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

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

interpretation of porphyroblast inclusion trails limitations imposed by growth kinetics and strain rates porphyroblast inclusion trails provide important information about the tectonometamorphic evolution of a metamorphic rock however there remains considerable controversy over whether porphyroblasts rotate during bulk non coaxial deformation with reference to an area of the scandinavian caledonides and utilizing existing data from theoretical and experimental modelling this study demonstrates that both straight and s shaped inclusion trails are consistent with an interpretation in terms of syndeformational porphyroblast growth in a regime approximating to newtonian simple shear at crustal strain rates of 10 14 s 1 and porphyroblast growth times of 0 1 1 0 ma it is shown that a maximum of 5d 9d angular rotation would occur during growth at faster strain rates of 10 12 s 1 e g those in a shear zone porphyroblast angular rotations of 90d are shown to occur in 0 1 0 25 ma i e times comparable with or faster than porphyroblastesis in view of this s shaped inclusion trails are to be expected in porphyroblasts growing in active shear zones or other situations of high shear strain whereas straight inclusion trails can be interpreted as static overgrowth of an existing fabric or as syndeformational porphyroblastesis at low strain rates

the three dimensional geometry of simulated porphyroblast inclusion trails inert marker viscous flow models a fluid dynamic model for a rigid spherical porphyroblast growing in a viscous fluid matrix undergoing simple shear deformation predicts an arrangement of captured inert inclusions that is remarkably similar to the spiral pattern observed in synkinematic rolled garnets the model assumes only creeping stokes flow of the matrix and a kinetic model for the growth of the porphyroblast of the form rm km where r is the crystal radius is the amount of shear induced rotation and m and k are kinetic parameters for rotation less than 18d j b thompson and j l rosenfeld s simple ring model provides a surprisingly accurate description of the three dimensional geometry of the central inclusion surface in the simulated porphyroblasts although marker planes originally parallel to the shear plane are deflected around the porphyroblast their intersections with its surface remain approximately circular for much larger amounts of rotation than anticipated by rosenfeld the polar coordinate equation t r of the double spiral formed by the trace of the central inclusion surface in a section through the centre of simulated porphyroblasts is also surprisingly simple as follows rm 6 5k m this relationship was observed over the complete range of rotation angles investigated 0 720d and could form a reasonable basis for estimating the growth rotation history of rolled garnets from the shape of their spiral inclusion trails two dimensional sections through the simulated porphyroblasts parallel to the rotation axis yield clamshell rosenfeld s term inclusion geometries similar to the controversial millipede patterns observed in many natural porphyroblasts

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

progressive metamorphism in a crustal scale shear zone an example from the leon region north west brittany france this paper describes the progressive metamorphism and deformation of a series of metasediments le conquet schists and their higher grade equivalents which occur as tectonically emplaced screens within a sequence of foliated gneisses the gneiss de brest and gneiss de lesneven the sequence exhibits a steep south to north increase in metamorphic grade from garnet staurolite schist to sillimanite gneiss and sillimanite k feldspar migmatite the relationship of mineral growth to foliation development has been established for individual screens at least five phases of deformation d1 d5 are preserved analysis of porphyroblast inclusion trails is used to demonstrate sequential mineral growth during the successive development of orthogonal foliations s1 s4 porphyroblasts continued to grow during the subsequent development of c s mylonite fabrics and extensional crenulation cleavages which are genetically related to a series of high strain zones d5 mineral assemblages phase relations and mineral chemistry are consistent with porphyroblast growth being the result of continuous reactions microstructure porphyroblast relations are used to show that although mineral growth proceeded during continuous reactions these only operated episodically phase relations mineral chemistry and p t estimates are used to constrain p t trajectories and these are linked to the deformation histories within individual screens a comparison between the resulting pressure temperature deformation paths is used to demonstrate that the metamorphic peak occurred progressively later and at successively lower pressures with increasing metamorphic grade it is suggested that the early evolution of the belt is the result of crustal thickening by overthrusting the subsequent history is one of progressive heating and unroofing of the higher grade rocks in a dextral strike slip transtensional shear zone

