mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

formation mechanism and tectonic significance of millipede microstructures in the nw himalaya abstract this article focuses on the origin and tectonic significance of millipede inclusion trail microstructures preserved within garnet porphyroblasts of typical barrovian type schist from the swat region of the himalayas in pakistan two possible kinematic models are assessed envisaging bulk coaxial shortening and progressive simple shear 3d microstructural analysis of three samples hosting the millipedes reveals a poly phase deformation history that produced three suborthogonal foliations called sx sy sz successive inclusion of sx and sy in garnet porphyroblasts produced two sets of millipedes with distinctive subvertical and subhorizontal orientations respectively this kinematic reconstruction is supported further with a numerical simulation using finite difference code of millipede formation in a spherical porphyroblast subjected to a superposition of two orthogonal shortening directions measurements of inclusion trail curvature axes or fia foliation intersection inflexion axes in porphyroblasts in 61 additional samples from the study area which contain straight sigmoidal or spiral inclusion trails reflect two principle orogenic episodes during the first episode fias developed with wnw ese set 1 and e w set 2 trends associated with thrusting and collision tectonics a younger fia set 3 with nne ssw trend developed as a result of subsequent bulk e w crustal shortening and associated regional scale folding sigmoidal or straight inclusion trails defining this fia set can be correlated with the millipede microstructures based on their similar trends and timing relative to matrix fabrics these results are consistent with the meso and macro scale structures in the region and significant in terms of establishing relative timing of formation of different structures in the region well developed preferred vertical and horizontal orientations of both sigmoidal and millipede inclusion trails imply that their development involved limited or no relative rotations between porphyroblasts sigmoidal trails formed in zones where bulk shearing caused progressively reoriented matrix foliations around stable garnets while millipedes formed simultaneously in local coaxial strain pods

the use of porphyroblasts to resolve the history of macro scale structures an example from the robertson river metamorphics north eastern australia the successions of inclusion trail asymmetries defining foliation intersection inflection axes fias preserved in porphyroblasts document the geometry of deformation associated with folding and fabric development during discrete episodes of bulk shortening and suggest that deformation histories are commonly more complex than previously considered a succession of four fias trending ene wsw e w n s and ne sw has been distinguished based upon relative timing plus inclusion texture and orientation in the proterozoic robertson river metamorphics georgetown inlier qld australia the successions of asymmetries formed around these fias bear no relationship to the geometry of macroscale folds present in the area suggesting that these folds predate porphyroblast growth the widespread metamorphism and matrix fabric development the onset of regional macro scale folding may have begun soon after the deposition at around 1655 ma in the georgetown inlier as opposed to the previously suggested date of 1570 ma these folds were then amplified overturned and refolded during nnw sse n s e w and nw se directed regional bulk shortening earlier deformations were erased from the matrix because of bedding induced shearing reactivation on the limbs of pre existing macro scale folds four foliations s1 s4 identified in the matrix provided information about the youngest deformations preserved in these rocks these results also suggest that existing tectonothermal models regarding the ne australian craton should be reconsidered

porphyroblast inclusion trail geometries in the grand canyon evidence for non rotation and rotation abstract porphyroblasts in paleoproterozoic supracrustal rocks of the grand canyon grew during a progressive deformation that involved shortening and heterogeneous development of ne striking subvertical foliation s2 that crenulates and transposes an earlier foliation s1 inclusion trails in syn s2 porphyroblast cores show a remarkably consistent nw strike across the regional transect suggesting a lack of appreciable porphyroblast rotation relative to the trace of the ne striking s2 foliation these apparently non rotated porphyroblast cores thus preserve a regional nw striking s1 orientation porphyroblast rims and younger porphyroblasts locally overgrow ne striking s2 indicating that porphyroblast growth and continued non rotation spanned s2 apparent non rotation of most porphyroblasts across the transect is interpreted to reflect a dominance of coaxial shortening in contrast single thin sections show domains of these non rotated garnets with sigmoidal inclusion trails adjacent to domains where the characteristic sigmoids are rotated by about 45 this documents local rotation of garnets relative to s2 during a heterogeneous late or post s2 reactivation of the foliation thus these types of studies should continue to look for both rotational and non rotational behaviors at all scales and continue to specify the frame of reference

recycling of foliations during folding bedding on the limbs of early formed regional folds generally lies oblique to the major directions of bulk shortening in the crust that is the horizontal and vertical during subsequent deformations matrix foliations on at least one limb of these folds may start to form but then be destroyed by reactivation of the bedding causing decrenulation and rotation into parallelism with the compositional layering consequently the schistosity parallel to bedding s0 s1 in multiply deformed rocks contains the relics of many deformation events and the 2 or 3 oblique foliations record only the very youngest history this lengthy early history is preserved as inclusion trails within porphyroblasts recycling of foliations depends on the shear sense acting on any newly developing foliation and the orientation of this foliation relative to s0 s1 for some orientations and combinations of shear senses both limbs of a pre existing fold can be reactivated from the commencement of a new deformation event this can result in the decrenulation and obliteration of a new foliation that is beginning to form before it shows any significant degree of development for other combinations one limb of a pre existing fold will behave in this manner whereas a new foliation does develop fully on the other limb however subsequent phases of deformation switch which limb shears versus develops an oblique new cleavage rotating the earlier formed oblique foliation into parallelism with s0 s1

δ objects as a gauge for stress sensitivity of strain rate in mylonites abstract our understanding of the flow properties of deforming rocks in the earth s lithosphere is mainly based on theoretical work and on the extrapolation of high strain rate experimental data to the low strain rates of rock deformation in nature the geometry of structures in naturally deformed rocks can be an additional source of information on the rheology of the lithosphere flow experiments show that the geometry of a mantle of recrystallised material around a rigid object can be used to distinguish between a linear or power law relation of differential stress and strain rate in strongly deformed rocks such as mylonites this means that it is possible to use geometrical patterns in deformed rocks to constrain aspects of the rheology and to check the applicability of theoretical models and the extrapolation of laboratory data on creep in rocks to nature

shear sense a new approach that resolves conflicts between criteria in metamorphic rocks investigation of microstructural relationships in major movement zones in metamorphic rocks where the sense of displacement is known from regional geological relationships indicates numerous problems with current concepts of shear sense criteria and their application the direction of apparent shearing commonly conflicts from one criterion to another e g from the symmetry of quartz c axis orientation diagrams to the asymmetry of extensional crenulation cleavages this implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous a new approach to interpreting shear sense involving the use of strain fields resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear sense histories extending back before the last shearing event for any foliated metamorphic rock it also provides a powerful tool for determining the structural metamorphic path that a rock has followed within an orogen for determination of the shear sense on the last foliation developed in a rock this approach uses geometries developed around competent heterogeneities such as quartz pebbles pegmatite pods veins porphyroclasts porphyroblasts and breccia clasts a shear sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows

x ray computed micro tomography of spiral garnets a new test of how they form abstract garnet porphyroblasts with spiral shaped inclusion trails of two micaschist samples have been investigated with x ray computed tomography for each garnet crystal the orientation of its internal foliation intersection inflexion axes fia was measured its volume and shape orientation the latter via calculation of best fit ellipsoids additionally image analysis of inclusion trails was performed in slices passing through garnet centres oriented normal to fia our data demonstrate subvertical and subhorizontal preferred orientations of the inclusion trails and of the porphyroblast crystals themselves in both samples this supports a genetic model in which garnets periodically overgrew a succession of vertical and horizontal foliations during alternating crustal shortening and gravitational collapse without significantly rotating the fact that both samples come from areas previously identified as regional shear zones supports the idea that porphyroblasts are able to maintain stable positions even in such environments our study exemplifies the strength of x ray computed micro tomography compared to previous techniques based on serial thin sections for distinguishing multiple fia sets present in single samples

shear zone vs folding origin for spiral inclusion trails in the canton schist the three dimensional geometry of spiral inclusion trails from the canton schist were measured to determine whether the spirals were a product of porphyroblast rotation within a shear zone or porphyroblast growth during a series of overprinting fold events the spiral inclusion trails are composed of three separate sub planar inclusion trail surfaces occupying texturally distinct parts of the porphyroblasts these surfaces are correlated across a 10 km2 area using textural criteria and relative timing measured patterns of inclusion trail orientation and asymmetry suggest they did not form by porphyroblast rotation within a non coaxial shear zone rather the porphyroblasts grew during three successive overprinting fold events f2 f4 and the spiral inclusion trails represent the accumulated curvature associated with folding of successive axial plane foliations the data show that spiral garnets are not peculiar to shear zones and can form by overprinting crenulations and folds this is consistent with the common occurrence of spiral garnets in multiply deformed regionally metamorphosed fold belts

timing and climate forcing of volcanic eruptions for the past 2 500 years volcanic eruptions contribute to climate variability but quantifying these contributions has been limited by inconsistencies in the timing of atmospheric volcanic aerosol loading determined from ice cores and subsequent cooling from climate proxies such as tree rings here we resolve these inconsistencies and show that large eruptions in the tropics and high latitudes were primary drivers of interannual to decadal temperature variability in the northern hemisphere during the past 2 500 years our results are based on new records of atmospheric aerosol loading developed from high resolution multi parameter measurements from an array of greenland and antarctic ice cores as well as distinctive age markers to constrain chronologies overall cooling was proportional to the magnitude of volcanic forcing and persisted for up to ten years after some of the largest eruptive episodes our revised timescale more firmly implicates volcanic eruptions as catalysts in the major sixth century pandemics famines and socioeconomic disruptions in eurasia and mesoamerica while allowing multi millennium quantification of climate response to volcanic forcing

