how do lineations reflect the strain history of transpressive shear zones the example of the active alpine fault zone new zealand abstract lineations within mylonites exhumed in the hanging wall of new zealand s active alpine fault zone have a complicated relationship to contemporary plate kinematics the shear zone is triclinic and macroscopic object lineations are not usually parallel to the simple shear direction despite high total simple shear strains 150 this is mostly because the lineations are inherited from pre mylonitic fabrics and have not been rotated into parallelism with the mylonitic stretching direction which pitches c 44 in the fault plane furthermore some lineations have been variably rotated depending on whether they are present in shear bands or microlithons which accommodated bulk strains with different vorticities total strains required to obtain parallelism between the finite maximum principal stretching direction calculated from transpression models and these mylonitic lineations are pure shear stretch s 1 3 5 simple shear 11 7

are we studying deformed rocks in the right sections best practices in the kinematic analysis of 3d deformation zones abstract a standard procedure in kinematic analyses of deformed rocks is studying samples only on the foliation perpendicular lineation parallel section based on the assumption that the rotational component of deformation is restricted to that plane the theoretical background for this procedure is that deformation is plane strain which is often useful but fails to account for all relevant features observed in many natural cases more general 3d kinematic models reproduce several of these features and help to better understand 3d deformation likely predominant in nature so they are a more appropriate starting point for deformation zone analysis theoretical and natural 3d deformation examples suggest that the section with highest degree of asymmetry the vorticity normal section vns and the corresponding vorticity vector are independent from the finite strain fabric and thus may present varied orientations with respect to the tectonic foliation and lineation the vns orientation depends on the kinematics resulting from boundary conditions whereas the strain fabric orientation also depends on accumulated strain the orientation of the vns is itself an essential element of shear zone kinematics hence it cannot be pre assumed but must be determined independently

applying a general triclinic transpression model to highly partitioned brittle ductile shear zones a case study from the torcal de antequera massif external betics southern spain abstract oblique convergence and subsequent transpression kinematics can be considered as the general situation in most convergent and strike slip tectonic boundaries to better understand such settings progressively more complex kinematic models have been proposed which need to be tested against natural shear zones using standardized procedures that minimise subjectivism in this work a protocol to test a general triclinic transpression model is applied to the torcal de antequera massif tam an essentially brittle shear zone our results given as kinematic parameters of the transpressive flow transpression obliquity extrusion obliquity and kinematic vorticity number wk suggest that the bulk triclinic transpressive flow imposed on the tam was partitioned into two different flow fields following a general partitioning type as such one flow field produced narrow structural domains located at the limits of the tam where mainly dextral strike slip simple shear dominated transpression took place outer domains ods in contrast the remaining part of the bulk flow produced pure shear dominated dextral triclinic transpression at the inner part of the tam inner domain id a graphical method relating internal wk to far field dip of the shear zone boundary angle of oblique convergence transpression parameters is proposed to obtain the theoretical horizontal velocity vector v which in the case of the tam ranges between 099 and 118 these results support the applicability of kinematic models of triclinic transpression to brittle ductile shear zones and the potential utility of the proposed protocol

quantitative finite strain and kinematic flow analyses along the zagros transpression zone iran abstract the ghouri area in southwest iran exposes a cross section through the zagros orogenic belt the area provides an opportunity to investigate quantitative finite strain rs kinematic vorticity number wk proportions of pure shear and simple shear components sense of shear indicators steeply plunging lineations and other moderate to steeply plunging stretching lineations in a transpressional zone based on a classical strain analysis of deformed microfossils with oblate strain ellipsoid shape the zagros orogenic belt is classified as a pure shear dominated zone of transpression but asymmetry of shear sense indicators suggests that a significant component of simple shear was involved along the deformation zone boundaries the long axes of the microfossils and stretched pebbles of a deformed conglomerate were used to indicate the stretching direction in this zone the stretching lineations have a steep to moderate plunge but a constant strain magnitude characteristics of dextral inclined transpressional kinematics in the zagros continental collision zone were quantified and indicate an estimated k value

