antarctic snow accumulation mapped using polarization of 4 3 cm wavelength microwave emission different parts of antarctica receive different amounts of snowfall each year in this paper we map the variations of the mean annual snow accumulation across the ice sheet we also quantify the uncertainty in our estimates more objectively than has been possible for earlier maps the new map is produced using observations from satellites and ground based measurements after a logarithmic transformation these are combined using the geostatistical method of continuous part universal kriging to give an estimate of the snow accumulation within each cell of a rectangular grid covering antarctica we also derive spatial averages over the major drainage systems of the ice sheet along with their confidence intervals we obtain a value of 143 4 kg m 2 a 1 for the average rate of snow accumulation upon the grounded ice sheet of antarctica

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

the effects of quartz dauphine twinning on strain localization in a mid crustal shear zone abstract we use microstructural electron backscatter diffraction ebsd and crystal vorticity axis cva analyses to evaluate the effects of quartz dauphine twinning on strain localization we compare a granodiorite mylonite and a tonalite mylonite from the grebe shear zone a mid crustal transpressional shear zone in fiordland new zealand ebsd analysis reveals identical quartz grain boundary migration dynamic recrystallization microstructures in both samples with abundant dauphine twinning only in the granodiorite mylonite bulk cva patterns further distinguish the untwinned and dauphine twinned samples indicating a transition from simple shear to pure shear dominated transpression respectively the quartz cva pattern from the dauphine twinned sample indicates that twinned and untwinned grains record a transition from simple shear to pure shear dominated deformation respectively pole figures of the dauphine twinned sample reveal high temperature slip systems in the untwinned and dauphine twinned grains we conclude that dauphine twinning formed early in transpression and rendered the twinned grains less deformable relative to the untwinned grains over time the effectiveness of dauphine twinning in localizing strain is therefore strongly influenced by its timing relative to the evolving shear zone kinematic deformation geometry

kinematic evolution of the ama drime detachment insights into orogen parallel extension and exhumation of the ama drime massif tibet nepal abstract the ama drime massif is a north south trending antiformal structure located on the southern margin of the tibetan plateau that is bound by the ama drime and nyonno ri detachments on the western and eastern sides respectively detailed kinematic and vorticity analyses were combined with deformation temperature estimates on rocks from the ama drime detachment to document spatial and temporal patterns of deformation deformation temperatures estimated from quartz and feldspar microstructures quartz c axis fabrics and two feldspar geothermometry of asymmetric strain induced myrmekite range between 400 and 650 c micro and macro kinematic indicators suggest west directed displacement dominated over this temperature range mean kinematic vorticity estimates record early pure shear dominated flow 49 66 pure shear overprinted by later simple shear 1 57 pure shear high strain 36 50 shortening and 57 99 down dip extension dominated flow during the later increments of ductile deformation exhumation of the massif was accommodated by at least 21 42 km of displacement on the ama drime detachment samples from the nyonno ri detachment were exhumed from similar depths we propose that exhumation on the nyonno ri detachment during initiation of orogen parallel extension 11 13 ma resulted in a west dipping structural weakness in the footwall that reactivated as the ama drime detachment

determining vorticity axes from grain scale dispersion of crystallographic orientations kinematic shear sense indicators are critical tools for tectonic interpretation of shear zones the appropriate plane in which to interpret shear sense indicators is the vorticity normal surface which is often inferred using fabric foliation lineation orientation strain modeling however suggests that such fabrics can be unreliable for determining a kinematic framework we demonstrate the application of a new quantitative method crystallographic vorticity axis cva analysis that utilizes rotation statistics to calculate dispersion axes from crystallographic orientations at the grain scale we apply cva analysis to samples from three shear zones that exhibit distinct kinematics and deformation geometries in all cases the aggregate of calculated grain scale dispersion axes yields a preferred axis of crystallographic vorticity at the specimen scale that is coincident with the independently determined bulk vorticity axis this new method allows the position of vorticity axes to be evaluated independently of foliation and lineation information and without assumptions about the kinematics of deformation

