rare earth element mobility during prograde granulite facies metamorphism significance of fluorine mafic gneisses occur as lenses or thin layers in spatial association with tonalitic leucosomes in a granulite zone of the quetico subprovince of the superior province ontario canada and exhibit concentric zoning with a biotite rich margin orthopyroxene rich outer zone clinopyroxene rich central zone and occasionally patches of relict amphibolites within the clinopyroxene rich zone the granulites biotite orthopyroxene and clinopyroxene rich zones in the mafic gneisses are characterized by significant amounts of rare earth element ree bearing fluorapatite 1 10 vol and other ree rich minerals allanite monazite and zircon fluorapatite shows an increase in modal abundance from the biotite and orthopyroxene rich zones to the clinopyroxene rich zone but is rare in the relict amphibolites textural evidence and element partitioning indicate that fluorapatite and other ree rich minerals was part of the peak metamorphic assemblages whole rock geochemical analyses confirm that the granulites in the mafic gneisses contain anomalously high contents of ree and high field strength elements hfse whereas the relict amphibolites are geochemically typical of tholeiitic basalts mass balance calculations reveal that ree and hfse were introduced into the mafic gneisses during the prograde granulite facies metamorphism pointing to ree mobility under granulite facies metamorphic conditions the presence of high f contents in the ree rich minerals and their associated minerals e g biotite and hornblende suggests that ree and hfse may have been transported as fluoride complexes during the granulite facies metamorphism this conclusion is supported by previously published results of hydrothermal experiments on the partitioning of ree between fluorapatite and f rich fluids at 700 c and 2 kbar

quartz fabrics in the alpine fault mylonites influence of pre existing preferred orientations on fabric development during progressive uplift strong quartz crystallographic preferred orientations cpos were developed during dislocation creep in the mylonite zone exposed in the hanging wall of the dextral reverse alpine fault zone new zealand the cpos have a consistent asymmetry indicating a high ratio of simple to pure shear strain with a shear sense of dextral up to the nw consistent with the mesoscopic shear sense indicators and with slip on the active fault there is a transition from y maxima and asymmetric single girdles in mylonites and ultramylonites within 300 m of the present fault trace to cross girdle fabrics in the protomylonites further from the fault the strong y maxima or single girdle cpos are ascribed to high ductile shear strains under amphibolite facies conditions while the cross girdle patterns found in the protomylonites are interpreted to represent deformation under lower temperature conditions however the observed fabric transition cannot logically be attributed to variations in temperature during the last increment of deformation the highly oriented y maximum fabrics formed at high temperature contain very few grains suitably oriented for basal slip so that the slip systems activated during subsequent shear at lower temperatures during exhumation were prism or rhomb further from the fault where shear strains under high temperature conditions were lower weaker fabrics developed under these conditions were modified at higher levels in the crust into crossed girdle patterns this interpretation implies that intense localisation of shear strain along the fault zone within the lower crust must have occurred in order for the high temperature fabrics to become sufficiently intense to be preserved this in turn implies that deep seated localised shear was taking place early during the evolution of the current oblique slip plate boundary fault the results of this study also indicate that the current model that operation of certain slip systems in quartz is mostly a function of temperature is incorrect future work should consider the effects of deformation history total strain and the presence of other mineral phases

