patterns of strain localization in heterogeneous polycrystalline rocks a numerical perspective the spatial and temporal patterns of strain localization in materials with pre existing heterogeneities are investigated via a series of two dimensional numerical models models include i a dynamic feedback process to simulate rheological weakening in response to the transition from non linear flow dislocation creep to linear flow diffusion creep grain boundary sliding and ii a time dependent strengthening process counteracting the weakening process different load bearing framework geometries with 20 weak component are used to evaluate the impact of geometry on the strength of the material and its ability to localize strain into an interconnected weak layer iwl our results highlight that during simple shear if dynamic weakening with or without strengthening feedbacks is present strain is quickly localized into an iwl where an increasing proportion of weak material increases the interconnections between the iwls thereby increasing the anastomosing character of the shear zones we establish that not only bulk strain localization patterns but also their temporal patterns are sensitive to the dominance of the weakening or strengthening process consequently shear zones are dynamic in time and space within a single deformation event hence the pattern of finite strain can be an incomplete representation of the evolution of a shear zone network

flow laws for rocks containing two non linear viscous phases a phenomenological approach abstract mylonitic rocks and rock analogue materials reveal two basic types of structure 1 a load bearing framework lbf of strong phase contains isolated pockets of weak phase 2 interconnected layers of weak phase iwl separate boudins and clasts of strong phase aggregates with the lbf microstructure are characterized by nearly uniform strain rate stress is concentrated in the load bearing framework in aggregates with an iwl microstructure strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase the degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases based on these observations the rheology of two phase rock is modelled with separate functions for lbf and iwl microstructures a new flow law is derived for rock with iwl structure in which two phases undergo dislocation creep the flow law expresses composite creep strength in terms of temperature bulk strain rate and the volume proportions and creep parameters of the minerals in the rock strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate composite strengths predicted with the iwl flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates a hypothesis of viscous strain energy minimization is used to determine the relative stability of the lbf and iwl microstructures during steady state creep the iwl microstructure is predicted to be stable over a broad range of two phase compositions and mineral strength contrasts whereas the lbf microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts the iwl flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts

mechanisms of weak phase interconnection and the effects of phase strength contrast on fabric development abstract to determine how deformation mechanisms and fabrics evolve during the strain weakening and localization that accompany the transition from a load bearing framework to an interconnected weak phase shear experiments were conducted on a fine grained gneiss 58 quartz interconnected 28 plagioclase dispersed 13 biotite aligned but dispersed at 745 and 800 c 1500 mpa and two strain rates 2 10 5 and 2 10 6 s 1 in experiments with a high phase strength contrast hpsc 25 and 45 between dispersed biotite and framework quartz grains biotite grains become interconnected due to stress concentration at their tips that allow local semi brittle flow of intervening quartz and cataclasis of intervening plagioclase in the higher temperature hpsc samples the biotite partially dehydrated in high strain zones a single narrow shear zone formed because the psc between the quartz and biotite reaction product layers remained high in the lower temperature hpsc samples there was no reaction and a penetrative s c fabric formed the fabric is defined by the many multiply interconnected biotite strands that formed because the psc between the quartz and biotite decreased as the biotites kinked in slower strain rate experiments with a much lower psc 10 biotite interconnection occurs by shearing into quartz quartz boundaries where new weak strain free recrystallized grains form at low strain a weak s c fabric forms but it evolves to an s c fabric as the psc decreases with strain thus the magnitude of the strength contrast between a weak phase and its matrix influences the mechanism of weak phase interconnection the degree of strain localization and the fabric

tectono metamorphic evolution of a convergent back arc the famatinian orogen sierra de quilmes sierras pampeanas nw argentina

strain and vorticity analysis using small scale faults and associated drag folds abstract small scale faults with associated drag folds in brittle ductile rocks can retain detailed information on the kinematics and amount of deformation the host rock experienced measured fault orientation drag angle and the ratio of the thickness of deflected layers at the fault l and further away t can be compared with and l t values that are calculated with a simple analytical model using graphs or a numerical best fit routine one can then determine the kinematic vorticity number and initial fault orientation that best fits the data the proposed method was successfully tested on both analogue experiments and numerical simulations with basil using this method a kinematic vorticity number of one dextral simple shear and a minimum finite strain of 2 5 3 8 was obtained for a population of antithetic faults with associated drag folds in a case study area at mas rabassers de dalt on cap de creus in the variscan of the easternmost pyrenees spain

the eigenvalues of steady flow in mohr space abstract the eigenvalues of the velocity gradients tensor and of the position gradients tensor describing instantaneous flow or incremental deformations are easily determined in mohr space as the points of intersection between a mohr circle and the horizontal mohr axis intersections occur only if the tensor has real eigenvalues eigenvectors associated with the eigenvalues are directions of no rotation a line of material particles under an eigenvector does not rotate during steady flow although the line is instantaneously stretching at a rate equal to its corresponding eigenvalue for non spinning steady sub simple shearing there will always be two eigenvectors the angle between the eigenvectors is dependent on the ratio of pure to simple shearing rates this angle decreases as the pure shearing component decreases relative to the simple shearing component superimposed increments of pure shear and simple shear produce an equivalent set of non rotated material lines but the deformation histories of these lines are rotational the influence of eigendirections on geological structures is constrained by the rheology of a rock during deformation in homogenous steady flow no influence may be obvious spatially periodic material softening along eigenvectors during bulk hardening may result in a complex fabric of intersecting shear zones the geometry of the fabric elements provides important information on the bulk deformation of the rock particularly on the bulk shear sense