processes controlling vertical coupling and decoupling between the upper and lower crust of orogens results from fiordland new zealand the pre cenozoic configuration of western new zealand allows determination of the effects of magmatism and a changing lower crustal rheology on the evolution of a cretaceous orogen from upper to lower crustal levels 10 50 km beginning at 126 ma a composite batholith dominated by diorite was emplaced into the lower crust during emplacement deformation was partitioned into zones weakened by magma and heat leading to the development of two layer parallel shear zones at the upper and lower contacts of the batholith transient vertical decoupling of the crust above and below the batholith occurred from 126 ma until 120 ma as magma was emplaced into and moved through a weak thick lower crust by 116 ma however much of the batholith had crystallized and the lowermost crust had cooled from 750 c t 850 c to t 650 700 c cooling was aided by the juxtaposition of pre existing crust against hot new crust and by the efficient extraction of partial melts out of the lower crust cooling together with dehydration of the lower crust and mafic compositions led to the development of a strong dry lower crustal root by 116 ma a strong lower crust resulted in high degrees of vertical coupling between the upper and lower crust during contraction from 116 to 105 ma even as magma continued to be emplaced into the mid upper crust a narrow focused orogenic style in the upper crust at this time reflected a highly viscous lower crust through which compressional stresses were transferred vertically the results imply that changes in plate boundary dynamics rather than the thermal weakening of thick lower crust during convergence controlled the onset of regional extension at 108 105 ma

intra arc transpression in the lower crust and its relationship to magmatism in a mesozoic magmatic arc abstract structural observations and u pb geochronology from fiordland new zealand support a model of partitioned transpression within the lower crust of an early mesozoic magmatic arc called the median batholith we use this lower crustal section to test whether transpression was an efficient mechanism for transporting magma through the deep lithosphere a continentward migration of magmatic activity occurred within the margin of gondwana after 140 ma followed by a period of concentrated magmatism in a vertical 12 15 km wide lower crustal shear zone after 119 ma the shear zone named the indecision creek shear zone contains variably oriented dioritic intrusions and displays systematic variations in the three dimensional orientation of ductile structures from the margins to the center of the shear zone the pitch of stretching lineations on foliation surfaces changes from 10 35 to 55 82 with increasing finite strain this increase in pitch is accompanied by a steepening and counter clockwise rotation of foliation planes these and other structural patterns indicate that arc parallel sinistral oblique slip and strike slip displacements occurred at the shear zone margins and that deformation in its center was dominated by horizontal arc normal shortening and near vertical extrusion this style of partitioned transpression reflects the effects of rheological contrasts created by a heterogeneous pattern of magmatism within the arc field relationships and u pb dates on zircon suggest that the shear zone formed along the boundary between outboard older and inboard younger parts of the batholith and facilitated the transfer of small volumes of magma vertically through the lower crust until at least 111 ma when convergence and arc magmatism waned

tracking mechanical dauphine twin evolution with applied stress in axial compression experiments on a low grade metamorphic quartzite abstract the stress dependent evolution of mechanical dauphine twinning has been investigated in axial compression experiments on a low grade metamorphic quartzite applying both time of flight neutron diffraction and electron backscatter diffraction the data of the experimentally stressed quartzite samples were compared with those of the naturally deformed starting material to monitor dauphine twinning in relation to different experimental stress states this comparison shows that in the experimental conditions of 500 c temperature and 300 mpa confining pressure dauphine twinning initiates below 145 mpa differential stress and saturates between 250 mpa and 460 mpa differential stress a single grain orientation analysis sgoa has been developed based on the distinction of quartz grains free of dauphine twin boundaries dtbs and containing dauphine twin boundaries comparing pole figures and inverse pole figures of dtb free grains of the starting material with those of the experimentally stressed samples shows a significantly different orientation distribution of the positive 10 1 1 r and the negative 01 1 1 z rhombs in dtb containing grains the sgoa allows to distinguish between host and twin domains using dtb free grains the sgoa furthermore reveals a particular pattern with one of the r rhomb maxima parallel to the axial compressive stress direction and a girdle with two r rhomb submaxima perpendicular to it we believe that this relationship between the axial compressive stress direction and the rhomb orientation distribution shows the potential of the sgoa in the reconstruction of the paleostress state in naturally stressed quartz bearing rocks

