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

the motion of rigid ellipsoidal particles in slow flows abstract preferred orientations of fabric elements in tectonites are usually interpreted with reference to two continuum models namely those of march 1932 and jeffrey 1922 it is argued that the first of these is usually inappropriate if the fabric elements are rigid inhomogeneities embedded in a deformable matrix jeffrey s model is more applicable but until recently its use has been limited to problems involving axisymmetric particles and simple flow geometries in this paper jeffrey s equations have been solved numerically in order to investigate the behaviour of more general systems the motions of two particle shapes one a prolate spheroid the other a triaxial ellipsoid are considered in parallel for four specified flow geometries which involve pure or simple shear and combinations of both any departures from particle axisymmetry or from simple flow geometries have a significant effect on particle behaviour compared with that predicted by spheroid simple flow solutions

the limitations of three dimensional kinematic vorticity analysis abstract the kinematic vorticity number wk can be calculated for three dimensional as well as two dimensional geologic deformations for steady state deformations wk can be correlated to and analyzed in terms of finite strains the analysis shows that assumptions commonly made for two dimensional deformations are not applicable to three dimensional deformations a single wk describes an infinite number of three dimensional deformations further even knowledge of flow apophyses orientation instantaneous stretching axes orientation and or wk are not sufficient to describe deformation three dimensional deformations also require knowledge of the deformation type or boundary conditions of deformation e g transpression hence in addition to being difficult to estimate the value of knowing wk for three dimensional deformations is greatly reduced compared with plane strain the most useful methods of determining wk from naturally deformed rocks are presented

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

calcite textures examples from nappes with strain path partitioning abstract strain path partitioning partitioning of deformation into components of pure and simple shear in two alpine nappes is qualitatively indicated by a variation in orientation of foliation microkinematic criteria quartz textures and a comparison with strain path partitioning in theoretical nappe models the degree of non coaxial deformation relative to the total deformation is quantified by using the asymmetry of textures in calcite tectonites the texture interpretation is based on both polycrystal plasticity models and experimental data calcite tectonites in the graz paleozoic thrust sheet austria and the morcles nappe switzerland were deformed in a strain path with both pure and simple shear components texture asymmetry supports the strain path derived from geological observations as an exception a high strain ultramylonite deformed nearly exclusively in pure shear it is suggested that this represents a softening instability since deformation in pure shear is energetically more favorable than in simple shear the incompatibilities caused by heterogeneous pure shear are accommodated by detachments and heterogeneous simple shear adjacent to the pure shear zone

significance of transport parallel strain variations in part of the raft river shear zone raft river mountains utah usa this paper examines part of the raft river shear zone rrsz in northwestern utah that exhibits an extreme transport parallel increase in strain intensity coupled with a transition from flattening to constrictional strain detailed geologic mapping and finite strain quartz c axis fabric and kinematic vorticity analyses demonstrate local necking of the shear zone associated with an increase in transport parallel elongation accommodated by a stretching fault at the base of the shear zone the domain of intense deformation and necking of the shear zone is localized where the basal stretching fault cuts rheologically weak rocks this domain is characterized by strain in the constrictional field caused by transport perpendicular flow into the area of high transport parallel elongation where rocks cut by the stretching fault are rheologically strong the rrsz locally records flattening strain and lower strain intensities limiting the amount of stretching needed at the base of the shear zone in any one direction and or recording transport perpendicular flow into adjacent highly extended domains the rheology of rocks cut by the stretching fault directly controlled the amount and style of zone normal shortening and transport parallel elongation and these observations provide an example of deformation partitioning in a crustal scale structure driven by local rheological transitions