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

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

determining vorticity axes from grain scale dispersion of crystallographic orientations kinematic shear sense indicators are critical tools for tectonic interpretation of shear zones the appropriate plane in which to interpret shear sense indicators is the vorticity normal surface which is often inferred using fabric foliation lineation orientation strain modeling however suggests that such fabrics can be unreliable for determining a kinematic framework we demonstrate the application of a new quantitative method crystallographic vorticity axis cva analysis that utilizes rotation statistics to calculate dispersion axes from crystallographic orientations at the grain scale we apply cva analysis to samples from three shear zones that exhibit distinct kinematics and deformation geometries in all cases the aggregate of calculated grain scale dispersion axes yields a preferred axis of crystallographic vorticity at the specimen scale that is coincident with the independently determined bulk vorticity axis this new method allows the position of vorticity axes to be evaluated independently of foliation and lineation information and without assumptions about the kinematics of deformation

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

three dimensional reference deformations and strain facies abstract in an attempt to categorize three dimensional deformations the concepts of kinematic axes three dimensional reference deformations and strain facies are utilized we have chosen 12 reference deformations each being an idealized end member of deformation involving a simultaneous combination of a three dimensional coaxial component constriction flattening or pure shear and an orthogonal simple shear component velocity and displacement fields infinitesimal deformation parameters and finite deformation parameters can be calculated for each reference deformation assuming steady state deformation there are three possibilities for the orientation of foliation and three possibilities for the orientation of lineation depending on the relative contributions of the coaxial and non coaxial components the six emerging combinations of foliation and lineation orientations each give rise to a characteristic strain facies each of the strain facies is correlated to the reference deformations and thus deformation parameters which caused its formation however since the coaxial deformation component accumulates more effectively than the non coaxial component a change from one strain facies to another i e a change in the orientation of lineation or foliation is possible during steady state deformation the strain facies emphasize the boundary conditions of deformation and together with the reference deformations provide a framework for three dimensional deformations

implications of strain and vorticity of flow analyses to interpret the kinematics of an oblique convergence event zagros mountains iran abstract finite strain and vorticity of flow analyses were carried out within the deh vazir deformed conglomerates in the sanandaj sirjan metamorphic belt zagros mountains iran this belt is in a sequence of tectono metamorphic complexes made of low to high grade metamorphic rocks affected by a polyphase deformation history using rf technique for two dimensional strain analysis showed that the finite strain of xz plane is rs 3 7 0 70 the calculation of the harmonic mean from the axial ratios of the extracted pebbles and plotting these data in the flinn diagram resulted in k 0 7 0 38 i e oblate strain ellipsoid kinematic vorticity analysis of deformed pebbles showed that wk parameter is 0 59 0 10 i e pure shear dominated non coaxial flow the results quantitatively support the recognition of a partitioned dextral inclined transpressive regime with bulk triclinic symmetry along the sanandaj sirjan metamorphic belt resulted from the oblique collision between the african arabian continent and the iranian microcontinents

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

fabric attractors in general triclinic flow systems and their application to high strain shear zones a dynamical system approach high strain zones may deform by flow with a triclinic symmetry this paper describes triclinic flow in a reference frame where instantaneous stretching axes isa are fixed the operation of triclinic flow is described in two ways first in terms of flow and the nature of flow eigenvectors and in the second part of the paper in terms of finite strain in monoclinic flow at least one of the eigenvectors of the flow coincides with one of the isa and one or two of the eigenvectors act as attractors of foliation or lineation elements in triclinic flow some flow eigenvectors are undefined since the two largest eigenvalues controlling the flow are imaginary imaginary eigenvalues are particularly common at high kinematic vorticity and within flow with deviation of the vorticity vector of more than 20 from one of the isa strong deviation from monoclinic flow is therefore possible but this will not produce permanent foliations or lineations for triclinic flow that does produce permanent fabrics the angle between isa and the fabric is so small that it is unlikely that it can be recognised in nature a discussion of the potential application of such results within real shear zones is presented

