structural evolution of the sarandi del yi shear zone uruguay kinematics deformation conditions and tectonic significance the sarandi del yi shear zone is a crustal scale shear zone that separates the piedra alta terrane from the nico perez terrane and the dom feliciano belt in southern uruguay it represents the eastern margin of the rio de la plata craton and consequently one of the main structural features of the precambrian basement of western gondwana this shear zone first underwent dextral shearing under upper to middle amphibolite facies conditions giving rise to the reactivation of pre existing crustal fabrics in the easternmost piedra alta terrane afterwards pure shear dominated sinistral shearing with contemporaneous magmatism took place under lower amphibolite to upper greenschist facies conditions the mylonites resulting from this event were then locally reactivated by a cataclastic deformation this evolution points to strain localization under progressively retrograde conditions with time indicating that the sarandi del yi shear zone represents an example of a thinning shear zone related to the collisional to post collisional evolution of the dom feliciano belt that occurred between the meso to neoproterozoic 600 ma and late ediacaran lower cambrian times

the gem lake shear zone cretaceous dextral transpression in the northern ritter range pendant eastern sierra nevada california the gem lake shear zone is a northwest striking steeply dipping dextral transpressional shear zone that provides the first direct evidence for dextral deformation in wall rock pendants in the central part of the sierra nevada batholith the gem lake shear zone is a minimum of 30 km in length and extends at least from the north end of the northern ritter range pendant to the southeast edge of the ritter range pendant the amount of displacement on the zone is uncertain but matching fault slivers of pennsylvanian marble in the northern ritter range pendant to similar exposures north of the mount morrison pendant suggests a minimum dextral offset of 20 km the gem lake shear zone was active in early late cretaceous time from before 91 ma the age of the syntectonic granodiorite of kuna crest to at least 80 ma 40ar 39ar age of syndeformational mica from the shear zone in the ritter range pendant deformation in the gem lake shear zone is characterized by combined dextral simple shear and subvertical stretching which are variably partitioned in anastomosing high strain zones in the shear zone at gem lake predominantly dextral deformation is indicated by porphyroclast asymmetries s c fabric and asymmetric crenulations a component of subvertical stretching is indicated by a moderately to steeply plunging stretching lineation in a segment of the shear zone at kuna crest a strongly developed stretching lineation indicates predominantly subvertical stretching with a lesser component of dextral strike slip the gem lake shear zone is considered to be part of a proposed regional system of shear zones in the eastern sierra nevada the sierra crest shear zone system this dextral transpressional system was active prior to and synchronous with intrusion of the late cretaceous sierra nevada batholiths deformation ceased shortly after pluton emplacement the sierra crest shear zone system includes possibly related shear zones as far north as saddlebag lake pendant and at least as far south as oak creek pendant and the owens valley indicating a possible strike length greater than 150 km

finite strains in transpression zones with no boundary slip abstract a mathematical model describing finite strains has been developed for transpression zones where no slip is allowed in any direction at the zone boundaries this has been adapted to model transpression zones where the boundaries are inclined the model can also be used to describe finite strain patterns where material is incorporated into the deforming zone from one or both sides after the initiation of deformation so that the deforming zone remains at a constant width thus allowing large transpressional strains to be accommodated without the deforming zone closing to zero thickness this model of steady state transpression can be used to explain some of the features of the south mayo trough a late silurian transpression zone in the west of ireland

progressive development of structures in a ductile shear zone in the singhbhum shear zone of eastern india successive generations of folds grew in response to a progressive ductile shearing during this deformation a mylonitic foliation was initiated and was repeatedly transposed the majority of fold hinges were formed in an arcuate manner at low angles to the y axis in an e w trending subhorizontal position and major segments of the fold hinges were then rotated towards the down dip northerly plunging x axis the striping and intersection lineations were rotated in the same manner the down dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations the consistent asymmetry of the folds the angular relations between c and s surfaces and the evidence of two dimensional boudinage indicate that the deformation was non coaxial but with a flattening type of strain with 1 2 the degree of non coaxiality varied both in space and time from the progressive development of mesoscopic structures it is concluded that the 2 3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale when a new set of shear lenses was superimposed on already sheared rocks the preexisting foliation generally lay at a low angle to the lenses no new folds developed where the acute angle was sympathetic to the sense of shear displacements where the acute angle was counter to the sense of shear the pre existing foliation lying in the instantaneous shortening field was deformed into a set of asymmetric folds

