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

strain analysis and vorticity of flow in the northern sardinian variscan belt recognition of a partitioned oblique deformation event abstract a field example of strain partitioning has been analysed along the nurra asinara transect of the nw sardinian variscan chain italy the section in the nurra asinara area is in a continuous sequence of tectono metamorphic complexes made of low to high grade metamorphic rocks affected by a polyphase tectonic history the principal fabric of the area is controlled by a d2 progressive deformation phase in which the strain is partitioned into folds and shear zone domains the d2 stretching lineation and shear sense show a clear change from south to north the principal meso and micro structures vorticity gauges and a quantitative kinematic analysis of local strain suggest that the d2 kinematic history could be envisaged as an oblique heterogeneous deformation similar to the transpressive systems described in ancient and modern settings elsewhere using a simple kinematic model we also propose that both a transpressive system followed by thrusting or a partitioned transpressive system could be responsible for the fabric distribution and strain accumulation described in the study transect

interannual modulation of east african early short rains by the winter arctic oscillation in the present study we analyzed the interannual linkage between the boreal winter arctic oscillation ao and east african early short rains when the indian ocean dipole and el nino southern oscillation variance are excluded by linear regression the boreal winter ao index is significantly correlated with the october east african precipitation over the domain of 5 n 5 s and 35 45 e for the period 1979 2014 r 0 46 the upper ocean heat content likely acts as a medium that links the ao and east african precipitation significant subsurface warming and positive upper ocean heat content anomalies occur over the western indian ocean during the autumn following positive ao winters which enriches the atmospheric moisture intensifies convection and enhances precipitation oceanic dynamics play a key role in causing this subsurface warming winter ao related atmospheric circulation creates anomalous wind stress which forces a downwelling oceanic rossby wave between 60 75 e and 5 10 s where the thermocline significantly deepens this rossby wave propagates westward and accompanies significant subsurface warming along the thermocline the rossby wave arrives at the western indian ocean in the late summer significantly warming the region to the west of 55 e at a depth of 60 100 m this warming remains significant through october correspondingly the upper ocean heat content significantly increases by approximately 2 3 108 j m 2 in the region west of 60 e between 5 and 10 s the role of these oceanic dynamics in linking the winter ao and anomalous subsurface warming was tested by numerical experiments with an oceanic general circulation model the experiments were performed with the forcing of ao related wind stress anomalies over the indian ocean in the winter the oceanic rossby wave generated in the central indian ocean during boreal winter the consequent subsurface warming and the anomalous upper ocean heat content in october over the western indian ocean were all adequately reproduced the winter ao can serve as a precursor of east african short rain anomalies

mylonites from the towaliga fault zone central georgia products of heterogeneous non coaxial deformation abstract retrograde mylonitic rocks occur in the segment of the towaliga fault southeast of jackson georgia mylonite protoliths are heterogeneous quartz feldspathic gneisses upper amphibolite facies sillimanite zone from the pine mountain and piedmont terranes protoliths contain no distinctive lithological markers that can be used to determine fault displacement shear sense can only be determined using microstructural criteria and type porphyroclast systems shear bands asymmetric microfolded quartz ribbons subgrain fabrics in quartz ribbons the most common porphyroclast system developed in mylonites and ultramylonites is type type systems do not have readily identifiable tails but do produce distinctive asymmetrical pile ups in the matrix such that the angle of intersection between the perturbed zone and the mylonitic foliation points in the shear direction differences in morphology between and type systems can be related to differences in the recrystallization rate deformation rate ratio r reflecting changes in ductility contrast between matrix and porphyroclast that controls the amount of recrystallized material in a porphyroclast mantle that is potentially available to form tails in early stages of mylonitization with low ductility contrasts and large r ratios large mantles occur around porphyroclasts and tailed systems develop in advanced stages of mylonitization with large ductility contrasts and small r ratios either very small or partially or totally missing mantles occur and type systems develop microstructural criteria ubiquitously record a right lateral strike slip motion component to the deformation textural evidence e g unidirectional shear bands consistent vorticity recorded by porphyroclasts and asymmetrical microfolds suggest that the mylonites are the products of non coaxial deformation the local presence of fabrics with coaxial symmetry result from heterogeneity in the deformation the right lateral offset on the towaliga fault most likely occurred during the late paleozoic alleghanian orogeny

