local deformation in compacting flows development of pressure shadows abstract small scale structures produced by deformation and compaction are common in igneous metamorphic and sedimentary rocks and are often used to define foliation and lineation directions analytic solutions for models of flow round a rigid stationary sphere are obtained using the equations that describe viscous compaction the finite deformation associated with these flows shows many of the features observed in deformed rocks some of which resemble those generated when the ratio of the bulk viscosity to the shear viscosity is small under these conditions the particle paths of the matrix are almost unaffected by the presence of the sphere it is encouraging that the continuum compaction equations can describe the observed behaviour even though the length scale on which the deformation occurs is only mm to cm it is unlikely that the observations can be used to distinguish between d arcy flow of the low viscosity phase and mass transport by diffusion

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

fold axis parallel extension in an arcuate fold and thrust belt the case of the helvetic nappes abstract the helvetic nappes of western switzerland are discussed as an example of an arcuate foreland fold and thrust belt in which active fold axis parallel stretching occurred fold axis parallel extension is recorded by 1 1 incremental strain data from pressure shadow fibres the significance of pressure shadow fibres for the determination of the deformation history of a region is discussed pressure shadows are used to quantify the amount of and to describe the distribution of fold axis parallel extension occurring in the helvetic nappes 2 2 the extension directions of conjugate systems of en echelon veins it is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern the relative age of the conjugate en echelon vein systems in the helvetic deformation history can be assessed the geometry of the conjugate sets relative to the local anisotropy plane is described and the significance of the preferred orientation of their extension directions is discussed 3 3 fold axis parallel sections a comparison of the regional distribution of the fold axis parallel strain with the shape of the helvetic nappes in fold axis parallel sections shows that the fold axis parallel strain cannot be related to the footwall topography of the nappes it is concluded that the fold axis parallel extension in the helvetic nappes was induced by a change of direction of overthrust shear this change occurred late in the deformation history and was superposed on the already formed nappes the changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting alpine nappe pile relative to the european plate a rotation which lead to the arcuate shape of the western alps

complex fringes around magnetite porphyroclasts growth and deformation history abstract deformed strain fringes in iron formation rocks show complex quartz fiber patterns that grew alongside magnetite porphyroclasts embedded in a matrix of quartz and iron oxides during coaxial to non coaxial deformation these rocks have been deformed by a combination of processes involving microfracturing pressure solution and dislocation glide at temperatures of approximately 300 c detailed microstructural observation and crystallographic analysis show that quartz fiber growth is not controlled by the crystal faces quartz c axis orientations of fibrous quartz indicate that these grains initially grew with their c axes parallel to the fiber length late solid state deformation of crystallized fibers comprised dislocation glide along basal plane and subsequent recrystallization this produces aggregates of recrystallized grains consisting of equant to elongated quartz grains with straight and orthogonal grain boundaries

kinematic analysis of a large scale leading edge fold lost river range idaho abstract the mahogany creek buck creek structure within the lost river range idaho is a large scale leading edge fold which developed in late paleozoic outer shelf stratigraphy during the sevier orogeny incremental strain histories determined from antitaxial fibrous pressure shadows are used to quantify temporal variations in the magnitude and orientation of elongation as a function of structural position around the leading edge of this northward and eastward propagating blind thrust plane strain is indicated by coaxial up dip extension on cleavage planes on the backlimb of the structure the bluebird mountain formation a calcareous sandstone has deformed by flexural flow whereas the forelimb of the structure records spin through a fixed steeply plunging incremental extension direction the flat limb ahead of the anticline exhibits top to the foreland simple shear in contrast the underlying surrett canyon formation a thick bedded limestone has experienced flexural flow localized within thin zones on both limbs of the anticline samples from the anticlinal hinge zone suggests kinematic partitioning with pin lines which were variably distributed within each unit and which were temporally transient finite elongations are greatest in the hinge and above the foreland flat in the bluebird mountain formation but are negligible in the surrett canyon formation except within the mechanically active interbeds of the hanging wall and within the footwall adjacent to the fault along the northernmost exposures strain histories in the hanging wall do not vary significantly along strike while the footwall varies from undeformed in the south to penetratively deformed in the north thus the strain data are consistent with self similar along strike fold development in the hanging wall during strike parallel ramp propagation forward thrust propagation may have been blunted by a combination of fold accommodated shortening and footwall deformation to the north once the fold had tightened and fold shortening required greater differential stress forward fault propagation resumed transporting the anticline onto the upper flat with little change in hanging wall fold geometry or strain distribution

formation of orogen perpendicular thrusts due to mechanical contrasts in the basal decollement in the central external sierras southern pyrenees spain two series of analogue models are used to explore the effect of ductile frictional contrasts of the basal decollement on the development of oblique and transverse structures during thin skinned shortening these models simulate the evolution of the central external sierras ces southern pyrenees spain which constitute the frontal emerging part of the southernmost pyrenean thrust the ces are characterised by the presence of n s to nw se anticlines perpendicular to the pyrenean structural trend and developed in the hanging wall of the thrust system they detach on unevenly distributed triassic materials evaporitic dolomitic interfingering the models simulated the effect of adjacent ductile versus frictional decollements during shortening model series a tests the thickness ratio between overburden and the ductile layer whereas model series b tests the width perpendicular to the shortening direction of frictional decollement model results confirms that deformation reaches further in areas detached on a ductile layer whereas above frictional decollement areas shortening is accommodated by additional uplift and penetrative strain this replicates the structural style of the ces higher structural relief of n s anticlines with regard to orogen parallel structures absence of a representative ductile decollement in the core plunging towards the hinterland and foreland side closure not thrusted by the south pyrenean thrust