influence of a permanent low friction boundary on rotation and flow in rigid inclusion viscous matrix systems from an analogue perspective abstract natural occurrences and recent experimental work show that a low friction inclusion matrix boundary can be responsible for antithetical rotation and development of stable shape preferred orientations in simple shear the flow of a viscous matrix around a rigid inclusion to which it may or may not be adherent is still not well studied but it is relevant to the understanding of the behaviour of structural elements in mylonite zones we used two dimensional 2 d analogue experiments to address these issues the experimental results with a permanent low friction inclusion matrix boundary nonadherent mode show the following 1 the rotation behaviour of inclusions in this mode is markedly different from the theoretical predictions and experimental results for the adherent mode 2 inclusions with aspect ratio equal to 1 rotate synthetically at a rate that depends upon inclusion shape and orientation 3 ellipse rectangle and lozenge shaped inclusions rotate antithetically when starting with their greatest axis parallel to the shear direction 4 back rotation is limited in all cases studied and the angle between the inclusion greatest axis and the shear direction represents a stable orientation which depends on inclusion aspect ratio and shape 5 a metastable orientation exists which is strongly dependent upon inclusion shape and aspect ratio and separates fields of antithetic and synthetic rotation our experimental results show that the overall flow pattern is bow tie shaped in adherent and nonadherent modes however there are major differences in the way the matrix flows near the inclusion i in the nonadherent mode the nearby matrix flows past the inclusion whereas in the adherent mode the nearby matrix flows with and follows the inclusion therefore in the adherent mode passive marker lines parallel to the shear direction and streamlines show considerable deflections at the inclusion crests in marked contrast with their straight character in the nonadherent mode ii stagnation points or closed flow lines near the inclusion were not observed in the nonadherent mode which means that there is no closed separatrix around the inclusion in this mode despite the fact that the overall flow shape is bow tie iii in the adherent mode the line of flow reversal is stable throughout deformation but in the nonadherent mode it changes position and orientation with progressive shearing this shifting of the line of flow reversal can be an important factor controlling rotation behaviour in the nonadherent mode iv in the nonadherent mode the inclusion behaviour is similar to that observed in confined flow v the flow pattern in the nonadherent mode provides an explanation for the observed lack of drag folds associated with small scale rigid inclusions in mylonites

kinematic vorticity and strain rate patterns associated with ductile extrusion in the chelmos shear zone external hellenides greece abstract kinematic vorticity strain rate finite strain and structural data have been used to describe the heterogeneous nature of ductile deformation in an extruding crustal scale shear zone the chelmos shear zone csz of the external hellenides the csz has a curved shape which is marked by a frontal flat and a steep zone the spatial variation in the kinematic vorticity number is strongly correlated with the geometry and the structural depth of the shear zone the pure shear component of deformation decreases from back to front and from top to bottom the ductile thinning normal to the shear zone s boundaries is higher in the steep zone and decreases westwards quartz grain size and strain rate in the csz is distributed into two approximately homogeneous domains the higher strain rate is recorded in the steep zone and the base of the frontal flat zone with lower rates in the upper parts of the frontal flat zone based on mesoscopic structural data and the spatial distribution of the shear strain rate the existence of a west dipping back flow zone in the boarder zone between these two domains is inferred the overall pattern of deformation in the csz implies that tectonic extrusion from the steep zone occurred at the final stages of ductile deformation and was associated with the formation of the backflow zone in the front of the shear zone ductile deformation of rocks probably started close to simple shear on a planar low angle shear zone progressively became penetrative and more general shear occurred as the csz steepened up this progressive increase in pure shearing may have occurred in a heterogeneous manner and was higher close to both the eastern margin of the steep zone and the top of the frontal flat zone solid state extrusion in the csz was also associated with a heterogeneous and progressive increase in strain rate

applications of inclusion behaviour models to a major shear zone system the nordfjord sogn detachment zone in western norway rigid inclusion models have reached a stage where one should be able to use them to obtain quantitative values from ductile shear zones we used natural data collected in three sites and combined analogue and theoretical modelling to assess vorticity strain nature of rigid inclusion matrix interface and confinement in the large scale ductile nordfjord sogn detachment zone nsdz of the caledonides of western norway our study shows that 1 the observed shape preferred orientation spo at higher structural levels of the nsdz at site 1 gjervika can be explained by simple shear pure shear simple shear ratio sr 0 associated with a slipping inclusion matrix interface 2 the observed spo at deeper structural levels of the nsdz at site 2 near sandane can be produced by simple shear associated with a significant amount of shortening across the shear zone sr z 1 acting upon rigid inclusions in slipping contact with the enclosing matrix 3 observed back rotated boudins deeper in the nsdz at site 3 biskjelneset can form in confined flow associated with a considerable amount of shortening across the shear zone sr 0 4 4 the observed tails of porphyroclasts indicate a minimum at least local strain of ca g z 20 5 the clasts in the studied shear zones strongly depart from jeffery s model jeffery g b 1922 the motion of ellipsoidal particles immersed in a viscous fluid proceedings of the royal society of london a102 161e179 the large scale extensional nsdz under investigation shows evidence of strain partitioning rocks vary from protomylonites to ultramylonites and the simple shear and pure shear components are heterogeneously distributed therefore we conclude that flow in the nsdz was very heterogeneous both at the kilometre and the metre scale however the present study suggests that the amount of shortening across the shear plane throughout the nsdz increases with depth and the flattening component contributes to exhumation of the eclogite facies rocks in its footwall 2007 elsevier ltd all rights reserved