high strain deformation of conglomerates numerical modelling strain analysis and an example from the wutai mountains north china craton abstract conglomerates have been widely used to investigate deformation history and rheology strain vorticity and viscosity previous studies reveal that several factors such as pebble shapes and concentrations as well as material properties affect conglomerate deformation however how pebble concentration and interaction between pebbles affect deformation is not understood very well we use the 2d numerical modelling platform elle coupled to the full field crystal visco plasticity code vpfft to simulate the deformation of conglomerates with various viscosity contrasts between pebbles and matrix and different pebble concentrations with both linear stress exponent n 1 and power law n 3 viscous rheologies under simple shear conditions up to a shear strain of ten pebbles can behave as effectively passive deformable or effectively rigid an increase in pebble concentrations viscosity contrasts enhances pebble deformation but reduces their rotation a mean aspect ratio rf orientation plot is proposed to gain an estimate of pebble deformation behaviour and the amount of bulk strain closely spaced rigid or deformable pebbles can form clusters that mechanically act as single inclusions rigid clusters rotate and survive for only short strain increments whereas the more stable deformable ones keep on elongating with minor rotation we provide a natural example of deformed conglomerates from the wutai mountains north china craton these consist of banded iron formation bif pebbles embedded in a schistose matrix using the mean rf plot a finite strain of 6 under simple shear could be determined the viscosity of the pebbles is estimated at about 5 8 times that of the matrix for a linear rheology n 1 or 2 to 5 times if a power law rheology with n 3 is assumed

vorticity determination distribution partitioning and the heterogeneity and non steadiness of natural deformations abstract an investigation on vorticity in general heterogeneous and non steady flows is presented the bulk vorticity of a flow is solely determined by the imposed boundary conditions and it exerts no constraints on local flow the distribution of bulk vorticity in different rheological domains and the partitioning of the distributed vorticity into shear induced vorticity and spin are both time dependent this indicates that heterogeneous flows are inevitably non steady in a quantitative example it is demonstrated that even though the imposed boundary movement is a constant simple shear local flow can be non steady spinning and non coaxial super simple shear flow with internal kinematic vorticity number greater than i occurs but the finite strain does not necessarily pulsate the geological implications of this investigation are discussed the significance of heterogeneity and non steadiness in natural deformations are emphasized it is concluded that vorticity distribution and partitioning is the link between different scale structures the internal kinematic vorticity number is not a criterion of finite strain oscillation

multiscale modeling of ice deformation behavior understanding the flow of ice in glaciers and polar ice sheets is of increasing relevance in a time of potentially significant climate change the flow of ice has hitherto received relatively little attention from the structural geological community this paper aims to provide an overview of methods and results of ice deformation modeling from the single crystal to the polycrystal scale and beyond to the scale of polar ice sheets all through these scales various models have been developed to understand describe and predict the processes that operate during deformation of ice with the aim to correctly represent ice rheology and self induced anisotropy most of the modeling tools presented in this paper originate from the material science community and are currently used and further developed for other materials and environments we will show that this community has deeply integrated ice as a very useful model material to develop and validate approaches in conditions of a highly anisotropic behavior this review by no means exhaustive aims at providing an overview of methods at different scales and levels of complexity

time for anisotropy the significance of mechanical anisotropy for the development of deformation structures hr acknowledges financial support by the china scholarship council csc grant nr 201506400014 egr acknowledges the support of the beatriu de pinos programme of the government of catalonia s secretariat for universities and research of the department of economy and knowledge 2016 bp 00208 we thank bruce hobbs and an anonymous reviewer for their suggestions to improve this article

coupled deformation and metamorphism in the roof of a tabular midcrustal igneous complex the 370 377 ma mooselookmeguntic igneous complex was emplaced at 14 km depth into steeply dipping metaturbidites that were folded and metamorphosed ca 400 405 ma during the acadian orog eny gravity and drill hole data isograd geometry thermal modeling and struc tural measurements all indicate that the eastern aureole of the complex represents an areally extensive roof above the gently east dipping tabular shaped intrusion this roof preserves a classic example of low pressure high temperature metamorphism caused by the underlying intrusion ther mal modeling indicates that the roof is no more than 1000 m thick over an area exceed ing 100 km2 unlike other roofs described in the literature the mooselookmeguntic igneous complex roof preserves a thick 600 m emplacement related strain gradi ent with a deformational fabric that evolved through all stages of crenulation cleavage to become an intense nondifferentiated folia tion approximately parallel to the intrusive contact and perpendicular to the steeply dip ping regional acadian age foliation fabric evolution through all stages of crenulation cleavage requires the vertical growth of the tabular intrusion to have been largely accommodated by dissolution precipitation creep which is a linear viscous deformation mechanism that can occur at lower differ ential stress than for example dislocation creep as the crenulation cleavage evolves a stage is reached where further deformation requires a change in deformation mecha nism which may lead to hardening of the rock development of crenulation cleavage requires a mica rich foliation at an initially high angle to the emplacement related fl at tening plane and this confi guration may be the primary reason why the strain aureole in the mooselookmeguntic igneous complex roof is so well developed compared to other published examples of midcrustal roofs