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

permo triassic changes in bulk crustal shortening direction during deformation and metamorphism of the taebaeksan basin south korea using foliation intersection inflection axes implications for tectonic movement at the eastern margin of eurasia during the songrim indosinian orogeny abstract the permo triassic songrim indosinian orogeny in south korea was a major tectonic event involving complicated continental collisions at the eastern margin of eurasia previous studies have examined the structural and metamorphic features of the songrim orogeny in each of the paleozoic terranes of the orogenic belt i e the taebaeksan basin the okcheon basin and the imjingang belt but correlations of these features among the terranes remain uncertain the aim of this paper is to reveal deformation history including bulk crustal shortening directions in the taebaeksan basin and to correlate the tectono metamorphic evolution of the taebaeksan basin with other phanerozoic mobile belts in eastern asia based on a combined analysis of foliation intersection inflection axes fia trends and metamorphic p t and t t time paths the orientations and relative timing of fia preserved as inclusion trails within porphyroblasts of andalusite chloritoid garnet and staurolite reveal two age groups of inclusion trails in the pyeongan supergroup at the northeastern margin of the taebaeksan basin these microstructures indicate the development of early nnw nne trending structures and fabrics followed by later e w trending ones these observations suggest a change in the orientation of bulk crustal shortening from e w to n s during the songrim orogeny based on the similar microstructures and temperature time paths of the three paleozoic terranes we interpret that the e w bulk crustal shortening influenced the eastern part of the korean peninsula during the early stages of the songrim orogeny presumably related to amalgamation between the proto japan terrane and the eastern margin of eurasia whereas the n s bulk crustal shortening was stronger in the western part of the peninsula during the later stages of the orogeny related to collision between the south and north china blocks

distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral staircase and sigmoidal inclusion trails in garnet schists from the appalachian orogen in south east vermont have undergone multiple phases of garnet growth these phases can be distinguished by the trend and relative timing of foliation inflexion or intersection axes fias of foliations preserved as inclusion trails in garnet porphyroblasts the relative timing of different generations of fias is determined from samples containing porphyroblasts with two or three differently trending fias developed outwards from core to rim multi fia porphyroblasts schists from south east vermont show a consistent pattern of relative clockwise rotation of fia trends from oldest to youngest four populations or sets of fias can be distinguished on the basis of their relative timings and trends from oldest to youngest the four sets have modal peaks trending sw ne w e nnw sse and ssw nne these peaks show that each of the four fia sets has a statistically consistent trend at all scales across a 35 125 km area containing numerous mesoscopic and macroscopic folds the fias of set 4 are defined by inclusion trails that are continuous with matrix foliations have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures conversely sets 1 to 3 are oblique to and pre date most matrix foliations and folds all four fia sets occur in siluro devonian rocks and must have formed in the acadian orogeny the lack of statistically significant differences in the distribution of fia trends across the study area and their consistent relative timings in multi fia porphyroblasts despite a complex regional deformation history involving numerous phases of folding at all scales suggest the porphyroblasts have not rotated relative to one another the change in fia trend with time resulted from rotation of the kinematic reference frame of bulk flow possibly as a consequence of the reorganization of lithospheric plates responsible for acadian orogenesis recognition of distinct generations of fias provides a means of distinguishing different phases of porphyroblast growth four periods of garnet porphyroblast growth occurred in the schists of south east vermont this growth was heterogeneously distributed on the cm2 m2 scale no single porphyroblast records all stages of growth and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth factors that may have been responsible for switching porphyroblast growth on and off at this scale include i subtle differences in bulk chemical composition ii oscillating levels of heat owing to the buffering effect of endothermic garnet forming reactions iii channelized infiltration of fluids with localized fluid buffering of bulk composition and iv cyclic controls on the rates of diffusion and material transport of reactants either by channelized fluid flow or by a changing pattern of microfracturing during foliation development consistency in fia trend and relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events or even phases of metamorphism within a progressive tectonothermal event along and across orogens using a consistent pattern of core to rim changes in fia trend multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished allowing correlation of different phases of growth around and across macroscopic folds the relative timing of growth of different porphyroblastic minerals can also be quantitatively determined using fia data and correlated around and across macroscopic folds conceptually the paragenetic history preserved in each generation of porphyroblast growth in the form of chemical zoning and the minerals in inclusion trails could be combined to produce a more detailed p t t deformation path than previously determined

