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

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

deformation partitioning shear zone development and the role of undeformable objects abstract the formation of a mylonite zone by homogeneous progressive simple shear is highly unlikely because heterogeneous strain of grains with different compositions causes deformation partitioning and results in progressive inhomogeneous simple shear or non coaxial progressive bulk inhomogeneous shortening however the presence of rigid bodies in a zone of rock undergoing bulk simple shear spreads and homogenizes the stress field such that the instantaneous displacement field is essentially homogeneous at a scale up to at least four times the diameter of the rigid object that is the portion of rock surrounding the rigid body deforms by homogeneous progressive simple shear and the rigid object is forced to rotate this has considerable implications for the rotation or lack thereof of unstrained porphyroclasts and porphyroblasts and hence the information such grains preserve about the sense of shear on their margins or through inclusion trails geometric relationships between porphyroclasts or porphyroblasts and the surrounding matrix that are superficially similar can indicate exactly the opposite sense of shear depending on whether the porphyroclasts or porphyroblasts have or have not rotated leading to considerable problems in interpretation of the bulk movement in a shear zone resolution of these relationships enables ready distinction of whether the deformation history involved progressive bulk inhomogeneous shortening or progressive simple shear in particular these results have considerable implications for the origin of blueschist minerals preserved in garnet porphyroblasts via the roles of strain softening and hardening during mylonitization due to the destruction of feldspar porphyroclasts and growth of garnet porphyroblasts respectively

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

sub millimeter heterogeneity of yttrium and chromium during growth of semi pelitic garnet x ray maps of yttrium and chromium in two garnets from a semipelitic schist from central nepal reveal parallel spiraling bands of high and low concentration 10 30 m wide this zoning mimics similar concentration bands defined by foliation in the matrix outside the garnets the garnets apparently incorporated the matrix zoning intact during growth at conditions up to lower amphibolite facies these observations and interpretations indicate that yand cr concentrations remained heterogeneous at the scale of tens of micrometers during breakdown of matrix minerals and growth of garnet this conclusion extends results of previous work which showed that y and cr do not homogenize at the centimeter to millimeter scale during metamorphism to amphibolite facies the width of the concentration bands in the garnets places a constraint on the maximum effective intracrystalline diffusion coefficient for both y and cr in garnet of about 10 21 cm 2 s for peak metamorphic conditions of about 9 kbar at 5708c different concentrations of yand cr at different positions around the rim of each garnet indicate disequilibrium for these elements between garnet and matrix phases the presence of an internal foliation defined by the yand cr concentration bands rules out pre kinematic garnet growth most useful in conjunction with other indicators zoning of these elements in garnet is thus a tool for deciphering the relative timing of deformation and garnet growth

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

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

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

deformation history of the otago schists new zealand from progressively developed porphyroblast matrix microstructures uplift collapse orogenesis and its implications abstract foliation overprinting relationships preserved in porphyroblasts and the surrounding matrix in the otago schists indicate development of up to six penetrative foliations during the late paleozoic mesozoic rangitata orogeny s1 s3 and s5 formed with steep dips and s2 s4 and s6 formed with gentle dips s1 and s2 are preserved only as inclusion trails in porphyroblasts because of matrix foliation reworking during progressive deformation s3 s6 are preserved in the matrix as well as in porphyroblasts and are generally correlatable from one to the other inclusion trail asymmetries define the location of the orogenic uplift core during the rangitata orogeny when the porphyroblasts grew the location of this core is consistent with macroscale geological and structural observations alternating steep and gently dipping foliations are inferred to indicate alternating horizontal compression and extension predictions made by an orogenic model compatible with continually alternating horizontal compression and extension compare favorably with geometries observed in the schists reconstruction of the gross geometry of the otago schists during the rangitata orogeny based on the three dimensional geometry of porphyroblast inclusion trails is similar to reconstructions based on regional geological considerations and plate tectonic calculations