numerical study of winter water formation in the chukchi sea roles and impacts of coastal polynyas winter water formation is examined in the chukchi sea for the winters of 1992 2006 using a primitive equation ocean model forced by ncep wind and surface salinity flux derived from ssm i thin ice thickness estimates the model is also forced by an external inflow of 0 8 sv through the bering strait the model successfully reproduces the oceanic circulation on the chukchi shelf thus providing numerous insights into behaviors of salt enriched water produced on the shelf the experiments show that under northeasterly winds northward throughflow across barrow canyon is reduced this results in salinity buildup under freezing conditions and ultimately in greater enhancement of salinity of the waters carried into the arctic basin the flow and salinity enhancement of the flow through herald canyon is less extreme but more steady than through barrow canyon together with moored salinity in the bering strait the model results estimate the actual salinity to be 32 9 0 8 psu and 32 7 0 3 psu respectively for waters moving through the barrow and herald canyons both estimates are less than 33 1 psu that is typically observed for the cold halostad layer in the canada basin suggesting the importance of diapycnal mixing with saltier atlantic origin water

fabric transitions in quartz via viscoplastic self consistent modeling part i axial compression and simple shear under constant strain quartz is a common crustal mineral that deforms plastically in a wide range of temperatures and pressures leading to the development of different types of crystallographic preferred orientation cpo patterns in this contribution we present the results of an extensive modeling of quartz fabric transitions via a viscoplastic self consistent vpsc approach for that we have performed systematic simulations using different sets of relative critical resolved shear stress of the main quartz slip systems we have performed these simulations in axial compression and simple shear regimes under constant von mises equivalent strain of 100 1 73 assuming that the aggregates deformed exclusively by dislocation glide some of the predicted cpos patterns are similar to those observed in naturally and experimentally deformed quartz nevertheless some classical cpo patterns usually interpreted as result from dislocation glide e g y maxima due to prism slip are clearly not developed in the simulated conditions in addition we reported new potential preferred orientation patterns that might happen in high temperature conditions both in axial compression and simple shear we have demonstrated that cpos generated under axial compression are usually stronger that those predicted under simple shear due to the continuous rotation observed in the later simulations the fabric strength depends essentially on the dominant active slip system and normally the stronger cpos result from dominant basal slip in followed by rhomb and prism c slip whereas prism slip does not produce strong fabrics the opening angle of quartz 0001 fabric used as a proxy of temperature seems to be reliable for deformation temperatures of 400 c when the main slip systems have similar behaviors

plastic anisotropy and fabric evolution in sheared and recrystallized quartz single crystals 1 the effect of a lattice preferred orientation on the flow strength of quartz aggregates dynamically recrystallized from single crystals of synthetic quartz was investigated using general shear experiments in a griggs apparatus experiments were conducted at shear strains up to 5 at a temperature of 900 c confining pressure of 1 5 gpa and shear strain rate of 10 5 s 1 three starting orientations of crystal were used to activate three slip systems basal a prism c and prism a although slip induced rotation of the crystal axes in the first two orientations led to the activation of additional slip systems for crystals with higher water contents basal a and prism a orientations are relatively weak and prism c orientations are stronger all three initial crystal orientations undergo dynamic recrystallization with increasing shear strain although the strain required for 100 recrystallization varies 2 for prism c slip 3 8 for basal a slip and 5 for prism a slip for all three starting orientations distinct domains of recrystallized grains develop with c axes parallel to y of the strain ellipsoid ymax replacing recrystallized grains of other orientations the ymax domains increase in size with increasing strain in addition strain markers show that strain is highly localized within the ymax domains indicating geometrical softening of up to an order of magnitude in effective viscosity