flow variation in layered rocks subjected to bulk flow of various kinematic vorticities theory and geological implications abstract flow variation is studied analytically in rocks with layers of varied competence and varied orientation with respect to constant bulk flow fields of given kinematic vorticity 0 w k 1 the results explicitly demonstrate that flow varies significantly from layer to layer and with time in any individual layer even though the bulk flow is constant any flow regime can occur and the instantaneous stretching axes isa spin all the time reverse sense of non coaxiality can occur in the same layer at different times and in different layers at the same time this study further reveals the significance of heterogeneity and non steadiness in natural flows the assumption of a time independent flow history for natural deformation is highly unrealistic the spinning of the instantaneous stretching axes isa and the variation of flow regimes have bearings on the progressive development of fabrics and structures such as folded boudins refolded folds in shear zones and double rotation of porphyroclasts that are often interpreted as being due to multi deformational events

fold mechanisms in the canton schist constraints on the contribution of flexural flow abstract fold mechanisms operating in the canton schist have been resolved using the geometric relationship between folds and spiral inclusion trail geometries of the four end member fold mechanisms tangential longitudinal strain folding and slip folding are unable to produce the observed inclusion trail and fold geometries but a combination of flexural flow and pure shear folding is consistent with the geometric constraints the maximum flexural flow component during each fold event was determined from the geometric data during f 3 and f 4 flexural flow produced 27 and 37 of measured limb rotation respectively which corresponds to a maximum of 24 limb rotation by flexural flow in each fold event to satisfy the geometric constraints the remainder of limb rotation must be a product of pure shear folding the maximum possible component of flexural flow folding during f 3 and f 4 increases with increased variation in vorticity between layers in the rock mass in the flexural flow pure shear model a maximum of 28 of inclusion trail curvature is produced by rotation of porphyroblasts relative to irrotational fold limbs with a minimum 72 curvature due to rotation of fold limbs relative to irrotational porphyroblasts all model solutions produce less than 8 syn folding porphyroblast rotation relative to geographic coordinates

rotated staurolite porphyroblasts in the littleton schist at bolton connecticut usa staurolite porphyroblasts 1 5 8cm in length and 0 3 2cm in width in the littleton schist at bolton connecticut contain curved quartz inclusion trails which document synkinematic rotations of at least 135 the orientations of long axes of these staurolite crystals define a weak preferred orientation in a plane approximately parallel to the external foliation serial sections of four differently orientated crystals and u stage measurements of the orientations of their inclusion trails demonstrate that the inflection hinge line and the statistical symmetry axis characterizing the foliation within a porphyroblast are unrelated to the orientations of external crenulations and are in all cases parallel to the long axis of the porphyroblast the cumulative rotation reflected in the curvature of the inclusion trails is a maximum in a c axis section through the initial core of a crystal the amount of rotation about the c axis decreases linearly along the length of the crystal away from the nucleation site the sense and amount of rotation recorded by a porphyroblast is related to its orientation a tightly constrained transition from clockwise to anticlockwise rotation defines a slip direction that coincides with the preferred orientation of the staurolite c axes the total rotation reflected by the inclusion trails increases as a function of the angle between the c axes of the staurolite crystals and the slip direction initially random staurolite porphyroblasts rotated during growth as a consequence of laminar shear in the surrounding viscous matrix this interpretation is quantitatively consistent with the staurolite preferred orientation its coincidence with the apparent slip direction the correlation between both the sense and the amount of rotation and the orientation of the long axis of the porphyroblast and the twisted conical shape of the family of surfaces defined by the inclusion trails