do smoothly curving spiral shaped inclusion trails signify porphyroblast rotation smoothly curving spiral shaped inclusion trails may form in porphyroblasts during the overprinting of near orthogonal foliations in schists that are relatively quartz rich and in general graphite poor the smooth curvature results from the combined effects of 1 the protection from progressive shearing provided by projecting crystal faces and 2 greater competency and generally wider zones of progressive shearing in these rock types than in ones that are less quartz rich and more graphtic therefore lack of truncations or abrupt changes in compositional zoning profiles and the presence of smoothly curving spiral shaped inclusion trails do not necessarily signify that the porphyroblast containing them rotated as it grew this has considerable significance for shear sense determinations in deformed and metamorphosed rocks

crystallographic orientation chemical composition and three dimensional geometry of sigmoidal garnet evidence for rotation studies of the microstructure crystallographic orientation chemical composition and three dimensional shape of sigmoidal garnet reveal a number of features that can be used to discuss its origin and in particular to distinguish between rotational versus non rotational models crystallographic orientation mapping of sigmoidal garnet shows no evidence of subgrain formation or of being polycrystalline suggesting that neither ductile nor brittle deformation is significant chemical mapping shows that the garnet grew during a single metamorphic event arguing against ideas that sigmoidal garnet forms as the result of a series of independent growth events the chemical mapping also reveals anisotropic growth of garnet with the long axis of the ellipsoidal grain rotating in the same direction but to a lesser degree than the inclusion trails this is best explained as the result of syn growth rotation of the garnet with respect to the foliation and maximum growth direction high resolution x ray ct scanning shows that the inclusion trails have a complex three dimensional spiral geometry this type of geometry is predicted by rotational models of sigmoidal garnet formation

a 3d study of aligned porphyroblast inclusion trails across shear zones and folds inclusion trails si within 741 equant garnet porphyroblasts show a consistent orientation across a 100 m2 outcrop of precambrian metapelite with later d2 folds and shear zones in posio finland variation in si orientations is not systematic with respect to these later folds and shear zones but instead represents microfolding and warping of s1 before the formation of d2 structures the development of the d2 folds and shear zones is investigated by comparing the pattern of si in the outcrop to those predicted for porphyroblasts in tangential longitudinal strain axial slip flexural flow pure shear partitioned flexural flow and partitioned pure shear folds si patterns are also compared with models for non coaxial and coaxial shear zones along with corroborating evidence from structures in the outcrop and matrix the comparisons indicate that d2 folds and shear zones in the outcrop formed in response to nw se axially symmetric shortening during which garnet porphyroblasts did not rotate relative to each other the axial plane of f2 folds and d2 shear zone boundaries

snow and firn permeability at siple dome antarctica the nature of air snow exchange processes depends upon both the physical characteristics of the snow and forcing from the atmosphere an understanding of snow air transport processes and models of these processes are important for interpreting ice cores and for predicting remotely sensed snow surface characteristics accurate modeling requires knowledge of the physical characteristics of the snow and firn in this paper measurements of snow and firn permeability from the surface down to 12 m depth at siple dome antarctica are presented the permeability varies greatly as a function of layer and depth generally increasing to approximately 3 m and generally decreasing below that the maximum permeability 280 10 10 1 0 m2 occurs at approximately 3 m depth the lowest permeability in the top 12 m of snow and firn 10 10 10w m2 was measured in the surface wind pack the measurements show that layering and microstructure have large effects on permeability a mathematical expression for the overall variation of permeability with depth is presented it is shown that snow density is a poor indicator of permeability quantitative microscopy may be helpful in relating observed snow characteristics to permeability and useful in modeling firnification and chemical species transport

a new spin on non rotating porphyroblasts implications of cleavage refraction and reference frames abstract it has been claimed that rigid porphyroblasts which grow before or during folding and concurrent cleavage development do not rotate with respect to the geographical reference frame grf even if the straining is non coaxial bell 1985 bell and johnson 1990 the explanation offered is based on strain partitioning it is argued that the initial orientations of early fabrics included as internal foliations s i in the porphyroblasts have been preserved after polyphase deformation and even after successive orogenies according to the strain partitioning model the porphyroblasts are fixed in domains of coaxial straining microlithons and are isolated from the non coaxial straining associated with the enveloping septa s e this hypothesis and also its discussions both pro and contra suffer from insufficient attention to reference frames we therefore attempt to demonstrate a the need for rigorous treatment of reference frames in geological interpretations b that in a folding situation grains that do not rotate with respect to their immediate matrix generally rotate with respect to the grf c that lack of porphyroblast rotation with respect to the grf demands a rare folding mechanism slip fold model and d that the non rotation hypothesis is in conflict with heterogeneous deformation cleavage refraction finally we question the validity of the evidence in a study by fyson 1980 cited in support of non rotation with respect to the grf during folding fyson reported orientations of s i that are constant after folding over a large area this scenario is a product of selective data acquisition in summary our investigation shows that the lack of porphyroblast rotation with respect to a grf during folding while possible is not universal the development of microstructures e g curved s i is only related to the local deformation path the characterisation of which does not rely on the grf

recent accumulation variability in northwest greenland from ground penetrating radar and shallow cores along the greenland inland traverse accumulation is a key parameter governing the mass balance of the greenland ice sheet several studies have documented the spatial variability of accumulation over wide spatial scales primarily using point data remote sensing or modeling direct measurements of spatially extensive detailed profiles of accumulation in greenland however are rare we used 400mhz ground penetrating radar along the 1009 km route of the greenland inland traverse from thule to summit during april and may of 2011 to image continuous internal reflecting horizons we dated these horizons using ice core chemistry at each end of the traverse using density profiles measured along the traverse we determined the depth to the horizons and the corresponding water equivalent accumulation rates the measured accumulation rates vary from 0 1mw e a in the interior to 0 7mw e a near the coast and correspond broadly with existing published model results though there are some excursions comparison of our recent accumulation rates with those collected along a similar route in the 1950s shows a 10 increase in accumulation rates over the past 52 years along most of the traverse route this implies that the increased water vapor capacity of warmer air is increasing accumulation in the interior of greenland

eemian interglacial reconstructed from a greenland folded ice core efforts to extract a greenland ice core with a complete record of the eemian interglacial 130 000 to 115 000 years ago have until now been unsuccessful the response of the greenland ice sheet to the warmer than present climate of the eemian has thus remained unclear here we present the new north greenland eemian ice drilling neem ice core and show only a modest ice sheet response to the strong warming in the early eemian we reconstructed the eemian record from folded ice using globally homogeneous parameters known from dated greenland and antarctic ice core records on the basis of water stable isotopes neem surface temperatures after the onset of the eemian 126 000 years ago peaked at 8 4 degrees celsius above the mean of the past millennium followed by a gradual cooling that was probably driven by the decreasing summer insolation between 128 000 and 122 000 years ago the thickness of the northwest greenland ice sheet decreased by 400 250 metres reaching surface elevations 122 000 years ago of 130 300 metres lower than the present extensive surface melt occurred at the neem site during the eemian a phenomenon witnessed when melt layers formed again at neem during the exceptional heat of july 2012 with additional warming surface melt might become more common in the future

formation of massive sulfide lenses by replacement of folds the hercules pb zn mine tasmania the hercules pb zn deposit consists of a number of isolated ellipsoidal massive sulfide pods hosted in a tuffaceous slate unit of middle cambrian age these pods are aligned roughly parallel to the main cleavage and have completely discordant to moderately folded bedding mineralization is localized in the short limb zones of parasitic f 2 folds from the micro to the macroscopic scale antithetic shearing induced along the bedding during d 3 caused localized dilation and fracturing of some of the short limb zones this resulted in alteration and replacement late in d 3 such that sa was overprinted by the mineralization the following paragenetic replacement sequence exists pyrite i quartz chlorite pyrite ii sphalerite galena chalcopyrite pb isotope age data are consistent with the derivation of the deposit from dispersed primary sulfides in the mount read volcanics by synmetamorphic mobilization into structural traps at higher levels in the crust

