a history of cleavage and folding an example from the goldenville formation nova scotia temporal relationships between cleavage formation and folding in sedimentary rock sequences can vary from either phenomena preceding the other to both occurring simultaneously knowledge of this relationship however is required to understand fully the relationship of cleavage orientation to finite strain and the origin of cleavage refraction across competent incompetent layers evidence from the goldenville formation provides an example of cleavage formed before folding by 50 pressure solution removal of material resulting in cleavage planes normal to bedding during active folding this layer parallel shortening was accompanied by excess pore pressure that in some cases exceeded lithostatic pressure causing formation of bedding parallel quartz veins with crack seal textures folds developed by a mechanism of differential flexural flow with no loss of coherence across bedding surfaces en echelon veins formed sequentially across some beds during active folding and subsequently were deformed by various increments of bedding parallel shear after folds reached 80 limb dips continued horizontal shortening was accommodated by additional pressure solution but no recognizable new cleavage was formed because cleavage in the goldenville formation formed perpendicular to bedding before folding and was passively rotated and extended by bedding parallel simple shear during fold amplification the resulting cleavage planes cannot lie in the xy plane of the finite strain ellipsoid differential shear between metagray wacke and slate beds accounts for the observed cleavage refraction and the bedding cleavage angle provides a good estimate of the shearing across these beds during active folding if cleavage can be shown to postdate folding in other examples it is possible that cleavage planes will lie in the xy plane of the finite strain ellipsoid

alleghanian reactivation of the acadian fold belt meguma zone southwest nova scotia shear zones and northwest verging folds define a 30 km wide belt of deformation that overprints the acadian fold belt and telescopes isograds in the meguma zone in southwest nova scotia the shear zones appear to form a linked system that accommodated convergence dominated transpression of the meguma zone against an irregular avalon boundary available geochronological data indicate a mid carboniferous alleghanian variscan age for the overprinting deformation the mid carboniferous basement reactivation in southwest nova scotia is likely coeval with deformation of carboniferous strata and reactivation of basement structures meguma group in the northern meguma zone together these mid carboniferous structures may define a wide belt of alleghanian variscan deformation across the northwest cratonward margin of the meguma zone

cambrian ordovician acritarchs in the meguma terrane nova scotia canada resolution of early paleozoic stratigraphy and implications for paleogeography organic walled microfossils acritarchs provide age constraints for the previously poorly dated cambrian and lower ordovician turbiditic goldenville and halifax groups in the northern appalachian meguma terrane of nova scotia and address controversies about stratigraphy and provenance the oldest exposed formation of the 8 km thick goldenville group contains the trace fossil oldhamia which suggests an age of late early cambrian whereas the coticule bearing uppermost formation yielded an acritarch species consistent with middle cambrian epoch 3 age the conformably overlying 5 km thick halifax group includes basal pyritiferous units that yielded a late cambrian furongian assemblage of acritarch species providing further confirmation that the underlying manganese bearing formations are of cambrian epoch 3 age and not ordovician as recently claimed overlying nonpyritiferous formations contain the early ordovician graptolite rhabdinopora flabelliformis flabelliformis and acritarchs of similar age samples collected up section from the graptolite occurrence yielded acritarch species that are indicative of the later tremadocian and floian the new fossil data confirm a gap in age of 30 m y between the halifax group and the overlying silurian to lower devonian rockville notch group the new ages are consistent with stratigraphic units defined in regional mapping and support recent interpretation of fundamental differences in depositional and tectonic environments between the northwestern and southeastern parts of the meguma terrane they also provide new constraints on paleogeography of the terrane by confirming age similarities to stratigraphy in north wales with which correlation has been proposed previously based mainly on lithological similarities

petrology age and tectonic setting of the white rock formation meguma terrane nova scotia evidence for silurian continental rifting the white rock formation in the yarmouth area of the meguma terrane of southern nova scotia consists mainly of mafic tuffaceous rocks with less abundant mafic flows epiclastic and clastic sedimentary rocks and minor intermediate and felsic crystal tuff it is divided into seven map units that appear to young from west to east inconsistent with a previously assumed synclinal structure the white rock formation is flanked on both northwest and southeast by mainly the cambrian to lower ordovician halifax formation the western contact is interpreted to be a sheared disconformity whereas the eastern contact appears to be a major brittle fault and shear zone that juxtaposes different crustal levels the granitic brenton pluton forms a faulted lens within the eastern shear zone a felsic tuff from the upper part of the white rock formation yielded a u pb zircon age of 438 3 2 ma identical within error to published ages for the brenton pluton and felsic volcanic rocks near the base of the white rock formati