simultaneous pure and simple shear the unifying deformation matrix abstract any simultaneous combination of finite simple shear and finite pure shear is a linear transformation which can be expressed as a single transformation matrix for two dimensions the matrix is upper triangular with an off diagonal term called the effective shear strain is a simple function of the pure and simple shear components for three dimensions a simultaneous combination of thrusting in the x direction thrusting in the y direction and a wrench in the x direction in addition to 3 orthogonal components of coaxial strain can also be represented by a 3 3 upper triangular matrix here three off diagonal terms xy x y and y z occur xy is a simple function of the horizontal coaxial strain values and thrusting in the x direction yz depends on the coaxial strain components in the y and z directions and the thrusting in the y direction while xz is related to all six strain components the matrix also allows for volume change either homogeneously or preferentially in a single direction a method of decomposing the deformation matrix into a series of incremental deformation matrices where each incremental deformation records the same kinematic vorticity number as the finite deformation is shown the orientation and magnitude of the finite strain ellipsoid ellipse is easily and accurately found at any increment during the deformation

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

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

planar non planar and refolded sheath folds in the phulad shear zone rajasthan india abstract progressive ductile shearing in the phulad shear zone of rajasthan india has produced a complex history of folding with development of planar non planar and refolded sheath folds there are three generations of reclined folds f 1 f 2 and f 3 with a striping lineation l 1 parallel to the hinge lines of f 1 the planar sheath folds of f 1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices l 1 swerves harmoniously with the curving of f 1 hinge line there is a strong down dip mineral lineation parallel to the striping lineation in most places but intersecting it at apices of first generation sheath folds both the striping and the mineral lineation are deformed in u patterns over the hinges of reclined f 2 and f 3 folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non planar sheaths the reclined folds of all the three generations were deformed by a group of subhorizontal folds each generation of fold initially grew with the hinge line at a very low angle with the y axis of bulk non coaxial strain and was subsequently rotated towards the down dip direction of maximum stretching the patterns of deformed lineations indicate that the stretching along the x direction was extremely large much in excess of 6000 percent

planar dispersion of folds in ductile shear zones and kinematic interpretation of fold hinge girdles abstract as indicated by theory and multiple case studies hinge lines of minor folds in ductile shear zones are dispersed in a plane that is incompletely rotated toward mylonitic foliation this yields a geometric relationship between the fold hinge girdle and mylonitic foliation that can be used to determine sense of shear statistical study of published data from various shear zones and of new data from the blue ridge thrust complex reveals a small to moderate angle between the best fit plane to hinge lines and mean mylonitic foliation this angle is 4 9 in eight of 12 test cases moderate angles 22 34 are associated with nappes kinematic analyses of 12 fold hinge girdles indicate the sense of shear otherwise determined for host shear zones a theoretical model of contemporary fold rotation in simple shear closely simulates the geometric relationships described above for natural folds using this model large shear strains 10 are implied by the 4 9 angle between the fold hinge girdle and mylonitic foliation

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

interfering folds in constrictional deformation abstract experiments on buckle folding indicate that in pure constriction 2 3 2 3 there is an association of domes and basins with nonplane noncylindrical folds the areas of domes and basins are reduced with increasing deformation there is a dominant fold trend perpendicular to 3 with a less dominant trend perpendicular to 3 folds at an angle to these two sets may also occur the divergent folds link up in smooth arcs or join up at a dome or a basin over a relatively large domain the interference pattern produced by general constriction can be recognized by the association of domes and basins with nonplane noncylindrical folds by occurrence of hair pin bends of hinge lines of open folds by occurrence of amoeboid outcrop patterns and by absence of a consistent overprinting relation among different sets of folds layers subparallel to the 3 axis of general constriction may give rise to two or more sets of coaxial cylindrical folds with orthogonal axial surfaces