crustal thickening of the early cretaceous paleo pacific gondwana margin abstract the cause of 7 to 25 km of early cretaceous thickening of parts of the new zealand gondwana margin has been argued to be due to tectonic or magmatic processes new insights into this event have been gained from examination of the grebe mylonite zone an extensive amphibolite facies early cretaceous ductile shear zone that separates volcanic arc rocks of the outboard median batholith from metasedimentary and intrusive rocks of the western province inboard median batholith at lake manapouri in southwest new zealand ar ar and u pb geochronology shows that here exhumation of the outboard median batholith began 128 ma syn kinematic magmatism was taking place at 121 ma and measurable displacement had ceased by 116 ma although the mylonite zone is currently subvertical a combination of thermochronological cooling profiles lineation orientations and back rotation of tilted tertiary unconformities indicate that it most likely formed as an oblique sinistral reverse fault that dipped 60 e and therefore displaced the outboard median batholith over the margin of the western province however the high original angle of this and other compressional shear zones in analogous structural positions the apparent absence of a regional thrust or nappe sheet s and the subtly different ages of thickening for different areas indicate that tectonic loading can have played only one part in the crustal thickening event another factor was the contemporaneous emplacement of 30 000 km 3 of mafic to felsic magma which was likely accommodated by vertical depression of the middle and lower crust tectonic and magmatic processes leading to crustal thickening were therefore not mutually exclusive

the recrystallized grain size piezometer for quartz an ebsd based calibration we have re analyzed samples previously used for a quartz recrystallized grain size paleopiezometer using electron backscatter diffraction ebsd recrystallized and relict grains are separated using their grain orientation spread gos which acts as a measure of intragranular lattice distortion and a proxy for dislocation density for ebsd maps made with a 1 m step size the piezometer relationship is d 103 91 0 41 1 41 0 21 for rms mean diameter values we also present a sliding resolution piezometer relationship d 104 22 0 51 1 59 0 26 that combines 1 m step size data at coarser grain sizes with 200 nm step size data at finer grain sizes the sliding resolution piezometer more accurately estimates stress in fine grained 10 m samples the two calibrations give results within 10 of each other for recrystallized grain sizes between 10 m and 100 m both piezometers match the original light optical microscopy quartz piezometer within error

deformation thermometry based on quartz c axis fabrics and recrystallization microstructures a review abstract for quartz rich tectonites two types of deformation thermometer are currently commonly employed 1 the quartz c axis fabric opening angle thermometer that provides an estimate of deformation temperatures when fabrics were locked in during dislocation creep and dynamic recrystallization 2 the quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization a critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development however fabric opening angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening in this paper the development of these thermometers is reviewed and their potential sensitivities to competing factors such as temperature strain rate water weakening and in the case of opening angles 3d strain type are discussed examples of the application of these potential thermometers to naturally deformed quartz rich rocks are given and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology based thermobarometers are highlighted in the review attention is focused on problems associated with applying these thermometers to natural deformation and examples of such problems are discussed

deformation path in high strain zones with reference to slip partitioning in transpressional plate boundary regions abstract the current status of the kinematics and strain geometry of high strain zone studies is briefly summarized a general high strain zone has a triclinic deformation path and monoclinic shear zones are special end member cases fabrics observed in natural shear zones and theoretical considerations based on continuum mechanics are compatible with this conclusion non steady deformation paths remain difficult to deal with and may ultimately rely on a realistic mechanical treatment of high strain zones which may be possible when our knowledge of the mechanical behavior of rocks under natural deformation conditions is improved an examination of the phenomenon of slip partitioning in transpressional plate boundary regions shows that the bulk deformation path in the forearc area trench parallel high strain zone is generally triclinic the alpine fault in the south island of new zealand provides an example of a currently active triclinic shear zone the southern knee lake shear zone of manitoba canada provides an archean example of a triclinic shear zone