origin of kinkbands and shear band cleavage in shear zones an experimental study specimens of artificial kcl mica schist have been deformed in simple shear with different initial orientations 10 intervals between 0 and 170 of the mica schistosity s1 relative to the imposed simple shear plane for most experiments the angular shear strain was 30 the strain rate 4 8 10 4s 1 and the stress normal to the imposed shear plane 20 mpa all experiments were conducted at room temperature deformation of the specimens is achieved largely by slip parallel to the s1 mica foliation however two new types of slip surfaces also develop as either a single orientation set or as a pair of conjugate sets these new slip surfaces are kinkbands and a shear band cleavage the type and number of sets of new slip surfaces which develop depends on the orientation of s1 relative to the shortening and extension fields of the instantaneous strain ellipsoid our experiments indicate that kinkbands or shear band cleavage form in shear zones in which s1 is inclined to the margin of the zone which structure forms and whether it occurs as a single set of structures or as a conjugate pair depends on the initial orientation of s1 in situations where s1 is parallel to the margins of the zone neither structure develops if the deformation is a simple shear the presence of kinkbands or shear band cleavage in such zones where s1 is parallel to the margins is an indication that the zones were transtensional or transpressional respectively we conclude that shear band cleavage is not only an excellent indicator of shear sense but also a qualitative indicator of the ratio of s1 normal shortening to s1 parallel shear conjugate shear bands indicate that s1 normal shortening was relatively strong and single shear band sets indicate that s1 parallel shear was relatively strong

a model for the development of crenulations in shear zones with applications from the southern appalachian piedmont abstract two sets of crenulations are associated with a major alleghanian dextral shear zone which deformed stratigraphic and structural boundaries in the eastern piedmont of south carolina both sets of crenulation planes are oblique to the boundaries of the shear zone the morphologies and orientations of the crenulation sets and their spatial distributions indicate that they are related to slip along foliation planes and that they serve to compensate for displacement components of foliation slip normal to the overall movement direction in the shear zone the crenulations act to maintain the initial thickness of the shear zone our evaluation of the recent literature on shear zones suggests that crenulations related to foliation slip are abundant and constitute a reliable sense of shear indicator

reprint of extensional shear band development on the outer margin of the alpine mylonite zone tatare stream southern alps new zealand abstract schistose mylonitic rocks in the central part of the alpine fault af at tatare stream new zealand are cut by pervasive extensional c shear bands in a well understood and young natural ductile shear zone the c shears cross cut the pre existing mesozoic aged foliation displacing it ductilely synthetic to late cenozoic motion on the af using a transect approach we evaluated changes in geometrical properties of the mm cm spaced c shear bands across a conspicuous finite strain gradient that intensifies towards the af precise c attitudes c foliation dihedral angles and c s intersections were calculated from multiple sectional observations at both outcrop and thin section scales based on these data the direction of ductile shearing in the alpine mylonite zone during shear band activity is inferred to have trended 20 clockwise down dip of the coeval pacific australia plate motion indicating some partitioning of oblique slip motion to yield an excess of dip slip relative to plate motion azimuth or some up dip ductile extrusion of the shear zone as a result of transpression or both constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary szb across all examined parts of the shear zone the mean dihedral angle between the c shears and the mylonitic foliation s remains a constant 30 1 1 the aggregated slip accommodated on the c shear bands contributed only a small bulk shear strain across the shear zone 0 6 0 8 uniformity of per shear slip on c shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain hardening rheology such that they locked up at a finite shear strain inside c bands of 12 15 shear band boudins and foliation boudins both record extension parallel to the szb as do the occurrence of extensional shear band sets that have conjugate senses of slip we infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with w k

shear fractures in anisotropic ductile materials an experimental approach abstract analogue models have been used to investigate the influence of planar mechanical anisotropy on the orientation of shear fractures in an elastoviscous brittle material multilayered models consisting of a mixture of plasticine vaseline and preferentially oriented paper flakes have been coaxially deformed for this purpose the evolution of the orientation length and connectivity of fracture sets has been systematically analysed with progressive coaxial deformation the experimental results show that the orientation of fractures with respect to the deformation axes depends on the orientation of transverse anisotropy two symmetrical sets of shear fractures are formed in models with layering parallel to the extension axis x while the fracture network is asymmetrical with respect to the deformation axes for cases with oblique anisotropy the average dihedral angles between fracture sets are higher than 100 with the obtuse bisector between the two sets oriented perpendicular to layers the stress fields calculated from fracture data differ from the boundary conditions applied by the deformation apparatus this misorientation is related to the degree of anisotropy in models with high oblique anisotropy both fracture arrays rotate in a dextral sense together with layers towards the x axis indicating that the presence of a strong anisotropy controls their evolution with progressive deformation