meguma terrane orocline u pb age and paleomagnetism of the silurian mavillette gabbro nova scotia canada

mass transfer and preferred orientation development during extensional microcracking in slate belt folds elura mine australia microstructures in slate belt rocks at the elura mine near cobar south eastern australia indicate that volume loss by syntectonic dissolution is coupled with mass accretion by reprecipitation of the dissolved material in dilational sites the mass accretion is sustained primarily by repetitive tensile microfracturing at high pore fluid pressures oriented growth in the inter and intragranular microcracks is locally host controlled creating lattice and shape preferred orientations the grain scale crack seal features throughout the rock reflect rhythmic fluid pressure fluctuations a balance is achieved between the fracture induced permeability and consequent flushing rates and the rate of fluid build up in a relatively sealed environment instability in the balancing factors can lead to localization and intensification of tensile failure and hence tension vein formation in the grain aggregate growth of veins by crack seal also reflects a steady state but with more localized fluctuations of fluid flow on the aggregate scale still larger imbalances between flushing and fluid accumulation i e pressure variations induce breccia veining the larger pressure gradients over greater distances associated with dilation localization from pervasive microfracturing to spaced breccia domains allow fluid channelling with an increased potential for chemical fluid rock disequilibrium therefore large breccia vein systems tend to be sites of extensive fluid rock interaction and replacement as spectacularly illustrated by the syntectonic sulphide orebodies at elura the huge amounts of silicate carbonate and sulphide accumulated during folding at elura illustrate the large scale of source and sink couples possible in solute mass transfer

finite strain heterogeneity and volume loss in slates of the taconic allochthon vermont u s a abstract slates of the taconic allochthon have ellipsoidal reduction spots which have been used to determine the finite state of strain at seven sites the strain is highly heterogeneous at all scales ranging from several centimeters to many kilometers variations in x y ratios is small but variations in y z ratios is large and this pattern is seen at all scales the heterogeneity can be used to determine a strain path which is nearly horizontal on a flinn diagram we propose that uniaxial flattening associated with late stage cleavage development was accommodated by volume loss and that the most highly strained sites have experienced an average 55 volume loss at a minimum explaining our results without volume loss requires a subhorizontal extension and creates strain compatibility problems consideration of the strain history of these rocks suggests that folding related strains have been modified by later cleavage related events the last of which was uniaxial flattening accommodated by volume loss the finite strains are reflective almost exclusively of the latest cleavage producing strains

volume loss during cleavage formation in the meguma group nova scotia canada abstract diffusive mass transfer pressure solution is a widely recognized cleavage forming mechanism under low grade conditions the amount of accompanying volume change during cleavage development however is difficult to quantify and remains controversial the distance between sites of mineral dissolution and reprecipitation and transport mechanisms also are controversial the structural history of the cambrian ordovician meguma group nova scotia canada is well known and abundant strain markers provide an opportunity to assess geometrically volume changes accompanying cleavage development during diagenesis about 70 compaction in muds and 15 in silts and sands occurred the pelitic to psammitic meguma group was cleaved prior to being folded pressure solution was the dominant cleavage forming deformation mechanism under chlorite grade metamorphic conditions during the early devonian acadian orogeny measurement of tectonic strain in principal directions x y z shows that bedding parallel coaxial shortening z is 60 and there is no evidence of fold hinge parallel extension y estimation of tectonic vertical extension x in the cleavage plane is complicated by diagenetic compaction corrections to primary strain markers but is insufficient to compensate for the observed 60 shortening in z indicating 40 60 volume loss during cleavage formation because cleavage formed at least 50 ma after deposition and under approximately 6 km of silurian to early devonian strata porosity of the sediments upon onset of cleavage was likely less than 15 pore space reduction therefore cannot account for the apparent volume loss during cleavage formation cleavage related volume loss applies regionally and to a stratigraphic thickness of several thousand meters implying that a large amount of material was removed and transported large distances when combined with 50 shortening by folding a bulk tectonic shortening of 80 is estimated for the meguma group during the acadian orogeny