a unified kinematic model for the evolution of detachment folds abstract detachment folds represent a major structural element in a number of fold belts they are common in the jura mountains the zagros fold belt the central appalachian fold belt the wyoming fold belt the brooks range the parry islands fold belt and parts of the subandean belt these structures form in stratigraphic packages with high competency contrasts among units the competent upper units exhibit parallel fold geometries whereas the weak lower unit displays disharmonic folding and significant penetrative deformation two distinct geometric types disharmonic detachment folds and lift off folds have been recognized however these structures commonly represent different stages in the progressive evolution of detachment folds the structures first form by symmetric or asymmetric folding with the fold wavelength controlled by the thickness of the dominant units volumetric constraints require sinking of units in the synclines and movement of the ductile unit from the synclines to the anticlines continuing deformation results in increasing fold amplitudes and tighter geometries resulting from both limb segment rotation and hinge migration initially limb rotation occurs primarily by flexural slip folding but in the late stages of deformation the rotation may involve significant internal deformation of units between locked hinges the folds eventually assume tight isoclinal geometries resembling lift off folds variations in the geometry of detachment fold geometry such as fold asymmetry significant faulting and fold associated with multiple detachments are related to variations in the mechanical stratigraphy and pre existing structure

the use of syntectonic fibres to determine strain estimates and deformation paths an appraisal abstract fibrous pressure shadows developed adjacent to rigid brittle inclusions are familiar features in many tectonites belief that these structures reflect the deformation path as once adjacent grains move apart has led to the derivation of a variety of techniques to determine both incremental and finite strain estimates these methods which can be divided into rigid fibre models e g durney and ramsay and deformable fibre models e g ellis display potentially serious problems when applied to field examples recent studies of microstructures within veins and pressure shadows support the concept that syntectonic fibre complexes can reflect deformation paths and are therefore useful markers of incremental and finite strain the problems mentioned therefore stem directly from the application of the techniques worked examples using these techniques give a variety of strain values dependent upon the extent to which a particular technique allows for volume change or fibre deformation this is useful locally however if the results are to be meaningful on a regional scale then it is imperative that the same technique be applied in the analysis of all fibres regardless of the local deformation path in such studies the merits of a particular technique in determining a correct strain value come second to allowing the spatial distribution of strain to be determined semi quantitatively and thus allowing comparison of similarly derived results with one another

strengthening the link between geochronology textures and petrology time must be integrated with other geologic parameters such as pressure temperature p t composition or structural textural information in order to quantify the dynamics of geologic processes like mountain building faulting metamorphism or magmatism in the past this connection was weak due to large sample sizes required in geochronology however recent developments open up exciting new possibilities texturally controlled in situ dating of minerals utilizing uv laser ablation 40ar 39ar or rb sr microsampling facilitates the crucial link between ages and textures at high spatial resolution in situ u pb ages of accessory minerals can now be linked to the corresponding p t evolution via their characteristic trace elemental signatures indicative of mineral reactions in rocks garnet dating by sm nd and more recently lu hf can greatly benefit from both laser ablation inductively coupled plasma mass spectrometry icpms screening and improved leaching strategies improving the link between time and p t such integrated approaches can yield the duration and rates of deformation in shear zones the rates of burial and exhumation of subducted rocks or the duration of entire metamorphic loops to name a few potential for further development is identified in diverse areas dating of carbonate minerals related to brittle faulting using u pb could provide previously unavailable constraints on rates and duration of brittle deformation in the earth s upper crust in situ rb sr dating applicable to micas could become available using either laser ablation dynamic reaction cell icpms or microbeam accelerator mass spectrometry other future developments and trends and their limitations are discussed briefly