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

rotation of rigid elliptical cylinders in viscous simple shear flow analogue experiments we used experiments to investigate the behaviour of rigid elliptical cylinders embedded in a viscous fluid of finite thickness and for very small ratios s between shear zone width z and the shortest principal axis e2 of the ellipse that constitutes the base of the elliptical cylinder 3 5 s z e2 1 2 former theoretical and experimental work on the rotation of rigid inclusions immersed in a viscous matrix considered infinite shear zone thickness and or a very large ratio between shear zone thickness and inclusion size s 1 and concluded that inclusions rotate continuously and synthetically with the applied bulk simple shear flow except for inclusions with infinite axial ratio our results depart greatly from the analytical predictions experiments were carried out at constant shear zone thickness but variable s and degree of coupling at the inclusion matrix interface the results show that 1 confined inclusion rotation always deviates from theoretical predictions for infinite shear zone thickness even for synthetic rotation 2 the deviation in the angular velocity and or in the sense of rotation increases as s approaches 1 and antithetic rotation is possible from a position with ellipse longest axis parallel to the shear plane and 3 a slipping inclusion matrix interface greatly enhances deviation from theoretical predictions with antithetic rotations to as much as 20

asymmetric quartz c axis fabrics and flow vorticity a study using rotated garnets abstract the development of a type i crossed girdle c axis fabric with an asymmetric density distribution and an internally asymmetrical skeletal outline is examined in a specimen of garnet bearing mylonitic quartzite from the betic movement zone in southern spain the garnets are pre mylonitic skeletal porphyroblasts that rotated during mylonitic deformation but preserved a strain free equilibrium microstructure and a random c axis fabric it follows that the asymmetry of the density distribution must result from the deformation history itself the sense of asymmetry of the c axis population and that of the skeletal outline are consistent with the sense of vorticity inferred from extensional crenulations and asymmetric microboudinage and also with the sense of rotation of the garnets analysis of the rotation angles with aspect ratio of the garnet porphyroblasts demonstrates that a pure shear component was involved in an essentially non coaxial flow this may explain the internal asymmetry of the fabric skeleton and confirms the idea that a spectrum exists of type i fabrics intermediate between the theoretically predicted fabrics for pure and simple shear a tentative reconstruction of the kinematic framework suggests that the central segment of the fabric skeleton is linked to the extensional apophysis of the flow

a critique of vorticity analysis using rigid clasts abstract estimating the kinematic vorticity numbers from rock fabrics presents many problems in this paper we use numerical modeling to investigate the reliability of those widely used vorticity analysis methods using rigid clasts we use very simple flows steady state homogeneous and having monoclinic symmetry to represent plane straining zones and transpressional zones and assume that the rotation of rigid clasts perfectly obeys the theory of jeffery 1922 proceedings of the royal society of london a102 161 179 these are assumptions made in current vorticity analysis using rigid clasts even with these simple assumptions our modeling shows that the current methods have intrinsic uncertainties so large that it is pointless to use the estimated vorticity numbers to constrain shear zone boundary conditions and kinematics it is perfectly consistent with numerical modeling results if the currently reported vorticity numbers estimated from rigid clasts in the range of 0 50 0 85 are all interpreted as being from natural shear zones with close to simple shearing flows the large uncertainties arise because the motion of rigid clasts is intrinsically a three dimensional problem