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

numerical modelling and comparison of the temporal evolution of mantle and tails surrounding rigid elliptical objects in simple shear regime under stick and slip boundary conditions abstract structures associated with rigid inclusions are a rich source of evidence to understand the local deformation regime the behaviour of rigid objects in modelled here as being immersed in a linear newtonian fluid with either i a stick boundary condition continuity of stress and velocity across the boundary or ii a slip boundary condition continuity of boundary normal stress and velocity across the boundary with zero shear stress at the boundary of particular interest are the types of structures developed in a concentric region adjacent to the object termed the mantle a model of the displacement of points around the inclusion comprises a set of ordinary differential equations which are solved numerically a comprehensive set of simulations for a variety of mantle sizes object aspect ratios initial orientations as well as different boundary conditions has been performed a comparison between natural examples and model output indicates a level of consistency the resulting structures differ in detail and in a broader sense in general type structures only develop when stick boundary conditions are in operation in contrast type structures at high strain are restricted to slip boundary conditions slip conditions also tend to be the source of complex mantle types involving more than one generation of mantle structures or wings furthermore our model indicates that using asymmetry of orientation of objects relative to the shear direction may be problematic when used alone particularly if stick boundary conditions prevail but that together with mantle structures there is less chance of confusion

rolling structures at large shear strain abstract at large shear strain shear criteria are often obliterated or become ambiguous from examination of both natural examples and experimental models we describe a new criterion called rolling structure widely represented in sheared rocks a typical rolling structure is composed of a rigid or competent object e g porphyroclast boudin fossil etc with two tails asymmetrically disposed around it in most cases tails are of the same material as that of the rotating object and result from strain softening and grain size reduction at the object periphery forming a mantle z and s asymmetries of rolling structures represent dextral and sinistral senses of shearing respectively tails must not be confused with pressure shadows which usually present an opposite asymmetry for a given shear sense besides the determination of the sense of shear the occurrence of these structures allows a minimum estimate of the strain intensity in strongly sheared rocks since rolling structure length is proportional to shear strain

shear criteria in the grenville province ontario canada abstract the grenville province of canada is a major proterozoic orogenic belt mapping and structural analysis shows that the central ontario segment of this belt is composed of a stack of imbricated thrust nappes separated by mylonite zones the mylonite zones were generated at high metamorphic grades and contain a distinctive assemblage of mesoscopic and microscopic asymmetric structures these structures include rotated tectonic inclusions rotated boudins and pinch and swell structures sheath folds rotated single crystal porphyroclasts single crystal fish and shear band foliation the shear asymmetry indicated by these structures is highly consistent between members of the assemblage and over a large area of the grenville province and shows that the thrust nappes were transported to the northwest the consistency of the structures is thought to be due to cyclical dynamic recrystallization leading to steady state foliations in the mylonites

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

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

flow mixing object matrix coherence mantle growth and the development of porphyroclast tails abstract the kinematic stability fields of common porphyroclast types have been defined using a computer program that simulates low reynolds number newtonian shearing flow in the vicinity of a rigid spherical object the principal variables governing the geometry of porphyroclast tails at a given finite shear strain are 1 porphyroclast matrix coherence and 2 mantle recrystallization history both of which affect the extent to which mantle material becomes mixed with the matrix as deformation progresses

