inclusions as the old grains but no enrichment in graphite while grain boundaries between new grains are smooth and can be inclusion free or contain arrays of fluid inclusions we interpret these structures to have formed by a series of complex interactions between grain boundaries migrating at different velocity and the fluid inclusions differences in mobility and grain boundary velocity can result in different variations of drag and drop interaction while chemical differences lead to phase separation during grain boundary fluid interaction migration of grain boundaries into the old grains was accompanied by significant redistribution of fluids and graphite along the grain boundary together with oxidation of graphitic inclusions to co2

strain localisation in an orthogneiss laccolith the pinet massif aveyron southern france abstract sheared granitoids of the pinet laccolith southern france display strain gradients from undeformed protolith to s c mylonites and ultramylonites that formed at amphibolite facies of metamorphism we study shear strain distribution at a regional scale and use image processing techniques to measure strain across continuous shear gradients local perturbations of strain intensity seem to represent early stages of shear strain localisation parallel to the main mylonitic foliation our study suggests that localisation of shear strain shows a periodicity at regional and outcrop scales

deep fluid circulation in alpine shear zones pyrenees france field and oxygen isotope studies a combined field stable isotope and whole rock chemical study was made on late cretaceous to tertiary metasomatic shear zones cutting hercynian gneisses in the aston massif pyrenees france mylonitisation occurred during the early stages of alpine compression under retrograde conditions at 400 450 c and about 10 km depth whole rock 18o values of 11 to 12 in the gneisses was lowered to 5 to 9 in the shear zones with the quartz muscovite 18o 16o fractionations of about 2 to 4 essentially unchanged these 18o 16o systematics together with d muscovite 40 to 50 indicate that large volumes of formation waters or d rich meteoric waters passed through the shear zones during deformation the same fluids also redistributed major elements as shown by the correlation of 18o shift with muscovitisation and albitisation reactions in granitic wall rocks however even though 18o was universally lowered within the shear zones the 18o 16o ratios were not homogenised nor do they correlate in detail with the presence or absence of muscovitisation suggesting that fluid flow was probably fracture controlled and episodic field mapping shows that along the length of a particular shear zone muscovitisation of granite gneiss dies out 150m above the contact with underlying sillimanite gneiss thus muscovitisation and albitisation of granite gneiss in shear zones and their wall rocks probably occurred during re equilibration of acidic chloride rich aqueous fluids that had previously moved upward within the shear zones through underlying sillimanite gneiss extremely high material balance fluid rock ratios 103 are required to explain the extent of muscovitisation along this shear zone implying integrated fluid mass fluxes of about 108 kg m2 this is probably close to the maximum value for other shear zones in the network similar volumes of a more chemically evolved fluid must have passed through the unmuscovitised mylonites showing that the absence of alteration cannot necessarily be used to infer low values of fluid flux for reasonable pressure gradients and time scales of fluid movement effective permeabilities of 10 15 to 10 17 m2 are required such values can be accounted for by short lived widely spaced cracks produced during seismic activity a model is presented in which formation waters were seismically pumped down an underlying shallow southward dipping decollement and then upward through the steeply dipping shear zone network