are crenulation cleavage zones mylonites on the microscale abstract mylonites commonly show characteristic structures such as s c fabric and c type shear bands in the present paper the presence of similar structures on the microscale is reported from the cleavage zones of differentiated crenulation cleavage in garnet biotite schists belonging to the lunavada group of proterozoic metasedimentary rocks india these rocks have experienced three episodes of deformation a differentiated crenulation cleavage s 2 characterized by alternating cleavage zones and microlithons developed during d 2 by microfolding of the s 1 foliation although the schists under investigation do not show any macroscopic or mesoscopic scale evidence of mylonitization they show the presence of shear structures within the cleavage zones the fabric resembling s c and c shear bands within these zones indicates shearing within them during d 2 deformation a model incorporating shearing along the cleavage zones is proposed to explain the genesis of shear structures within them accordingly it is invoked that solution transfer and grain rotation are important deformation mechanisms during the early stages of crenulation and this results in the migration of quartz from the limbs to the hinges of the microfolds at the later stages of crenulation the phyllosilicates micas forming the limbs of the microfolds are at an oblique angle to the direction of shortening and most of the mobile material like quartz has already been removed from the limbs by solution transfer therefore the stress conditions are ideal for shearing and intracrystalline crystal plastic deformation to occur along the limbs during the later stages of crenulation it is proposed that the fabric resembling s c embryonic c type shear bands and well developed c in that order develop with increasing strain and shearing within the cleavage zones at still higher strains the shear bands may rotate into parallelism with the domain boundary between the cleavage zones and the microlithons composition of muscovite constituting cleavage zones and microlithons is discussed and it is concluded that the deformation mechanisms that operate during the later stages of crenulation especially under upper greenschist to lower amphibolite conditions are similar to those during mylonitization

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

measurement and correlation of microstructures the case of foliation intersection axes recent studies have used the relative rotation axis of sigmoidal and spiral shaped inclusion trails known as foliation inflexion intersection axis fia to investigate geological processes such as fold mechanisms and porphyroblast growth the geological usefulness of this method depends upon the accurate measurement of fia orientations and correct correlation of temporally related fias this paper uses new data from the canton schist to assess the variation in fia orientations within and between samples and evaluates criteria for correlating fias for the first time an entire data set of fia measurements is published and data are presented in a way that reflects the variation in fia orientations within individual samples and provides an indication of the reliability of the data analysis of 61 fia trends determined from the canton schist indicate a minimum intrasample range in fia orientations of 30 three competing models are presented for correlation of these fias and each of the models employ different correlation criteria correlation of fias in model 1 is based on relative timing and textural criteria while model 2 uses relative timing orientation and patterns of changing fia orientations and model 3 uses relative timing and fia orientation as correlation criteria importantly the three models differ in the spread of fia orientations within individual sets and the number of sets distinguished in the data relative timing is the most reliable criterion for correlation followed by textural criteria and patterns of changing fia orientations from core to rim of porphyroblasts it is proposed that within a set of temporally related fias the typical spread of orientations involves clustering of data in a 60 range but outliers occur at other orientations including near normal to the peak distribution consequently in populations of fia data that contain a wide range of orientations correlation on the basis of orientation is unreliable in the absence of additional criteria the results of this study suggest that fias are best used as semiquantitative indicators of bulk trends rather than an exact measurement for the purpose of quantitative analyses

garnet rotation and the development of axial plane crenulation cleavage rotated helicitic garnets syntectonically overgrowing axial plane crenulation cleavage on the limbs of folds show the opposite sense of rotation to that undergone by the folded foliation during the development of the fold the garnet rotation is inconsistent with simple minded shear models for the development of the crenulation cleavage and it is useful to consider this problem in order to understand the development of crenulation cleavage better various models are considered in an attempt to explain both garnet rotation and cleavage morphology earlier models were based on the assumption that garnet rotation is driven by shear parallel to s2 they were mostly unsatisfactory but one involving migration of the crenulation hinges is viable when combined with other models described here the new models proposed here are based on the assumption that garnet rotation is driven by shear on the crenulated surface s1 the new foliation s2 is considered to be a passive structure at least in the early stages of its development these models satisfactorily explain garnet rotation and development of the cleavage including differentiation and they can be used to predict the morphological evolution of the cleavage microstructure as it should be preserved in garnets comparison of observed and predicted helicitic structures strongly supports the proposed models evidence is presented to show that where a garnet has an se that is a schistosity and an si that is a crenulation cleavage the schistosity can have developed by unfolding of the crenulations