mixing in flow perturbations a model for development of mantled porphyroclasts in mylonites abstract the presence of a rigid object such as a porphyroclast in simple shear flow induces a flow perturbation with a relatively simple geometry domains of closed and open flow lines are bounded by a separatrix that surrounds the porphyroclast the concept of a separatrix to describe heterogeneous flow gives rise to some new insights in the development of porphyroclasts with a mantle of recrystallized material in mylonites the final shape of the recrystallized mantle depends on separatrix shape but also on the degree of intersection of the mantle and the separatrix and therefore on relative mantle width this model can explain the coexistence of several types of mantled porphyroclast in a single mylonite sample the occurrence of porphyroclasts that lack wings at high strain values and the presence of symmetric winged mantled porphyroclasts in ductile shear zones

winged inclusions pinch and swell objects during high strain simple shear abstract in this study we compare natural examples of isolated pinch and swell objects which have been deformed in simple shear with results of high strain numerical models such structures which have geometrical similarities with clast systems and rolling structures have been called winged inclusions we suggest a new mechanical explanation for the evolution of winged inclusions which form when pinch and swell shaped objects consisting of a core and pre existing wings rotate out of the shear plane the viscosity ratio the stress exponent and the shape of the winged inclusion have a significant influence on the rotation rate the rotational behaviour of winged inclusions differs significantly from the rotational behaviour of simple elliptical objects with comparable aspect ratios during the early stages of formation winged inclusions may resemble mirror images of sigmoids and misinterpretations may lead to a wrong determination of the shear sense during progressive shear to large strains the structures may be approximately described as consisting of a pulsating rotating core and thinning wings that rotate with different rates during rotation of the structure the core of the winged inclusions records an overall decrease of the aspect ratio influencing the rotation rate of the inclusion the wings are subject to ptygmatic folding when they rotate through the field of instantaneous shortening and may unfold again in the field of instantaneous stretching during ongoing shearing the wings on both sides of the core rotate with the core may change their positions and finally unfold after rotation about 180 resulting again in a pinch and swell shaped object therefore winged inclusions record ambiguous information about the finite strain rotating winged inclusions create a significant flow perturbation in the matrix resulting in nucleation of folds refolding and propagation of shear zones rootless intrafolial folds with opposing closures have strong geometrical similarities with winged inclusions and we therefore speculate that the formation of winged inclusions might be an efficient mechanism to produce rootless intrafolial folds

computer modelling of mantled porphyroclasts in newtonian and non newtonian simple shear viscous flows abstract in computer simulations based on creeping motion equations the difference in stress sensitivity of strain rate between newtonian rheology and non newtonian power law rheology does not greatly influence the types of mantled porphyroclast developed in simple shear viscous flows porphyroclast type is determined by the radius of an initial spherical mantle when a thin mantle lies within the separatrix 2 objects develop at small then complex porphyroclasts appear with increasing when the mantle is thicker and transected by the separatrix objects form at small and 2 objects appear with increasing during the same deformation the transition from to 2 objects is gradual the transition strain being dependent on the initial radius of the mantle our simulations did not produce 1 or objects

modelling of mantled porphyroclasts using non newtonian rock analogue materials abstract the development of type porphyroclasts was modelled experimentally in a circular transparent shear rig using crystalline rock analogue materials rectangular camphor objects embedded in a matrix of octachloropropane were deformed in simple shear flow the camphor deformed into objects with progressive deformation up to a shear strain of 100 the development of these objects and of gradients of flow parameters in the matrix was followed from step to step during the deformation the objects in this experiment show stair stepping of wings that did not occur in earlier experiments with newtonian fluids analysis of the flow pattern indicates that this is due to an unusual flow perturbation geometry around the relatively rigid camphor objects which rotate more slowly than equivalent objects in newtonian flow the non newtonian rheology of the experimental materials may be responsible for this deviant behaviour this implies that non newtonian fluids are better analogues than newtonian fluids to study the development of structures in rocks with non newtonian rheology since the geometry of the resulting structures can be different