alteration associated with gold deposition at the getchell carlin type gold deposit north central nevada wall rock alteration at the getchell underground deposit was examined to determine the effects of au bear ing fluids on host lithologies and the relationship between k bearing alteration minerals and au deposition the major minor and trace element geochemistry of highly altered and mineralized to unmineralized rocks from the getchell deposit was quantified for more than 50 samples collected along 13 transects through calcareous siltstone and carbonaceous limestone and along one transect through a rhyodacite dike each transect in sedimentary rocks was collected along a single homogeneous bed that could be followed from high grade ore to moderately altered rock or waste rock analyses were obtained for 39 elements 10 oxides and loss on ignition using multiple techniques petrographic studies were integrated with geochemistry and x ray diffraction and electron microbeam analyses to identify ore and alteration minerals and to correlate mineralogy with geochemical fluxes geochemical analyses indicate that ti al zr and th behave as immobile elements immobility isocon diagrams show that significant hg sb se te tl and cs are typically added to the wall rocks with au fe is im mobile in some transects added to wall rocks in others and removed from rhyodacite host rocks minor s and w are usually added to the rock whereas minor cu and mo are variably added or removed sio 2 appears to be either added during alteration or remain constant elements that are consistently removed during alteration include ca mn sr and sc and minor mg ba and k na was significantly removed from the rhyodacite au correlates positively with ag hg sb se si te and tl and negatively with ca mn and sc element fluxes correspond with observations of mineral abundances and show that sulfidation and decar bonitization are spatially associated with gold mineralization au hg sb tl cu and possibly te and w were incorporated in trace element rich pyrite as host rock fe was sulfidized fe was added to mineralized rocks in some transects indicating that pyritization occurred in addition to sulfidation cs is a component of galkhaite a trace mineral that is relatively common in some parts of the getchell system decarbonitization removed as much as 95 wt percent calcite from mineralized rocks in some transects and is responsible for loss of ca mn sr and sc although sio 2 as and s are significantly added to carlin type systems they do not appear as major added components in the isocon diagrams this is because the halos for these components are much larger than the areas examined by the sample transects x ray diffraction analyses identified minerals in ore and waste samples and quantified clay minerals including illite and montmorillonite kaolinite was not identified or is present in low abundances in both mineralized and unmineralized rocks montmorillonite does not correlate with gold and textures are consistent with pore filling deposition of montmorillonite following au deposition illite abundance exhibits a positive correlation with au r 2 0 80 intergrowths of illite with ore stage pyrite support a genetic relationship between the two minerals and indicate that illite formed as part of the au related hydrothermal event mineral abundances and textures suggest that illite formed by alteration of k feldspar during au deposition results show that dating of k bearing phases in sedimentary rocks other than illite would provide ages unrelated to gold deposition

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation its role has generally been insufficiently emphasized because evidence is best preserved where por phyroblasts which contain inclusion trails are present reactivation occurs when progressive shearing operating in a synthetic anastomosing fashion parallel to the axial planes of folds changes to a combination of coarse and fine scale zones of progressive shearing some of which operate antithetically relative to the bulk shear on a fold limb reactivation of earlier foliations occurs in these latter zones reactivation decrenulates pre existing or just formed crenulations generating shearing along the decrenulated or rotated pre existing foliation planes partitioning of deformation within these foliation planes such that phyl losilicates and or graphite take up progressive shearing strain and other minerals accom modate progressive shortening strain causes dissolution of these other minerals this results in concentration of the phyllosilicates in a simi lar but more penetrative manner to the for mation of a differentiated crenulation cleavage except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds reactivation can generate bedding parallel schistosity in multideformed and meta morphosed terrains without associated folds heterogeneous reactivation of bedding gen erates rootless intrafolial folds with sigmoidal axial planes from formerly through going struc tures reactivation causes rotation or refrac tion of axial plane foliations forming in the same deformation event causing reactivation in those beds or zones in which an earlier foli ation has been reactivated and results in destruction of the originally axial plane foli ation at high strains reactivation also provides a simple explanation for the apparently wrong sense but normally observed rotation of gar net porphyroblasts whereby the external foli ation has undergone rotation due to antithetic shear on the reactivated foliation alter natively the rotation of the external foliation can be due to its reactivation in a subsequent deformation event porphyroblasts with inclusion trails com monly preserve evidence of reactivation of earl ier foliations and therefore can be used to ident ify the presence of a deformation that has not been recognized by normal geometric methods because of penetrative reactivation reac tivation often reverses the asymmetry between pre existing foliations and bedding on one limb of a later fold leading to problems in the geo metric analysis of an area when the location of early fold hinges is essential the stretching lineation in a reactivated foliation can be rad ically reoriented potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events other aspects of reac tivation however can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized for example d grown porphyroblasts may be dis solved against reactivated s and hence appear to have grown syn d

strain variations in an ancient accretionary complex implications for forearc evolution strain magnitudes and orientations and incremental strain histories are characterized throughout the kodiak formation a portion of a late cretaceous early tertiary accretionary prism the results of these analyses are used to evaluate the history of underplating penetrative strain and unroofing in the forearc of the southwest alaskan convergent margin five major deformation events are recognized in the kodiak formation 1 layer parallel extension and layer parallel shear d1 related to underthrusting 2 seaward verging fold and thrust deformation d2 with development of both a slaty cleavage s2 and a downdip stretching lineation l2 related to underplating and early penetrative strain within the wedge 3 development of a regional scale antiform d3 4 development of a shear zone along the northwest limb of the d3 antiform d4 and 5 right lateral strike slip faulting and penetrative right lateral simple shear in the seaward half of the kodiak formation d5 based on sharp differences in structural style and the magnitude and orientation of strain associated with d2 the kodiak formation has been subdivided into three structural units informally called the landward central and seaward belts the structural variations observed in these three belts correspond to differences in structural relief of 1 5 km associated with the d3 antiform the lowest d2 structural level is in the core of the antiform central belt and progressively higher structural levels are exposed on both limbs landward and seaward belts the regional distribution of incremental strain histories associated with d2 d3 d4 and d5 has been quantified by using syntectonic fibers in pressure shadows around spherical pyrite framboids from the highest to the lowest structural level d2 strain is characterized by 1 an increase in the magnitude of finite strain 2 a change in the orientation of finite extension from nearly vertical to gently northwest plunging 3 a change in the incremental extension direction from nearly vertical to moderately northwest plunging and finally 4 a change in the strain history from coaxial to noncoaxial with a sense of rotation consistent with southeast directed thrusting we interpret these d2 strain patterns to reflect a strain field that was fixed relative to the basal decollement with coaxial subhorizontal shortening in the interior of the prism and noncoaxial strain probably simple shear near the base subsequent variations in the orientation of incremental extension associated with d3 are consistent with relatively rapid vertical translation of the accreted sequences relative to the d2 strain field we specifically propose two periods of wedge thickening in response to a combination of underplating and penetrative strain these two wedge thickening events correspond with episodes of subaerial exposure and erosional unroofing in the late cretaceous and middle tertiary