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

numerical simulations of spiral shaped inclusion trails can 3d geometry distinguish between end member models of spiral formation numerical 3d simulations of the development of spiral inclusion trails in porphyroblasts were conducted in order to test the proposals that a 3d spiral geometry differs between the rotation and nonrotation end member models of spiral formation proposed in the literature and b 3d spiral geometry can be used as a criterion to distinguish between the two end member models in rocks four principal differences are identified between the two sets of simulations smoothness of spiral curvature spacing of foliation planes alignment of individual foliation planes either side of the sphere representing the porphyroblast and spiral asymmetry with respect to matrix shear sense of these differences only spiral asymmetry and possibly the alignment of individual foliation planes are diagnostic criteria for distinguishing between the end member models in the absence of a readily applied test to distinguish the end member models interpretation of spiral inclusion trails is problematic it is necessary to determine complementary evidence to distinguish porphyroblast rotation or nonrotation during spiral formation

from adria to africa driven orogenesis evidence from porphyroblasts in the betic cordillera spain abstract in order to unravel the kinematic evolution of the betic rif orogen we investigated the geometry of porphyroblast inclusion trails in the nevado filabride complex a high pressure unit situated at the base of the betic nappe stack a large number of quantitative orientation data for these microstructures and their mutual overprinting relationships reveal two age groups of inclusion trails in the study area with distinctive geographic trends coupled to a comprehensive compilation of field data in the study area these microstructures are shown to coherently witness the development of early ne sw trending structures and fabrics followed by younger nw se trending ones whereby the former experienced clockwise reorientations a major change in the direction of bulk crustal shortening from nw se to ne sw is inferred from these data and tentatively linked to the plate tectonic history of the mediterranean region paleomagnetic data indicate that prior to about 35 ma nw se convergence between adria eurasia and iberia produced orogenesis along a belt bordering greater iberia including sardinian and corsica and probably continuous in the alps brianconnais domain the roughly ne sw trend of this precursor appears to be preserved within porphyroblasts that grew during this period presumably due to the partitioning of deformation around them africa iberia relative motion had been mainly transcurrent until 35 ma azores transform but then rapidly changed to head on nne ssw convergence this gave rise to a second set of nw se trending structures which accompanied deformation of the precursor orogen into several oroclines western alps gibraltar and calabria arcs orogenic collapse and crustal extension initiated shortly after this transition in tectonic regime suggesting a causal relationship we propose that the waning of compressive stresses transmitted by adria and their substitution by a near perpendicular maximum stress direction resulting from africa eurasia convergence could have enhanced the sinking of a thick lithospheric root or roll back of a subducted slab which in turn would have provoked crustal extension and basin formation

experimental investigation of the effects of fluid heterogeneity upon the motion of rigid ellipsoidal inclusions during bulk inhomogeneous shortening abstract an experimental study of the motions of rigid ellipsoidal inclusions within deforming homogeneous and heterogeneous fluid systems indicated that the time behaviour of such objects measured relative to the experimental apparatus can vary significantly between the two system types during inhomogeneous time dependent flow initially aligned ellipsoidal inclusions within a deforming homogeneous fluid were rotated in the same sense as an adjacent passive strain marker introduction of heterogeneity by partially enclosing the ellipsoidal inclusions with a less viscous fluid resulted in three different rigid body behaviours during deformation the behaviours observed for initially aligned ellipsoids in the system containing fluid heterogeneities were 1 opposite sense rotation to that of the strain marker 2 negligible rotation of the objects and 3 same sense of rotation as that of the strain marker a computerized time based analysis of the deformation possible through the incorporation of marker particles into each of the two different deforming systems revealed similar deformation kinematics for the two experimental types at both the system and sub system scale results of the experiments indicate that significant differences can occur in rigid body behaviour between apparently similarly deforming homogeneous and heterogeneous systems the results suggest that fluid flow theory as derived for rigid body behaviour in homogeneous fluids may be unsuitable for the purpose of qualitatively predicting rigid object behaviour within deforming rock systems that are highly anisotropic at the porphyroblast porphyroclast scale this notion is supported by an examination of the assumptions and simplifications upon which the theory relating to rigid body behaviour in homogeneous fluids was originally based