very high strains recorded in mylonites along the alpine fault new zealand implications for the deep structure of plate boundary faults abstract oblique displacement on the alpine fault which forms the principal structure along the australian pacific plate boundary in south island new zealand has resulted in exhumation of a kilometre wide mylonite zone in the hanging wall adjacent to the current brittle fault trace the mylonites formed under amphibolite facies conditions at depths of ca 25 km and have been uplifted during the past 5 ma a suite of 65 70 ma pegmatite veins in the hanging wall alpine schists has been progressively deformed within the mylonite zone and sheared out over a strike length of ca 100 km measurements of the thickness distribution of the pegmatite veins within the non mylonitised schists and at three localities within the progressively strained mylonites have been used to estimate strain values within the mylonites the thicknesses approximate a log normal distribution with a mean value that is progressively reduced through the protomylonites mylonites and ultramylonites by assuming that the thickness distribution currently observed in the schists was the same for the pegmatites within the mylonites before strain a model of deformation incorporating simple shear and simultaneous pure shear is used to strain the undeformed veins until a fit is obtained with the strained distributions shear strains calculated range from 12 to 22 for the protomylonites 120 to 200 for the mylonites and 180 to 300 for the ultramylonites corresponding to pure shear values of 1 3 in each case these values are compatible with the strains predicted if most of the surface displacement on the fault over the past 5 ma were accommodated within a 1 2 km wide mylonite zone through the middle and lower crusts the results suggest that processes such as erosional focussing of deformation and thermal weakening may cause intense strain localisation within the lower crust with plate boundary deformation restricted to narrow zones rather than becoming increasingly distributed over a widening shear zone with depth

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

grain boundary migration of water and carbon dioxide during uplift of garnet zone alpine schist new zealand deformed quartz veins in garnet zone schist adjacent to the active alpine fault new zealand have fluid inclusions trapped along quartz grain boundaries textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins many of the inclusions are empty but some contain carbon dioxide with densities that range from 0 16 to 0 80 g cm 3 no water nitrogen or methane was detected the inclusions are considerably more co2 rich than either the primary metamorphic fluid 5 co2 or fluids trapped in fracture related situations in the same or related rocks 50 co2 enrichment of co2 is inferred to have resulted from selective migration wicking of saline water from the inclusions along water wet grain boundaries after cooling induced immiscibility of a water co2 mixture inclusion volumes changed after loss of water non wetting co2 remained trapped in the inclusions until further percolation progressively removed co2 in solution this mechanism of fluid migration dominated in ductile quartz rich rocks near but below the brittle ductile transition at deeper levels hydraulic fracturing is also an important mechanism for fluid migration whereas at shallower levels advection through open fractures dominates the fluid flow regime

hydrologic segmentation of high temperature shear zones structural geochemical and isotopic evidence from auriferous mylonites of the renco mine zimbabwe abstract combined structural mineralogical and geochemical observations in auriferous mylonites of the renco mine hosted by late archaean high grade metamorphic granitoids in southern zimbabwe are used to describe the spatially heterogeneous fluid flow and metasomatism that occurred synchronous with deformation at mid to upper amphibolite facies metamorphic conditions significantly the narrow on average 1m wide mylonitic shear zones are internally zoned reflecting a pronounced hydrologic segmentation during deformation shear zones typically consist of two distinct domains i anastomosing quartz feldspar biotite hornblende mylonites and or quartz mylonites and ii tabular shaped pods referred to as lithons that are enveloped by mylonites and that exhibit evidence of transient episodes of brittle fracturing and ductile creep whole rock geochemistry and mass balance calculations indicate dramatic element volume up to 100 and associated mass gains for the brittle ductile lithons that are mineralogically reflected in a volumetrically abundant sulphide mineralisation and the formation of a pervasively developed silicate alteration paragenesis in contrast mylonites have experienced only minor element and volumetric changes and minor alteration 18 o values for whole rocks and quartz are enriched in lithons compared to wall rocks and enveloping mylonites which implies the influence of externally derived fluids these results indicate a strongly domainal fluid flow and mass transfer fluid advection was dominated by microscopic and macroscopic fracture permeabilities related to periods of coseismic dilatancy in lithons the patchy but partly interconnected distribution of lithons enveloped by mylonites indicates that deformation in the high temperature shear zones was characterised by transient periods of seismic slip rather than continuous aseismic creep based on the well preserved internal mineralogical and textural development of the renco shear zones a more general model applicable for fluid advection coupled with the rheological behaviour of mid crustal shear zones is presented