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

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

structures and fabrics in a crustal scale shear zone betic cordillera se spain abstract a broad zone of dominantly ductile high strain deformation lies beneath the aguilon nappe in the sierra alhamilla southern spain it forms part of a crustal scale movement zone traceable through much of the betic cordillera which separates the higher betic nappes from the underlying nevado filabride complex the zone is characterized in outcrop by a distinctive platy foliation and a strong nne trending stretching lineation microstructural characteristics include quartz ribbons mica fish augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz and extensional crenulation cleavages narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone deformation occurred under lower greenschist facies conditions and was accompanied by retrogression of earlier higher grade mineral assemblages structures in the movement zone developed in a temporal sequence beginning with isoclinal folding and transposition of older foliations this was followed by the formation of extensional crenulation cleavages and the progressive localization of strain into the ultramylonite bands mylonitic foliation in these bands is deformed by syn mylonite folds restricted to the bands all these structures were then deformed by s to se vergent small scale folds restricted to the movement zone as a whole cataclasis associated with alteration is localized along the ultramylonite bands and indicates a transition to late stage brittle deformation the lower boundary of the movement zone is gradational strain decreases recrystallized grainsize and the degree of recrystallization of quartz increases and pressure solution becomes the dominant deformation mechanism in mica schist asymmetric quartz fabrics in the movement zone indicate a nne sense of shear but variations in the degree of asymmetry suggest that flow was partitioned with the ultramylonite bands taking up much of the shear strain and the intervening rocks deforming more slowly and with a lower degree of non coaxiality diffuse fabrics in the fine grained ultramylonite bands may indicate a switch to a grainsize sensitive deformation mechanism and an overall downward increase in the opening angle of crossed girdle fabrics may reflect increased water activity at depth

sequential porphyroblast growth during progressive deformation and low p high t lpht metamorphism cooma complex australia the use of microstructural analysis to better understand deformation and metamorphic histories abstract detailed microstructural analysis has been conducted in the cooma complex which is one of the most thoroughly studied areas structurally and metamorphically in australia this was done to demonstrate the usefulness of microstructural analysis for better understanding deformation and metamorphic histories and the results add significant new information about deformation and metamorphism of this area microstructural relationships in the schists and gneisses indicate that four coaxially superposed foliations formed with alternating subhorizontal s 1 and s 3 and subvertical s 2 and s 4 orientations and the meso and macrostructural relationships are easily understood within this deformational framework three stages of sequential porphyroblast growth involve cordierite and andalusite in the schists and cordierite andalusite and k feldspar in the gneisses the porphyroblasts overgrew and preserved progressive stages in the structural development of these rocks with each period of growth occurring during a different deformation event d 2 d 3 and d 4 the sequential timing of these porphyroblasts is consistent with kfmash reaction curve topology and indicates a prograde p t t path convex to the temperature axis combining this with isobaric cooling inferred from previous studies gives an anti clockwise p t t path subvertical and subhorizontal foliations in porphyroblasts contrast with the variable orientations of the same foliations measured in the field however careful field work and microstructural analysis demonstrate that matrix foliation rotation rather than porphyroblast rotation relative to geographic coordinates is largely responsible for the contrasting observations even where bedding has been overturned after porphyroblast growth

two phase syntectonic gold mineralization and barite remobilization within the main ore body of the golden giant mine hemlo ontario canada abstract structural analysis of the central part of the hemlo gold deposit in the hemlo heron bay greenstone belt has revealed four generations of deformation structures d1 d4 the second d2 and fourth d4 deformations significantly influenced the orebody and controlled the localization of gold mineralization development of d3 dextral shearing during retrogressive conditions resulted in strain partitioning around the feldspathic orebody such that this deposit appears to be controlled by the hemlo shear zone however the orebody dominantly preserves vertical d2 parasitic folds rather than d3 structures the earliest occurrence of gold is with molybdenite as s1 inclusion trails in vanadium muscovite grains that grew during d2 this suggests that the gold deposit was emplaced by at least early d2 syngenetic banded barite was partially dissolved transported and redeposited during each deformation event and in particular in association with brittle deformation that occurred during d4 microstructural relationships indicate two distinct generations of gold the first generation is associated with molybdenite and potassic alteration and was deposited before or early in d2 during progressive amphibolite facies metamorphism the second generation formed by redistribution and concentration of gold together with stibnite in ductile brittle d4 structures after peak metamorphic conditions

archaeological evidence for holocene landslide activity in the eastern carpathian lowland abstract landslides are widespread phenomena that contribute to shape the landscape assessing the time sequence of landslide activity during the holocene can help i better frame the present day landslide distribution in the wider context of climate change and ii better define landslide hazard to take adequate mitigation measures to preserve the elements at risk such as archaeological heritage and currently used structures and infrastructures rigorous image interpretation criteria applied to the interpretation of remote sensing images can be a valuable tool to derive information on landslide spatial and temporal distribution however it only allows to broadly estimate the relative age of landslides based on their morphologic signature in this work we investigate the topological relations between landslides and archaeological sites for nine selected settlements in the moldavian plateau situated on ridges and hillslopes landslides and sites were mapped using high resolution lidar dems and extensive field validation activities landslides were classified as very old relict old and recent according to their morphologic appearance we argue the possibility of i assigning a relative age to the three main classes of landslides as they appear on the present day topography and ii assessing the landslide activity during the holocene using this information we set up a model of landslide evolution during the holocene for the lowland of eastern carpathians based on collected data we cannot exclude the pleistocene age for some very old landslides whereas the old and recent landslides appeared during the holocene we think this approach can be extended to other archaeological sites of the study area and to other areas furthermore similar studies can prove useful for landslide hazard analyses helping to adopt adequate protection and mitigation measures framed in a climate change scenario