the formation of antitaxial calcite veins with well developed fibres oppaminda creek south australia abstract of the several vein types with elongate crystals we distinguish a class of veins with well developed fibres exposed at oppaminda creek south australia these calcite veins are characterised by very limited competition between growing crystals which leads to the development of fibrous crystals with smooth boundaries and which can attain extreme length width ratios fibres are often curved and track the opening trajectory the veins are also characterised by their outwards antitaxial growth starting from a narrow median zone these median zones contain wall rock inclusions and a distinctly different non fibrous internal microstructure such veins continue growing as long as extension is suitably oriented relative to the vein orientation multiple cross cutting vein sets can thus grow simultaneously and can together track over 90 rotation of the fibre growth direction the veins formed by calcite precipitation on both their outer surfaces we suggest that the term antitaxial should be redefined as applying only to those veins that have two simultaneous growth planes at their outer surfaces antitaxial veins grow without further fracturing which sets them apart from other veins that grew by precipitation inside a crack and which only have one single growth plane at the time the veins at oppaminda creek are about 585 ma in age and therefore formed at an estimated 4 6 km depth scanning electron microscope observations revealed about 1 m sized structures which we interpret as fossils of microbes that lived inside the veins at the time of their formation and which may even have played a role in the vein formation these structures would represent the first finds of fossilised life forms of such a combined age and depth

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement abstract this paper discusses the differences in mechanical properties and kinematics between fold and thrust belts detaching on evaporitic decollements and those detaching on stronger detachments physical experiments are described that model shortening of a thick brittle cover overlying a weak viscous decollement to gain a better understanding of these differences we tested the influence of 1 the decollement layer thickness and 2 the presence of a deformable backstop on the hinterland side and of a decollement pinch out on the foreland side because of the very low shear strength of the viscous decollement folds and thrusts did not propagate according to a piggy back sequence but by centripetal back and forth propagation additional shortening was accommodated by growth of all existing structures fold symmetry and thrust vergence varied between experiments models confined between two rigid vertical endwalls always deformed by symmetric folding and thrusting in models having a deformable backstop and a foreland decollement pinch out forethrusting initially dominated folds were asymmetric and fault blocks rotated in models having a thick decollement layer folds kept growing asymmetrically diapirism also depended on the initial decollement geometry diapirs formed only in models having a deformable backstop and were restricted to the hanging wall of a fault related fold finally the implications of such kinematics on the thermal history are discussed as well as the likelihood of hydrocarbon maturation and preservation

structural style of the appalachian plateau fold belt north central pennsylvania abstract new seismic and well data from hydrocarbon exploration and development activity associated with the marcellus formation shale gas play in north central pennsylvania provide insight to the structural style of the appalachian plateau fold belt in the region north and northwest of the allegheny structural front in potter tioga bradford sullivan lycoming clinton and centre counties the plateau fold belt in this area developed over a detachment in upper silurian salina group evaporites during the permian alleghanian orogeny in response to north northwest directed shortening at the allegheny structural front a deep detachment in cambrian shales that underlies the valley and ridge province to the south southeast ramps up section through cambro ordovician carbonates and lower middle silurian clastics to a shallow detachment in upper silurian evaporites at the northeastern plunge of the nittany anticline south and east of williamsport pa only a small amount of slip is interpreted to have been transmitted into the foreland on the shallow upper silurian detachment instead most slip was consumed in fault propagation folds immediately north of the allegheny structural front the plateau fold belt developed above the upper silurian evaporites can be divided into structural domains based on fold characteristics domain 1 folds have short wavelengths and low amplitudes domain 2 salt cored anticlines have long wavelengths and large amplitudes domain 3 comprises large synclines located between domain 2 anticlines halite originally beneath domain 3 synclines is interpreted to have been mobilized or evacuated into the cores of adjacent domain 2 anticlines during folding seismic data indicate that the base of the salt detachment underlying plateau folds is a non planar stepped surface possible scenarios for the development of the non planar detachment include 1 mobilization of halite from an evaporite sequence that contained an originally non uniform distribution of halite 2 it is an erosion surface that existed prior to deposition of the evaporite sequence or 3 it developed in response to buckle folding of the stratigraphic layer overlying the evaporite sequence