sense and non sense of shear reloaded abstract this paper reviews the geometrical and mechanical evolution of selected groups of kinematic indicators that are frequently used for inferring large scale geodynamics but are prone to ambiguous shear sense interpretations i flanking folds and shear bands are complex structures because under general shear conditions co rotating secondary shear zones can experience either syn or antithetic slip leading to extensional or contractional offsets on the other hand counter rotating secondary shear zones can exhibit either extensional or contractional offsets but always experience synthetic slip ii boudinaged layers may display both syn and antithetic offset across the interboudin surfaces in addition to the flow type the initial orientation of interboudin surfaces strongly influences the sense of slip under coaxial flow conditions and with high strength contrast between the boudinaged layer and the matrix domains of layer oblique parallel interboudin surfaces can form by chance and their local occurrence may hence lead to misinterpretations of shear zone kinematics in particular if the number of observable boudins is small if the viscosity contrast between the boudinaged layer and the matrix is low domino boudins may strongly deform internally during shearing so that in contrast to flanking structures antithetic slip along interboudin surface may lead to a marked normal drag resulting in shear band boudinage geometries that appear to have formed during flow with an opposite shear sense iii winged inclusions are pinch and swell shaped objects rotating into the shear direction after several revolutions these structures can resemble b type clasts or clasts with stair stepping that formed during shear flow with the opposite shear sense in this paper new and published examples of these ambiguous shear sense indicators are discussed and interpreted we give some guidelines for the correct interpretation of these structures with the caveat that if marker lines and their deflections are not well preserved these shear sense criteria are not always reliable kinematic indicators

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

toward a unified conceptual framework for shear sense indicators abstract the growing lexicon for shear sense indicators has created a need for a unified conceptual system of classification many common shear structures can be described by linear combinations of two kinematic variables which quantify the relative importance of objects vs foliations in controlling the shear zone velocity field

controls on the geometry of tails around rigid circular inclusions insights from analogue modelling in simple shear abstract we used analogue modelling to investigate the factors controlling tail geometry in porphyroclast systems results show that 1 inclusions can develop in both slipping and non slipping modes but inclusions only form in the latter 2 inclusions develop when the mantle production rate is constant and the mantle is transected by the separatrix inclusions form when the mantle is initially outside the separatrix and later comes inside this line 3 in the slipping mode the wedge shaped tail of inclusions always has a straight external surface parallel to the shear plane whereas in the non slipping mode the external surface is curved inwards external embayments 4 together with earlier theoretical results and inclusions always show stair stepping of tails when embedded in a viscous matrix under homogeneous simple shear deformation 5 maximum stair stepping occurs in the slipping mode and is at least equal to the inclusion diameter if our models bear significant similarity to nature then i the straight or curved character of inclusions could mean that they had respectively a slipping or non slipping interface with the surrounding recrystallized matrix and ii inclusions may result from shear deformation under retrogressive metamorphic conditions in thrust systems

an on line analysis system for three component seismic data method and preliminary results this paper presents the basic method and algorithm of a system for automatic on line analysis of seismic data at three component stations and gives preliminary experimental results data processing consists of four steps 1 preliminary detection and estimation of the signal arrival time by using the energy based ydetector 2 refinement of signal parameters arrival time amplitude period and coda duration 3 estimation of the azimuth and incident angle of the seismic ray by using polarization analysis and 4 approximate estimation of event parameters epicenter coordinate origin time and magnitude from teleseismic p waves for a constrained focal depth the software package which is called seismostansia has been tested in an on line environment at an experimental seismic station in eastern kazakhstan testing results show that the system can handle an input flow of up to 100 signals in a 24 hr period comparison of the results of automatic detection with data of the station bulletin obtained by visual analysis of analog seismograms shows that 1 with the present threshold setting the automatic system detects practically all teleseismic signals seen by the analyst together with typically 10 to 15 signals per day not confirmed by visual analysis 2 in 70 per cent of the cases the residual between the arrival time obtained from the automatic and subjective procedures is 0 5 sec or less for snr 10 and 1 sec or less for snr 10 3 in 70 per cent of the cases the residual between the signal period obtained from the automatic and subjective procedures is 0 2 sec or less analysis of data indicates that an effective means of identifying false alarms with low snr can be the level of polarization rectilinearity of p and s waves by analyzing azimuth estimates for a suite of explosions at the nevada test site it is shown that a systematic bias forms the main contribution to the residual relative to true azimuth in the general case the azimuth residuals are due to the features of the medium both at the epicenter and at the station and consequently depend on both azimuth and epicentral distance an analysis of azimuth residuals points to a periodic dependence on azimuth the value of the period is about 120 additional information azimuth and angle of incidence reported from threecomponent stations to data centers is shown to improve significantly the reliability of phase association in the processing of data from a global seismic network