the big flush paleomagnetic signature of a 70 ma regional hydrothermal event in displaced rocks of the northern canadian cordillera the 70 ma carmacks group a subaerial volcanic succession which once covered much of central southwest yukon has a paleomagnetic remanent direction which passes the fold test and the reversal test a new collection of 13 sites combined with 13 sites from a previous study renders a pole 088 6 e 78 4 n a95 7 8 which is far sided with respect to the pole for cratonic north america and implies a displacement from the south of 1900 700 km late triassic mandanna member red beds and early jurassic nordenskild formation tuffs deformed in the late jurassic fail the fold test and conglomerate test but pass a contact test with eocene dykes the postdeformational remanent direction is identical to that isolated from the carmacks group the magnetic signature contained in these older formations is probably an overprint produced by an extensive hydrothermal system active during carmacks extrusion geological work indicates that the carmacks group is plume related given its paleomagnetic latitude and geological nature we hypothesize that the carmacks group is a displaced segment of the yellowstone hot spot track and the hydrothermal system which remagnetized the older rocks was established by mantle upwelling below the region resume le groupe de carmacks go de 70 ma est une succession volcanique subaorienne qui couvrait jadis la majeure partie du centre sud ouest du yukon il possde une direction paloomagnotique romanente certifioe par le test du pli et le test des inversions une nouveau groupe de 13 sites combino 13 sites diune otude antorieure fournit un pule 088 6 e 78 4 n a95 7 8 qui est considorablement oloigno par rapport au pule du craton de liamorique du nord et qui implique un doplacement partir du sud de 1900 700 km les couches rouges du membre de mandanna du trias tardif et les tufs du formation de nordenskild du jurassique prococe qui subirent une doformation durant le jurassique tardif ont ochouo le test du pli et le test de conglomorat mais ils ont passo avec succs le test de contact avec les dykes de li ocne la direction romanente post tectonique est identique celle isoloe du groupe de carmacks la signature magnotique qui caractorise ces formations plus anciennes est probablement une roaimantation produite par un systme hydrothermal otendu qui fut actif durant liextrusion de carmacks liotude goologique rovle que le groupe de carmacks est apparento un panache tant donno sa latitude paloomagnotique et sa nature goologique nous avanaons lihypothse que le groupe de carmacks reprosente un segment doplaco du traco du point chaud de yellowstone et que le systme hydrothermal qui a roaimanto les roches les plus anciennes est ds liascension du manteau sous la rogion otudioe

the structural evolution of sheath folds a case study from cap de creus abstract it has long been recognised that within zones of intense non coaxial deformation fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds however there is a surprising lack of detailed and systematic field analysis of such evolving sheath folds this case study therefore focuses on the sequential development of cm scale curvilinear folds in the greenschist facies el llimac shear zone cap de creus spain this simple shear dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite increasing amounts of fold hinge curvature are marked by hinge segments rotating into sub parallelism with the mineral lineation lm whilst the acute angle between the axial planar hinge girdle and foliation also displays a sequential reduction although lm bisects the noses of sheath folds it is also clearly folded and wrapped around the sheath hinges lm typically preserves a larger angle with the fold hinge on the lower limb l compared to the upper u limb l u suggesting that lm failed to achieve a steady orientation on the lower limb adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature or alternatively opposing directions of antithetic curvature such patterns reflect original buckle fold geometries coupled with the direction of shearing the ratio of long short fold limbs decreases with increasing hinge curvilinearity indicating sheath folds developed via stretching of the short limb rather than migrating or rolling hinge models this study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge line curvilinearity compared to adjacent return hinges this may provide a useful guide to bulk shear sense