tectonometamorphic evolution of the koralm complex eastern alps constraints from microstructures and textures of the plattengneis shear zone abstract lattice preferred orientations lpo of quartz have been investigated along a south north oriented section across the plattengneis of the koralm complex eastern alps the plattengneis forms an important shear zone within the austroalpine nappe complex of the eastern alps which has developed during the cretaceous evolution of the alpine orogen the quartz c axes form small circle distributions in the southernmost parts of the koralm complex which represent the uppermost structural level of the plattengneis further to the north two maxima between the y and z directions of the finite strain can be interpreted in terms of preferred slip on the rhomb planes these fabrics continuously grade into type i and type ii crossed girdle distributions in a northward direction a strong maximum near the y axis with the tendency to be distributed along a single girdle with three corresponding maxima of a axes near the margin of the pole figure can be observed in the central and northern parts such lpo are characteristic for both high grade metamorphic conditions and high finite strain the microstructures display that the deformation within the plattengneis shear zone was synmetamorphic a continuous increase of peak temperatures and pressure from approximately 550 c to approximately 750 c from the south to the central parts can be inferred from geothermometric calculations the temperatures then decrease to approximately 650 c from the central parts to the north the related pressures increase from 8 to 16 kbar and then decrease to 10 kbar the lpo changes that have been observed in the study area are best interpreted in terms of temperature dependence of the activation of glide systems within quartz aggregates the temperature and pressure evolution may indicate that the central parts of the koralm complex have been exhumed by larger amounts than the northern and southern parts this is also documented by the lpo evolution therefore we assume that the plattengneis shear zone formed during the exhumation of the koralm complex and is related to the exhumation of high pressure units in the footwall of this shear zone accordingly the kinematics of the plattengneis shear zone is rather extensional than thrust related the implications for the structural evolution of the eastern alps are shortly discussed

the effect of dislocation density on the aqueous solubility of quartz and some geologic implications a theoretical approach the internal energy associated with dislocations is calculated in quartz as a function of dislocation density these calculations show that at dislocation densities of 109 1010 1011 and 1012 cm dislocation cm3 the energy stored per mole of quartz is 2 2 19 7 170 and 1426 j respectively the density of free dislocations in natural quartz rarely exceeds 109 line cm cm3 and the resulting increase in free energy is probably too small to affect any geochemical processes significantly this applies to the overwhelming volume of rocks in the earth s crust the density of dislocations in tangles may exceed 1011 line cm 2 which is significant but the volume of quartz containing tangles is a very small fraction of most quartz grains nevertheless where fluid rock ratios are low the aqueous activity of sio2 may be significantly increased by the dissolution of quartz containing dislocation tangles such dissolution could be geologically significant in active greenschist facies fault zones and mylonites where repeated cold working would produce a sustained source and large population of dislocation tangles in such a setting the locally high internal energies of quartz containing dislocation tangles would lead to strain solution the resulting high activity of aqueous silica would enhance the stability of feldspars and would establish chemical potential gradients of sio2 which would drive the diffusive mass transfer of sio2 out of the fault zone

deformation mechanisms in second phase affected microstructures and their energy balance based on the relationship zener parameter z second phase size second phase volume fraction vs calcite grain size dg second phase controlled aggregates and microstructures that are weakly affected by second phases are discriminated the latter are characterized by large but constant grain sizes high calcite grain boundary fractions and crystallographic preferred orientations cpo while calcite grain size and calcite grain boundary fraction decrease continuously and cpo weakens with decreasing z in second phase controlled microstructures these observations suggest that second phase controlled microstructures predominantly deform via granular flow because pinning of calcite grain boundaries reduces the efficiency of dynamic recrystallization favoring mass transfer processes and grain boundary sliding in contrast the balance of grain size reduction and growth by dynamic recrystallization maintains a steady state grain size in microstructures that are only weakly affected by second phases promoting a predominance of dislocation creep with increasing temperature the relationship between z and dg persists but the calcite grain size increases continuously based on microstructures the energy of each modifying process is calculated and its relative contribution is compared with energies of the competing processes surface energy dragging energy dynamic recrystallization energy the steady state microstructures result from a temperature dependent energy minimization procedure of the system