polyphase late paleozoic tectonothermal evolution of the southwestern meguma terrane nova scotia evidence from 40ar 39ar mineral ages 40ar 39ar incremental release ages of hornblende muscovite and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late paleozoic tectonothermal evolution for the southwestern meguma terrane regional d1 folding with cleavage formation under greenschist lower amphibolite facies m1 metamorphic conditions occurred at ca 400 410 ma and was followed by emplacement of a series of granitic stocks ranging in age between ca 375 and 315 ma these were emplaced at relatively shallow crustal levels and developed contact metamorphic aureoles of variable grade these are locally superposed on m1 regional metamorphic assemblages and result in a complex isograd pattern 40ar 39ar mineral ages suggest episodes of contact metamorphism occurred at 1 360 375 ma possibly related to emplacement of the south mountain batholith or temporal equivalents 2 350 356 ma around the port mouton pluton and northeastern shelburne pluton 3 ca 315 325 ma near the wedgeport pluton and in

the role of fluids in syntectonic mass transport and the localization of metamorphic vein type ore deposists abstract in low to medium grade metamorphic environments the mobilization of species is closely associated with the operation of solution precipitation creep processes during foliation development such deformation processes involve dissolution of material from source sites which are usually discrete dissolution surfaces having orientations controlled by the applied stress and displacement fields mass transfer occurs via solute diffusion in an essentially static fluid network or by transport of dissolved species in a migrating fluid precipitation of material removed from dissolution sites occurs in a variety of sink sites including pore surfaces grain scale microcracks and veins sink sites may be close to or distantly removed from source sites the transport of material during deformation involving solution precipitation creep is largely controlled by the mobility of metamorphic fluids in regions of high fluid mobility significant mass transfer may occur over large distances fluid flow mass transfer paths and the development of sink sites in such cases are constrained by the structurally controlled development of high fluid pressure domains and the consequent mechanical enhancement of permeability by hydraulic fracture processes and the development of grain scale dilatancy these processes may result in focussed fluid flow and precipitation of mobilized species in restricted regions a proportion of syntectonically mobilized material is precipitated to form vein type deposits in hydraulic fracture arrays which develop in high fluid pressure environments the microstructures developed in minerals deposited in these sites reflect the interaction of a range of nucleation and growth processes which usually involve multiple generations of fracture growth and mineral crystallization the formation of syntectonic gold quartz vein deposits in central victoria australia illustrates the importance of structural controls on both the large scale migration of metamorphic fluids and the development of sites suitable for the precipitation of metamorphically mobilized species

deformed graptolites finite strain and volume loss during cleavage formation in rocks of the taconic slate belt new york and vermont u s a abstract an analysis of graptolites in the taconic slate belt of eastern new york and western vermont u s a shows that they are nearly ideal strain markers for the three species used in this study orthograptus whitfieldii orthograptus calcaratus and climacograptus bicornis the spacing of thecae is constant except for the first five or so thecae in the proximal part of the fossil rhabdosome further the thecal apertures are perpendicular to the long axis of the stipe observations of the thecal spacing in deformed rocks leads to a determination of extension e l f l 0 l 0 and a measure of the angle between thecal aperture and stipe axis yields a direct determination of angular shear strain in practice we find it is most straightforward to use length changes to determine the magnitude of principal strains in taconic slates e 1 ranges from 1 0 to 0 24 e 2 ranges from 0 23 to 0 43 and e 3 ranges from 0 56 to 0 74 thus we find that the absolute finite strain in these slates is constrictional at three sites plane strain at another and a true flattening at only one site an examination of volume changes based on strain results in determinations of between 81 volume loss and 7 volume gain with volume losses between 28 and 81 in 9 out of 10 calculations these conclusions are in accord with previous determinations of volume loss based on reduction spot analyses and are consistent with the observation that pressure dissolution was a common grain scale deformation process in cleavage formation but that these slates lack abundant veins fibrous overgrowths or other identifiable sites of reprecipitation

asymmetric slip partitioning in the sea of marmara pull apart a clue to propagation processes of the north anatolian fault between 1939 and 1999 the north anatolian fault naf experienced a westward progression of eight large earthquakes over 800 km of its morphological trace the 2000 km long north anatolian transform fault has also grown by westward propagation through continental lithosphere over a much longer timescale 10 myr the sea of marmara is a large pull apart that appears to have been a geometrical mechanical obstacle encountered by the naf during its propagation the present paper focuses on new high resolution data on the submarine fault system that forms a smaller pull apart beneath the northern sea of marmara between two well known strike slip faults on land izmit and ganos faults the outstandingly clear submarine morphology reveals a segmented fault system including pull apart features at a range of scales which indicate a dominant transtensional tectonic regime there is no evidence for a single continuous purely strike slip fault this result is critical to understanding of the seismic behaviour of this region of the naf close to istanbul additionally morphological and geological evidence is found for a stable kinematics consistent both with the long term displacement field determined for the past 5 myr and with present day anatolia eurasia motion determined with gps however within the sea of marmara region the fault kinematics involves asymmetric slip partitioning that appears to have extended throughout the evolution of the pull apart the loading associated with the westward propagation process of the naf may have provided a favourable initial geometry for such a slip separation