remagnetization in the tennessee salient southern appalachians usa constraints on the timing of deformation article i nfo the appalachian fold thrust belt is characterized by a sinuous trace in map view creating a series of salients and recesses the kinematic evolution of these arcuate features remains a controversial topic in orogenesis primary magnetizations from clastic red beds in the pennsylvania salient show pennsylvanian rotations that account for about half of the curvature while kiaman aged permian remagnetizations display no relative rotation between the limbs the more southern tennessee salient shows a maximum change in regional strike from 65 in virginia to 10 in northern georgia paleomagnetic results from thirty two sites in the middle to upper ordovician chickamauga group limestones and twenty sites from the middle cambrian rome formation red beds were analyzed to constrain the relative age of magnetization as well as the nature of curvature in the tennessee salient results from three sites of the silurian red mountain formation were added to an existing dataset in order to determine whether the southern limb had rotated after thermal demagnetization all three sample suites display a down and southeasterly direction albeit carried by different magnetic minerals the syn tilting direction of the chickamauga limestones lies on the pennsylvanian segment of the north american apparent polar wander path apwp indicating that deformation was about half completed by the late pennsylvanian the rome and red mountain formations were also remagnetized during the pennsylvanian both the chickamauga limestones and rome red beds fail to show a correlation between strike and declination along the salient suggesting either that the salient was a primary non rotational feature or that secondary curvature occurred prior to remagnetization as it did in pennsylvania moreover remagnetized directions from the red mountain sites show no statistical difference between the southern limb of the salient and the more northeasterly trending portion of the fold thrust belt in alabama thus all of the studied units in the tennessee salient are remagnetized and show no evidence for rotation this confirms that remagnetization was widespread in the southern appalachians and that any

water loss and the origin of thick ultramylonites hydrolytic weakening has been suggested as a major process facilitating strain localization in line with many studies that found a positive correlation between water content and intensity of deformation we examine the role of water in the unusually thick ultramylonite of the el pichao shear zone northwestern argentina we used fourier transform infrared spectroscopy to measure water content in quartz and feldspar comparing ultramylonitic rocks to mylonites and weakly deformed rocks quartz and feldspar in ultramylonites contained half the water of weakly deformed rocks contrary to findings in previous studies we propose that the kilometer thick ultramylonite formed in three stages 1 localized deformation and recrystallization caused release of intracrystalline water to grain boundaries which promoted grain boundary sliding forming the ultramylonite 2 high pressure in the shear zone continuously expelled intercrystalline water to the surroundings drying the boundaries and leading to strain hardening and 3 water migrated to less deformed rocks causing hydrolytic weakening repeating the cycle and widening the ultramylonite

numerical modelling of porphyroclast and porphyroblast rotation in anisotropic rocks abstract the rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field experimental and numerical modelling studies often centred on the question whether objects rotate or not in non coaxial deformation with numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations originally shape and orientation were investigated while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century due to improved numerical techniques anisotropic rheology has become the focus of most recent studies indicating that it is a primary factor in the rotation behaviour of objects we present new simulations investigating the role of anisotropy on different scales relative to the object and show how this influences the rotation rate as well as the inclusion patterns in case of syntectonically growing porphyroblasts these simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects the variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field

strain softening and microstructural evolution of anorthite aggregates and quartz anorthite layered composites deformed in torsion torsion experiments of anorthite an aggregates and layered composites with equal volume fractions of quartz qtz and an were performed in a gas medium apparatus at a confining pressure of 400 mpa temperatures from 1373 to 1473 k and twist rates from 1 0 x 10 4 to 3 0 x 10 4 rad s dense specimens were fabricated from an glass and qtz crystalline powder using hot isostatic pressing hip techniques both an aggregates and qtz an layered composites show a continuous strain weakening from a peak stress at gamma 0 2 0 3 to gamma 3 2 and steady state flow has not reached under the experimental conditions the weakening is even more pronounced in the layered composites than the monolithic aggregates suggesting channeling or localization of flow into the weak material between strong layers the sheared an specimens developed pervasively c s c structures which are similar to those observed in natural ductile shear zones tem and electron backscattering diffraction ebsd fabric analyses suggest that grain boundary migration recrystallization accommodated dislocation creep with 010 100 as the dominant slip system was operating in the an the strain softening may be due to the development of crystallographic preferred orientation cpo the operation of dynamic recrystallization and the formation of extremely fine grained recrystallized material in the narrow c shear bands c 2004 elsevier b v all rights reserved

nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions abstract in the neves area the pre alpine intrusive protolith of the zentralgneise unit tauern window eastern alps is well preserved in a kilometric scale low strain domain without pervasive alpine deformation it is compositionally heterogeneous consisting predominantly of granodiorites with lesser leucocratic granites and different generations of lamprophyres and aplites the intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes incipient alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities resulting in the development of several different types of shear zones fractures without alteration haloes initially accommodate slip entirely on the fracture itself with increasing deformation a foliation is progressively developed in the adjacent host rock eventually producing a single heterogeneous ductile shear zone with the typical sigmoidal foliation pattern strong layers aplite dykes and bleached alteration haloes developed to either side of precursor fractures localize shear on their boundaries to produce characteristic paired shear zones shearing is more evenly distributed within weak layers lamprophyres and quartz veins with a marked discontinuity in shear strain against the adjacent little deformed granodiorite shear zone development was accompanied by the formation of new fractures and quartz rich veins in the host rock which in turn also localized shear magmatic contacts fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations the kinematics of the overprinting strike slip shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis 1 here oriented ca 345 discontinuities of almost all orientations show shear reactivation even in the case of very low resolved shear stress indicating an overall viscous response of the system without a specific yield stress the geometry and kinematics of the shear zone network suggest that the overall deformation in low strain domains was close to coaxial during deformation along the shear zone network compatibility was maintained by fracturing with the development of new quartz veins and by distributed ductile deformation of the host rock especially within contractional domains of the shear network and at shear zone intersections deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities shear zone development in intact rock is always preceded by fracturing which localizes subsequent shear reactivation