late caledonian extension in the ofoten area northern norway abstract structural observations along the base of the caledonian nappe pile in the ofoten region of northern norway demonstrate a complex interaction of compressional and extensional structures top to the west extensional structures are consistently superimposed on top to the east southeast compressional structures and control the basement cover relationships across the orogen extensional shear zones offsetting the basement cover contact appear to be restricted to the area east of the ofoten synform the eastern flank of the synform is an approximately 1 km wide w dipping shear zone with more than 30 km of dip slip offset the basement culimination roll over west of the synform is most likely controlled by the listric geometry of this shear zone at depth this major shear zone and other minor extensional shear zones within the rombak window have a ductile shear fabric with post kinematic recovery fabrics developed under middle to upper greenschist facies conditions suggesting that the footwall and hanging wall rocks were still at a mid crustal level some time after the extensional event the extensional shear zone flattens upsection and splays into a network of anastomosing detachment zones eastwards cutting both upsection and downsection in the direction of westward tectonic transport development of steeply dipping extensional basement faults and subhorizontal detachment faults in the cover have resulted in a complex basement cover relationship and reworking of the nappe stratigraphy established during the preceeding compressional event these relationships make restoration of the caledonian nappe pile to a pre orogenic configuration uncertain and shortening estimates difficult the recognition of the extensional event as a distinct post contractional tectonic event suggests a link with the devonian extensional collapse in southern norway a major difference between southern and northern norway however is the lack of brittle extensional features in the often area suggesting different amounts of extension and basement uplift in the two regions

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

rotation behaviour of rigid inclusions in multiple association insights from experimental and theoretical models many rocks are representative of inclusion matrix systems where rigid inclusions float on a ductile matrix the geological analogues of such inclusion matrix systems are theoretically modelled with the help of jeffery s 1922 theory in recent years there have been a number of reinvestigations on the rotational motion of rigid inclusions to account for some geological observations that are not yet predicted in models based on jeffery s theory adding to this effort this paper investigates the effects of 1 inclusion concentration and 2 the degree of coherence at the inclusion matrix interface on the rotation behaviour of rigid inclusions in a multiple inclusion system during simple shear deformation shear box experiments were run on models containing multiple identical elliptic cylinders of wax mimicking rigid inclusion embedded within putty representing ductile matrix two sets of experiments were performed with and without lubrication at the inclusion matrix interfaces each model had an isolated inclusion to reveal the variation of rotations of single and multiple inclusions increase in inclusion concentration results in contrasting rotation behaviour of isolated and multiple inclusions initially oriented parallel to the shear direction when the interfaces were not lubricated inclusions in multiple association rotated antithetically whereas the isolated one almost remained stationary in models wherein the inclusion matrix interfaces were lubricated with liquid soap both isolated and multiple inclusions rotated antithetically but the isolated inclusion rotated at a lower rate with increasing inclusion concentration the difference in the rates of antithetic rotation between isolated and multiple inclusions tended to be larger in both lubricated and non lubricated conditions the sense of rotation of isolated as well as multiple inclusions initially oriented perpendicular to the shear direction was always synthetic irrespective of the nature of the interface lubricated or non lubricated however the instantaneous rotation rates of inclusions in multiple association were higher than that of the isolated inclusions disposed in the same orientation rotation of inclusion in an inclusion matrix system is basically induced by the traction exerted by the flowing matrix on the surface of the inclusion during deformation employing a hydrodynamic model it is shown that mutual mechanical interaction among inclusions which is a function of inclusion concentration modifies the stresses at the inclusion matrix interface to a large extent moment calculations reveal that inclusions oriented parallel to the shear direction experience an antithetic moment in response to the normal stress components the magnitude of which increases with increasing inclusion concentration this implies that rotation of shear parallel inclusions in antithetic sense is favoured by higher inclusion concentration on the other hand inclusions oriented perpendicular to shear direction experience a moment that induces synthetic rotation the magnitude of the synthetic moment is larger for larger inclusion concentration leading to increase in the rate of synthetic rotation using the theoretical model the moments are calculated as a function of the aspect ratio of inclusions and the inferences based on this moment calculation are complemented with experimental findings