strain localization in the sea of marmara propagation of the north anatolian fault in a now inactive pull apart 1 the sea of marmara is presently the site of pure dextral strike slip faulting that connects the izmit fault in the east to the ganos fault in the west along a linear strike slip fault that is part of the northwestern branch of the north anatolian fault this active main marmara fault mmf cuts across a succession of depocenters and highs analysis of a large data set that includes multichannel seismic lines sparker and deep towed high definition seismics shows that most of the n120 e trending normal faults framing these depocenters are now inactive as they are sealed by an average thickness of 300 m of sediments the highs correspond to a series of ne sw trending anticlines and reverse faults blanketed by a thin cover of undeformed sediments we interpret these structures as inverted former transfer faults of the now inactive sea of marmara pull apart system depocenters and highs are the relicts of a former pull apart basins and transfer faults system that accommodated the north anatolian fault motion within the sea of marmara prior to the formation of the mmf we propose that a relatively short a few hundreds of thousands of years period of basin inversion accompanied the localization of strain along the present main marmara fault until it was fully formed 200 100 kyr ago the preexisting pull apart basin system accounts for about 30 km of motion along this fault system and thus probably appeared sometime in late pliocene or early pleistocene as a reactivated portion of the old thrace basin during the plio quaternary the sea of marmara has been the site of progressive localization of strain

when do dragfolds not develop into sheath folds in shear zones abstract using a unified model for high strain zones rotation paths of material lines in different flow types are investigated and presented as stereographic projections a material fabric element such as a mineral lineation or a fold axis can be plotted on such stereograms and the rotation path revealed using this method we examine the rotation of folds and the development of sheath folds in general three dimensional zonal deformation we show that the evolution of dragfold geometry in a high strain zone depends on the flow type and the initial perturbation of the fold axis in simple shear zones and thickening zones monoclinic or triclinic dragfolds will evolve into sheath folds in monoclinic thinning zones with biaxially stretching boundaries if the shear direction is parallel to the maximum stretching direction of the boundary sheath folds will develop if the shear direction is parallel to the minimum stretching direction of the boundary dragfolds become more cylindrical as deformation advances in thinning zones with boundaries equally stretched in all directions the likelihood of sheath fold development depends on the ratio of the shear strain rate to the thinning rate in triclinic thinning zones fold evolution depends on the details of the flow and the value of initial fold axis perturbation

fission track thermochronology vertical kinematics and tectonic development along the western extension of the north anatolian fault zone 1 we have investigated the low temperature history of pre neogene basement areas adjacent to the western extension of the north anatolian fault zone nafz by apatite fission track thermochronology of 33 samples taken from marmara island kapidag peninsula both in the sea of marmara samothrake island and chalkidike peninsula both in the north aegean region together with already published apatite fission track data from 30 other sampling localities of the region these data 63 in total have been evaluated mainly with regard to the transitional period from the mesohellenic orogeny eocene oligocene to the plate tectonical individualization of the aegean antolian microplate to gain a better understanding of the tectonical chronology that has led to modern east mediterranean plate tectonics apatite fission track investigations of the area under discussion reveal two major events of cooling and exhumation of pre neogene rocks a first frequency maximum of cooling ages between 35 and 25 ma late eocene to early oligocene is due to postorogenic regional erosion after the climax of the mesohellenic orogeny which was caused by continental collision of the pelagonian platform and the rila rhodope zone after the final closure of the axios vardar ocean a second frequency maximum of cooling ages between 17 and 11 5 ma early to middle miocene depicts more localized uplift and exhumation in consequence of transpressive and or transtensive movements along early structural discontinuities that gave rise to the later fault systems in the western extension of the nafz the earliest fault structures of this system have developed shortly after the orogenic collapse and crustal extension of the rila rhodope area but quasi simultaneously with the onset of subduction along the modern hellenic trench the regional distribution of fission track ages of the miocene age group 17 to 11 5 ma and corresponding thermochronological calculations based on frequency distributions of confined track lengths suggest that an early fault propagation has occurred from east to west during the miocene the onset of subduction along the hellenic trench and subsequent gravitational retreat of the subducting oceanic slab are considered as a major cause for transtension of the aegean crust especially with regard to the development of north aegean pull apart basins such early fault structures have facilitated the westward propagation of a continuous nafz that took place during the pliocene from the easternmost marmara region to the gallipoli peninsula and to the north aegean trough