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

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

a possible geodetic anomaly observed prior to the loma prieta california earthquake monthly measurements since mid 1981 of distance from a geodetic station located 11 km from the epicenter of the loma prieta earthquake m sub s 7 1 october 17 1989 to three stations 30 to 40 km distant provides an unusually complete record of deformation in the epicentral region in the years prior to an earthquake roughly 1 3 years before the earthquake at about the time of the first magnitude 5 foreshock the rate of change in line length for two of the lines appears to change the rate for the third line does not change other similar though smaller changes in rate are apparent in the eight year record thus there is marginal evidence for a change in deformation rate about one year before the loma prieta earthquake but that change need not be a precursor

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

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

distribution of pre folding linear indicators of movement direction around the spring hill synform vermont significance for mechanism of folding in this portion of the appalachians abstract three distinctly oriented sets of pre folding and one set of syn folding axes of curved inclusion trails are preserved in garnet porphyroblasts in 50 samples around the doubly plunging spring hill synform in southeast vermont over one third of the samples contain consistent changes in the trend of these axes from the core to rim since the core grew before the rim this enabled the relative timing of each set of axes to be determined from the oldest to the youngest as ne sw e w nnw sse and nne ssw the youngest trend is parallel to the axial plane of the regional folds only those samples with the latter trend have their inclusion trails connected continuously to the matrix foliation the three pre folding sets of axes have the same orientation on both limbs this consistency in orientation has significant implications for the processes operating during folding and three mechanisms are presented that could potentially explain it these involve the classic card deck model of shear folding de sitter s model of clay bricks shortening as they shear past one another and the progressive bulk inhomogeneous shortening model the relative merits of each of these models are discussed

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

crust and upper mantle structure of wanganui basin and southern hikurangi margin north island new zealand as revealed by active source seismic data

unravelling the spirals a serial thin section study and three dimensional computer aided reconstruction of spiral shaped inclusion trails in garnet porphyroblasts seventy seven spatially orientated serial thin sections cut from a single rock reveal changes in the geometry of spiral shaped inclusion trails sslts in garnet porphyroblasts the observed sits are doubly curved non cylindrical surfaces with total inclusion trail curvature decreasing systematically from the cores to the rims of porphyroblasts the three dimensional geometry of the ssits reconstructed with the aid of computer graphics shows that the orientations of spiral axes defined by the ssits are not related in any expected nor predictable way to the main foliation in the matrix this suggests continued deformation after or during the latest stages of porphyroblast growth which has important implications for the use of ssits as shear sense indicators whether the formation of ssits involves significant porphyroblast rotation with respect to a geographically fixed reference frame cannot be determined from the available data key wocds computer graphics garnet porphyroblasts serial thin sections spiral shaped inclusion trails three dimensional reconstruction

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

multi stage mountain building vs relative plate motions in the betic cordillera deduced from integrated microstructural and petrological analysis of porphyroblast inclusion trails abstract the tectonic evolution of the betic cordillera has been investigated through integrated microstructural and petrological analysis of 93 samples of garnetiferous grt cld plg ky st phengite schists from its lowermost allochthon the nevado filabride domain porphyroblast inclusion trails in these samples exhibit well developed preferred vertical and horizontal orientations and truncational relationships these features indicate a complex history of multiple crenulation cleavage development during alternating contractional and gravity induced deformations associated foliation intersection axes preserved in porphyroblasts fia were determined separately for 2 groups of samples using the asymmetry technique bell et al 1995 and the fitpitch method aerden 2003 respectively broadly consistent results establish a succession of 4 fia sets with specific trends which from the oldest to the youngest fia set are ne sw fia set 1 nw se fia set 2 w e fia set 3 and nnw sse fia set 4 this trend sequence is also reflected in the orientations of successive fold generations and associated lineations more remarkably the fia trend sequence mimics known changes in the plate motions of africa and adria relative to iberia in the 50 10 ma interval this is also the time span indicated by published radiometric ages garnet isopleth thermobarometry applied to selected samples and compositional zoning of garnets indicate that the earliest 3 fia sets formed along a prograde metamorphic path from ca 5 kb 500 c to 10 kb 550 c whereas the latest stages of fia set 3 and fia set 4 developed during exhumation exhumation involved a period of heterogeneous reheating that was spatially and temporally linked to emplacement of the overlying alpujarride complex