vertical and fold axis parallel extension within a slate belt in a transpressive setting northern appalachians abstract an incremental and finite strain study of a mid paleozoic slate belt in a transpressive setting in gaspe canadian appalachians has been undertaken using syntectonic fibers from pyrite type pressure shadows regionally strain estimates are quite consistent and reflect only slightly higher strain in the central part of the anticlinorial structure defining the slate belt extensions of up to 160 vertically and 110 parallel to the fold axis were recorded in the slate belt combined with an estimated overall regional shortening of 80 the fibers first rotated toward the final position of the cleavage within a vertical section normal to the cleavage and then rotated from a vertical to a horizontal orientation this change in fiber orientation observed in cleavage parallel sections indicates an abrupt change in the incremental stretching direction from sub vertical to sub horizontal fold axis parallel the vertical extension is related to folding and cleavage development during coaxial flattening the horizontal extension reflects the strain imposed on the rocks during regional simple shear which produced strike slip faulting and further tightening of the folds folding cleavage development fold axis parallel extension and faulting are all related to oblique convergence during the middle devonian acadian orogeny in this part of the canadian appalachians and clearly indicate an overall transition from a pure shortening deformation to a simple shear dominant deformation in an overall transpressive setting

porphyroblast non rotation during crustal extension in the variscan lys caillaouas massif pyrenees abstract the dominant foliation s 2 in the variscan lys caillaouas massif central pyrenees france spain formed in a subvertical orientation during crustal thickening d 2 subsequent non coaxial crustal extension produced a subhorizontal crenulation cleavage s 3 whereby s 2 was folded and rotated andalusite staurolite cordierite and biotite porphyroblasts grew very early during this extension d 3 and included s 2 as straight curved and weakly crenulated inclusion trails these inclusion trails exhibit a strong subvertical preferred orientation which is independent of the local magnitude of d 3 strain the dip angle of the external main foliation s 2 or the aspect ratio and shape orientation of porphyroblasts this indicates that porphyroblasts did not rotate in the d 3 deformation field and hence preserve the orientation of s 2 at the time it was trapped in the porphyroblast relative to the bulk d 3 flow plane porphyroblasts could maintain a stable orientation in the d 3 flow due to complete accommodation of the bulk flow vorticity by heterogeneous shear strain the lack of vorticity of local volumes of zero shear strain such as porphyroblasts was balanced by concentration of shear strain in planar zones anastomosing around them it is likely that s 2 maintained a subhorizontal orientation throughout the d 3 crustal extension which would imply that porphyroblasts also remained stationary relative to the earth s surface during d 3 whereas s 2 in the deforming matrix experienced shear induced rotation a late brittle ductile folding event d 4 involving differential rigid block rotations dispersed the orientation of inclusion trails to some degree these late rotations could be corrected for by artificial unfolding of the f 4 folds

thermochronology of a convergent orogen constraints on the timing of thrust faulting and subsequent exhumation of the maladeta pluton in the central pyrenean axial zone abstract the pyrenees mountains of spain and france are a collisional orogen cored by a south vergent antiformal stack of paleozoic basement thrust sheets the axial zone thrusting accommodated shortening due to convergence between iberia and europe granitic plutons intruded during the hercynian orogeny form an important component of the basement units thermochronology of granodioritic samples collected from a vertical age elevation profile from the south central maladeta pluton provide new constraints on the timing of thrust faulting and rock exhumation of the pyrenean orogen the maladeta massif lies within the orri thrust sheet presently occupying the immediate footwall of the gavarnie thrust a major axial zone alpine age thrust fault new biotite and k feldspar 40ar 39ar and apatite u th he ahe data combined with previous apatite fission track aft thermochronology and structural studies of the maladeta pluton constrain the thermal evolution of this massif biotite weighted mean plateau and k feldspar maximum 40ar 39ar ages from the highest elevations of the maladeta pluton 2850 m are 280 ma close to the age of intrusion and are interpreted to date the timing of rapid cooling in the hercynian subsequently the maladeta massif remained close to the surface as indicated by the presence of unconformable triassic continental sediments k feldspar 40ar 39ar multiple diffusion domain thermal models from two samples at lower elevations 1780 m and 1400 m indicate heating initiated at 65 ma reaching maximum temperatures of 270 to 280 c these data are interpreted to record thrust burial of the maladeta pluton to depths of 6 10 km in the footwall of the gavarnie thrust burial and heating were followed by a period of relatively rapid cooling beginning at 50 ma recorded in three k feldspar thermal models from samples at different elevations 2850 m 1780 m and 1400 m aft thermal models in conjunction with the aft and apatite ahe age elevation profile interpretation reveal a lower temperature portion of the thermal history these data indicate the onset of rapid cooling that began at 30 35 ma slowed dramatically at 25 30 ma and continued at a reduced rate until 15 ma finally aft thermal models from the lowest elevation samples record rapid cooling in the late miocene pliocene most likely due to re excavation of the southern flank of the pyrenean orogen following burial by syn tectonic conglomerates the integrated thermochronologic data set from the age elevation profile within the maladeta pluton illustrates the potential for obtaining age constraints related to processes operating within the interior portions of convergent orogenic systems e g the onset and or duration of thrust fault activity that are otherwise unobtainable