lack of porphyroblast rotation in noncoaxially deformed schists from petrel cove south australia and its implications abstract inclusion trails representing an s 1 cleavage demonstrate the lack of porphyroblast rotation during subsequent highly non coaxial deformations the pelitic schists of the kanmantoo group at petrel cove south australia contain two generations of porphyroblasts the first one consists of cordierite porphyroblasts that formed early in d2 and contain straight to slightly sigmoidal inclusion trails of s 1 the second generation consists of andalusite porphyroblasts that overgrew crenulated s 2 late during d3 several hundred inclusion trail traces from cordierites measured from oriented specimens taken throughout a strongly folded area show a horizontal great circle distribution when plotted and contoured on a stereographic projection hence s 1 was planar and horizontal prior to d2 s 1 measurements on limbs and hinges of a mesoscale d2 foldpair show that folding had little effect on porphyroblast orientation as the s 1 orientation remained constant and subhorizontal around the fold hence porphyroblasts have not rotated during any of the non coaxial deformations accompanying and following their growth this is interpreted as a result of the partitioning of the deformation around them s 1 in the matrix has been totally destroyed by the formation of s 2 as a fully differentiated crenulation cleavage what has previously been regarded as inconsistent senses of shear recorded by porphyroblasts around folds is resolved by the fact that the matrix foliation rotated rather than the porphyroblasts due to the effects of deformation partitioning the presence of a subhorizontal s 1 foliation suggests horizontal movements e g thrusting or detachment faulting during the earliest phase of the adelaidian orogeny

foliation boudinage control on the formation of the rosebery pbzn orebody tasmania abstract the rosebery pb zn ag au massive sulphide deposit occurs within the middle cambrian mount read volcanics of western tasmania it is hosted by a tuffaceous slate lens that is surrounded by felsic volcanics the volcanic belt and its ordovician cover were deformed and metamorphosed during an early to middle devonian orogeny tabberabberan orogeny during this orogeny d 3 e w shortening was partially accommodated by reverse bedding parallel shearing within the slate unit and immediately underlying volcanic rocks this caused reactivation and intensification of an early foliation s 2 and the development of asymmetric chocolate tablet foliation boudinage structures with boudin necks parallel to the x and y directions the mineralization occurs along extensional shear planes that are associated with foliation boudinage mineralogical zoning in the orebody can be directly related to dilational bends in shears associated with foliation boudin necks it is proposed that these structures gaped during the d 3 deformation allowing infiltration of metalliferous hydrothermal fluids along shear plane and cleavage directions into the fractured wall rock progressive metasomatic replacement of the wall rock resulted in the formation of a sulphide sulphate silicate orebody the orebody is completely discordant to folds in the host rock this observation confirms a structurally controlled origin and refutes earlier interpretations of the deposit as a tightly folded chemical sediment isotope data indicate that the orebody was probaby derived from volcanogenic sulphides within the mount read volcanics the rosebery orebody is best regarded as a metamorphogenic deposit that formed by dissolution upward transport concentration and re precipitation of primary sulphides the proposed macroscopic model complements separate microstructural evidence by the author indicating a syn d 3 timing of mineralization

dissolution solution transfer diffusion versus fluid flow and volume loss during deformation metamorphism dissolution and solution transfer during deformation metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries progressive shortening is accommodated by the cores of most other minerals including sulphides these minerals develop strain and hence dislocation density gradients on their rims due to progressive shearing along grain boundaries these gradients are particularly large when the mineral abuts phyllosilicate or graphite the resulting chemical potential gradients between the core and rim drive dissolution causing removal of the highly strained grain margins removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation interlayers and interfaces on boundaries lying at a low angle to the direction of shearing and oriented relative to the sense of shear such that they can open gape by small amounts water present in these interlayer spaces becomes destructured considerably enhancing diffusion rates along the foliation penetrative volume loss especially in deforming metamorphosing pelitic rocks is large at all metamorphic grades increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies transference of material by fluid flow from deep to high levels in the earth s crust is precluded because thousands to tens of thousands of rock volumes of fluid are required necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells unless some extremely large scale form of fluid channelling is possible reassessment of diffusion mechanisms and hence rates during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required the amount of fluid can be reduced sufficiently to allow large volume losses by a one way flow of fluid to the earth s surface in deforming metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure deformation partitioning controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation metamorphism because matrix minerals other than phyllosilicates and graphite are preferentially removed the large size of porphyroblasts if present tends to preserve them from dissolution hence the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew in rocks in which bedding is still well preserved

a new method for predicting transport properties of polar firn with respect to gases on the pore space scale abstract in this study a powerful tool to investigate the permeabilities and effective diffusion coefficients of polar firn is presented using a combination of an experimental method for three dimensional pore structure reconstruction and two models to simulate advective and diffusive transports of gases through the pore space the reconstruction follows a semi automated digital analysis of serial surface sections the simulations are based on a three dimensional lattice boltzmann formulation they separately solve the navier stokes equation and the diffusive transport equations in a first application effective diffusion coefficients and permeabilities are calculated from firn samples of a core drilled during the north greenland traverse 1993 95 the estimated relationships of diffusivity and permeability to the open porosity are expressed by power law functions with exponents 2 1 and 3 4 respectively

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

correlating deformation in variscan nw iberia using porphyroblasts implications for the ibero armorican arc abstract 3 d microstructural analysis of 30 porphyroblastic samples from the variscan orogen in nw iberia has revealed four sets of inclusion trails with distinctive geographic trends and relative timing the chronological and orientational consistency of these microstructures provides a reference frame for the correlation of macroscopic structural successions the two younger inclusion trail sets correlate with the main map scale structures of middle to upper carboniferous age whereas the two older sets correspond to mesoscopic structures preserved locally in low strain pods and dating back as far as 365 ma upper devonian inclusion trail orientations appear uninfluenced by a major orocline in the study area ibero armorican arc outlined by late stage folds and thrusts with variable trends it is proposed that the orocline developed by wnw ese trending transpression superposed on originally n s to ne sw trending structures during a late stage of the orogeny upper carboniferous domains of low or zero strain e g porphyroblasts are inferred to have maintained relatively constant orientations in the zone of transpression presumably due to deformation partitioning effects consequently they preserve a succession of pre orocline structural trends the collected microstructural data also indicate moderate preferred vertical and horizontal orientations of inclusion trails in the analysed samples as reported earlier in other variscan regions this probably witnesses a dynamic equilibrium between tectonic forces and gravity during four periods of differently oriented crustal shortening that generated the four recognised sets of inclusion trails

the age of the air in the firn and the ice at summit greenland in the framework of the eurocore drilling project 1989 at summit central greenland air samples were collected from a drill hole in the firn at several depth levels the samples have been analyzed for 85kr co2 ch4 cfc s f 11 and f 12 contents and for the isotopic composition of nitrogen and oxygen the measured data are compared with the results of a diffusion model co2 for example takes 12 years with a standard deviation of 7 5 years to diffuse from the surface to the firn ice transition under the present climatic conditions at summit the difference between the age of the ice and the mean age of the occluded air is approximately 210 years an enrichment of the heavier isotopes due to separation by gravity has been observed molecular diffusion alone suffices to explain the observed concentration trends in the firn column we can however not exclude the existence of a convective zone below the surface we have also measured the density and closed porosity along the firn core that has been retrieved when drilling the hole the porosity data reveal that the air is occluded into bubbles mainly between 65 and 80 m corresponding to densities of 790 and 830 kg m3 respectively

hercynian low pressure high temperature regional metamorphism and subhorizontal foliation development in the canigou massif pyrenees france evidence for crustal extension a detailed study has been made of the structural and metamorphic history of part of the hercynian low pressure high temperature regional metamorphic terrane exposed in the canigou massif france microtextural relations between and compositional zoning within porphyroblast phases in pelitic rocks indicate that the pressure temperature time p t t trajectory followed by the massif during metamorphism involved decompression under prograde and retrograde conditions clockwise p t t trajectory the geometric relations between inclusion trails within these porphyroblasts and the structures in the adjacent matrix indicate that metamorphism was synchronous with the development of the regional subhorizontal foliation s3 the s3 foliation formed in response to noncoaxial bulk strain the absence of a marked hiatus in sedimentation in the pyrenean region during hercynian metamorphism and deformation and the widespread preservation of low grade hercynian metasedimentary rocks suggest that low p high t metamorphism occurred in an extensional tectonic environment and that extension manifested itself in asymmetric top to the northwest shear during d3

foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation its role has generally been insufficiently emphasized because evidence is best preserved where por phyroblasts which contain inclusion trails are present reactivation occurs when progressive shearing operating in a synthetic anastomosing fashion parallel to the axial planes of folds changes to a combination of coarse and fine scale zones of progressive shearing some of which operate antithetically relative to the bulk shear on a fold limb reactivation of earlier foliations occurs in these latter zones reactivation decrenulates pre existing or just formed crenulations generating shearing along the decrenulated or rotated pre existing foliation planes partitioning of deformation within these foliation planes such that phyl losilicates and or graphite take up progressive shearing strain and other minerals accom modate progressive shortening strain causes dissolution of these other minerals this results in concentration of the phyllosilicates in a simi lar but more penetrative manner to the for mation of a differentiated crenulation cleavage except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds reactivation can generate bedding parallel schistosity in multideformed and meta morphosed terrains without associated folds heterogeneous reactivation of bedding gen erates rootless intrafolial folds with sigmoidal axial planes from formerly through going struc tures reactivation causes rotation or refrac tion of axial plane foliations forming in the same deformation event causing reactivation in those beds or zones in which an earlier foli ation has been reactivated and results in destruction of the originally axial plane foli ation at high strains reactivation also provides a simple explanation for the apparently wrong sense but normally observed rotation of gar net porphyroblasts whereby the external foli ation has undergone rotation due to antithetic shear on the reactivated foliation alter natively the rotation of the external foliation can be due to its reactivation in a subsequent deformation event porphyroblasts with inclusion trails com monly preserve evidence of reactivation of earl ier foliations and therefore can be used to ident ify the presence of a deformation that has not been recognized by normal geometric methods because of penetrative reactivation reac tivation often reverses the asymmetry between pre existing foliations and bedding on one limb of a later fold leading to problems in the geo metric analysis of an area when the location of early fold hinges is essential the stretching lineation in a reactivated foliation can be rad ically reoriented potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events other aspects of reac tivation however can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized for example d grown porphyroblasts may be dis solved against reactivated s and hence appear to have grown syn d