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

strain distribution and fabric development modeled in active and ancient transpressive zones 1 a model based on kinematics of transpression allows the measurable internal parameters simultaneous strain fabric and vertical elevation to be simulated in relation to the macroscopically determined external parameters of transpressive zones i e zone width velocity angle of convergence and zone depth we present a diagram of convergence angle against time for homogeneous transpression where isolines of strain intensity d strain symmetry k and vertical elevation of rock samples are superposed however the distribution of internal strain parameters is sensitive to three types of strain partitioning 1 discrete partitioning results in general decrease of finite strain accumulations and in increase of pure shear component 2 ductile partitioning splits the transpressional domain into a pure shear zone where strain accumulations decreases and in a wrench dominated zone where strain accumulations increases and 3 viscosity partitioning is marked by different strain rates in zones of different viscosity and therefore by different strain parameters

strain partitioning in banded and or anisotropic rocks implications for inferring tectonic regimes abstract among the many concepts that have contributed to the development of structural geology in the last half century two are particularly important these are i the link between the symmetry of the structure the vorticity of the associated deformation process and the tectonic regime and ii the widespread partitioning of strain however a clear understanding of these concepts and the links between them are not always apparent in structural analyses carried out to determine the prevailing tectonic regime of an area through the use of structural symmetries and kinematic indicators this contribution based on field studies from cap de creus experimental data and theoretical concepts highlights some of the problems encountered when attempting to deduce the type of strain and associated regional tectonic regime from field structures the relation between the symmetry of a structure and its associated kinematics is not unique for example it is demonstrated that in mechanically anisotropic materials symmetry variations do not necessarily reflect marked vorticity variations but rather the angular variations between the kinematic framework and the anisotropy planes because of widespread strain partitioning local and general kinematics do not generally match this mismatch is especially accentuated in rheologically heterogeneous rocks nevertheless correct interpretations are possible by performing continuous multi scale structural analyses in which the impact of strain partitioning and of the presence of material anisotropies and heterogeneities are considered

forward modeling of non steady state deformations and the minimum strain path reply structural analyses of shear zones often rely on an assumption of steady state behavior i e that the ratio of pure shear strain rate s to simple shear strain rate s remains fixed throughout deformation however geological deformations are not necessarily steady state non steady state deformation paths can be theoretically modeled if certain deformation parameters such as strain or offset are specified we have analyzed a two dimensional case of specified offset and geometry termed the minimum strain path the minimum finite strain needed to produce a fixed offset across a shear zone is neither simple shear nor pure shear but a combination of the two the minimum strain path if this deformation accumulates with a steady state deformation the kinematic vorticity number w of the minimum strain path varies with the amount of finite offset although w approaches 0 7 at high offset values because of this relation between w and finite offset the minimum strain path is better modeled as a non steady state deformation in which case deformation history starts close to simple shear but rapidly changes to a more pure shear dominated deformation it is expected that the minimum strain path is applicable to geological deformation zones with relaxed boundary conditions such as basal parts of spreading nappes or extensional detachment systems 0 1997 elsevier science ltd

using along strike variation in strain and kinematics to define the movement direction of curved transpressional shear zones an example from northwestern superior province manitoba a review of current understanding of structures in shear zones reveals that correlating the orientation of stretching lineations with movement direction once a common practice based on the simple shear model is not generally applicable to transpressional zones it is difficult to determine the movement direction of a transpressional zone based on internal structures we show that the movement direction can be better defined by considering the variation in strain geometry and kinematics of differently oriented and kinematically related shear zones or different segments of a curved shear zone application of this method to a shear zone in the northwestern superior province of manitoba shows that the movement direction there is approximately north south in contrast to a commonly held view for similarly oriented shear zones in the superior province that the movement direction is approximately east west