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

near orthogonal foliation development in orogens meaningless complexity or reflection of fundamental dynamic processes abstract orogenic belts are geometrically complex owing to repeated deformation within this complexity there is evidence that may suggest a common pattern of sequential steeply dipping and gently dipping foliations seven possible explanations are presented for the sequential development of these foliations which can probably be reduced to four of general importance 1 the passage of thrust sheets over flats and ramps 2 switching of the maximum and minimum compressive stress orientations during orogenesis 3 reversal of structural development owing to strong rheological anisotropy and 4 back rotation of crenulation hinges during crenulation cleavage development it is suggested that all four of these reflect fundamental dynamic processes at work to build an orogen and therefore that sequentially overprinted steeply and gently dipping foliations also reflect such fundamental processes this illustrates that detailed geometrical analysis at the meso and microscales can provide valuable input to dynamic models of orogenesis

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

foliation intersection axes preserved in garnet porphyroblasts from the swat area nw himalaya a record of successive crustal shortening directions between the indian plate and kohistan ladakh island arc abstract we have investigated the geometry and tectonic significance of porphyroblast inclusion trails in 61 oriented samples from the swat region of north pakistan nw himalaya the studied samples represent the indian plate cover sequence deformed and metamorphosed to medium and high grade during the himalayan orogeny two complementary techniques were applied to measure the orientation of foliation intersection inflection axes fia in porphyroblasts in the studied samples and reveal the existence of three principal fia sets in the study area measuring approximately 60 30 km each fia set is characterized by a distinctive regionally consistent trend and relative timing from old to young ese wnw fia set1 e w fia set 2 and broadly nne ssw fia set 3 this sequence is inferred to record an anticlockwise rotation of the direction of tectonic transport or bulk crustal shortening perpendicular to fia through time either progressively or in a stepwise fashion consistent with previous kinematic and geochronological data regarding collisions between the indian plate kohistan ladakh island arc klia and eurasia between ca 55 and 40 ma fia set 1 and 2 correlate well with early formed ese wnw and e w trending thrust related structures in the study area respectively whereas the trend of fia 3 matches that of relatively young regional scale folds and shear zones with a conspicuous n s trend hence the fia succession has successfully resolved the relative timing of formation between the e w versus n s trending structures in the region which was a longstanding question in the tectonics of the nw himalaya in addition a comparison of orientation data for symmetrical millipede inclusion trails versus asymmetrical curved trails indicates that porphyroblasts experienced very limited differential rotations in the region despite the complex polyphase deformation history recorded perhaps the most fascinating aspect of our findings is the direct relationship that is suggested between fia trends and successive vectors of relative indian plate motion since collision with the klia

regional slaty cleavage formation and fold axis rotation by re use and reactivation of pre existing foliations the fiery creek slate belt north queensland abstract a deformation history comprised of six separate deformation events of differing intensity has affected the rocks of the south palmer river region of the the hodgkinson province north queensland within this region a zone of pervasive slaty cleavage herein termed the fiery creek slate belt has developed as a result of the superposition of fabrics formed during several of these events the most important processes in the formation of this composite cleavage were the re use and reactivation of the favourably oriented steep n s trending s2 foliation by the intense fourth deformation event d4 this produced micro meso and macroscopic folds in an originally shallow s3 foliation produced during the intervening d3 deformation with an axial planar s2 s4 foliation the d4 stretching lineation l44 plunges subvertically to steeply north and indicates that shear during d4 was oriented steeply north south in the fiery creek slate belt d2 fold axes are interpreted to have formed in much shallower orientations than their present moderately n s plunging to subvertical orientations we consider this to be a result of d4 shear which caused variable degrees of rotation of d2 fold axes toward the d4 stretching lineation due to subparallelism of the bulk shortening directions of the d2 and d4 events near total destruction of the pre d4 foliations during slaty cleavage formation has produced a misleading impression of a simple deformation history there is no relationship between metamorphic grade and intensity of slaty cleavage development