an sem study of porosity and grain boundary microstructure in quartz mylonites simplon fault zone central alps a backscattered and secondary electron sem study of the grain boundary microstructure in quartz mylonites sampled along the length of the retrograde simplon fault zone established three characteristic components 1 fine isolated pores 1 m diameter are scattered across two grain interfaces preferentially concentrated on surfaces in extension pores are uncommon on three grain junctions and there is no evidence for fluid interconnectivity along three and four grain junctions the fine porosity may develop by accumulation of original mainly intragranular fluid inclusions to the grain boundary during deformation and recrystallization and by cavitation of grain boundaries during grain boundary sliding dynamic cavitation implies that the ductile mylonitic deformation is at least locally dilatant and therefore pressure sensitive 2 large vug like pores up to mm scale extend along multi grain boundaries observed in all samples they are most common in the higher initial temperature coarse grained samples with a microstructure dominated by grain boundary migration recrystallization grains bordering this connected porosity develop perfect crystal faces undecorated by fine pores or pits the irregular lobate optical microstructure of many migrating grain boundaries actually consists of a series of straight crystal faces the coarse porosity is probably due to accumulation during dynamic recrystallization of co2 rich fluid with a high wetting angle against quartz 3 in one sample interconnected sinuous ridges 0 2 m high are observed to follow three and four grain junctions and disjoint into more isolated worms and spheroidal globules on two grain interfaces these are transitional to more branching vein like or convoluted brain like forms the brain like and globular forms have been observed with varying frequency through the range of samples with the globules attaining sizes of up to 60 m vein structures have also been observed on intragranular fractures these topologies do not match across adjoining surfaces and must have developed into free space the ridge vein brain spheroid structure is distinctly different to that previously observed on experimentally healed microcracks and its origin is not unequivocally established they could represent unstable meniscus necking of a thin grain boundary phase of low viscosity developed due to quasi adiabatic shear and or local stress induced dilatancy during microcracking

inclusion localised crystal plasticity dynamic porosity and fast diffusion pathway generation in zircon abstract a population of oscillatory zoned igneous zircon grains in a javanese andesite contains fluid and mineral inclusions up to 10 m across trapped during zircon growth orientation contrast imaging and orientation mapping by electron backscatter diffraction reveal that crystal plastic deformation overprints growth zoning and has localized around 1 10 m pores and inclusions cumulative crystallographic misorientation of up to 25 around pores and inclusions in zircon is predominantly accommodated by low angle 010 slip a distinctly different population of sub micron pores is present along subgrain boundaries and their triple junctions these are interpreted to have formed as a geometric consequence of dislocation interaction during crystal plasticity dislocation creep microstructures are spatially related to differences in cathodoluminescence spectra that indicate variations in the abundance of cl active rare earth elements the extent of the modification suggests deformation related fast pathway diffusion distances that are over five orders of magnitude greater than expected for volume diffusion this enhanced diffusion is interpreted to represent a combination of fast diffusion pathways associated with creep cavitation dislocations and along low angle boundaries these new data indicate that ductile deformation localised around inclusions can provide fast pathways for geochemical exchange these pathways may provide links to the zircon grain boundary thus negating the widely held assumption that inclusions in fracture free zircon are geochemically armoured once they are physically enclosed

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

shallow level metamorphic fluids in a high uplift rat metamorphic belt alpine schist new zealand fluids some of which are co2 rich up to 40 mol co2 and some of which are highly saline up to 18 wt nacl equivalent are trapped as fluid inclusions in quartz calcite metallic minerals veins which cross cut the pumpellyite actinolite to amphibolite facies rocks of the alpine schist fluids were commonly trapped as immiscible liquid vapour mixes in quartz and calcite showing open space growth textures fluid entrapment occurred at fluid pressures near 500 bars possibly as low as 150 bars at temperatures ranging from 260 to 330 c saline fluids may have formed by partitioning of dissolved salts into an aqueous phase on segregation of immiscible fluids from a low density co2 rich fluid calcite deposited by these fluids has 13c ranging from 8 4 to 11 5 and 18o from 4 to 13 isotopic data fluid compositions and mode of occurrence suggest that the fluids are derived from high grade metamorphic rocks fluid interaction with wall rock has caused biotite crystallization and or recrystallization in some rocks and retrogression of biotite to chlorite in other rocks fluid penetration through the rock is almost pervasive in many areas where permeability probably related to alpine fault activity has focussed fluids on a regional scale into fractured rocks the fluid flow process is made possible by high uplift rates in excess of 10 mm year bringing hot rocks near to the surface