conjugate shear folding a model for the relationships between foliations folds and shear zones abstract microstructural mapping of whole thin sections cut from two samples of micaschist containing cm scale folds plus garnet porphyroblasts has provided new insight in the relationships between folding shearing and foliation development the garnets exhibit coherent inclusion trail patterns that place important constraints on the kinematic development of both samples which are shown to be representative of coaxial versus non coaxial deformation in rocks containing a pre existing schistosity a comparison of crenulations cleavages geometries in both samples and a review of the geometry of natural and experimental multilayer folds leads to the conclusion that folding involves conjugate shearing at different scales at microscopic scales crenulation cleavages nucleate as conjugate kink or shear instabilities and develop further as a function of the macroscopic partitioning of deformation in fold hinge domains bulk coaxial deformation results in equal development of conjugate crenulations that progressively coalescence into symmetrical crenulation patterns so that macroscopically parallelism is achieved between foliation fold axial planes and long axes of strain ellipses fold limb domains represent a system of conjugate shear zones where single sets of crenulation instabilities with synthetic shearing component preferentially develop producing oblique relationships between the aforementioned elements cleavage fanning is inferred as a direct consequence of this conjugate shear origin of folds the model implies that crenulation cleavages and s c fabrics in shear zones form by analogous processes in both cases involving a component of shearing along foliation planes the development of conjugate sets of foliation planes surrounding porphyroblasts during early relatively coaxial stages of deformation explains continued gyrostatic behaviour during more advanced non coaxial stages as indicated by consistently oriented inclusion trails in the studied samples

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

kinematics of decollement folding in the lost river range idaho decollement folding of massive 2 m thick carbonate beds of the scott peak formation within the willow creek anticline northern lost river range idaho was associated with the development of thin 0 25 m widely spaced deformation zones incremental strain histories determined from fibrous overgrowths in strain fringes from the deformation zones provide a basis for kinematic analysis of the anticline plane strain is indicated by coaxial up dip extension on cleavage xy planes and a lack of extension in the y direction in xz sections coaxial incremental strain histories characterize fold hinges and document pinned hinges during folding the sense of rotation on opposing limbs indicates that the fiber curvature records primarily the spin of layers through fixed moderately steep 66 12 dg l diverging extension directions the magnitude of the external rotation for each sample is typically less than the bedding dip suggesting that the external rotation also includes a component of internal rotation related to layer parallel shear toward the upper flat panel or the hinge pin line the magnitude of this internal rotation varies within and between layers

computer experiments to investigate complex fibre patterns in natural antitaxial strain fringes antitaxial non deforming strain fringes from lourdes france show complex quartz calcite and chlorite fibre patterns that grew around pyrite in a slate during non coaxial progressive deformation development of these fringes was modelled using a computer program fringe growth 2 0 which can simulate incremental growth of crystal fibres around core objects of variable shape it uses object centre paths as input which are obtained from fibre patterns in thin section the numerical experiments produced fibre patterns that show complex intergrowth of displacement controlled face controlled and intermediate fibres similar to those in the natural examples the direction of displacement controlled growth is only dependent on the relative movement between core object and fringe so that core object rotation with respect to the fringe influences the fibre patterns and produces characteristic asymmetric fibre curvature object centre paths should be used for kinematic analysis of strain fringes instead of single fibres since these paths represent the fringe as a whole the length along the path can be interpreted in terms of finite strain and path curvature in terms of rigid body rotation of fringes with respect to an external reference frame

thrust related strain gradients and thrusting mechanisms in a chevron folded sequence southeastern australia an e directed imbricate fan thrust system within a chevron folded monotonous quartz rich turbidite succession shows marked gradients in strain from the upper to lower parts of individual thrust sheets upper parts of thrust sheets show low to moderate strains xz strains 30 1 tight inclined chevron folds ila 30 with predominantly w dipping axial surfaces intense phyllitic cleavage with a strong down dip mineral stretching lineation and long curved tapering quartz pressure shadows on pyrite such differences in fold geometry the intensity of the cleavage and the geometry of quartz fibres in pressure shadows require thrust sheet emplacement to be synchronous with the internal deformation of sheets thrust sheets show a change from an approximately constant volume plane strain coaxial deformation in their upper parts to non coaxial high strain deformation with incremental i between those expected for pure shear and simple shear i e 0 i 5 45 in their basal parts observed geometries and intensities reflect partitioning of strain and deformation mechanisms within individual thrust sheet volumes both during the initiation and propagation of subsurface detachment faults and with their emplacement to shallower crustal levels strain softening related to chevron folding and more intense cleavage development within a 500 m basal zone has controlled the emplacement of these turbidite dominated thrust sheets transport and internal deformation of the thrust sheets did not involve gravitationally induced spreading but required either a push from the rear or some form of underthrusting

modeling of thrust fronts above ductile and frictional detachments application to structures in the salt range and potwar plateau pakistan series of scaled sandbox models are used to simulate the development of thin skinned simultaneous shortening above adjacent ductile and frictional substrates these models simulate the evolution of the potwar plateau and salt range in pakistan where pale