quartz c axis texture mapping of a variscan regional foliation malpica tui unit nw spain abstract mineral assemblages developed under high pressure and low to intermediate temperature conditions h p li t are found in eclogites orthogneisses and metasediments of the malpica tui unit mtu either defining or surrounded by a regional foliation that was mostly re equilibrated under amphibolite to greenschist facies conditions this regional tectonic foliation tracking the exhumation path of the mtu in the context of the variscan collision in sw europe has been studied using the microstructural record in quartz from the different rock types most of the quartz c axis patterns show two distinct asymmetries contained in perpendicular planes which are related to two different non coaxial components of the finite fabric i e the regional foliation and the lineation the asymmetry of deformation inferred from quartz c axis textures suggests that fabric accumulation during exhumation occurred under 3 d general flow departing strongly from the simple shear type this has significant tectonic implications for the interpretation of regional stretching lineations which in the present geodynamic setting of irregular collisional tectonics are not parallel to the direction of general tectonic transport and ultimately to the plate convergence direction

triclinic transpression zones with inclined extrusion data from natural transpressional shear zones reveal a large range of lineation orientations that can spread along complete girdles one possible way to account for such a type of fabrics is variation in the extrusion direction this work presents a new theoretical model of oblique transpression with inclined extrusion the model is kinematic and the flow is considered as homogeneous and steady the angle between the simple shear direction and the extrusion direction is one of the main parameters controlling the final fabric low values of 40 can generate strain ellipsoids very close to the abscissa axis of the flinn plot depending on the vorticity of the flow the finite strain and the actual value of the angle the model predicts the generation of lineation fabrics ranging from point maxima to patterns with opposite plunge senses these patterns can display monoclinic nearly monoclinic or clearly triclinic symmetry lineation patterns spreading along complete girdles can be obtained by allowing the extrusion direction to deviate less than 30 about the vertical in either direction

strain patterns within part of the willard thrust sheet idaho utah wyoming thrust belt abstract the willard thrust sheet located in northern utah usa contains a 10 15 km thick sequence of strata that were internally deformed and translated about 50 km ese upper levels of the sheet are relatively little deformed lower levels have minor folds and cleavage that increase in intensity downward and rotate into parallelism with the thrust and the basal fault zone contains mylonite and ultracataclasite finite strain displays systematic changes with structural position and lithology basal levels have strain ratios r r xz of 3 5 and angles between stretch directions and the thrust of r of 1 5 3 and of 10 35 with overall correlations between increasing r decreasing and greater mica content overlying micaceous quartzite has relatively low strain with r of 1 3 2 and 50 principal stretching x directions trend perpendicular to oblique to minor fold axes which are locally oblique to the overall transport direction reflecting partial decoupling of internal deformation from thrust slip principal axial ratios have r yz r xy but microfabrics indicate only limited extension in the intermediate stretching direction y corresponding to volume losses of about 10 40 strain is factored into components of thrust perpendicular thinning and thrust parallel shear that increase downward and in mica rich layers minor thrust parallel shortening to extension minor longitudinal extension and minor longitudinal wrench shear these patterns reflect litho mechanical layering of the thrust sheet which includes a very weak basal fault zone that formed at elevated fluid pressures a weak mica rich lower level with widespread crystal plastic and mass transfer deformation and a stronger upper part with limited internal deformation

pure shear dominated transpression and vertical extrusion in a strike slip fault splay from the itapirapua shear zone ribeira belt brazil abstract the itapirapua shear zone is a major ne trending dextral transcurrent zone from the southern ribeira belt brazil which was active during the neoproterozoic brasiliano pan african orogeny related with west gondwana assembly macroscopic sigmoid shaped blocks in its northernmost splay termination present a ne trending subvertical tectonic foliation and subvertical stretching lineation suggesting a transpressional tectonics this paper investigates this splay termination using integration of data from geological mapping thermodynamic modeling microtectonics quartz c axis fabrics kinematic vorticity and finite strain analysis distinct rock assemblages are placed in tectonic contacts along the splay termination including statherian sillimanite biotite paragneiss and granitic orthogneiss apiai mirim complex statherian phyllites bairro dos prestes formation calymmian chlorite epidote actinolite metabasites fazenda velha suite ediacaran phyllites metalimestone and quartzite itaiacoca group and ediacaran granite tres corregos suite isochemical phase diagrams constrain synkinematic metamorphic conditions of 490 510 c and 11 12 5 kbar chlorite biotite phengite phyllite 520 560 c and 9 10 kbar biotite phengite phyllite and 680 690 c and 5 6 kbar sillimanite biotite paragneiss with isothermal decompression paths microstructural evidence suggests that increase in metamorphic temperatures were accompanied by changes in the dominant deformation mechanism in quartz aggregates from subgrain rotation to grain boundary migration recrystallization significant contribution of dissolution precipitation creep indicates water abundant deformational conditions strain ellipsoids are oblate shaped and present ne trending subvertical xy planes and subvertical x axes in agreement with field tectonic fabric quartz c axis fabrics in xz sections of the strain ellipsoid are dominated by symmetrical crossed girdles while the fabrics present dextral asymmetry in horizontal yz sections consistent with macroscopic structural deflection indicating a subvertical vorticity axis vorticity results indicate 48 59 of pure shear contribution the data indicate that strain in the itapirapua shear zone northern termination was accommodated by a pure shear dominated transpression with vertical extrusion at mid crustal conditions which contrasts with a transtensional deformation in the southeastern block