experimental modelling of mantled porphyroclasts abstract mantled porphyroclasts are commonly used as sense of shear indicators in mylonites analogue experiments have been carried out to model the development of such objects rigid spheres mantled by a viscous plasticene putty mixture with power law behaviour were suspended in an optically transparent newtonian polymer matrix the samples were deformed between two cylinders one placed within the other and with coinciding axes this induces a non coaxial flow between the cylinders and allows the achievement of high finite strain the viscous mantle around the rigid sphere remained undeformed deformed into an ellipsoid or developed wings the final geometry depends mainly on the induced viscosity contrast between the mantle and the matrix which is a function of the flow induced stress on the surface of the mantle winged mantles only develop if the induced viscosity contrast between the mantle and the matrix polymer is sufficiently small if this critical induced viscosity contrast is exceeded the mantled sphere remains undeformed or obtains a stable ellipsoidal shape even at high finite strain shaped winged mantles only develop in simple shear flow if the mantle around a rigid sphere is thin

progressive development of mantle structures around elongate porphyroclasts insights from numerical models abstract this paper presents a generalized theoretical approach towards two dimensional numerical modeling of the mantle geometry of inequant porphyroclasts of varying shapes within a newtonian matrix during progressive general type of bulk deformation the analysis takes into account the effects of synkinematic size reduction of the porphyroclast with concomitant mantle development in response to dynamic recrystallization numerical simulations reveal that the principal factors governing the geometry of mantle structures are 1 the initial aspect ratio of the porphyroclast a b 2 the rate of clast size reduction and 3 the ratio of the rates of pure shear and simple shear sr or the kinematic vorticity wk in the general type of non coaxial deformation in general porphyroclasts develop and finally type mantle structures as the rate of clast size reduction is progressively increased the tails of equant porphyroclasts tend to be characterized by wings with increase in bulk shear during progressive deformation in contrast inequant objects a b 1 develop composite tails with multiple wings even at low finite shear strains however with increase in aspect ratio geometry tends to dominate the overall mantle structure porphyroclasts with a large aspect ratio a b 3 form tails with overturned wings as described in passchier c w simpson c 1986 porphyroclast system as kinematic indicators journal of structural geology 8 831 844 in general the type of non coaxial deformation with decrease in kinematic vorticity or increase in sr porphyroclasts irrespective of their initial shapes tend to form atypical like tails that do not cross the central reference plane

shear criteria and structural symmetry abstract during the last decade it has been shown that most relevant shear criteria within ductile rocks are asymmetric structures e g pressure shadows shear bands c s structures fabrics tension gashes folds veins the correspondence between coaxial or non coaxial deformation and symmetric or asymmetric particle velocity fields respectively accounts for the use of structural symmetry as an indicator of strain history the application of this symmetry concept to various field examples emphasizes that i the degree of symmetry of a given structural pattern reflects the bulk strain regime irrespective of the size and the mechanical behaviour of the considered system and ii the strain regime can also be inferred from the order of appearance and dominance of structures which contribute to the total deformation pattern even where the progressive deformation results in a complex pattern which cannot be directly interpreted