the geological significance of the western termination of the merens fault at port vell central pyrenees abstract a detailed local structural map of the merens fault zone at port vell vall ferrera reveals that the main fault related mylonite band splits westwards into a network of mylonite zones these consist of bands of highly strained rocks exhibiting tightened folds transposed foliations and extensive obliteration of pre existing structures evidence of continuity of marble layers across the port vell mylonite zone and structural and metamorphic similarities on both sides of the port vell mylonite band indicate that there is no significant throw across it the mylonite foliation forms parallel to the axial planes of tight folds in or near the mylonite zone which can be related to more open folds in adjacent areas the port vell mylonite band corresponds to a pinched w e axial trending late fold in order to estimate the importance of these results a review of the controversial geological significance of mylonite belts in the pyrenees with special emphasis on the merens fault introduces the paper

hercynian crustal scale extensional shear zone in the pyrenees detailed structural analysis of metasedimentary rocks and gneisses in the hospitalet massif pyrenees has revealed that upright structures in low grade rocks are in a structurally higher position and older than gently inclined to flat lying structures in higher grade rocks the steep structures were sub vertically flattened during formation of the flat lying structures indicating that the higher grade rocks formed as part of a shear zone system a similar conclusion can be drawn from the relation between deformation and metamorphism in the higher grade rocks we suggest that the shear zone may have accommodated crustal extension

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

rotation of biotite porphyroblasts in pelitic schist from the nukata area central japan abstract the detailed microstructures of biotite porphyroblasts in pelitic schist from the nukata area central japan indicate that the porphyroblasts rotated in an external reference frame during non coaxial deformation the amount of rotation and rotation axis are controlled by the 001 orientation of the biotite porphyroblast with respect to the schistosity and the lineation concurrently with rotation the biotite porphyroblasts suffered shape change by solution and deposition in the shortening and dilating fields around the porphyroblast respectively the close co occurrence of inclusion free domains newly deposited biotite domains and pressure shadows along the 001 faces suggests that continuously opening fractures along the 001 faces caused biotite deposition as well as pressure shadow aggregates the role of pressure shadow formation for porphyroblast rotation is discussed in terms of stresses applied on the porphyroblast surface it is concluded that although the formation of deformation partitioning induced pressure shadows tends to suppress porphyroblast rotation that of fracture induced pressure shadows pressure fringes promotes porphyroblast rotation the present biotite porphyroblasts would have rotated at rates depending not only on their shape and shape orientation but also on the 001 orientation

porphyroblast nucleation growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development in regional metamorphic rocks the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain rate gradients across the boundaries between the partitioned zones these generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain because of their uniquely layered crystal structure however most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients and hence are susceptible to dissolution and solution transfer as a consequence zones of progressive shearing become zones of dissolution of most minerals and of concentration of phyllosilicates and graphite exceptions are mylonites where strain rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing as they would be dissolved by the effects of shearing strain on their boundaries however they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation this includes a build up of stored strain energy relative to zones of progressive shearing in which dissolution is occuring porphyroblast growth is generally syndeformational and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account porphyroblasts in a contact aureole do not grow statically either as microfracturing associated with emplacement allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening the criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning reactivation of deforming foliations and a general lack of rotation of porphyroblasts with the spectacular exception of genuinely spiralling garnet porphyroblasts

non rotation of garnet porphyroblasts and 3 d inclusion trail data an example from the imjingang belt south korea abstract we describe non rotation of garnet porphyroblasts in non coaxially deformed metapelites of the imjingang orogenic belt in the middle korean peninsula the three dimensional geometry of inclusion trails within 1 2 mm garnet porphyroblasts was analyzed by serial polishing and reflected light microscopy most of the garnet porphyroblasts have straight inclusion trails of quartz and some plagioclase s i oblique to an external foliation s e serial polishing of two samples normal to mineral elongation lineation a plane and normal to s e and parallel to the lineation b plane with twenty intervals of about 0 1 mm reveals that the straight s i is a planar structure measurement of the angles between s i and s e on both planes from samples of sixteen outcrops shows that s i orientation of individual garnets is fairly constant in each sample the attitude of s i in geographic space determined by using its pitch angles on the a and b planes is more or less constant throughout the area over 6 5 km traversing normal to the strike of regional foliation the sense of asymmetry of strain shadows at the margin of garnet porphyroblasts and the s i s e relationship indicate rotation of s e rather than garnet porphyroblasts with respect to the geographic reference frame rotation of s e is also supported by a sense of shear inferred from other shear criteria all indicating a top up to the south shearing

kinematics of orogenic collapse in the variscan pyrenees deduced from microstructures in porphyroblastic rocks from the lys caillaouas massif in the pyrenees a phase of bulk vertical shortening d3 is indicated by a horizontal crenulation cleavage s3 which is developed in a pre existing schistosity s2 porphyroblasts of the lys caillaouas massif regionally preserve 1 1 the initial pre d3 orientation of s2 in the form of a vertical and e w trending preferred orientation of their inclusion trails 2 2 a vertical l2 stretching lineation defined by the elongation of the inclusions these data are consistent with n s compression and crustal thickening during d2 the angle between the inclusion trails s2 i and the external schistosity s2 e records the subsequent rotation of s2 from vertical to gently dipping orientations at higher crustal levels s2 remained steep suprastructure and little d3 deformation is evidenced with depth d3 deformation progressively intensifies and zones can be distinguished with opposite dip directions of s2 opposite apparent porphyroblast rotation senses and opposite f3 crenulation asymmetries these micro structures on a n s section through the lys caillaouas and the garonne dome massifs record a regional partitioning of bulk coaxial vertical shortening this deformation partitioning was probably controlled by a not exposed gneiss dome which behaved relatively rigid as compared to its metasedimentary cover in zones of non coaxial d3 deformation s2 was extended reactivated and forms the dominant schistosity where d3 was locally more coaxial s2 was shortened isoclinally crenulated and s3 forms the macroscopic cleavage the fact that d3 extension in the lys caillaouas massif immediately followed d2 compression and that similar extension is recorded in large parts of the variscan foldbelt suggests gravitational collapse of a thickened orogen this is consistent with the flattening type of d3 strain recognized in a number of pyrenean variscan massifs peak high t low p metamorphic conditions coincide with the onset of orogenic extension when very little crustal thinning could have been accomplished yet this suggests that heat induced strain softening within the orogen may have triggered its collapse

development of a chevron cleavage pattern and porphyroblast rotation in graded metaturbidites slave structural province northwest territories canada abstract chevron cleavage pattern displays opposite cleavage vergence across a psammite pelite contact or within a single graded couplet in this example graded metaturbidites preserve an s2 cleavage with dextral vergence in their competent psammitic bases and an s3 crenulation cleavage with sinistral vergence in their less competent semi pelitic tops bedding parallel shear partitioned into tops of graded couplets resulted in buckle folding of s2 cleavage s3 is also a compositionally differentiated cleavage displaying alternating quartz rich microlithons q and muscovite rich cleavage domains m together q and m domains define s microfolds related to d3 biotite porphyroblasts developed during static metamorphism before d 2 s 2 contain quartz inclusion trails sii with three orientation patterns that are related to their location within graded beds porphyroblasts in psammitic bases of graded couplets are not affected by d 3 s 3 in comparison sii in porphyroblasts in semi pelitic tops of beds are on average about 5 clockwise in m domains and about 90 counter clockwise in q domains corresponding to flat and steep limbs of s shaped microfolds in s2 from these observations it is apparent that biotite porphyroblasts within q domains rotated about 90 counter clockwise during microfolding within m domains the sii data suggest that porphyroblasts rotated little relative to their orientation in the base of the beds

the rotation of garnet porphyroblasts around a single fold lukmanier pass central alps abstract the orientation of the straight internal foliation s i within large 5 mm garnet porphyroblasts has been measured relative to the orientation of the external foliation s e around a single antiform of 0 5 m wavelength which folds the dominant regional foliation the internal foliation is not constant in orientation but varies consistently both with position around the fold and with the porphyroblast ellipticity the dip of s i hinge dip taken as zero is consistently less than the dip of s e it increases with increasing dip of s e and with increasing ellipticity of the porphyroblasts s i effectively defines a fold with an opening angle greater than that in the external foliation the opening angle of this fold in s i decreases with increasing porphyroblast ellipticity the observed variation in the orientation of s i can be explained qualitatively by a flattened flexural flow model for fold development as could be expected for folding of a pre existing strongly anisotropic foliation the measurements clearly demonstrate that rotation of porphyroblasts relative to geographical co ordinates did occur during the development of this fold and that a model based on the classical theories of rotation of stiff inclusions in a weaker viscous matrix is most appropriate