the initiation and evolution of the transpressional straight river shear zone central fiordland new zealand abstract structural data and u pb geochronology on zircon from central fiordland new zealand show the role of pre existing structural heterogeneities in the kinematic evolution of a newly discovered zone of transpression the straight river shear zone consists of steep zones of high strain that are superimposed onto older fabrics across a 10 80 km region the older foliation formed during two periods of tectonism contraction and magmatism of mostly carboniferous 312 306 ma age and early cretaceous batholith emplacement ending by 113 4 1 7 ma followed by extension that ceased by 88 4 1 2 ma the primary mechanism for the formation of steep shear zone foliations was the folding of these older fabrics conjugate crenulation cleavages associated with the folding record shortening at high angles to the shear zone boundaries fold axial surfaces and axial planar cleavages strike parallel to the shear zone with increasing strain as they progressively steepened to subvertical in most areas shear sense flips from oblique sinistral east side down component to oblique dextral west side down across zones of intermediate and high strain high strain zones display subvertical mineral lineations steep strike slip faults and shear sense indicators that record strike slip motion across the steep lineations these patterns reflect triclinic transpression characterized by narrow zones of mostly strike slip deformation and wide zones of mostly contraction zones of high strain align with offshore traces of late tertiary strike slip faults suggesting that a previously undocumented component of late tertiary shortening and strike slip motion is accommodated within fiordland

conflicting shear sense indicators in shear zones the problem of non ideal sections abstract deflection of pre existing planar structures such as foliations or veins by ductile shear zones is geometrically very similar to the curvature of newly developed shear zone restricted foliations in zones that cut a rock with a random fabric sense of curvature of shear zone restricted foliations can be used to determine shear sense but the deflection of pre existing planar structures is less reliable two examples are presented of shear zones in australia where both types of structures seem to represent conflicting shear sense this conflict can be attributed to a geometrical effect which causes deflection of older structures in an opposite direction to shear sense on outcrop surfaces oblique to the displacement vector if the orientation of the displacement vector cannot be accurately determined in shear zones e g because of limited outcrop or unclear lineations most shear sense indicators can still be used to determine shear sense but pre existing planar structures are notoriously unreliable

deformation path in high strain zones with reference to slip partitioning in transpressional plate boundary regions abstract the current status of the kinematics and strain geometry of high strain zone studies is briefly summarized a general high strain zone has a triclinic deformation path and monoclinic shear zones are special end member cases fabrics observed in natural shear zones and theoretical considerations based on continuum mechanics are compatible with this conclusion non steady deformation paths remain difficult to deal with and may ultimately rely on a realistic mechanical treatment of high strain zones which may be possible when our knowledge of the mechanical behavior of rocks under natural deformation conditions is improved an examination of the phenomenon of slip partitioning in transpressional plate boundary regions shows that the bulk deformation path in the forearc area trench parallel high strain zone is generally triclinic the alpine fault in the south island of new zealand provides an example of a currently active triclinic shear zone the southern knee lake shear zone of manitoba canada provides an archean example of a triclinic shear zone