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

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

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

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

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

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

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

garnet porphyroblast timing and behaviour during fold evolution implications from a 3 d geometric analysis of a hand sample scale fold in a schist detailed 3 d analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand sample scale tight upright fold has revealed a complex deformation history the fold dominated by interlayered quartz mica schist and quartz rich veins preserves a crenulation cleavage that has a synthetic bulk shear sense to that of the macroscopic fold and transects the axis in mica rich layers garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural discontinuities garnet porphyroblast cores and rims pre date the macroscopic fold and preserve successive foliation inflection intersection axes fias which have the same trend but opposing plunges on each limb of the fold and trend nne ssw and ne sw respectively the fias are oblique to the main fold which plunges gently to the wsw inclusion trail surfaces in the cores of idioblastic porphyroblasts within mica rich layers define an apparent fold with an axis oblique to the macroscopic fold axis by 32 whereas equivalent surfaces in tabular garnet adjacent to quartz rich layers define a tighter apparent fold with an axis oblique to the main fold axis by 17 this potentially could be explained by garnet porphyroblasts that grew over a pre existing gentle fold and did not rotate during fold formation but is more easily explained by rotation of the porphyroblasts during folding tabular porphyroblasts adjacent to quartz rich layers rotated more relative to the fold axis than those within mica rich layers due to less effective deformation partitioning around the porphyroblasts and through quartz rich layers this work highlights the importance of 3 d geometry and relative timing relationships in studies of inclusion trails in porphyroblasts and microstructures in the matrix

el nino la nina and the nonlinearity of their teleconnections the paradigm of an atmospheric system varying linearly with respect to extreme phases of the el nino southern oscillation is questioned it is argued that the global response to tropical pacific sea surface temperature forcing will be inherently nonlinear a physical basis for this intrinsic nonlinearity is the thermodynamic control on deep convection climate statistics for warm and cold events of the tropical pacific are analyzed separately for the northern winter periods during 1950 96 composite analysis of 500 mb heights reveal planetary scale teleconnection patterns as noted in earlier studies a new result is the evidence for an appreciable 358 longitude phase shift between the warm and cold event circulation composites and the two wave trains appear to have different tropical origins a large nonlinear component in north american surface climate anomalies is also found which is consistent with such a phase shift in teleconnections in the tropics rainfall anomalies also show evidence of nonlinear behavior the maximum rain anomalies along the equator are located east of the date line during warm events but west of the date line during cold events the interpretation of this behavior is complicated however by the fact that composite warm event sst anomalies are not the exact inverse of their cold event counterparts idealized atmospheric general circulation model agcm experiments are performed in order to test the question of whether the observed nonlinearity is an intrinsic property of the atmospheric system the model is forced with a composite sst anomaly that undergoes a realistic seasonally varying enso life cycle as described by e rasmusson and t carpenter both positive and negative phases of the sst anomaly are used and a 40 member ensemble of warm and cold event model simulations is conducted a nonlinear climate response in the agcm is found that closely resembles the observed composites including a shift in the equatorial positions of the maxmium rain responses and a phase shift of teleconnection patterns in the upper troposphere barotropic model experiments indicate that the inherent nonlinearity in the tropical rain response may itself be responsible for the phase shift in the extratropical teleconnection patterns

kinematics of the alpine arc and the motion history of adria the alps have been interpreted traditionally in terms of north or northwest directed convergence between africa and europe1 with adria the continental underpinnings of the adriatic sea and northern italy acting as a promontory of the african continent2 this has been supported by recent analyses of africa europe relative plate motions3 5 which indicate dominantly north south directed convergence during the tertiary phase of intracontinental shortening we show here however that kinematic indicators from alpine tectonites are difficult to reconcile with this motion history figs 1 and 2 and are explained best by roughly west northwest directed adria europe motion since about 70 million years ago combined with an arc normal pattern of motion driven by local body forces this implies that adria has moved independently of both africa and europe since at least the late cretaceous

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

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