lateral extrusion in transpression zones the importance of boundary conditions the homogeneous transpression strain model formulated by sanderson and marchini journal of structural geology 6 449 458 1984 has proved to be a useful tool in the analysis of complex three dimensional deformation patterns however some of the boundary conditions introduced in the mathematical model may be unrealistic and unnecessarily restrictive in this paper a strain matrix for unconfined transpression and transtension is derived in which material is allowed to move not only vertically but also horizontally in and out of the deforming zone parallel to its length lateral extrusion three end member plane strain components are defined wrench simple shear pure shear in xy lateral stretch and pure shear in yz vertical stretch these biaxial strains can be viewed as the apices of a strain triangle for unconfined transpression or transtension the edges of the triangle correspond to triaxial pure shear non coaxial biaxial lateral extrusion and the triaxial confined transpressional or transtensional strain of sanderson and marchini during unconfined transpression the orientation and in particular the geometry k value of the finite strain ellipsoid depends upon not only the amount of shortening across the zone and the amount of strike slip parallel to the zone but also upon the ratio of vertical to lateral stretch this can present serious difficulties when attempting to use finite strains to infer the direction and magnitude of zone boundary displacements examples of transpression zones in which there is evidence of a component of lateral extrusion are described from sw cyprus and central scotland these examples illustrate that antithetic strike slip shearing is a kinematic requirement of laterally unconfined transpression implying that synchronous shear sense indicators may give opposite senses of movement in shear zones specific geometric and mechanical boundary conditions together with internal fault zone rheologies may favour the lateral extrusion of material kinematic partitioning can occur to form fault bounded domains in which end member biaxial and or non coaxial strains are developed analysis of such domains can give a clearer understanding of regional scale triaxial deformations these findings illustrate the importance of establishing displacement boundary conditions when qualitatively or quantitatively analysing crustal deformation zones

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

patterns of deformed early lineations over later folds formed by buckling and flattening abstract the patterns of deformed early lineations l1 over later folds f2 can be classified into several morphological types depending on the nature of variation of l1 f2 over the folds the field relations indicate that the folds under consideration are neither shear folds nor parallel folds modified by flattening the lineation patterns are therefore interpreted in terms of an empirical model of simultaneous buckling and flattening in which it is assumed that i the central surface of the folded layer remains a sine curve in transverse profile ii the ratio of rates of buckle shortening to homogeneous strain is proportional to sin 2a with a as the dip angle and iii the progressive deformation is coaxial with the z axis of bulk strain parallel to the planar segments of the early folds the model gives an insight into the relative importance of different physical factors which control the development of dissimilar lineation patterns not all lineation patterns are explicable by this simplified model thus complex patterns with variable l1 f2 along the fold axis may develop by a progressive rotation of the geometrically defined fold hinge through successive material lines the theoretical results have been applied to interpret the lineation patterns in central rajasthan india it is concluded that l1 was initially very close to the e ese trending subhorizontal z axis of bulk deformation during f2 folding and that the x axis was subhorizontal or gently plunging with a n nne trend

age and implications of the phosphatic birmania formation rajasthan india abstract the birmania inlier in western rajasthan india contains important phosphate deposits the depositional age of which is poorly constrained here we provide the first direct age constraints for the phosphate bearing birmania formation using a combination of paleontological and detrital zircon u pb data the occurrence of the multicellular algal fossil wengania exquisita in phosphatic chert of the birmania formation suggest that it was deposited during the ediacaran detrital zircon age distributions contain prominent populations of 1 7 1 9 ga grains with subordinate younger grains that range from 650 to 980 ma the distribution broadly resembles those from neoproterozoic strata from both cratonic and himalayan india but like zircon age distributions from the marwar group lacks 1 0 1 2 ga grains which may suggest that both areas shared similar local sources the lack of zircon grains younger than 650 ma is consistent with an ediacaran depositional age because almost all cambrian or younger strata from india have yielded cambrian or latest neoproterozoic age grains these findings raise the possibility of a previously unrecognized late neoproterozoic episode of phosphogenesis on the india craton the presence of wengania exquisita further supports strong palaeogeographic affinity between the india and south china during the neoproterozoic and cambrian