relationships between kinematic indicators and strain during syn deformational exhumation of an oblique slip transpressive plate boundary shear zone the alpine fault new zealand abstract in order to assess how different elements of plate boundary scale shear zone tectonite fabrics reflect the boundary kinematics and strain history we have examined a range of such structures within the active alpine fault mylonite zone new zealand in this oblique slip zone we find that quartz cpo fabric symmetry and individual mica grain lineations reflect incremental stretches but develop stable orientations parallel to the finite stretching direction s 1 this s 1 trends oblique to the plate motion vector because the shear zone experiences some extrusion toward the free surface conversely asymmetric structures such as shear bands and mica fish reflect simple shear dominated deformation late in the shear zone history parallel to the imposed relative plate motion vector finally linear fabrics are a poor record of the mylonitic deformation in this zone because 1 they are dominantly inherited and have been only partially re oriented during mylonitisation and 2 macroscopically they become less visible at higher strains due to fabric homogenisation generally these results indicate that particularly in heterogeneous polyphase shear zones a full understanding of the movement history can only be obtained by considering kinematic information from as many different sources as possible

sustainable transpression an examination of strain and kinematics in deforming zones with migrating boundaries most orogenic belts owe their development to oblique convergence and commonly have many orogen parallel transpressional high strain zones to constrain the tectonic history of orogenic belts by structural and fabric analysis of rocks it is desirable to understand quantitatively the relationship between the boundary conditions and the resulting strain distribution and kinematics in these zones most current models for transpression assume homogeneous deformation confined by boundaries that are fixed to material planes this creates a strain compatibility problem at the margins of the active deforming zone and also requires that the strain rate normal to the zone boundaries increase to implausibly high values soon after the onset of oblique convergence transpressional motion the latter contradicts with the observation that transpressional motion can be sustained throughout an orogeny the assumption that zone boundaries are fixed to material planes is unrealistic the outstanding problems of current transpressional models are resolved in this paper by allowing the zone boundaries to migrate through the rock material the consequence of zone boundary migration for the strain field and kinematics within a transpressional high strain zone is investigated mathematically the implications of the modeling for fabric interpretation are discussed the modeling makes general predictions consistent with observed planar and linear fabric patterns in natural transpressional high strain zones it predicts that foliations in transpressional high strain zones are subparallel to the zone boundaries regardless of variation in the imposed boundary conditions lineations cluster along the great circle girdle subparallel to the average foliation the spread of the lineations may vary from point maxima to complete girdles

testing the model of oblique transpression with oblique extrusion in two natural cases steps and consequences abstract kinematic models of various types of transpression have been used to explain fabric features and strain in many natural deformation studies here a mathematical model that encompasses all monoclinic and triclinic transpressional deformations including triclinic deformation with inclined simple shear and or inclined extrusion orientations can be tested using a step by step approach with available field evidence two cases are presented the first case from the wabigoon quetico boundary in the archean superior province utilizes both fabric orientation and quantified strain data the best fit of the field evidence to the model indicates that deformation likely took place along subvertical shear zones via transpression with subhorizontal simple shear 0 20 and variable inclined extrusion direction extrusion can be either east or west and typically indicates extrusion orientations between 0 and 50 from vertical the second case of the south iberian shear zone has fabric orientation data but no quantifiable strain possibilities the best fit of the field evidence to the model indicates that deformation likely took place along a moderately inclined shear zone via transpression with subhorizontal simple shear 0 20 and variable inclined extrusion direction values between 0 and 80 from the true dip of the shear zone using this protocol in other examples of natural deformation will allow further constraints to be applied to kinematic models

constraints on deformation path from finite strain gradients we present a methodology for analyzing finite strain gradients in high strain zones in order to place constraints on the zone s possible deformation path to accomplish this the high strain zone is divided into several sub zones within each of which finite strain is described by a single ellipsoid based on knowledge of the spatial and temporal evolution of deformation strain observed in less deformed sub zones is then mathematically removed from more deformed sub zones to calculate incremental finite strains the mean kinematic vorticity number of each of these increments must be constant if deformation was steady state we use the methodology to analyze two natural examples of high strain zones at different scales both natural examples have non steady state likely deformation paths which may be a common characteristic of high strain zones although results of the methodology are not unique identification of likely paths considerably narrows the range of possible deformation paths consequently useful kinematic information can be extracted from finite strain data even in instances where the detailed spatial and temporal evolution of a high strain zone cannot be determined conclusively