kinematics of rock flow and the interpretation of geological structures with particular reference to shear zones abstract current interpretations of structures are generally based on homogeneous and steady deformation models despite the fact that both the heterogeneity of rocks materially rheologically and geometrically and the time dependence of imposed geological conditions give rise to significant heterogeneous and non steady flow in concert with field observations we emphasize that the expectation of heterogeneity and non steadiness is the key to understanding natural deformation and that in order to carry out successful structural analysis and tectonic interpretation it is necessary to recognize the first order distinction between imposed boundary conditions typically used to define the tectonic regime e g transcurrent transpression and the response recorded by rocks within the zone structures and fabrics using s c fabric as an example it is demonstrated how flow with a non zero spinning component resulting from the rheological contrasts and or geologically realistic time dependent boundary displacement can drastically change the ideal geometric and kinematic relations between the fabric and the host zone in agreement with both theoretical analysis and field observation it is shown that natural flow regimes range from pure shear to pure rotation including super simple shear in consideration of the heterogeneity and non steadiness of natural deformation kinematic analysis is justifiable only within a homogeneous domain and steady period flow kinematics and mechanisms are interrelated in that firstly mechanisms provide internal constraints on kinematics ensuring that only certain flows are possible and secondly flow kinematics will favour development of certain mechanisms

kinematic vorticity and tectonic significance of superposed mylonites in a major lower crustal shear zone northern fiordland new zealand abstract new structural metamorphic finite strain and kinematic vorticity data from a 4 km wide subvertical shear zone in fiordland new zealand reveal a history of deformation reflecting different tectonic regimes an analysis of ductile fabrics within the shear zone and its wall rocks shows two distinctive stages of amphibolite facies mylonitic deformation dasz2 and dasz3 that are superimposed on older paleozoic or early mesozoic structures variations in strain intensity and well defined shear zone boundaries have allowed us to examine the progressive development of lasz2 sasz2 and lasz3 sasz3 fabrics and compare the types kinematics and conditions of deformation that produced them mineral assemblages defining lasz2 sasz2 provided calculated peak conditions of 11 9 1 1 kbar and 581 34 c indicative of lower crustal depths mineral assemblages defining lasz3 sasz3 provided calculated peak conditions of 8 7 1 2 kbar and 587 42 c finite strain and kinematic vorticity studies show that dasz2 involved ductile normal faulting and crustal thinning leading to decompression and exhumation of lower crustal rocks at some time during the cretaceous rifting of ancestral new zealand from gondwana dasz3 represents an episode of mid crustal dextral transpression that may have resulted from latest mesozoic or cenozoic oblique convergence reactivation of the subvertical sasz3 foliation by cataclastic shear zones and brittle faults dasz4 was accompanied by limited recrystallization at greenschist facies conditions dasz4 shear zones record upper crustal dextral strike slip faulting that resembles late tertiary deformation patterns associated with the australian pacific transform plate boundary

dynamic recrystallisation of ice aggregates during co axial viscoplastic deformation a numerical approach results of numerical simulations of co axial deformation of pure ice up to high strain combining full field modelling with recrystallisation are presented grain size and lattice preferred orientation analysis and comparisons between simulations at different strain rates show how recrystallisation has a major effect on the microstructure developing larger and equi dimensional grains but a relatively minor effect on the development of a preferred orientation of c axes although c axis distributions do not vary much recrystallisation appears to have a distinct effect on the relative activities of slip systems activating the pyramidal slip system and affecting the distribution of a axes the simulations reveal that the survival probability of individual grains is strongly related to the initial grain size but only weakly dependent on hard or soft orientations with respect to the flow field dynamic recrystallisation reduces initial hardening which is followed by a steady state characteristic of pure shear deformation

the ephemeral development of c shear bands a numerical modelling approach abstract c shear bands are ubiquitous structures in shear zones but their development is poorly understood previous research has determined they mostly occur in rocks with a high mechanical strength contrast using numerical models of viscoplastic deformation we studied the effect of the proportion of weak phase and the phase strength contrast on c shear band development during simple shearing to a finite strain of 18 we found that c shear bands form in models with 5 weak phase when there is a moderate or high phase strength contrast and they occur in all models with weak phase proportions 15 contrary to previous research we find that c shear bands form when layers of weak phase parallel to the shear zone boundary rotate forwards this occurs due to mechanical instabilities that are a result of heterogeneous distributions of stress and strain rate c shear bands form on planes of low strain rate and stress and not in sites of maximum strain rate as has previously been suggested c shear bands are ephemeral and they either rotate backwards to the c plane once they are inactive or rotate into the field of shortening and thicken to form x and triangle shaped structures

shear bands crenulations and differentiated layering in ice mica models abstract thin sheets of composite ice mica have been deformed in order to simulate the development of cleavages in quartz mica rocks a strong initial mica preferred orientation was variably oriented to the shortening direction deformation parallel to the foliation results in a crenulation type cleavage developing from shear bands initiated after a component of pure shear deformation oblique to the foliation produces a differentiated cleavage and involves a large component of shear strain subparallel to the original anisotropy the strain is accommodated by intra and intercrystalline processes that produce extensive grain elongation and rearrangement of the ductile matrix thereby forming ice vs mica rich regions on the other hand there is no drastic morphological change when a sample is shortened perpendicular to an original foliation that is where the micas lie in the plane of no shear strain instead the mica fabric is strengthened and the grains in the ductile matrix are flattened two models are presented for the initiation propagation and evolution of the observed crenulation versus differentiated cleavage types these depend on mica stacking and orientation relative to the transverse properties of the sample and also on the direction of anisotropy to the xy plane of the bulk strain ellipsoid the models invoke shear on planes of high shear strain and rotation of the shear bands and rigid mica grains into a direction approximately parallel to the bulk extension direction