porphyroblast non rotation during crustal extension in the variscan lys caillaouas massif pyrenees abstract the dominant foliation s 2 in the variscan lys caillaouas massif central pyrenees france spain formed in a subvertical orientation during crustal thickening d 2 subsequent non coaxial crustal extension produced a subhorizontal crenulation cleavage s 3 whereby s 2 was folded and rotated andalusite staurolite cordierite and biotite porphyroblasts grew very early during this extension d 3 and included s 2 as straight curved and weakly crenulated inclusion trails these inclusion trails exhibit a strong subvertical preferred orientation which is independent of the local magnitude of d 3 strain the dip angle of the external main foliation s 2 or the aspect ratio and shape orientation of porphyroblasts this indicates that porphyroblasts did not rotate in the d 3 deformation field and hence preserve the orientation of s 2 at the time it was trapped in the porphyroblast relative to the bulk d 3 flow plane porphyroblasts could maintain a stable orientation in the d 3 flow due to complete accommodation of the bulk flow vorticity by heterogeneous shear strain the lack of vorticity of local volumes of zero shear strain such as porphyroblasts was balanced by concentration of shear strain in planar zones anastomosing around them it is likely that s 2 maintained a subhorizontal orientation throughout the d 3 crustal extension which would imply that porphyroblasts also remained stationary relative to the earth s surface during d 3 whereas s 2 in the deforming matrix experienced shear induced rotation a late brittle ductile folding event d 4 involving differential rigid block rotations dispersed the orientation of inclusion trails to some degree these late rotations could be corrected for by artificial unfolding of the f 4 folds

energy regimes for aeolian sand grain surface textures abstract an experimental study of aeolian sand grain surface texture development was undertaken with an air driven grain recirculating desktop apparatus scanning electron microscope analysis of resulting textures indicated that different texture types can be associated with distinct zones in a grain shape grain speed matrix in particular for subrounded and rounded grains low and high energy transport can be unequivocally distinguished by the occurrence of upturned plates and hertzian frustra respectively textural development does not have a simple relationship to grain velocity but appears to relate to the energy expended per unit area within the contact zone generated by elastic deformation during impact hertzian theory was adapted to irregular sand grain shapes and close agreement was found between experimental results and theoretical predictions for textural development results of this study improve our ability to reconstruct palaeoaeolian environments and therefore our ability to determine grain provenance in particular the latter is shown to have direct relevance to forensic inquiries and terrorism investigations

firn air δ15n in modern polar sites and glacial interglacial ice a model data mismatch during glacial periods in antarctica the phase lag between atmospheric composition air bubbles and temperature water isotopes can be quantified from ice cores provided that the age difference between entrapped air and the surrounding air can be correctly estimated this difference depends on the lock in depth lid when air no longer mixes with the atmosphere the lid can be estimated from firnification models or from the air isotopic composition 15n and 40ar both methods give consistent results for greenland and one coastal site in antarctica byrd new firn measurements in greenland northgrip and antarctica berkner island bas depot dome c confirm that firnification models correctly reproduce the present lid over a large range of surface conditions however a systematic mismatch is observed for the last glacial maximum lgm in east antarctic sites vostok dome c dome f questioning the model s validity here we use new 15n measurements from two coastal antarctic sites kohnen station and law dome providing depth estimates again distinct from firnification model calculations we show that this discrepancy can be resolved by revising the estimate of past accumulation rates 15n measurements can therefore help to constrain past accumulation rate and improve ice core dating

the kinematic interpretation of obliquely transected porphyroblasts an example from the trois seigneurs massif france abstract obliquely transected porphyroblasts otps contain a straight inclusion pattern that is continuous with the external foliation but oblique in orientation otps are interpreted to form by porphyroblast growth between two phases of deformation the obliqueness of the inclusion pattern and external foliation is due to the second deformation phase and has been variously explained by porphyroblast rotation in the kinematic reference frame of bulk flow or by foliation rotation around a stationary porphyroblast the two models imply opposite sense of vorticity also loosely referred to as sense of shear to produce the same geometry and their relevance should be known if otps are to be used as sense of vorticity indicators andalusite otps in the trois seigneurs massif french pyrenees have a spool shape in three dimensions that is interpreted to result from passive overgrowth of andalusite on a spaced s 2 cleavage subsequent non coaxial d 3 flow led to development of an s 3 crenulation cleavage and the otp geometry although the porphyroblasts rotated rotation of the foliation in the kinematic reference frame of bulk flow was mainly responsible for the otp geometry this example shows that otps can indeed develop by foliation rotation in non coaxial flow similar looking otps can apparently develop in flows with opposite sense of vorticity depending on whether foliation or porphyroblast rotation is dominant the dominance of either development mechanism depends on the importance of flow partitioning and solution transfer during progressive deformation

conjugate shear folding a model for the relationships between foliations folds and shear zones abstract microstructural mapping of whole thin sections cut from two samples of micaschist containing cm scale folds plus garnet porphyroblasts has provided new insight in the relationships between folding shearing and foliation development the garnets exhibit coherent inclusion trail patterns that place important constraints on the kinematic development of both samples which are shown to be representative of coaxial versus non coaxial deformation in rocks containing a pre existing schistosity a comparison of crenulations cleavages geometries in both samples and a review of the geometry of natural and experimental multilayer folds leads to the conclusion that folding involves conjugate shearing at different scales at microscopic scales crenulation cleavages nucleate as conjugate kink or shear instabilities and develop further as a function of the macroscopic partitioning of deformation in fold hinge domains bulk coaxial deformation results in equal development of conjugate crenulations that progressively coalescence into symmetrical crenulation patterns so that macroscopically parallelism is achieved between foliation fold axial planes and long axes of strain ellipses fold limb domains represent a system of conjugate shear zones where single sets of crenulation instabilities with synthetic shearing component preferentially develop producing oblique relationships between the aforementioned elements cleavage fanning is inferred as a direct consequence of this conjugate shear origin of folds the model implies that crenulation cleavages and s c fabrics in shear zones form by analogous processes in both cases involving a component of shearing along foliation planes the development of conjugate sets of foliation planes surrounding porphyroblasts during early relatively coaxial stages of deformation explains continued gyrostatic behaviour during more advanced non coaxial stages as indicated by consistently oriented inclusion trails in the studied samples

time constraints on deformation and metamorphism from epma dating of monazite in the proterozoic robertson river metamorphics ne australia epma dating of monazite confirms the relative timing of a succession of foliation intersection axis trends in porphyroblasts fias and two periods of progressive metamorphism intervened by retrogression in the robertson river metamorphics ne australia a complete pressure temperature deformation time p t d t path has been deciphered involving an early clockwise p t loop followed by high temperature low pressure metamorphism successive generations of monazite were identified using microstructure and fia trends and then dated the succession of four fias trending ene wsw fia1 e w fia2 n s fia3 ne sw fia4 plus four matrix foliations s1 s4 were accompanied by three periods of garnet and two periods of staurolite growth monazite grew in response to most of these events isopleth thermobarometery using p t pseudosections in the mnnckfmash system for garnet porphyroblasts selected based on fia trends in their core reveal that pressure increased progressively from 3 4 kb at 530 550 c to 6 7 kb at 600 620 c during medium pressure temperature metamorphism this was accompanied by changing bulk shortening directions from nnw sse to n s to e w perpendicular to fias 1 2 and 3 the development of fia1 and fia2 occurred between 1655 2 2 ma and 1625 15 ma fia3 developed over 30 50 ma periods from 1625 15 ma to 1586 6 ma retrogressive metamorphism decompression and exhumation occurred over a not vert similar45 ma period further metamorphism accompanied regional granite intrusion and generated lower pressure higher temperature metamorphism during the development of fia4 nw se bulk shortening at 1542 8 ma this period possibly extended for another 30 50 ma based on the monazite ages obtained from the youngest foliations in the matrix correlation of these data with other ne australian proterozoic inliers suggests that they were all once part of a single orogen that developed from ca 1650 to 1500 ma