states of stress and slip partitioning in a continental scale strike slip duplex tectonic and magmatic implications by means of finite element modeling abstract orogenic belts at oblique convergent subduction margins accommodate deformation in several trench parallel domains one of which is the magmatic arc commonly regarded as taking up the margin parallel strike slip component however the stress state and kinematics of volcanic arcs is more complex than usually recognized involving first and second order faults with distinctive slip senses and mutual interaction these are usually organized into regional scale strike slip duplexes associated with both long term and short term heterogeneous deformation and magmatic activity this is the case of the 1100 km long liquine ofqui fault system in the southern andes made up of two overlapping margin parallel master faults joined by several ne striking second order faults we present a finite element model addressing the nature and spatial distribution of stress across and along the volcanic arc in the southern andes to understand slip partitioning and the connection between tectonics and magmatism particularly during the interseismic phase of the subduction earthquake cycle we correlate the dynamics of the strike slip duplex with geological seismic and magma transport evidence documented by previous work showing consistency between the model and the inferred fault system behavior our results show that maximum principal stress orientations are heterogeneously distributed within the continental margin ranging from 15 to 25 counter clockwise with respect to the convergence vector in the master faults and 10 19 clockwise in the forearc and backarc domains we calculate the stress tensor ellipticity indicating simple shearing in the eastern master fault and transpressional stress in the western master fault subsidiary faults undergo transtensional to extensional stress states the eastern master fault displays slip rates of 5 to 10 mm yr whereas the western and subsidiary faults show slips rates of 1 to 5 mm yr our results endorse that favorably oriented subsidiary faults serve as magma pathways particularly where they are close to the intersection with a master fault also the slip of a fault segment is enhanced when an adjacent fault kinematics is superimposed on the regional tectonic loading hence finite element models help to understand coupled tectonics and volcanic processes demonstrating that geological and geophysical observations can be accounted for by a small number of key first order boundary conditions

anisotropic grain growth in quartz aggregates under stress and its implication for foliation development fine grained quartz aggregates show significant grain growth during high temperature 700 1000 c and high pressure 400 mpa deformation experiments when water is present flattening of grains that exceeds the deformation induced flattening and a resultant clear foliation are found in the samples recrystallized at relatively low temperatures and high strain rates i e high deviatoric stresses the distribution of water as inferred from the distribution of pore space is also anisotropic in these samples in contrast the samples recrystallized at relatively high temperatures and low strain rates i e low deviatoric stresses show small grain flattening and almost isotropic water distribution the observed very large flattening under high stress indicates a significant contribution of anisotropic grain growth because the anisotropy of grain growth is correlated with the anisotropy of water distribution we suggest that the observed foliation under high stress is due to anisotropic grain growth caused by enhanced mass transport through water foliation caused by this mechanism will be related to the stress orientation

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

quantitative modeling of fault fluid discharge and fault dilation induced fluid pressure variations in the seismogenic zone the rise of fluid pressure along a fault plane can trigger an earthquake as shear strength decreases recent models have thus suggested that the rate of interseismic fluid pressure increase may control earthquake recurrence in electric analog model simulations incorporating new experimental data on rock properties within the lower seismogenic zone resultant fluid pressure fluctuations are restricted to the fault plane and are strongly attenuated during their slow propagation into the country rock fluid pressure drops triggered by coseismic fault dilation require days to hundreds of years to propagate over distances 1 m into low permeability country rock 10 17 m2 the equilibration of fluid pressure between the faulted and the intact country rock requires tens to hundreds of years if the fault seals prior to this equilibration the fluid pressure recovers instantaneously to near the prefailure value these results may imply that if elevated fluid pressure weakens transcrustal fault zones they are likely to remain weak after earthquakes

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

synkinematic microscopy of transparent polycrystals abstract deformation of weak polycrystalline materials in thin section on the stage of a petrographic microscope offers insight into the origin of optical microstructures similar to microstructures in naturally deformed rocks sample materials used to date include several soft minerals ice carnallite bischofite nitratite and various organic plastic materials they can be simple sheared or pure sheared at temperatures to 300 c at a variety of strain rates using relatively simple and inexpensive apparatus the complete microstructural evolution can be recorded photographically along with the complete kinematic history research applications have focussed on dynamic recrystallization and progress has been made towards understanding how the average grain size is maintained constant in steady state recrystallization flow records have been obtained of the development of various crystal plastic microstructures and of lattice reorientation trajectories during flow teaching applications have begun to be developed and several videotapes are available future technical developments are expected to permit stress measurement or control during experiments and synkinematic recording of lattice reorientation numerous research areas invite attention these include investigation of the microstructural developments that precede faulting the behaviour of deforming crystal melt systems and the behaviour of dynamically reactive multi phase systems