secondary cleavages in ductile shear zones abstract secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone these are the orientation of a single secondary cleavage oblique to the shear zone boundaries conjugate sets with opposite senses of shear and multiple sets with the same sense of shear these features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation s coaxial stretching along the foliation and spin

weakening and strain localization produced by syn deformational reaction of plagioclase there are many observations in naturally deformed rocks on the effects of mineral reactions on deformation but few experimental data in order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase h2o more albitic plagioclase zoisite kyanite quartz we utilized fine grained 2 6 m plagioclase aggregates of two compositions an54 and an60 both dried and with 0 1 0 4 wt h2o present in shear deformation experiments at two sets of conditions 900 c 1 0 gpa in the plagioclase stability field and 750 c 1 5 gpa in the zoisite stability field dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep but samples with 0 1 to 0 4 wt water deformed in the zoisite stability field showed extreme strain localization into very narrow 1 3 m shear bands after low shear strain in these samples the microstructures of reaction products in the matrix differ from those in the shear bands in the matrix large up to 400 m zoisite crystals grew in the direction of finite extension and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic in the shear bands the reaction products albitic plagioclase zoisite white mica and traces of kyanite form polyphase aggregates of very fine grained 0 1 m dislocation free grains most of the sample strain after 2 has occurred within the shear bands within which the dominant deformation mechanism is inferred to be diffusion accommodated grain boundary sliding dagbs the switch from dislocation creep in dry samples deformed without reaction to dagbs in reacted samples is associated with a decrease in flow stress from 800 to 200 mpa these experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine grained polyphase assemblage leading to a switch in deformation mechanism and significant weakening thus localization of deformation in polyphase rocks may occur on any pressure p temperature t path where the equilibrium composition of the constituent minerals changes

sense and non sense of shear in flanking structures abstract the deformation of marker horizons across slip surfaces can lead to the development of several types of flanking structures the development of such structures was investigated using a numerical model to simulate the flow around a slip surface in a viscous medium the modelled structures can be classified on the basis of three criteria 1 the extensional or contractional offset of markers 2 the co or counter shearing sense along the slip surfaces and 3 the normal or reverse drag of markers relative to the shear sense along the slip surfaces as a function of the kinematic vorticity of the flow and the initial orientation of the slip surface with respect to the shear zone boundaries three main types of structures can develop 1 shear bands developed along co shearing extensional slip surfaces 2 a type flanking folds developed along counter shearing slip surfaces and 3 s type flanking folds developed along co shearing contractional slip surfaces all these structures can occur with either normal or reverse drag effects shear bands and a type flanking folds recording opposite shear senses can be geometrically very similar and consequently lead to kinematic misinterpretations however correctly identified flanking structures can provide quantitative information about the kinematic vorticity of the flow

frictional viscous flow in mylonite with varied bimineralic composition and its effect on lithospheric strength abstract a theoretical composite flow law is presented for mylonite containing interconnected layers of a weak mineral undergoing power law creep and porphyroclasts of a stronger mineral undergoing fracture and frictional sliding such mylonite termed clastomylonite is said to undergo frictional viscous fv mylonitic flow its bulk strength is expressed as a function of bimineralic composition temperature effective pressure and shear strain rate as well as the material parameters for the constituent minerals the fv flow law predicts that the rheology of clastomylonite is predominantly non linear viscous and only slightly pressure sensitive for most bimineralic compositions fv mylonitic flow is shown to occupy a depth interval between cataclastic flow involving adhesive wear in the upper crust and fully viscous mylonitic flow at greater depths the transition from adhesive wear to fv mylonitic flow is related to the onset of dislocation creep glide plus climb in the weakest phase and is inferred to coincide with a crustal strength maximum a peak strength of about 80 mpa is calculated with a combination of byerlee s constants for frictional sliding of granite and the fv flow law for granitic clastomylonite 30 vol quartz at elevated fluid pressures this value falls within the range of flow stresses independently obtained from quartz palaeopiezometry in many greenschist facies granitic mylonite zones

geometry classification and kinematics of s c and s c fabrics in the mushandike area zimbabwe planar fabrics in small dextral shear zones in the mushandike area of the zimbabwe archaean craton consist of a sigmoidal primary foliation s displaced by discrete shears c or c these planar fabrics can be divided into porphyroclastic megacrystic and banded types depending mainly on the values of the angles between s and c or c surfaces the structures that define s surfaces and the spacing of c or c surfaces the values of the angles between the foliations are proposed as the basis for a general classification of planar fabrics in shear zones the values of the angles depend on microstructures and strain microstructural evidence shows that slip did not occur on s surfaces but only on c or c surfaces a model for the development of the fabrics proposes that s surfaces lie parallel to the xy plane of the local finite strain ellipsoid in domains of simple shear between c or c surfaces c surfaces may form in the orientation of a coulomb failure surface the field evidence suggests that an increase in bulk strain is at least partly accommodated by an increase in local strain in domains between c or c surfaces as the s surfaces rotate with the local finite strain ellipsoid