evidence for regional dauphine twinning in quartz from the santa rosa mylonite zone in southern california a neutron diffraction study preferred orientation in granitic mylonites from the santa rosa mylonite zone in southern california is investigated with time of flight neutron diffraction quartz and biotite display strong preferred orientation whereas feldspar alignment is weak for quartz a c axis maximum in the intermediate fabric direction is consistent with dynamic recrystallization pole figures for positive and negative rhombs such as rz 10 11 and zz 01 11 are distinct over a wide region suggesting that the pattern reflects tectonic deformation and was not

the link between strength of lattice preferred orientation second phase content and grain boundary migration a case study from the alpine fault zone new zealand abstract we analyse the microstructure and quartz lpos of 36 layers of varying composition from a several meter thick sequence of amphibolite facies metacherts and related mica garnet plagioclase quartz bearing schists from the central southern alps in the mylonite zone related to the alpine fault quartz contents vary from 10 to 100 and all of the lpo fabric skeletons are similar featuring an asymmetric single girdle of c axes inclined 30 away from the zy plane lpo strength is typically low at quartz contents

the down stress transition from cluster to cone fabrics in experimentally deformed ice abstract during plastic deformation of polycrystalline ice 1h ice crystals become crystallographically aligned due to dislocation glide primarily on the basal slip system such crystallographic preferred orientation cpo introduces a viscous anisotropy in ice and thus strongly influences the kinematics of the flow of glaciers and ice sheets two key mechanisms exert different controls on cpo in axial compression recrystallization dominated by lattice rotation yields a cluster of c axes parallel to compression and recrystallization dominated by grain boundary migration gbm yields a cone shaped distribution of c axes with the cone axis parallel to compression the transition between these dominant mechanisms of cpo formation has not been well quantified in this study we explore how this transition varies with stress ice deformation experiments were conducted using a high pressure gas medium apparatus to prevent fracturing of samples at relatively high stresses samples were deformed in uniaxial compression at a temperature of 10 c and a confining pressure of 10 mpa fabricated ice samples with starting average grain sizes of either 0 23 mm or 0 63 mm were each deformed to an axial strain of 0 2 at a nominally constant strain rate in the range 1 2 10 6 to 2 4 10 4 s 1 yielding flow stresses of 1 17 to 4 31 mpa high quality electron backscatter diffraction reveal the grain size shape subgrain structure and cpos formed at different stresses all deformed samples have strong non random cpos with c axes concentrated in cones the cone angle and cpo strength are observed to decrease with increasing stress as stress increases the fraction of grains with highly curved or lobate grain boundaries decreases and the fraction of polygonal grains with straight grain boundaries increases based on these observations we propose that a transition in the dominant mechanism of cpo formation occurs with increasing stress from gbm which consumes grains with low schmid factors at low stress to lattice rotation caused by slip primarily on the basal slip system which causes c axes to rotate to become parallel to the shortening direction at high stress mapping out the transition from cluster rotation dominated to cone gbm dominated cpos as a function of stress this study and temperature future studies allows for a robust extrapolation to and a fundamental understanding of the cpos formed at glaciological stresses and temperatures