analogue modelling of the influence of shape and particle matrix interface lubrication on the rotational behaviour of rigid particles in simple shear abstract the rotational behaviour of a rigid particle embedded in a linear viscous matrix undergoing cylindrical simple shear couette flow was studied in 2d rock analogue experiments the influence of particle shape elliptical vs monoclinic aspect ratio and the nature of the matrix particle interface lubricated vs unlubricated was investigated both matrix pdms and lubricant liquid soap were linear viscous with a viscosity ratio of ca 10 4 without lubricant the rotational behaviour of all particles closely approximates the jeffery theory lubricated monoclinic particles with the long diagonal initially parallel to the shear direction show back rotation and approach a stable position lubricated elliptical particles initially parallel to the shear direction also show back rotation but only transiently stabilize weak planar zones in the matrix adjacent to unlubricated elliptical particles do not induce backward rotation in general for elliptical particles rotation rate as a function of orientation depends on axial ratio and thickness of the lubricant mantle for thick mantles initially 10 of the volume of the particle rotation rates are faster than jeffery theory for very thin mantles they are markedly slower compared with thick mantles particularly when the long axis is nearly parallel to the shear direction rotation rates are never strictly zero so true stabilization does not occur however for more elongate particles axial ratio 6 rotation rates are so slow that a very strong shape preferred orientation would develop in a lubricated elliptical particle population in experiments the volume of lubricant is constant and the thickness adjacent to the long side of the particle progressively decreases with increasing strain in natural examples of porphyroclast systems the weak mantle continually develops by recrystallization and or cataclasis of the rigid clast core and a steady state between production and thinning could be attained potentially leading to true stabilization for particles with a high axial ratio

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

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

kinematics and vorticity of flow associated with post collisional oblique transpression in the variscan inner zone of northern sardinia italy abstract quartz c axis fabrics vorticity of flow and deformation temperatures were analyzed in samples collected in the central sector of the variscan inner zone in sardinia italy in order to constrain the kinematics of exhumation of medium and high grade metamorphic rocks during d2 post collisional deformation we investigated two shear belts with opposing shear senses these shear belts are developed parallel to the trend of the inner zone which marks the boundary between two metamorphic complexes our data suggest that the sinistral shear belt initiated during the early stage of d2 deformation in a simple shear dominated regime whereas the dextral shear belt initiated later in a pure shear dominated regime we document a general non coaxial flow regime with contemporaneous pure and simple shear components geometrical relationships reveal the operation of oblique extrusion during a compressive regime where the coaxial component acted sub horizontal and orthogonal to the trend of the axial zone changes in the kinematics vorticity of flow and strain geometry that occurred during d2 deformation may be related to post collisional orogen parallel displacement caught up in the variscan orogeny

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

subduction of a buoyant plateau at the manila trench tomographic evidence and geodynamic implications we determined p wave tomographic images by inverting a large number of arrival time data from 2749 local earthquakes and 1462 teleseismic events which are used to depict the three dimensional morphology of the subducted eurasian plate along the northern segment of the manila trench dramatic changes in the dip angle of the subducted eurasian plate are revealed from the north to the south being consistent with the partial subduction of a buoyant plateau beneath the luzon arc slab tears may exist along the edges of the buoyant plateau within the subducted plate induced by the plateau subduction and the subducted lithosphere may be absent at depths greater than 250 km at approximate to 19 degrees n and approximate to 21 degrees n the subducted buoyant plateau is possibly oriented toward nw se and the subducted plate at approximate to 21 degrees n is slightly steeper than that at approximate to 19 degrees n these results may explain why the western and eastern volcanic chains in the luzon arc are separated by approximate to 50 km at approximate to 18 degrees n whereas they converge into a single volcanic chain northward which may be related to the oblique subduction along the manila trench caused by the northwestern movement of the philippine sea plate a low velocity zone is revealed at depths of 20 200 km beneath the manila accretionary prism at approximate to 22 degrees n suggesting that the subduction along the manila trench may stop there and the collision develops northward the taiwan orogeny may originate directly from the subduction of the buoyant plateau because the initial time of the taiwan orogeny is coincident with that of the buoyant plateau subduction