evolution and structure of the mesoscale convection and its environment a case study during the early onset of the southeast asian summer monsoon abstract the evolution and structure of mesoscale convection in the south china sea scs region are documented for the first time mainly using the dual doppler radar dataset collected during the south china sea monsoon experiment scsmex in 1998 in particular this study focuses on the convection associated with a subtropical frontal passage during the early onset of the southeast asian monsoon seam for the case of 15 may 1998 interaction between the tropical monsoon flow and frontal circulation played an important role in the evolution and structure of mesoscale convection in the prefrontal region the southwesterly monsoon flow converged with the southwesterly frontal flow to generate northeast to southwest oriented convection in the postfrontal region the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east to west orientation in addition the convergence between the southerly monsoon flow and the northerly postfrontal flow gener

a few remarks on the kinetics of static grain growth in rocks static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature given the relative simplicity of the protocol for grain growth experiments measurements of grain boundary mobility show surprising variations boundary mobilities during grain growth are affected by solute and impurity chemistry chemical fugacity of trace and major elements pore size and number pore fluid chemistry the presence of melts and the presence of solid second phases as well as temperature and pressure all of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments provided that the necessary kinetics parameters are known bounds may be placed on the interface mobility when pores partial melts or solutes are present to predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

a new estimate of the conditions for transition from basal a to prism c slip in naturally deformed quartz abstract quartz c axis fabrics of naturally deformed quartz in metacherts from the ryoke metamorphic belt in the yanai district southwestern japan suggest that the prism c prism a and basal a slip systems were activated during the plastic deformation under high temperature metamorphic conditions with increasing temperature quartz c axis fabric patterns change from girdle type through x maximum with a faint crossed girdle to x maximum the dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature closely correlated with the change in quartz c axis fabric the fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size moreover observations with transmission electron microscopy tem clearly identify the dominant activation of c slip through the presence of basal subgrain boundaries and free dislocations with the 0001 burgers vector in the quartz from the metachert samples exhibiting x maximum c axis fabric based on these observations it is concluded that the mechanism switch from basal a to prism c slip systems occurred with increasing temperature in the metacherts the transition temperature is petrologically estimated to be ca 550 600 c at the natural strain rate

grain boundary dissolution porosity in quartzofeldspathic ultramylonites implications for permeability enhancement and weakening of mid crustal shear zones abstract quartzofeldspathic ultramylonites from the alpine fault zone one of the world s major active plate boundary scale fault zones have quartz crystallographic preferred orientations cpo and abundant low angle 10 misorientation are decorated by faceted pores commonly with uniformly oriented pyramidal shapes only grain boundaries with 10 misorientation angles in polymineralic aggregates are decorated by pores mean grain boundary pore densities are 5 108 cm 2 grain boundary pores are dissolution pits generated during syn deformational transient grain boundary permeability nucleating on dislocation traces at dilatant grain boundary interfaces they have not been removed by subsequent grain boundary closure or annealing pore decoration could have led to grain boundary pinning triggering a switch in the dominant deformation mechanism to grain boundary sliding which is supported by evidence of cpo destruction in matrix quartz pore decorated grain boundaries have significantly reduced surface area available for adhesion and cohesion which would reduce the tensile and shear strength of grain boundaries and hence the bulk rock grain boundary decoration also significantly decreased the mean distance between pores potentially facilitating dynamic permeability consequently these microstructures provide a new explanation for strain weakening and evidence of fluid flow along grain boundaries in mylonites at mid crustal conditions