origin of striping lineation and transposition of linear structures in shear zones abstract striping lineation occurs as fine colour stripes on foliation surfaces of strongly sheared rocks that have a mylonitic colour banding although described as intersection lineation striping lineation in mylonites does not show two different intersecting s surfaces unlike ordinary bedding cleavage intersection striping lineation is a typical product of ductile shearing the colour stripes may have formed at a very early stage of deformation as intersections of a bedding or a colour banding on the foliation surface at a later stage the bedding or the colour banding ceased to be reoriented by external rotation the foliation became the only active s surface and the deformation of the colour stripes was controlled entirely by the nature of the strain after the appearance of a layer trace on the axial plane cleavage in an early stage of deformation the later history of its changing width and orientation becomes independent of changes in the thickness and orientation of the layer in strongly sheared rocks the nature of the original layering is greatly modified hence a colour striping can remain with very small width even when the foliation and bedding have become parallel the striping lineations in the phulad shear zone in western india and singhbhum shear zone in eastern india are often transposed and retransposed in response to folding sheath folding and strong stretching the process is very similar to the development of transposed foliation

vergence and facing patterns in large scale sheath folds the careful geometric analysis of minor structural detail elucidates the relationships and evolution of associated large scale curvilinear hinge geometries developed during wnw directed caledonian thrusting exposed in neoproterozoic moine psammites of the moine nappe reversals in the polarity of structural facing associated with minor folding mark the position of major sheath folds which parallel transport upwardly convex sheaths closing in the direction of thrust transport cored by older gneissose basement inliers are termed culminations whilst those opening in the transport direction and cored by moine psammites are termed depressions sheath folds are bisected by transport parallel and foliation normal culmination depression surfaces which separate not only the reversals in facing but also delineate zones of minor fold hinge obliquity into clockwise and anticlockwise domains relative to the transport direction the sense of obliquity of minor z and s folds is thus dependent on position with respect to the surfaces of culmination and depression and not the fold axial surfaces surfaces of culmination and depression may be superimposed on original overturned antiformal and synformal folds to produce a variety of dome culmination on antiform saddle depression on antiform inverted saddle culmination on synform and basin depression on synform configurations the curvilinear hinges of minor folds may also be asymmetrical about the transport direction and within the plane of the regional foliation to define patterns of fold hinge line vergence classical concepts of fold limb vergence may thus relate to larger antiformal and synformal hinges whilst the fold hinge line vergence defines major curvilinear hinges associated with culminations and depressions major sheath folds may therefore be interpreted in terms of both minor fold hinge line and limb vergence coupled with fold axis obliquity and reversals in the polarity of structural facing the ability to recognise consistent and reliable structural relationships between facing and hinge obliquity at small scales indicates that the regional deformation process forms a linked and coherent system through several orders of magnitude 1999 elsevier science ltd all rights reserved

sheath folds as discriminators of bulk strain type abstract nested elliptical closures defining eye folds represent classic y z cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks systematic analysis of 1425 such eye folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer r yz to the inner most r y z elliptical rings of individual sheaths the variation in overall aspect ratios from outer to inner rings is defined as r where r r yz r y z and may display a relative increase or decrease in ellipticity to define cats eye r bulls eye r 1 fold patterns respectively layer thicknesses may also be measured along the y axis parallel to the axial surface t y and at 90 to this along the z axis t z to define the ratio of t yz sheath folds generated during broadly simple shear deformation k 1 display mean r yz 4 61 t yz 3 31 and cats eye folds r 0 69 sheath folds generated during general shear also display cats eye folds with identical mean r 0 69 values but greater thickness variations and elliptical ratios t yz 4 35 r yz 5 76 thus within both simple and general shear dominated deformations the overall variation in layer thickness t yz and ellipticity of eye folds r yz increases with increasing deformation and a greater component of pure shear whilst the r value remains constant and reflects original fold patterns sheath folds formed during constrictional k 1 deformation display markedly lower aspect ratios r yz 2 42 and thickness variations t yz 2 94 together with distinctive bulls eye folds r 1 23 these empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types and allow sheath folds to act as both effective 95 consistent and robust discriminators of bulk strain type