an analytical solution in 2d for the motion of rigid elliptical particles with a slipping interface under a general deformation a mathematical model for rigid inclusions with a slipping interface immersed in a general 2d homogeneous deformation is developed under bulk pure shear inclusions are expected to rapidly approach the stretching axis when compared to the behaviour of inclusions with no slip at the interface the derived model predicts synthetic and antithetic motion into a stable orientation under simple shear and thereafter the inclusion makes an antithetic angle with the shear direction under simple shear rotation rates can be higher or lower than those of no slip inclusions depending on orientation a direct relationship between object inclination to the shear direction and the vorticity of the bulk flow is predicted the model compares well with published analogue and numerical experiments

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

high strain zones a unified model abstract where a shear zone is bounded by undeformed wall rocks and there is no volume change the deformation path within the zone on the bulk scale approximates a simple shear where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries the deformation path within the zone is not a simple shear but still has monoclinic symmetry both simple shear and general monoclinic deformation paths should be regarded as special cases in nature because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries in this general case the deformation path is triclinic we establish a model which includes this general case also taking into consideration volume change five independent parameters are required to characterize the rate of deformation of the flow four are sufficient to characterize the flow kinematics the model is generally triclinic and orthorhombic and monoclinic deformations are subgroups by varying the values of the characterizing parameters the various types of high strain zones can be represented the kinematics of flow within such zones and the accumulated finite deformation geometry are investigated many field observations such as for example lineations in transcurrent shear zones that vary between vertical and horizontal cleavage transected folds and the absence of sheath folds in some shear zones where folds have been significantly rotated can sometimes be better interpreted in the context of our general model

slip partitioning and inclined dextral transpression along the zagros thrust system iran abstract flexural duplex structures and shear zones are well developed in the heneshk area in southwestern iran the zagros thrust system in this area consists of eight sheets of nw striking ne dipping dextral strike slip duplexes that are linked with imbricate fans and oblique slip thrusts the zagros thrust system is a portion of the internal zone of the zagros orogenic belt this internal zone is characterized by penetrative plastic deformation and metamorphism based on the kinematic vorticity number w k 0 73 0 02 calculated from quartz porphyroclasts of the goshti shear zones along the zagros thrust system the estimated angle between the maximum instantaneous strain axis isa and the transpressional zone boundary is 33 the estimated angle of the plate convergence or the flow apophysis of the displacement field is 25 the presence of dominantly dextral shear sense indicators in the area is consistent with dextral inclined transpressional convergence the mean estimated finite deformation w m value indicates relative contributions of 47 pure shear and 53 simple shear for the deformation which involved both strike slip and oblique slip displacements in this inclined transpression about 40 strike slip partitioning was required to accommodate the finite strain and re orientation of instantaneous strain

a mohr circle construction to plot the stretch history of material lines abstract mohr circles for stretch can be used to find the spatial distribution of material lines which have been shortened extended shortened then extended and extended then shortened in a material plane which underwent area constant homogeneous deformation by steady state flow the distribution is mainly a function of the vorticity of the parent flow and can be used to determine sense of shear finite strain and even vorticity and orientation of finite strain axes the method may be useful in theoretical and experimental studies and can help to understand the complex deformation patterns in sets of cross cutting veins in deformed rocks

a fundamental problem with the kinematic interpretation of geological structures abstract non steady flows are ubiquitous in nature not only do imposed boundary conditions vary with time and rock rheology change during the course of deformation but also deformation is generally heterogeneous and all of these conditions lead to non steady flow histories in modern kinematic analysis flow apophyses the instantaneous stretching axes and the vorticity vector collectively referred to as the eigen directions of deformation are used in place of bruno sander s kinematic axes for deformation with a steady flow history this practice is well justified and has led to great advances in structural geology but for non steady flow histories the geometrical relationships among eigen directions flow pattern vary with time this makes it inappropriate to correlate an association of structures and fabrics with certain time invariant flow patterns and hence raises the question how should we interpret geological structures and fabrics kinematically without the assumption of homogeneous and steady deformation we suggest that the answer lies primarily in forward modeling of deformation based on a knowledge of rock properties