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

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

deformation of granitic rocks experimental studies and natural examples crustal deformation is inherently complex because the crust is inhomogeneous and anisotropic on a wide variety of scales advances in our understanding have come from a combination of field observations laboratory experiments and theoretical models what necessarily links all these investigations is a focus on process crystal plasticity dislocation creep is one of several grain scale deformation processes or mechanisms each process is dominant over a certain range of conditions in the crust has a particular form of the flow law and produces a characteristic set of microstructures characterizing the deformation mechanisms for different minerals over a range of conditions allows one to link the mechanical behavior or flow law associated with that process to the microstructures it produces the information on microstructures can be used to help interpret the thermomechanical history of naturally deformed rocks and flow laws can be used to create more realistic models predicting crustal behavior in different tectonic settings iterations among field theoretical and experimental approaches are critical for making progress in our understanding the deformation process of crystal plasticity is the main focus of this volume crystal plastic deformation of the crust is in many ways more complex than that of the mantle because most crustal rocks are polyphase aggregates and the strengths of common crustal silicates such as quartz feldspar and mica are very different at a given set of conditions yield strength envelopes for the crust as shown schematically in figure 1 are commonly plotted using the dislocation creep flow law for quartz which inherently assumes that quartz is the controlling phase and undergoes steady state flow but is the flow law for dislocation creep of quartz a good approximation for the deformation of a granite let alone a crustal plum pudding of regions and layers of different lithologies the crust

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

how ductile are ductile shear zones three premises of ductile deformation in the middle and lower crust are widely accepted 1 rocks flow with power law viscous rheology 2 localization in ductile shear zones involves strain softening and 3 fluid flows into and is channelized within ductile shear zones ductile viscous shear zones should therefore initially develop along planes of maximum shear stress strain soften and rotate as material planes into the field of progressive extension however the mean stress or pressure within a weak elongate viscous band being stretched is higher than the surrounding matrix and this is difficult to reconcile with premise 3 in contrast mohr coulomb brittle faults always have lower pressure within the fault zone flow of fluid and melt into high temperature shear zones therefore implies that ductile shear zones are not perfectly viscous but have a pressure dependent viscoplastic rheology the continued pressure dependence may reflect significant microcracking on the grain scale even when localized deformation does not produce larger scale discrete fractures

thermal regime in the central alpine fault zone new zealand constraints from microstructures biotite chemistry and fluid inclusion data abstract the thermal state within plate boundary fault zones is likely to influence the dimensions of large co seismic fault ruptures and strain localisation in underlying mylonitic shear zones vein microstructures biotite chemistry and fluid inclusion data offer new insights into the thermal structure of the mylonitic shear zone down dip of the alpine fault the main pacific australian plate boundary structure through southern new zealand quartz veins within the fault rocks were formed and deformed at temperatures ranging from 500 c to immediately above that of the brittle viscous transition 325 c illustrating that fluids are present throughout the fault zone microthermometric data from primary mixed co 2 h 2 o fluid inclusions in boudin neck quartz carbonate veins indicate that these inclusions were trapped at temperatures of 325 15 c and pressures of c 40 mpa vein microstructures indicate that fluid temperatures were equal to rock temperatures assuming hydrostatic fluid pressure the veins were trapped at depths of 2 o co 2 fluids in the shallower crust of 500 kg m 3 biotite grains in the mylonites are a mixture of porphyroclastic grains with high r ti mg inherited from the parent alpine schist and smaller low r grains that recrystallised during mylonitisation particularly those that are found within syn mylonitic shear bands assuming a simple step function model approximating diffusive equilibration the changes in biotite chemistry indicate t during mylonitic deformation of 340 c according to the ti in biotite thermometer of henry et al 2005 a best fit uplift p t path to existing data and these new results indicate that the uplift path p t gradient of fault zone rocks above the brittle viscous transition is of the order of 40 c km 1 below the brittle viscous transition the uplift path p t gradient is 10 c km 1 the p t profile within the fault rocks is comparable to that in the immediate hanging wall alpine schist indicating there is no peak in the crustal isotherms in the fault hanging wall as was indicated by previous numerical models and analyses that did not consider data from the fault rocks themselves