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

how useful are millipede and other similar porphyroblast microstructures for determining synmetamorphic deformation histories oppositely concave microfolds ocms in and adjacent to porphyroblasts can be classified into five nongenetic types type 1 ocms are found in sections through porphyroblasts with spiral shaped inclusion trails cut parallel to the spiral axes and commonly show closed foliation loops type 2 ocms commonly referred to as millipede microstructure are highly symmetrical the foliation folded into ocms being approximately perpendicular to the overprinting foliation type 3 ocms are similar to type 2 but are asymmetrical the foliation folded into ocms being variably oblique to the overprinting foliation type 4 ocms are highly asymmctrical only one foliation is present and this foliation is parallel to the local shear plane type 5 ocms result from porphyroblast growth over a microfold interference pattern types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths respectively during porphyroblast growth however theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing given particular initial foliation orientation and timing of porphyroblast growth type 3 ocms can also form during these intermcdiatc deformation paths and are commonly found in the same rocks as type 2 ocms type 4 ocms may indicate highly noncoaxial deformation during porphyroblast growth but may be difficult to distinguish from type 3 ocms thus types 1 3 and possibly 4 reflect the finite strain state giving no information about the rotational component of the deformation s responsible for their formation furthermore there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation s during the growth of porphyroblasts containing ocms type 2 ocms that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening but there is a lack of supporting physical or computer modelling it is possible that microstructures in the matrix around ocms formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another however determining degrees of noncoaxiality from rock fabrics is a major long standing problem in structural geology

the impact of accumulation rate on anisotropy and air permeability of polar firn at a high accumulation site the first three dimensional properties of polar firn obtained by x ray microtomography are used to study the microstructure of snow on a 15 m deep firn core from west antarctica the snow is found to undergo coarsening down to approximately 2 5 m depth before grain growth and densification become the prevalent mechanisms of microstructure change in contrast to previous assumptions distinct anisotropy of the ice and pore geometry is observed throughout the profile with a maximum at 2 5 m depth the air permeability and the degree of anisotropy vary with depth and can be linked to short term changes in accumulation rate via the residence time for which a certain snow layer stays in the uppermost 2 5 m patterns of the degree of anisotropy and air permeability of buried polar firn are relative indicators of past accumulation rates

on the impact of impurities on the densification of polar firn abstract understanding polar firn densification is crucial for reconstructing the age of greenhouse gas concentrations extracted from ice cores and for the interpretation of air in ice as a dating tool or as a climate proxy firn densification is generally modeled as a steady burial and sintering process of defined layers where the structure of the layering is maintained along the whole firn and ice column however available high resolution density data as well as firn air samples question this picture and point to a lack of understanding of firn densification based on analysis of high resolution density and calcium concentration records from antarctic and greenland ice cores we show for the first time that also impurities may have a significant impact on the densification analysis of firn cores shows a correlation between density and the calcium ion ca concentration and this correlation increases with depth the existence of this relationship is independent of the local climatic conditions at the core sites analyzed the strong positive correlation between the density and the logarithm of ca concentration indicates that impurities induce softening and lead to faster densification over a wide range of concentrations in one core the impurity effect manifests itself so strongly that the density develops a seasonal cycle closely following the seasonal cycle of ca our results clearly show that the structure of the firn layering changes with depth and suggest that the increased variability in density observed in deep firn recently described as a universal feature of polar firn may arise from the influence of ca and or other impurities the impurity effect is likely to have direct implications on our understanding of glacial firn densification and on glacial gas age estimates

microstructural analysis of the classical spiral garnet porphyroblasts of south east vermont evidence for non rotation new data strongly suggest that the classical spiral garnet porphyroblasts of south east vermont usa generally did not rotate relative to geographical coordinates throughout several stages of non coaxial ductile deformation the continuity of inclusion trails si in these porphyroblasts is commonly disrupted by planar to weakly arcuate discontinuities consisting of truncations and differentiation zones where quartz graphite si bend sharply into more graphitic si discontinuous tight microfold hinges with relatively straight axial planes are also present these microstructures form part of a complete morphological gradation between near orthogonally arranged discontinuous inclusion segments and smoothly curving continuous si spirals some 2700 pitch measurements of well developed inclusion discontinuities and discontinuous microfold axial planes were taken from several hundred vertically orientated thin sections of various strike from specimens collected at 28 different locations around the chester and athens domes the results indicate that the discontinuities have predominantly subvertical and subhorizontal orientations irrespective of variations in the external foliation attitude macrostructural geometry and apparent porphyroblast matrix rotation angles combined with evidence for textural zoning this supports the recent hypothesis that porphyroblasts grow incrementally during successive cycles of subvertical and subhorizontal crenulation cleavage development less common inclined discontinuities are interpreted as resulting from deflection of anastomosing matrix foliations around obliquely orientated crystal faces prior to inclusion most of the idioblastic garnet porphyroblasts have a preferred crystallographic orientation dimensionally elongate idioblasts also have a preferred shape orientation with long axes orientated normal to the mica folia within which epitaxial nucleation occurred truncations and differentiation zones result from the formation of differentiated crenulation cleavage seams against porphyroblast margins in association with progressive and selective strain induced dissolution of matrix minerals and locally also the porphyroblast margin non rotation of porphyroblasts relative to geographical coordinates suggests that deformation at the microscale is heterogeneous and discontinuous in the presence of undeformed relatively large and rigid heterogeneities which cause the progressive shearing rotational component of deformation to partition around them the spiral garnet porphyroblasts therefore preserve the most complete record of the complex polyphase tectonic and metamorphic history experienced in this area most of which was destroyed in the matrix by progressive foliation rotation and reactivation together with recrystallization

variscan extension in the pyrenees evidence for variscan extension in the pyrenees is summarized and used with other geological and geochronological data to reconstruct the variscan extensional geometry and kinematics at a crustal scale a regionally consistent pattern of ese wnw trending synmetamorphic stretching directions indicates variscan extension roughly parallel to the present alpine trend of the belt the spatial distribution of variscan low pressure high temperature metamorphic domes is interpreted as being possibly due to the activity of few moderately dipping crustal scale shear zones allowing heterogeneous ductile extension of the variscan middle and lower crust extension occurred synchronous with uplift and emergence of large parts of the crust and deposition during the upper carboniferous and permian of up to 2500 m of continental sediments and intercalated basaltic andesitic volcanics in fault bounded extensional halfgrabens the extensional structures and related metamorphism postdate a namurian to early westphalian stage of crustal thickening characterized by thrusting and steep folding it is suggested that variscan extension in the pyrenees was driven by the gravitational potential energy of an initially thickened lithosphere enhanced by detachment of a cold and gravitationally unstable lithospheric root which caused progressive uplift tectonic denudation and a highly anomalous thermal structure of the variscan crust leading to extensive lower crustal melting and granodioritic intrusion during the early permian

porphyroblast microstructures a review of current and future trends many recent papers show how porphyroblast microstructures play an important role in a wide range of structural and metamorphic studies this paper reviews ten current applications of these microstructures 1 porphyroblast growth timing criteria 2 tracking progressive foliation development relative to changing metamorphic conditions 3 timing of pluton emplacement relative to deformation and metamorphism 4 finite longitudinal strain determinations 5 kinematics and porphyroblast rotation 6 use of linear fabrics preserved in porphyroblasts 7 porphyroblasts and folding mechanisms 8 inclusion trail orientations and orogenic processes 9 inferring shear strain rates from porphyroblast growth rates and 10 in situ age determinations although there is still no concensus on the interpretation of some porphyroblast microstructures a bright future lies ahead as traditional and newly developed techniques of microstructural analysis are combined with modern chemical and microprobe techniques to provide an increased understanding of the relationships between deformation and metamorphism in a wide range of metamorphic settings

rotation of irregular staurolite porphyroblasts in a simple shear dominated shear zone controlled by initial growth orientation and aspect ratio abstract in a mica schist unit of the bossost dome central pyrenees spain idioblastic interkinematic staurolite porphyroblasts with straight inclusion trails s i have rotated with respect to the main schistosity in the same sense by different amounts orientation data of 600 porphyroblasts were obtained from planes parallel to stretching lineation xz perpendicular to lineation yz and parallel to schistosity xy interpenetrating growth twinning resulted in highly irregular shapes with aspect ratios r a ranging from 1 to 5 initial random porphyroblast growth is inferred from variable length axis la s i angles maximum rotation of 120 is observed in blasts with s i oriented orthogonal to the mineral long axis implying a shear strain of 4 3 in simple shear flow absence of back rotation is indicative of dominant simple shear flow w k 0 8 0 9 elongated porphyroblasts r a 3 with s i parallel to la have rotated very little because they were already in a stable position when shearing commenced rotation of staurolite porphyroblasts was possible because decoupling between blasts and matrix was weak and no significant strain partitioning had developed the wide variety of shapes with different aspect ratios formed by simple blasts and interpenetrating growth twins of staurolite are potential shear strain and vorticity gauges

porphyroblast inclusion trail orientation data eppure non son girate extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non coaxial ductile deformation of the matrix subsequent to their growth this has been demonstrated for all tectonic environments so far investigated the work also has provided new insights and data on metamorphic structural and tectonic processes including 1 the intimate control of deformation partitioning on metamorphic reactions 2 solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones and relative plate motion and 3 a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens

is there a time lag between the metamorphism and emplacement of plutons in the axial zone of the pyrenees detailed petrographic and geochemical studies conducted on zircons from the lys caillaouas pluton reveal their igneous and metamorphic affinities the igneous zircons constrain the emplacement of the pluton to 300 2 ma by contrast the metamorphic zircons yield an older age of 307 3 ma which probably dates the thermal peak of the ht lp variscan metamorphism therefore a short time lag of c 7 ma emerges between the metamorphic climax and emplacement of the pluton in the axial zone pyrenees