tracking the influence of a continental margin on growth of a magmatic arc fiordland new zealand using thermobarometry thermochronology and zircon u pb and hf isotopes 1 geothermobarometric radiogenic isotopic and thermochronologic data are used to track the influence of an ancient continental margin western province on development of an adjacent carboniferous cretaceous magmatic arc outboard median batholith in fiordland new zealand the data show a record of complicated mesozoic gondwana margin growth paragneiss within the outboard median batholith is of carboniferous to jurassic age and records burial to middle crustal depths in late jurassic early cretaceous during subduction related plutonism and arc thickening in contrast western province metasedimentary rocks in the area of study immediately west of the outboard median batholith are late cambrian early ordovician in age recrystallized at the amphibolite facies in the late devonian early carboniferous and exhibit no evidence for mesozoic textural or isotopic reequilibration a phase of deformation between 128 and 116 ma deformed exhumed and cooled the outboard median batholith to greenschist facies temperatures while large parts of the western province underwent 9 kbar metamorphic conditions zircon grains from mesozoic inboard plutons are isotopically more evolved hf t 2 3 to 4 0 than those in the outboard median batholith hf t 9 4 to 11 1 the contrasting zircon hf isotope ratios absence of s type plutons or proterozoic early paleozoic inherited zircon and the apparent absence of early paleozoic metasedimentary rocks indicates that the outboard median batholith is unlikely to be underlain by the western province continental lithosphere the new data are consistent with the outboard median batholith representing an allochthonous although not necessarily exotic arc that was juxtaposed onto the gondwana continental margin along the intervening grebe mylonite zone

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

a new perspective on paleopiezometry dynamically recrystallized grain size distributions indicate mechanism changes the dynamically recrystallized grain size is a material parameter associated with dislocation creep of crystalline solids that is especially important as a flow stress indicator via piezometer calibrations grain sizes have been measured in many studies of deformed rocks as well as metals and ceramics but global analyses of the frequency distribution of dynamically recrystallized grain sizes are lacking here we present the first systematic investigation of the recrystallized grain size distribution for quartz the grain diameters compiled from 555 samples of 31 studies of quartz mylonites deformed over a wide range of conditions extend from 3 m to 3 mm with distinct maxima at 10 20 m and 70 80 m and minima at 35 40 m and 120 m the frequency maxima correlate with distinct microstructures and the minima with the transitions between these microstructures which we interpret to result from the dominance of the recrystallization mechanisms of bulging subgrain rotation and grain boundary migration recrystallization these results demonstrate the necessity of distinct piezometer calibrations for different recrystallization mechanisms and highlight the importance of the recrystallized grain size for theoretical models of dynamic recrystallization

evolution of c axis pole figures and grain size during dynamic recrystallization results from experimentally sheared quartzite 1 quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation cpo with increasing shear strain up to 8 and degree of recrystallization up to 100 we used optical methods to determine the c axis cpo for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the cpos in five different orientation domains with increasing the c axis pole figure evolves from a broad peripheral maximum indicative of basal a slip to an inclined single girdle with two maxima indicative of rhomb a slip and finally an elongate single maximum at the girdle center indicative of prism a slip throughout this sequence the fabric skeleton rotates with the sense of shear the bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains grain boundary density analysis indicates that the size of prism a recrystallized grains is 1 5 times that of average and 2 times that of rhomb grains indicating that prism a slip is significantly easier at our experimental conditions this inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain similar progressive changes in c axis cpo are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates these results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization

texture development of recrystallised quartz polycrystals unravelled by orientation and misorientation characteristics abstract the development of microstructures and textures i e crystallographic preferred orientations during recrystallisation of naturally deformed quartz polycrystals has been studied via electron diffraction techniques in the scanning electron microscope in the investigated sample series of quartz rich rocks originating from different deformation regimes the microstructural and textural changes in quartz have been significantly influenced by dynamic recrystallisation based on microstructural observations paired with orientation and misorientation analyses down to the scale of grains and subgrains criteria could be established which characterise the dominant recrystallisation process and its influence on texture development it is shown that the texture development during dynamic recrystallisation is controlled by a differential activation of slip systems in grains of soft and hard orientations the analyses provide further evidence that specific grain orientations are preferred during crystal plastic deformation recrystallisation and grain growth the influence of twinning after the dauphine law was also investigated observations of a progressive reduction in the population of dauphine twin boundaries during recrystallisation and a penetrative deformation in both hosts and twins indicate a generation prior to deformation and recrystallisation a mechanical origin for twinning and possible influence on texture development was therefore discarded