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

a unified approach to measuring structures in orientated drill core a unified system of collecting structural data from drill core is proposed the system encompasses planes and planar fabrics lineations fold hinges and hinge surfaces faults and shear zones vorticity vectors shear directions and shear senses the system is based on standard measurements of angles in the reference frame of the core and angles which are easily carried out by means of core protractors or templates the methods for dealing with folds and kinematic analysis of shear zones have not been described previously but they follow logically from the standard methods for dealing with planes and lines

oblique plate motion and continental tectonics three dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins kinematic models of strike slip partitioning for transpression and transtension allow the exact relation among three critical parameters plate motion instantaneous strain axes and degree of strike slip partitioning to be calculated application to two end member tectonic environments characterized by a low south island new zealand and high central california degree of strike slip partitioning demonstrates a remarkable consistency among the three parameters suggesting that strike slip partitioned transpression is a valid model for deformation in these regions the extreme degree of strike slip partitioning in wrench dominated systems such as central california is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes

introduction to journal of structural geology special issue on deformation of the lithosphere how small structures tell a big story this special issue deformation of the lithosphere how small structures tell a big story is dedicated to professor henk zwart 1924 2012

kinematic analysis of the southern iberian shear zone and tectonic evolution of the acebuches metabasites sw variscan iberian massif 1 the acebuches metabasites constitute a slice of oceanic crust located at the variscan suture between the ossa morena and the south portuguese zones iberian massif the emplacement of the metabasites onto the accretionary prism was associated with the activity of the southern iberian shear zone careful measurements of the fabrics and structures related to this shear zone show consistent kinematic criteria pointing to the presence of a simple shear component with an oblique predominantly left lateral sense of displacement the data are consistent with existing models of triclinic or oblique shear zones considering coeval simple shear and pure shear components these models provide a satisfactory explanation of the observed spatial variations in the foliation lineation and fold hinge data the kinematics of other shear deformations older and younger than the southern iberian shear zone have also been established these results offer new insights into the tectonic evolution of this suture of the variscan belt and show how complex transpressional strains can arise during oblique crustal deformation associated with the motions of tectonic plates

testing models for obliquely plunging lineations in transpression a natural example and theoretical discussion theory predicts that stretching lineations in an ideal vertical transpressional zone should be either vertical or horizontal many field descriptions of transpressional zones however indicate a range of lineation orientations between these extremes several theoretical models have been developed to explain such departures from expected lineation orientation and we discuss these in the context of a field example from the archean superior province in the north american craton existing models are insufficient to explain obliquely plunging lineations in this example because 1 obliquely plunging lineations cannot be accounted for by shear zone boundary effects imposed by a no slip condition 2 foliations and lineations vary independently 3 the vorticity normal section is subhorizontal limiting possibilities for inclined simple shear 4 high vorticity is needed for finite strains and lineations to match previously proposed triclinic models but vorticity is relatively low and 5 juxtaposed east and west plunging lineations are unlikely in the previously proposed triclinic models because existing theoretical models are not applicable to our field example we contemplate a new model to explain obliquely plunging lineations within quasi homogeneous transpression

deformation path partitioning within the transpressional white mountain shear zone california and nevada most numerical simulations of transpression zones predict a change in finite stretching direction from subhorizontal to subvertical for simple shear dominated zones we provide a detailed description of a dextral transpression zone the white mountain shear zone wmsz with a range of lineation orientations and compare these natural data to numerical models the wmsz is characterized by steeply dipping foliations with dominant shallowly plunging lineations and coeval subordinate domains of steeply plunging lineations within shallowly lineated domains foliation geometry shear sense indicators and quartz c axis fabrics indicate a large component of simple shear while microstructural and quartz c axis fabric data from steeply lineated domains indicate a large component of pure shear geometric relationships between foliations and lineations and quartz c axis fabrics demonstrate that lineation orientation has remained constant during much of the deformation history comparison of numerical models with the data collected from wmsz shows that the shear zone geometry and the observed strain path partitioning do not match any of these models we propose a conceptual kinematic model for the wmsz involving stable segregated coeval kinematic domains of simple shear dominated fabrics and pure shear dominated fabrics that accommodate the transcurrent and contractional components of deformation respectively