mid crustal shear zone formation in granitic rocks constraints from quantitative textural and crystallographic preferred orientations analyses abstract this paper presents quantitative microstructural and crystallographic preferred orientation cpo analyses of an alpine amphibolite facies shear zone developed in the fibbia metagranite gotthard massif central alps the weakly deformed metagranite and orthogneiss at the margins of the shear zone are characterized by a bulk strain partitioning between harder coarse grained monomineralic aggregates derived from quartz and k feldspar porphyroclasts and softer fine grained plagioclase bearing shear bands a characteristic feature is a dilatant fracturing of strong quartz and feldspar aggregates cpos and microtextures suggest that quartz and k feldspar aggregates are dynamically recrystallized via dislocation creep while plagioclases show evidences of fluid assisted diffusive mass transfer and grain boundary sliding in the mylonite and ultramylonite shear zone core the porphyroclasts derived quartz and k feldspar layers are broken down to produce a polyphased matrix that is characterized by a homogeneous micron scale grain size and regular random distribution here the deformation of the whole aggregate occurs via a fluid assisted dissolution precipitation creep and grain boundary sliding referred as a fluid assisted granular flow we propose a model of shear zone formation associated with the nucleation of shear zone followed by lateral widening of the sheared domain the lateral broadening of the shear zone is driven by 1 the increase in fluid pressure in permeable albite oligoclase shear bands that results in expulsion of fluids to the shear zone margins and hydraulic fracturing of strong aggregates and 2 the thermodynamic re equilibration via metasomatic reactions of the shear zone walls

a tale of two viscosities abstract we describe a series of 12 large strain two dimensional simple shear numerical experiments on two phase linear and non linear viscous materials the modal percentage of the two phases their relative viscosities and the stress exponent of the dependence of viscosity on stress were systematically varied allowing us to observe the effects of these parameters on the flow behaviour of the system all experiments exhibited localisation but the extent and type of localisation varied considerably and the range of parameters allows both classical sc as well as sc structures to form the inter granular finite strain geometries also varied systematically although there is a tradeoff between non linearity of viscosity and the viscosity contrast that results in similar geometries for quite different mechanical behaviour the finite rotation of the harder phases was similar within any one experiment originally parallel passive marker lines thus remain sub parallel after deformation and rotation which may be mistaken for non rotation the internal deformability and interaction between grains allows meta stable grain elongation orientations to form without any additional consideration of external slip surfaces the slight variability of initial grain geometry and subsequent grain interactions leads to a wide variability in final grain shapes for the same initial viscosities

the deformational behaviour of bimineralic limestone halite aggregates abstract a compositional foliation banding develops within synthetic limestone halite aggregates experimentally deformed in axially symmetric compression and in simple shear the strength of the bulk aggregate decreases as the foliation becomes stronger foliation weakening in calcite rich specimens the development of foliation starts with the formation of cataclastic shear systems in calcite domains between isolated halite domains with increasing deformation halite is sheared into these systems the halite domains coalesce into halite rich shear systems and accommodate an increasing amount of the bulk deformation the bulk strength of bimineralic aggregates at a given amount of strain increases with decreasing halite content the influence of halite on the bulk strength is much greater than would be expected from its volume fraction and becomes dominant with increasing deformation due to the development of the foliation it is postulated that such processes also occur during natural deformation of polyphase rocks competent clasts within a weaker ductile matrix showing no recrystallization differ in shape and rotational behaviour from recrystallizing clasts such as feldspar augen these competent clasts denoted as type clasts may cause problems in deducing the sense and direction of shear when formed as almost exact mirror images of type recrystallizing clasts

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

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

strain localization and porphyroclast rotation it has been debated for decades whether rigid inclusions such as porphyroclasts and porphyroblasts do or do not rotate in a softer matrix during deformation experiments and numerical simulations with viscous matrix rheologies show ongoing rotation of circular inclusions whereas using mohr coulomb plasticity results in nonrotation because the rocks in which inclusions are found normally undergo deformation by dislocation creep we applied a full field crystal plasticity approach to investigate the rotation behavior of rigid circular inclusions we show that the inclusion9s rotation strongly depends on the anisotropy of the matrix minerals strongly anisotropic minerals will develop shear bands that reduce the rotation of inclusions inhibition of rotation can only occur after a significant amount of strain our results may help to explain why geologic rigid objects often show evidence for rotation but not necessarily in accordance with the viscous theory that is usually applied to these systems

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

structural analysis of mylonitic rocks in the cougar creek complex oregon idaho using the porphyroclast hyperbolic distribution method and potential use of sc type extensional shear bands as quantitative vorticity indicators abstract mylonitic rocks of the cougar creek complex of northeastern oregon and west central idaho provide an opportunity to document the deformational structures produced during general non coaxial shear within quartz feldspar mylonites and to explore the potential role of sc type extensional shear bands in vorticity analysis well developed feldspar porphyroclasts within six mylonite zones were utilized to estimate bulk kinematic vorticity w k using the porphyroclast hyperbolic distribution phd method w k values for the cougar creek mylonites range from w k 0 26 to w k 0 37 synthetic and antithetic shear band inclinations were measured relative to observed shear zone boundaries within five mylonite zones with estimated w k values and compared to the non coaxial flow field geometries and eigenvector orientations in each mylonite zone synthetic sc type shear band populations exhibit a range of inclination with maximum inclination lying approximately parallel to the acute bisector ab of the eigenvectors similarly antithetic shear band populations show a range of inclination near the obtuse bisector ob of the eigenvectors we infer that sc type extensional shear bands form initially parallel to ab and ob and rotate towards the flow plane with progressive deformation decreasing their inclination relative to the shear zone boundary ab and ob have significance in the strain field in that they represent orientations of maximum angular strain rate thus planes perpendicular to ab and ob are mechanically favorable for small zones of localized simple shear shear bands within the heterogeneous bulk strain of the mylonite orientation analysis of populations of sc type shear bands may provide a direct quantitative means of estimating w k