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

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

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

development of snowball structure numerical simulation of inclusion trails during synkinematic porphyroblast growth in metamorphic rocks abstract this paper presents a hydrodynamic analysis that simulates the development of snowball structures in porphyroblasts the porphyroblast is represented by a rigid sphere that grows at a constant volumetric rate while the matrix is represented by a newtonian fluid that deforms by simple shear well away from the sphere the marker particles used to track the motion of matrix and porphyroblast are arranged in a straight line before deformation those in the matrix move along flow lines that deflect around the sphere while those in the sphere passively rotate at a constant angular velocity those in the matrix are progressively captured and incorporated within the growing sphere the resultant trails of the marker particles in the sphere simulate the inclusion trails of synkinematic porphyroblasts three main types of trails are distinguished by the appearance of curvature in the sphere single double and triple rotation types single rotation types show very simple spiral trails formed when the initial angle between the marker line and the shear plane is between 0 and 45 double rotation types show a complicated pattern with the marker line showing a reversal of curvature from the porphyroblast center to its margin this type is formed when the angle between the marker line and the shear plane was initially between 45 and 135 when this angle exceeds 135 then triple rotation types appear which are characterized by two reversals of curvature of the marker line from center to the margin the calculated results are found to be very similar to some natural examples implying that such natural spiral patterns in the porphyroblast are produced in the similar way to what happens in the computer simulation

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

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

behaviour of rigid objects during deformation and metamorphism a test using schists from the bolton syncline connecticut usa the behaviour of spherical versus highly ellipsoidal rigid objects in folded rocks relative to one another or the earth s surface is of particular significance for metamorphic and structural geologists two common porphyroblastic minerals garnet and staurolite approximate spherical and highly ellipsoidal shapes respectively the motion of both phases is analysed using the axes of inflexion or intersection of one or more foliations preserved as inclusion trails within them we call these axes fias for foliation inflexion intersection axes for staurolite this motion can also be compared with the distribution of the long axes of the crystals schists from the regionally shallowly plunging bolton syncline commonly contain garnet and staurolite porphyroblasts whose fias have been measured in the same sample garnet porphyroblasts pre date this fold as they have inclusion trails truncated by all matrix foliations that trend parallel to the strike of the axial plane however they have remarkably consistent fia trends from limb to limb the fias trend 175 and lie 25 nnw from the 020 strike of the axial trace of the bolton syncline the plunge of these fias was determined for six samples and all lie within 30 of the horizontal eleven of these samples also contain staurolite porphyroblasts which grew before during and after formation of the bolton syncline as they contain inclusion trails continuous with matrix foliations that strike parallel to the axial trace of this fold the staurolite fias have an average trend of 035 15 ne from the 020 strike of the axial plane of this fold the total amount of inclusion trail curvature in staurolite porphyroblasts about the axis of relative rotation between staurolite and the matrix i e the fia is greater than the angular spread of garnet fias although staurolite porphyroblasts have ellipsoidal shapes their long axes exhibit no tendency to be preferentially aligned with respect to the main matrix foliation or to the trend of their fia this indicates that the axis of relative rotation between porphyroblast and matrix the fia was not parallel to the long axis of the crystals it also suggests that the porphyroblasts were not preferentially rotated towards a single stretch direction during progressive deformation five overprinting crenulation cleavages are preserved in the matrix of rocks from the bolton syncline and many of these result from deformation events that post date development of this fold staurolite porphyroblast growth occurred during the development of all of these deformations most of which produced foliations staurolite has overgrown and preserved as helicitic inclusions crenulated and crenulation cleavages i e some inclusion trail curvature pre dates porphyroblast growth the deformations accompanying staurolite growth involved reversals in shear sense and changing kinematic reference frames these relationships cannot all be explained by current models of rotation of either or both the garnet and staurolite porphyroblasts in contrast we suggest that the relationships are consistent with models of deformation paths that involve non rotation of porphyroblasts relative to some external reference frame further we suggest there is no difference in the behaviour of spherical or ellipsoidal rigid objects during ductile deformation and that neither garnet nor staurolite have rotated in schists from the bolton syncline during the multiple deformation events that include and post date the development of this fold