preferred orientation of planar microstructures determined via statistical best fit of measured intersection lines the fitpitch computer program the fitpitch computer program allows characterisation of preferred orientation of planar microstructures in a rock from the orientations of their intersection lines on different sections measured pitch or strike angles of such intersection lines are fitted to the theoretical intersection lines of a single model plane or a combination of two or three model planes the degree of fit is quantified in terms of the deviation between natural and theoretical intersection lines normalised by uniform data a first application is described concerning porphyroblast inclusion trails whose orientation in 28 samples from the variscan iberian massif was measured analysis of these data illustrates how the method can be used to quantify preferred orientation of relict foliations contained in a group of porphyroblasts and how this complements existing techniques for determining foliation intersection axes fia in porphyroblasts a second described application deals with previously published orientation data for microfracture trace lines in quartz grains of a variscan granite in the rhine graben qualitatively established best fit planes derived originally from this data are compared with the numerically established planes using the fitpitch program and discussed in terms of the advantages and potential limitations of the method q 2002 published by elsevier science ltd

probing the prodigious strain fringes from lourdes abstract we investigate the kinematics of classic sigmoidal strain fringes from lourdes france and review previous genetic models strain methods and strain rates for these microstructures displacement controlled quartz and calcite fibers within the fringes yield an average strain of 195 with the technique of ramsay and huber 1983 this agrees well with strains measured from boudinaged pyrite layers and calcite veins in the same rocks but conflicts with ca 675 strain in previous analogue models for the studied strain fringes produced by progressive simple shear we show that the detailed geometry and orientation of fiber patterns are insufficiently explained by simple shear but imply two successive differently oriented strain fields although all strain fringes have the same overall asymmetry considerable morphological variation resulted from different amounts of rotation of pyrite grains and fringes minor rotation led to sharply kinked fibers that record a ca 70 rotation of the kinematic frame larger up to 145 rotations accommodated by antithetic sliding on pyrite fringe contacts produced more strongly and smoothly curved fibers combined with published rb sr ages for the studied microstructures our new strain data indicate an average strain rate of 1 41 10 15 s 1 during ca 37 myr continuous growth

porphyroblast rotation eppur si muove in a number of recent papers the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates and can be used to determine the original orientation of older foliations complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis involving several alternating phases of horizontal shortening and extension critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws millipede structures used as proof for flow partitioning can also form by other flow geometries evidence quoted to support irrotational behaviour of porphyroblasts is unsound porphyrobl asts do occur in sets with a preferred orientation of the internal foliation trace but these cannot be shown to represent original orientations microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis they can however also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation finally porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations

microstructural chemical and textural records during growth of snowball garnet the growth history of two populations of snowball garnet from the lukmanier pass area central swiss alps was examined through a detailed analysis of three dimensional geometry chemical zoning and crystallographic orientation the first population collected in the hinge of a chevron type fold shows an apparent rotation of 360 the first 270 are characterized by spiral shaped inclusion trails gradual and concentric mn zoning and a single crystallographic orientation whereas in the last 90 crenulated inclusion trails and secondary mn maxima centred on distinct crystallographic garnet domains are observed microstructural geochemical and textural data indicate a radical change in growth regime between the two growth sequences in the first 270 growth occurred under rotational non coaxial flow whereas in the last 90 garnet grew under a non rotational shortening regime the second population collected in the limb of the same chevron type fold structure is characterized by a spiral geometry that does not exceed 270 of apparent rotation these garnet microstructures do not record any evidence for a modification of the stress field during garnet growth concentric mn zoning as well as a single crystallographic orientation are observed for the entire spiral electron backscatter diffraction data indicate that nearly all central domains in the snowball garnet are characterized by one 001 axis oriented sub parallel to the symmetry axis and by another 001 axis oriented sub parallel to the orientation of the internal foliation these features suggest that the crystallographic orientation across the garnet spiral is not random and that a relation exists among the symmetry axis the internal foliation and the crystallographic orientation

impacts of an accumulation hiatus on the physical properties of firn at a low accumulation polar site 1 recent field investigations of a megadune region of east antarctica provide evidence that differences in grain size thermal conductivity and permeability across a megadune profile are due to spatial accumulation variability in the absence of significant microclimate variations the megadunes are low amplitude 2 8 m long wavelength 2 5 km bands with perceptible but low accumulation less than 40 mm water equivalent weq yr 1 and accumulation hiatus within several kilometers proximity as determined by remote sensing surface feature classification and ground penetrating radar profiling our hypothesis that accumulation rate impacts the extent of temperature gradient driven metamorphic growth in low accumulation rate sites is supported by measurements of various firn physical properties relatively small differences in accumulation rate less than 40 mm weq yr 1 result in large differences in physical properties including grain size thermal conductivity and permeability which are apparent in satellite based microwave data from both passive and active sensors the differences in physical snow structure between low accumulation areas and accumulation hiatus areas in the near surface are sufficiently distinct that evidence of past accumulation hiatus should be observable in the physical and chemical properties of an ice core record

the pyrite type strain fringes from lourdes france indicators of alpine thrust kinematics in the pyrenees a new method is developed to reconstruct polyphase deformation histories from pyrite grains with displacement controlled fibrous strain fringes it decomposes the relative displacement between a pyrite grain and a fringe into incremental rotations and pull apart directions application to a classic specimen from lourdes france shows that the rotations determine the fibre curvature sense and strongly depend on local strain parameters as well as the initial orientation and shape of pyrite grains in contrast pull apart directions appear to be relatively constant and straight although curved pull apart trajectories were expected from the heterogeneous non coaxial deformation in the matrix the discrepancy is due to the localization of shear strain components around pyrite fringe complexes the complexes themselves represent ellipsoidal domains of relatively coaxial strain that parallel a regional crenulation cleavage even in zones lacking this cleavage due to stretching and shearing of a pre existing foliation s1 thus strain fringes consistently record bulk shortening orientations regardless of local variations in strain parameters and microstructural development the analysed pyrite fringe complexes from lourdes were previously considered as classic shear sense indicators but yield hook shaped pull apart trajectories in the new method this indicates two deformation events that can be related to a sub horizontal s2 and steep regional crenulation cleavage s3 the recorded polyphase deformation history is tentatively interpreted in terms of a change from gravitational spreading to a compressive emplacement of a thrust sheet in the north pyrenean foreland

tectonic evolution of the montagne noire and a possible orogenic model for syncollisional exhumation of deep rocks variscan belt france structural analysis in the montagne noire including a study of inclusion trail patterns in andalusite garnet and staurolite porphyroblasts reveals a succession of five foliations in alternately steep and flat lying orientations this is inferred to reflect multiple gravitational collapse phases that intervened during continuous plate convergence initial crustal thickening by foliation development and folding d1 was responsible for eclogite facies metamorphism in the gneissic basement subsequent thrusting and recumbent folding d2 generated a subhorizontal crenulation cleavage during namuro westphalian times while previous steep structures became rotated petrological and microstructural evidence indicate that thrust nappes were emplaced by a gravitational spreading mechanism while undergoing internal thinning an important amount of basement exhumation took place synchronously these observations form the basis for a new orogenic model in which thermal reequilibration of a previously thickened orogen induces gravitational instability in the upper crust collapse of the upper crust causes it to spreadout horizontally over continuously converging relatively cool plates thereby creating low angle thrust nappes the upper crustal thinning facilitates simultaneous extrusion of lower crust in vertical root zones at the orogen center which on reaching the higher part of the orogen can be incorporated in the nappes and displaced horizontally following such a mechanism of nappe tectonics renewed horizontal shortening d3 produced the montagne noire gneissic dome by upright folding of nappe related foliations with orogen parallel stretching previous crustal thinning had led to an increased heat flux between basement and cover so that during d3 cover sequences were still rising in temerature while basement rocks had already commenced their retrograde path during d2 peak metamorphic conditions in the cover triggered a second gravitational collapse during namurian times d4 producing a 2 km wide subhorizontal shear zone in which orogen parallel stretching continued at the surface deformation was accommodated by decollements normal faulting and basin formation gneissic rocks below the shear zone were not affected by the collapse because a stronger rheology and horizontal shortening was probably continuous here retrograde cooling of the cover restored gravitational equilibrium and caused its renewed upright folding during late variscan times d5 permian postorogenic extension d6 oriented at a high angle to the previous orogen parallel stretching completed exhumation of the gneiss dome the dome structure of the montagne noire metamorphic core complex was acquired during compressional tectonics and later modified flattened by synorogenic and postorogenic extensional events a similiar polyphase origin appears not to be excluded for other metamorphic core complexes with poorly constrained preextension histories

determining finite longitudinal strains from oppositely concave microfolds in and around porphyroblasts a new quantitative method abstract this paper describes a precise new method for determining finite longitudinal strains in porphyroblastic metamorphic rocks which makes use of oppositely concave microfolds ocms formed by heterogeneous strain of the matrix around porphyroblasts the initial spacing between two foliation surfaces is measured inside a porphyroblast and compared to the spacing between the same two surfaces in the matrix which results in a measure of extension e experienced by the rock during and or after porphyroblast nucleation a natural example is provided by the well known millipede plagioclase porphyroblasts from the robertson river metamorphics in queensland australia twenty four measurements were made from 22 serial thin sections cut parallel to both the x z and x y planes of finite strain giving an average extension of 1 72 parallel to the x direction of finite strain the least squares best fit line to a plot of initial length vs change in length gives an r 2 value of 0 998 a minimum estimate of maximum shortening negative e was also made by measuring the total lengths of s 1 folia that had been crenulated during ocm formation giving a value of 0 54 which falls short of the 0 63 expected for constant volume plane strain deformation because the ocm method is particularly suited to metapelites results may provide new insight into mechanisms of folding and crenulation cleavage development pressure temperature time deformation histories mass transport during deformation and metamorphism and kinematic studies of porphyroblast behavior rotation vs non rotation during ductile deformation