cape st marys shear zone and the halifax group rockville notch group disconformity southwestern nova scotia structural development and tectonic significance the cape st marys shear zone situated in a corridor of alleghanian reworking in the southwestern meguma terrane contains the deformed discordant contact of lower ordovician slate of the halifax group with the silurian white rock formation close to the contact the alleghanian cleavage s2 is parallel to the contact in both units with s0 in the white rock formation metavolcanic rocks and halifax slate parallel to and discordant to the contact the geometry of deformed neoacadian minor folds quartz fringes on sulphide grains and micro porphyroclasts demonstrate thrust sense shear white rock formation over halifax slate pure shear and volume loss are inferred as components of the strain path from s2 microstructure and estimates of strain in the halifax slate estimates of shear strain imply moderate displacements within the cape st marys shear zone during deformation of the northwestern limb of the cape st marys syncline the discordant contact of the halifax slate with the white rock formation

seismic models of detachment and faulted detachment folds abstract detachment folds constitute a common structural style in fold thrust belts and typically form in stratigraphic packages defined by relatively competent cover units underlain by ductile units pstm images through these structures contain artifacts resulting from lateral velocity variations between the cover sediments and the core units steep dips and complex fold fault relationships making them difficult to interpret understanding these artifacts and pitfalls in pstm seismic models enables improved interpretations of pstm data in natural structures we conducted 2d seismic modeling for detachment folds models to study the effect of these complexities on pstm data seismic pull ups and push downs of the strata underneath the salt or shale substrate are related to the relative velocities of the cover sediments and the core units a specific example of an appalachian plateau structure demonstrates how the relative velocities can influence the seismic image progressive evolution of the structures to disharmonic and lift off folds results in enhanced pull ups and poorer imaging of steep limb segments for faulted detachment folds faulted limbs and footwall zones are poorly imaged and marked by wide low reflectivity bands making fault interpretation difficult

development of antitaxial strain fringes during non coaxial deformation an experimental study strain fringes were modelled experimentally around wooden objects in a simple shear box containing polydimethylsiloxane pdms the fringes were made of paraffin wax that was periodically poured into dilated sites next to the core object and left to crystallise in the experiments fringes and core object rotated at similar decreasing rotation rates relative to extensional incremental stretching axes isa of flow and rotated relative to each other by small amounts fringes did not necessarily open parallel to isa due to interactions between fringes and core object therefore neither displacement controlled fibres nor object centre paths can be expected to record the exact orientation of the extensional isa but only give estimates existing methods to calculate finite strain were applied to the experimental fringes these methods generally underestimate the actual finite shear strain this study illustrates that different final fringe shapes can develop in the same shear zone depending on core object shape and its initial orientation with respect to the shear zone boundary a classification of final fringe shapes is presented based on the initial orientation of elongate core objects

the pyrite type strain fringes from lourdes france indicators of alpine thrust kinematics in the pyrenees a new method is developed to reconstruct polyphase deformation histories from pyrite grains with displacement controlled fibrous strain fringes it decomposes the relative displacement between a pyrite grain and a fringe into incremental rotations and pull apart directions application to a classic specimen from lourdes france shows that the rotations determine the fibre curvature sense and strongly depend on local strain parameters as well as the initial orientation and shape of pyrite grains in contrast pull apart directions appear to be relatively constant and straight although curved pull apart trajectories were expected from the heterogeneous non coaxial deformation in the matrix the discrepancy is due to the localization of shear strain components around pyrite fringe complexes the complexes themselves represent ellipsoidal domains of relatively coaxial strain that parallel a regional crenulation cleavage even in zones lacking this cleavage due to stretching and shearing of a pre existing foliation s1 thus strain fringes consistently record bulk shortening orientations regardless of local variations in strain parameters and microstructural development the analysed pyrite fringe complexes from lourdes were previously considered as classic shear sense indicators but yield hook shaped pull apart trajectories in the new method this indicates two deformation events that can be related to a sub horizontal s2 and steep regional crenulation cleavage s3 the recorded polyphase deformation history is tentatively interpreted in terms of a change from gravitational spreading to a compressive emplacement of a thrust sheet in the north pyrenean foreland

probing the prodigious strain fringes from lourdes abstract we investigate the kinematics of classic sigmoidal strain fringes from lourdes france and review previous genetic models strain methods and strain rates for these microstructures displacement controlled quartz and calcite fibers within the fringes yield an average strain of 195 with the technique of ramsay and huber 1983 this agrees well with strains measured from boudinaged pyrite layers and calcite veins in the same rocks but conflicts with ca 675 strain in previous analogue models for the studied strain fringes produced by progressive simple shear we show that the detailed geometry and orientation of fiber patterns are insufficiently explained by simple shear but imply two successive differently oriented strain fields although all strain fringes have the same overall asymmetry considerable morphological variation resulted from different amounts of rotation of pyrite grains and fringes minor rotation led to sharply kinked fibers that record a ca 70 rotation of the kinematic frame larger up to 145 rotations accommodated by antithetic sliding on pyrite fringe contacts produced more strongly and smoothly curved fibers combined with published rb sr ages for the studied microstructures our new strain data indicate an average strain rate of 1 41 10 15 s 1 during ca 37 myr continuous growth