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

successive development of plane noncylindrical folds in progressive deformation in the history of superposed deformations of the iron formations at the western border of the kolar gold field in s india an important event was the successive growth of broadly coaxial plane noncylindrical folds in course of a progressive deformation concomitant with development of ductile mesoscopic shear zones the noncylindrical folds were initiated as active folds by the creation of a buckling instability at successive stages on newly developed foliation surfaces the nucleation of noncylindrical folds and the subsequent axial plane folding of the tightened mature folds are explained by the mechanical inhomogeneity of the rocks and the heterogeneous character of strain the correlation between increasing tightness and increasing noncylindricity of the folds indicates that the initial curvatures of hinge lines were accentuated by an extension parallel to the subhorizontal stretching lineation from the patterns of deformed lineations over folds of varying tightnesses it is concluded that the passive accentuation of hinge line curvatures was mostly achieved when the folds had already become isoclinal or very tight

quartz fabric and strain partitioning in sheath folds an example from the voltri group western alps italy abstract a mesoscopic sheath fold defined by an almost pure quartzitic level in quartz mica schists of the voltri group southeastern border of the western alps was sampled and studied in detail microstructural analysis was carried out on thin sections from different structural domains in the quartzitic layer with the emphasis to analyse quartz shape fabrics and the crystallographic preferred orientation of the c axes of this mineral quartz was dynamically recrystallized but its degree of preferred orientation is not as high as it would be expected in a high shear strain regime required for the development of sheath folds this may suggest strain partitioning between the quartzitic level and the micaceous matrix crystallographic preferred orientation of c axes and grain shape fabric asymmetry indicate the same sense of shear on both fold limbs in the xz plane this is in agreement with a model of sheath fold development by the passive rotation of material lines clustering of the c axis poles shows a configuration indicative of a different strain state in different domains of the sheath fold and suggests an evolution of the fold characterized by combined simple shear and coaxial progressive deformation

analysis of transpressional deformation from geometrical evolution of mesoscopic structures from phulad shear zone rajasthan india abstract transpressional deformation in ductile shear zones may be analyzed by simultaneous wall parallel simple shear with strain rate xy along with three dimensional coaxial deformation with strain rates x y and z several types of transpression may be distinguished on the basis of relative values of strain rate ratios x a y b and z c it is possible to identify the nature of transpressional deformation from a detailed analysis of shear zone structures in the phulad shear zone of rajasthan india evidence of general flattening a thrusting sense of shear occurrence of transport parallel stretching lineation frequent occurrence of sheath folds with apical direction parallel to the stretching lineation and occurrence of u shaped lineation patterns indicate that the deformation was transpressional with b a ratio ranging between 1 and 2 the bulk deformation was not dominated by simple shear but involved both simple and pure shear with extrusion along the transport direction much greater than along the vorticity vector rotation of long tectonic clasts caused reorientation of monoclinic rolling structures so that strongly monoclinic structures with opposite senses of asymmetry in different domains appear on subhorizontal outcrops parallel to the vorticity vector although the component of simple shear is zero on these surfaces

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

rotation of long tectonic clasts in transpressional shear zones abstract the two dimensional analysis of rotation of rigid ellipsoidal inclusions is not applicable to situations in which either the bulk deformation deviates from plane strain or one of the principal axes of the inclusion is at an angle to the vorticity vector both these situations may occur in certain types of transpressional ductile shear zones the model presented here shows how long cylindrical or ellipsoidal tectonic clasts with the longest axis parallel to the walls of the shear zone but at an angle to the vorticity vector rotate in transpressional shear zones long clasts initially at a low angle to the vorticity vector and at a large angle to the stretching lineation will rotate and tend to become subparallel to the stretching lineation the rotation of the principal axes of the elliptical cross section around the cylinder axis cannot be unlimited for any inclusion including the inclusion with circular cross section r 1 if the deformation is sufficiently large there is always a stable position of orientation the model explains simultaneous occurrence of monoclinic rolling structures in sections parallel and perpendicular to the vorticity vector depending upon the initial orientation the long axes of different clasts in the foliation plane may rotate clockwise and counterclockwise consequently opposite senses of asymmetry of rolling structures may appear in sections parallel to the vorticity vector