the recrystallized grain size piezometer for quartz d 1 0 3 56 0 27 s 1 26 0 13 with no change in slope at the regime 2 3 transition and no effect of temperature or a b stability field another experimental piezometer relation for regime 1 of hirth and tullis 1992 differs in slope suggesting that different recrystallization mechanisms require different piezometer calibrations index terms 3902 mineral physics creep and deformation 5120 physical properties of rocks plasticity diffusion and creep 8030 structural geology microstructures 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere citation stipp m and j tullis the recrystallized grain size piezometer for quartz geophys res lett 30 21 2088 doi 10 1029 2003gl018444 2003

dynamic recrystallization near the brittle plastic transition in naturally and experimentally deformed quartz aggregates the electron backscattering diffraction technique ebsd was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains for the tonale mylonites we investigated a temperature range of 300 380 c 0 25 gpa confining pressure a flow stress range of 0 1 0 2 gpa and a strain rate of 10 13 s 1 were estimated experimental samples of black hills quartzite were analyzed which had been deformed in axial compression at 700 c 1 2 1 5 gpa confining pressure a flow stress of 0 3 0 4 gpa a strain rate of 10 6 s 1 and to 44 to 73 axial shortening using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains grain boundary bulges are not only formed by local grain boundary migration but they also display a lattice misorientation indicative of subgrain rotation dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation i e with an orientation unsuitable for easy basal slip in addition dauphine twins within porphyroclasts are preferred sites for recrystallization we interpret large misorientation angles in the experimental samples which increase with increasing strain as formed by the activity of fluid assisted grain boundary sliding

the effect of dauphine twinning on plastic strain in quartz we present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies t 300 400 c granitoid protomylonite from the arolla unit of the nw alps mechanical dauphine twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane z almost orthogonal to the compression direction z twin orientation twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb r nearly parallel to the compression direction r twin orientation in contrast we report the lack of twinning in two porphyroclasts already oriented with one of the r planes orthogonal to the compression direction one twinned porphyroclast has been investigated with more detail it shows the localization of much of the plastic strain into discrete r twins as a consequence of the higher amount of elastic strain energy stored by r twins in comparison to z twins the presence of dauphine twins induced a switch in the dominant active slip systems during plastic deformation from basal regions without twinning to and pervasively twinned regions dynamic recrystallization is localized along an r twin and occurred dominantly by progressive subgrain rotation with a local component of bulging recrystallization part of the recrystallized grains underwent rigid body rotation approximately about the bulk vorticity axis which accounts for the development of large misorientation angles the recrystallized grain size piezometer for quartz yields differential stress of 100 mpa the comparison of this palaeostress estimate with literature data suggests that mechanical dauphine twinning could have a potential use as palaeopiezometer in quartz bearing rocks

testing models for obliquely plunging lineations in transpression a natural example and theoretical discussion theory predicts that stretching lineations in an ideal vertical transpressional zone should be either vertical or horizontal many field descriptions of transpressional zones however indicate a range of lineation orientations between these extremes several theoretical models have been developed to explain such departures from expected lineation orientation and we discuss these in the context of a field example from the archean superior province in the north american craton existing models are insufficient to explain obliquely plunging lineations in this example because 1 obliquely plunging lineations cannot be accounted for by shear zone boundary effects imposed by a no slip condition 2 foliations and lineations vary independently 3 the vorticity normal section is subhorizontal limiting possibilities for inclined simple shear 4 high vorticity is needed for finite strains and lineations to match previously proposed triclinic models but vorticity is relatively low and 5 juxtaposed east and west plunging lineations are unlikely in the previously proposed triclinic models because existing theoretical models are not applicable to our field example we contemplate a new model to explain obliquely plunging lineations within quasi homogeneous transpression

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