tectonic evolution of the montagne noire and a possible orogenic model for syncollisional exhumation of deep rocks variscan belt france structural analysis in the montagne noire including a study of inclusion trail patterns in andalusite garnet and staurolite porphyroblasts reveals a succession of five foliations in alternately steep and flat lying orientations this is inferred to reflect multiple gravitational collapse phases that intervened during continuous plate convergence initial crustal thickening by foliation development and folding d1 was responsible for eclogite facies metamorphism in the gneissic basement subsequent thrusting and recumbent folding d2 generated a subhorizontal crenulation cleavage during namuro westphalian times while previous steep structures became rotated petrological and microstructural evidence indicate that thrust nappes were emplaced by a gravitational spreading mechanism while undergoing internal thinning an important amount of basement exhumation took place synchronously these observations form the basis for a new orogenic model in which thermal reequilibration of a previously thickened orogen induces gravitational instability in the upper crust collapse of the upper crust causes it to spreadout horizontally over continuously converging relatively cool plates thereby creating low angle thrust nappes the upper crustal thinning facilitates simultaneous extrusion of lower crust in vertical root zones at the orogen center which on reaching the higher part of the orogen can be incorporated in the nappes and displaced horizontally following such a mechanism of nappe tectonics renewed horizontal shortening d3 produced the montagne noire gneissic dome by upright folding of nappe related foliations with orogen parallel stretching previous crustal thinning had led to an increased heat flux between basement and cover so that during d3 cover sequences were still rising in temerature while basement rocks had already commenced their retrograde path during d2 peak metamorphic conditions in the cover triggered a second gravitational collapse during namurian times d4 producing a 2 km wide subhorizontal shear zone in which orogen parallel stretching continued at the surface deformation was accommodated by decollements normal faulting and basin formation gneissic rocks below the shear zone were not affected by the collapse because a stronger rheology and horizontal shortening was probably continuous here retrograde cooling of the cover restored gravitational equilibrium and caused its renewed upright folding during late variscan times d5 permian postorogenic extension d6 oriented at a high angle to the previous orogen parallel stretching completed exhumation of the gneiss dome the dome structure of the montagne noire metamorphic core complex was acquired during compressional tectonics and later modified flattened by synorogenic and postorogenic extensional events a similiar polyphase origin appears not to be excluded for other metamorphic core complexes with poorly constrained preextension histories

the determination of progressive deformation histories from antitaxial syntectonic crystal fibres abstract antitaxial syntectonic crystal fibres are modelled here as mechanically passive features the fibres representing displacement paths forming within a homogeneously deforming material evidence that many fibres develop in this way is widespread asymptotic curvature thinning of fibres and strained old fibres a measuring technique is proposed by which the complete deformation history excluding translation may be determined the method involves the iterative unstraining of two complete fibres whose development began and ended simultaneously a source of error is introduced by assuming that each of the two fibres developed at constant proportional rates although synthetic examples suggest this error to be minimal it is suggested that the kinematics of non passive fibres are unobtainable since the rheologies and boundary conditions of the material involved are relatively unknown and cannot be determined from geometry alone analysis of natural fibres found within the ordovician slates in new jersey u s a indicates that volume losses of up to 50 may occur during the formation of some slates general constraints concerning the geometrical analysis of syntectonic crystal fibres are briefly discussed and it is concluded that deformation histories must be interpreted with some modicum of caution

isotopic dating of strain fringe increments duration and rates of deformation in shear zones the time scales over which deformation in the earth9s crust remains localized in shear zones are poorly known as are the associated strain rates we have determined the longevity and rates of deformation using rubidium strontium rb sr microsampling dating of increments of fibrous strain fringes from a pyrenean shear zone the fibers grew quasi continuously through a protracted deformation history between 87 and 50 million years ago over a period comparable to that of an orogeny during a short interval between 66 and 62 million years a rise in strain rate from 1 1 10 15 to 7 7 10 15 seconds 1 occurred this acceleration correlates with an abrupt change in fiber growth direction and a stress field inversion from gravitational collapse to renewed horizontal crustal shortening

strain fringes and strain shadows at kamoa drc implications for copper mineralisation abstract the formation of strain shadows and strain fringes is described from diamictite and siltstone of the grand conglomerat unit nguba group at the kamoa copper deposit located within the drc at the western end of the central african copperbelt cacb fringes reveal a straight orientation of fibres during a coaxial deformation this supports the interpretation of a single phase of deformation during fringe growth within the deposit area the non deformed distal part of fringes and the straight orientation of fibres demonstrate the simple geometry of strain fringes quartz grains within strain fringes and strain shadows have generally straight or smoothly curved boundaries and lack undulose extinction in strain fringes muscovite and chlorite are generally observed on quartz grains where they define a foliation these characteristics suggest a static recrystallisation that developed under relatively high temperature and wet conditions a nw crustal shortening during the kolwezian deformation event of the lufilian orogeny is believed to be responsible for the coeval development of foliation strain shadows and strain fringes a syn orogenic origin of the copper sulphides during the lufilian orogeny is proposed