the transport of u and th series nuclides in sandy confined aquifers a comprehensive evaluation of the transport of u th ra and rn nuclides of the 238 u and 232 th decay series in an unconfined sandy aquifer long island ny was conducted groundwater data are compared with results of a theoretical transport model of weathering of aquifer grains and interaction with surface coatings to establish relationships between the concentrations of the radionuclide activities in the water and flow line distance the data provide estimates for geochemical parameters including weathering rates and chemical reactivities in both the vadose zone and the aquifer a theoretical treatment of the transport is presented that considers the reaction between the water and a reactive surface layer it is found that a model with chemical exchange between the surface layer and the water for all species is not valid and that the effects of saturation and irreversible precipitation of th is required the water table shows a relatively wide range in u activities the only element in the u th series for which vadose zone input is significant in the aquifer high weathering of u and recoil inputs of 234 u to the water occur in the upper 3 m of the vadose zone while lower weathering and removal of u from the water occur below the deeper aquifer has variable 238 u activities that can be accounted for by input from the vadose zone and is not a result of non conservative behavior the isotopic composition of u is shown to be directly related to the recoil rate relative to the weathering rate the wide range of 238 u in the aquifer waters is a reflection of diverse vadose zone inputs showing that dispersive mixing is not a dominant effect the higher values of 234 u in the aquifer reflect the recoil weathering input ratios from within the aquifer where the weathering rate is lower than the vadose zone both high u activities and high 234 u values cannot be obtained in the vadose zone or within reasonable flow distances in the aquifer radium isotopes are found to be in exchange equilibrium with the surface layer 224 ra 228 ra and 226 ra have comparable activities throughout the aquifer in the vadose zone the dominant input of ra to groundwater is weathering and recoil as found elsewhere the 222 rn in the water is a large fraction 5 of the rn produced in the aquifer rock this cannot be due to ra precipitation onto surface coatings in the aquifer as supported by present weathering with th in exchange equilibrium with the surface layer it is found that th is saturated in the waters under oxidizing conditions so that the weathering input is irreversibly precipitating onto surfaces however it is shown that under somewhat reducing conditions th activities are much higher and the th u ratio in the solution is approximately that of the rock we propose that under oxidizing conditions the source of rn is a surface coating enriched in 232 th and 230 th this th was precipitated in an earlier phase during rapid dissolution of readily weathered phases that contain 10 of the u th inventory of the rock with the associated u carried away in solution therefore the previously precipitated 230 th and 232 th produce daughter nuclides in the surface coating which are the dominant contributors of ra and rn to the ground water in particular rn is provided by very efficient losses by diffusion or recoil from the surface coating this then does not require recent large recoil losses from the parent rock or the presence of nanopores in the rock the first data of both long lived 232 th and short lived 234 th and 228 th in ground water is reported the th isotope activities indicate that desorption kinetics are slow and provide the first estimate based on field data of the th desorption rate from an aquifer surface the mean residence time of th in the surface coating is 3000 y while in the water it is 1 h ra is in partition equilibrium with the aquifer surface layer however the strong fixation of th on surface coatings is very susceptible to changes in oxidation state as is shown by a comparison of two adjacent aquifers this makes it difficult to define with certainty the retentive characteristics in natural systems in general it is shown that the distributions of naturally occurring nuclides can be used to calculate values for transport parameters that are applicable to the transport of anthropogenic nuclides

how do lineations reflect the strain history of transpressive shear zones the example of the active alpine fault zone new zealand abstract lineations within mylonites exhumed in the hanging wall of new zealand s active alpine fault zone have a complicated relationship to contemporary plate kinematics the shear zone is triclinic and macroscopic object lineations are not usually parallel to the simple shear direction despite high total simple shear strains 150 this is mostly because the lineations are inherited from pre mylonitic fabrics and have not been rotated into parallelism with the mylonitic stretching direction which pitches c 44 in the fault plane furthermore some lineations have been variably rotated depending on whether they are present in shear bands or microlithons which accommodated bulk strains with different vorticities total strains required to obtain parallelism between the finite maximum principal stretching direction calculated from transpression models and these mylonitic lineations are pure shear stretch s 1 3 5 simple shear 11 7