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

deformation mechanics in inclined brittle ductile transpression zones insights from 3d finite element modelling abstract most natural examples of transpression zones developed at oblique convergence regime are inherently 3d and have inclined boundaries a 3d finite element model with an elasto plastic rheology is used to investigate the structural and mechanical evolution of inclined transpression zones in a rock sequence above a frictional basal detachment inelastic constitutive relationships allow permanent strains to develop in response to the applied loads fe modelling results show that oblique convergence is accommodated by discrete deformation at the main pre existing inclined faults 70 and by distributed brittle and ductile deformation at active blocks oblique contraction at the active blocks resulted mainly in layer parallel shortening orthogonal to the model outer boundaries whereas thickening in the horizontal and vertical directions was accommodated via layer parallel fault strike parallel extension and up dip extrusion i e inclined extrusion lateral extrusion should have compensated the rest and or volume loss took place folding and thickening of the mobile backstop produced a non cylindrical asymmetric bi vergent anticline where permanent strains developed principally in the steep forelimb secondary conjugate fault zones also accommodate oblique slip and contribute to uplift displacement vectors within the transpression zone are rotated counter clockwise ca 20 30 with respect to vectors in the fixed backstop areas with higher rotation values seem to correlate with those showing higher ellipticity values the presence of pre existing faults favored strain partitioning from the onset of deformation fe modelling results compared with analytical natural example and analogue modelling results show that our mechanical modelling can overall match inclined transpression zones geometry that present different modes of strain partitioning and localisation

carbonate deposition during a time of mid latitude ocean cooling early permian subtropical sedimentation in the sverdrup basin arctic canada abstract carbonate rocks of the lower permian sakmarian to kungurian raanes and great bear cape formations accumulated in the sverdrup basin during a critical transition period when ocean waters cooled dramatically these rocks have many attributes similar to sediments deposited in modern subtropical zones the biota is dominated by a heterozoan assemblage of pelmatozoans bryozoans and brachiopods with the addition of photozoan elements such as fusulinids and colonial corals associated with the change in the sverdrup basin from a tropical to subtropical environment is the first occurrence of glauconite iron oxides and storm deposits the shift in depositional conditions is interpreted to reflect decreased seawater temperature that was brought about by a northward migration to a mid latitude position frequented by storms oceanographic change however was not unidirectional but was complicated by concomitant variations in sea level and oceanography recorded as three distinct phases determined by water depth and temperature 1 rocks of the raanes and lower great bear cape are deep water and heterozoan 2 the middle great bear cape records shallow water subtropical conditions with fusulinids and large colonial corals 3 the upper great bear cape is entirely heterozoan in all locations and by implication all depths all post raanes and great bear cape rocks of the sverdrup basin are entirely heterozoan taking caution for possible evolutionary changes subtropical carbonates can be recognized in rocks much older than previously believed the most important components for identifying subtropical carbonates of the past are large benthic foraminifers in an otherwise heterozoan assemblage and as such the ability to delineate this carbonate realm in the past is currently restricted to times when large benthic foraminifers grew in shallow marine environments

high frequency oscillations and their correlation with the wind forcing during the first sequal year two arrays of inverted echo sounders one along the equator and the other across the north equatorial countercurrent are analysed together with a wind record from st peter and st paul rocks the dominant high frequency feature along the equator is an energetic oscillation with a period of 5 8 days that shows strong coherence with the meridional component of the wind the sounder at 3 n 28 w on the other hand shows a high degree of coherence with the zonal component of the wind at a period of 4 5 days comparison of these measurements in 1983 with measurements made in 1979 suggest considerable non stationarity