full field predictions of ice dynamic recrystallisation under simple shear conditions understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large scale ice dynamics and thus our ability to predict future changes of ice sheets polar ice behaves anisotropically during flow which can lead to strain localisation in order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets we present a series of numerical simulations of ice polycrystals deformed under simple shear conditions the models explicitly simulate the evolution of microstructures using a full field approach based on the coupling of a viscoplastic deformation code vpfft with dynamic recrystallisation codes the simulations provide new insights into the distribution of stress strain rate and lattice orientation fields with progressive strain up to a shear strain of three our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure grain size and shape while the development of lattice preferred orientations lpo appears to be less affected activation of non basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration therefore the finite strain history is non directly reflected by the final microstructure masked strain localisation can be recognised in ice cores such as the edml from the presence of stepped boundaries microshear and grains with zig zag geometries

fast fourier based error propagation for the gravimetric terrain correction abstractwe have identified a gap in the literature on error propagation in the gravimetric terrain correction therefore we have derived a mathematical framework to model the propagation of spatially correlated digital elevation model errors into gravimetric terrain corrections as an example we have determined how such an error model can be formulated for the planar terrain correction and then be evaluated efficiently using the 2d fourier transform we have computed 18 3 billion linear terrain corrections and corresponding error estimates for a 1 arc second 30 m digital elevation model covering the whole of the australian continent

fabric controls on strain accommodation in naturally deformed mylonites the influence of interconnected micaceous layers abstract we present microstructural analyses demonstrating how the geometrical distribution and interconnectivity of mica influences quartz crystallographic preferred orientation cpo development in naturally deformed rocks we use a polymineralic qtz pl kfs bt ms grt tur mylonite from the zanskar shear zone a section of the south tibetan detachment nw himalaya to demonstrate how quartz cpo intensity decreases from quartz dominated domains to micaceous domains independently of whether or not quartz grains are pinned by mica grains we then use a bimineralic qtz ms mylonite from the main central thrust nw himalaya to show how increasing mica grain connectivity is concomitant with a systematic weakening of quartz cpo our results draw distinctions between cpo weakening due to i second phase drag leading to ineffective recovery in quartz and ii increased transmission and localisation of strain between interconnected mica grains in the latter case well connected micaceous layers take up most of the strain weakening the rock and preventing straining of the stronger quartz matrix our findings suggest that rock weakening in quartz rich crustal rocks is influenced not only by the presence of mica rich layers but also the degree of mica grain connectivity which allows for more effective strain localization through the entire rock mass

development of a shear band cleavage as a result of strain partitioning abstract microstructural analyses of shear band cleavages in a centimeter scale shear zone within a metasomatic biotite band in the teshima granite ryoke metamorphic belt southwest japan show that strain partitioning occurred between quartz and biotite feldspar domains within the shear zone pre tectonic hydrothermal alteration within the granite caused biotite replacement of both plagioclase and k feldspar resulting in the development of biotite feldspar domains where k feldspar mantles dominantly biotite plagioclase aggregates subsequently the altered granite was plastically deformed in simple shear so that intra layer shear band cleavages were passively developed within the biotite feldspar domains whereas intense dynamic recrystallization occurred in the quartz domains the rotation and orientation of the intra layer shear band cleavages can be explained by a finite strain ellipse model which shows that strain in the biotite feldspar domain requires only 10 20 of the bulk simple shear strain for the development of such cleavages so that most of strain could be accommodated by deformation in the quartz domain consequently the model suggests that the development of the shear zone resulted in strain partitioning between the quartz and the biotite feldspar domains due to compositional variations via hydrothermal alteration within the granite

the effects of quartz recrystallization and reaction on weak phase interconnection strain localization and evolution of microstructure abstract we conducted axial compression and general shear experiments at t 900 c and p 1 5 gpa on samples of banded iron formation bif and synthetic aggregates of quartz hematite and magnetite to investigate how dynamic recrystallization of quartz promotes strain localization and the role of weak second phases oxides on the rheology and microstructural evolution of the aggregates experiments showed strain localization into oxide rich layers and that the oxide content and oxide distribution are key factors for the strength of the aggregate only 2 10 wt hematite leads to pronounced weakening and increasing hematite content above 10 has only a minor additional effect where oxide grains are dispersed the initial strength contrast with quartz induces stress concentrations at their tips promoting high stress recrystallization accommodated dislocation creep of quartz fine recrystallized quartz reacts with oxide forming trails of fine reaction product ferrosilite fayalite leading to the interconnection percolation of a weaker matrix the strength contrast between the quartz framework and these fine grained trails promotes strain localization into micro shear zones inducing drastic strain weakening thus dynamic recrystallization of quartz promotes syn deformational reactions leading to a microstructurally controlled evolution of phase strength contrast it results in a rheologic transition from load bearing framework to a matrix controlled rheology with transition from s c to s c fabric with increasing strain