testing models for the development of spiral shaped inclusion trails in garnet porphyroblasts to rotate or not to rotate that is the question the formation of spiral shaped inclusion trails ssits is problematical and the two viable models for their formation involve opposite shear senses along the foliation in which the porphyroblasts are growing one model argues for porphyroblast rotation with respect to a geographically fixed reference frame whereas the other argues for no such porphyroblast rotation but instead rotation of the matrix foliation around the porphyroblast thus porphyroblasts with ssits cannot be used as shear sense indicators until it is conclusively determined which model best explains them any successful model must explain features associated with ssits including 1 foliation truncation zones 2 smoothly curving ssits 3 millipede microstructure 4 total inclusion trail curvature in median sections 5 porphyroblasts with ssits that have grown together 6 evidence for relative porphyroblast displacements 7 shear sense indicators inside and outside porphyroblasts 8 crenulations associated with porphyroblasts and 9 geometries in sections subparallel to spiral axes axes of rotation a detailed study of these features suggests that most if not all can be explained by both the rotational and non rotational models in spite of these models involving diametrically opposed movement senses therefore geometrical analysis of individual porphyroblast microstructures may not determine which model best explains ssits until the kinematics required to form these microstructures are better understood in particular the sense of shear along a developing crenulation cleavage specific tests for determining the shear sense along crenulation cleavages are proposed and results of such tests may conclusively resolve the debate over how ssits form

resolving complexities associated with the timing of macroscopic folds in multiply deformed terrains the spring hill synform vermont determining the timing of macroscopic folds of bedding in multiply deformed terrains is difficult especially for rocks that have undergone a succession of overprinting near orthogonal deformations the spring hill synform in southeast vermont is an example of such a fold the origin and timing of this structure has been the subject of several previous studies understanding its development is crucial to unraveling orogenesis in the vermont appalachians the fold formed during a deformation path that involved a succession of overprinting near orthogonal deformations that produced matrix fabrics s3 s4 and s5 these foliations developed with subvertical subhorizontal and subvertical orientations respectively before being rotated by the effects of younger deformations the spring hill synform is generally thought to have formed as a recumbent structure during regional nappe development s4 developing as an axial planar foliation however we demonstrate that the spring hill synform developed as a fold with a steeply dipping axial plane that was overprinted by s4 and s5 although this geometry and overprinting history are consistent with a d3 time of formation we can find no change in the asymmetry of pre s3 foliations across the fold we suggest that the synform may have formed at a much earlier stage in the orogen s history and was subsequently modified and rotated to its present geometry by the long history of west to east shortening that dominated the later stages of acadian orogenesis in southeast vermont

the internal inclusion trail geometries preserved within a first phase of porphyroblast growth the inclusion trail geometry within a first phase of porphyroblast growth can differ significantly from that preserved by further enlargement because the porphyroblast forms a rigid mass up against which the rock preferentially strains during ensuing events the geometry of the first overgrown inclusion trails is affected by their primary orientation including any pre existing curvature combined with any heterogeneous rotation of this foliation about the developing stretching lineation this can impact the apparent timing of foliation intersection inflection axes preserved within porphyroblasts fias that nucleated during the development of a sub horizontal foliation but is readily resolved 3 d computer analysis of sigmoidal inclusion trails reveals that the asymmetry method for fia determinations is unaffected by the cut location relative to the porphyroblast core significantly perfect spiral inclusion trail geometries can be produced from a sigmoidal shape in cuts up 30 away from the fia therefore since fias in most porphyroblasts bear no relation to matrix structures there is a 17 chance that thin sections cut relative to the foliation lie within 30 of a fia and could contain such an apparent spiral fias maintain consistent trends for the first phase of porphyroblast growth accompanying horizontal bulk shortening but may vary in plunge fias have sub horizontal plunges for porphyroblasts nucleating during gravitational collapse but may vary in trend for all periods of porphyroblast regrowth the data available indicates that fias remain consistently trending and sub horizontal until the relative direction of plate motion causing orogenesis changes

porphyroblast rotation during crenulation cleavage development an example from the aureole of the mooselookmeguntic pluton maine usa in the low pressure high temperature metamorphic rocks of western maine usa staurolite porphyroblasts grew at c 400 ma very late during the regional orogenesis these porphyroblasts which preserve straight inclusion trails with small thin section scale variation in pitch were subsequently involved in the strain and metamorphic aureole of the c 370 ma mooselookmeguntic pluton the aureole shows a progressive fabric intensity gradient from effectively zero emplacement related deformation at the outer edge of the aureole 2900 m map distance from the pluton margin to the development of a pervasive emplacement related foliation adjacent to the pluton the development of this pervasive foliation spanned all stages of crenulation cleavage development which are preserved at different distances from the pluton the spread of inclusion trail pitches in the staurolite porphyroblasts as measured in two dimensional 2 d thin sections increases nonlinearly from 16 to 75 with increasing strain in the aureole these data provide clear evidence for rotation of the staurolite porphyroblasts relative to one another and to the developing crenulation cleavage the data spread is qualitatively modelled for both pure and simple shear and both solutions match the data reasonably well the spread of inclusion trail orientations 40 75 in the moderately to highly strained rocks is similar to the spread reported in several previous studies we consider it likely that the sample scale spread in these previous studies is also the result of porphyroblast rotation relative to one another however the average inclusion trail orientation for a single sample may in at least some instances reflect the original orientation of the overgrown foliation

interference between pluton expansion and non coaxial tectonic deformation three dimensional computer model and field implications abstract structures in orogenic belts are greatly controlled by interactions between pluton related and tectonic related strain fields however many aspects of the geometry and evolution of these structures are poorly known for example 1 what is the geometry of these structures if regional deformation is coaxial 2 how does this geometry change with progressive deformation this paper describes a three dimensional simulation of strain shape and magnitude as well as foliation and lineation patterns formed by interactions between pluton expansion and homogeneous coaxial deformation this simulation suggests that interactions between pluton expansion and regional uniaxial shortening generate patterns that are distinct from the patterns produced by regional uniaxial extension or regional non coaxial deformation for example shortening produces doughnut shaped constrictional rings about the pluton whereas extension produces apple shaped flattening regions around the pluton in addition shortening produces one continuous doughnut shaped low strain region around the pluton on the other hand extension creates two separate ellipsoidal low strain regions at the ends of the pluton furthermore shortening forms foliation rings that have triangular two dimensional sections and lineation is concentric around the axis of maximum shortening in contrast extension produces two separate foliation and lineation cones at the ends of the pluton all these structural and strain elements migrate away from the pluton with increasing expansion rates and towards the pluton with increasing tectonic deformation rates although structural studies around plutons should always integrate microstructural radiometric and field measurements simulation generated patterns can be used to evaluate 1 origin geometry and evolution of structures around plutons 2 mechanisms that explain inclusion trails within porphyroblasts 3 contribution of pluton related strains to strains measured in the country rock and 4 type and three dimensional orientation of regional kinematic schemes

lack of porphyroblast rotation in the otago schists new zealand implications for crenulation cleavage development folding and deformation partitioning porphyroblasts of garnet and plagioclase in the otago schists have not rotated relative to geographic coordinates during non coaxial deformation that post dates their growth inclusion trails in most of the porphyroblasts are oriented near vertical and near horizontal and the strike of near vertical inclusion trails is consistent over 3000 km2 microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures this is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding several crenulation cleavage fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding in the light of accumulating evidence that porphyroblasts do not generally rotate the applicability of these models to deformed rocks is questionable whether or not porphyroblasts rotate depends on how deformation is partitioned lack of rotation requires that progressive shearing strain rotational deformation be partitioned around rigid heterogeneities such as porphyroblasts which occupy zones of progressive shortening or no strain non rotational deformation therefore processes operating at the porphyroblast matrix boundary are important considerations five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast a a thin layer of fluid at the porphyroblast boundary 2 grain boundary sliding 3 a locked porphyroblast matrix boundary 4 dissolution at the porphyroblast matrix boundary and 5 an ellipsoidal porphyroblast shadow unit

step wise growth of biotite porphyroblasts in pelitic schists of the western lys caillaouas massif pyrenees abstract detailed optical and electron microscope sem and tem examination of biotite blasts from pelitic schists of the western lys caillaouas massif in the pyrenees reveals a variety of microstructures suggesting growth of the blasts before and after a thermal peak during the d2 deformation in the early stages of growth the biotites had 001 at high angles to s1 and they appear to have existed as thin 001 parallel plates in many cases blastesis is suggested to have involved growth of the biotites into their own continuously dilating pressure shadows inclusions with dentate grain boundaries banded structures and radiator fin intergrowths suggest a mechanism analogous to the ramsay crack seal model for vein formation but here involving step wise dilation of a pressure shadow cavity into which the adjacent blast itself was growing