diffusion creep modelling of fibrous pressure shadows abstract grain scale deformation of a bimineralic composite by normal stress driven grain boundary diffusive mass transfer has been modelled using diffusion creep software that for the first time allows different diffusion coefficients for interphase boundaries and grain boundaries as well as for different components by selecting suitable coefficients we have obtained a fully coupled mechanical and mass transfer numerical solution for the formation of fibrous pressure shadows or strain fringes at the interface between a rigid object and a deforming rock matrix of different compositions the model is two dimensional and run in pure shear velocity mode to a stretch ratio of 1 4 the first results are reported which show dissolution and face controlled antitaxial growth of a soluble polycrystalline calcite matrix at the interface with a circular and relatively insoluble pyrite object the ratio of caco3 diffusion along calcite pyrite versus calcite calcite boundaries strongly influences the extent of pressure shadow growth and the pattern of strains in the matrix a low ratio produces little growth significant perturbation and deflection of total strain axes around the pyrite an intermediate ratio produces appreciable growth little perturbation and near parallel total strain axes a high ratio produces larger pressure shadows and total strain axes that bend in towards the pyrite as if it were a weak object sliding and rotation of matrix grains are also affected these results differ fundamentally from purely physical models of ductile flow around a rigid object and demonstrate the influence of interface diffusional properties on the resulting behaviour in particular assumptions such as perturbation of the flow by the object deflection of external foliation around pre deformational porphyroblasts and relationships of pressure shadow strain to bulk strain may need to be re examined in the light of this finding

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

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

u pb zircon ages and provenance of the white rock formation of the rockville notch group meguma terrane nova scotia canada evidence for the sardian gap and west african origin the white rock formation is the lowermost formation of the rockville notch group an assemblage of silurian devonian rocks preserved in five areas along the northwestern margin of the meguma terrane of nova scotia the formation consists mainly of mafic and felsic metavolcanic rocks interlayered with and overlain by marine metasedimentary rocks felsic metatuff has now been dated from four locations near both the bottom and top of the volcanic pile and yielded a narrow age range with errors of about 446 434 ma these dates confirm a 30 ma hiatus after deposition of the early ordovician hellgate formation in the underlying halifax group this hiatus is coeval with the sardian gap in the lower palaeozoic of peri gondwanan europe the metavolcanic metasedimentary assemblage is overlain by mainly metasiltstone with abundant quartzite and metaconglomerate lenses some of the latter were previously interpreted to be ordovician tillite an interpretation no longer viable new detrital zircon data from metas

intracontinental wedging and post orogenic collapse in the mesohellenic trough the mesohellenic trough is a 130 km long and about 30 km wide subsiding area which contains a thick sequence of well exposed late cenozoic post orogenic sediments this intermontane basin located at the contact between the apulian and pelagonian collided margins provides a good example of the characteristics needed to study the chronology of late orogenic intracontinental structures

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

apatite fission track data from central anatolian granitoids turkey constraints on neo tethyan closure 1 apatite fission track age versus elevation profiles and temperature time path modeling indicate an early to middle paleocene 57 62 ma rapid exhumation of the middle to late cretaceous granitoids in central anatolia and an oligocene 28 30 ma rapid exhumation of the eocene kosedag batholith in the ne sivas region part of eastern central pontides the early to middle paleocene rapid exhumation is thought to result from a regional compressional regime following the collision of the eurasian plate and the tauride anatolide platform at the closure of the izmir ankara erzincan branch of the northern neo tethys the oligocene accelerated exhumation of the kosedag batholith is contemporaneous with the oligo miocene closure of the southern neo tethys which juxtaposed the amalgamated eurasian and tauride anatolide platform and the african arabian plate along the bitlis zagros suture in southeast anatolia the compressional regime due to this collision affected a large area between the greater caucasus in the north and northern african arabian plate in the south

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

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

pressure temperature and composition history of syntectonic fluids in a low grade metamorphic terrane analysis of stable isotopes and fluid inclusions in centimeter scale fibrous quartz calcite strain fringes from the taconic thrust belt in vermont and new york allows reconstruc tion of fluid conditions and sources over the course of strain fringe growth oxygen isotope ratios in quartz show regular variations in d 18 o ranging between 19 2 and 20 0 in different parts of strain fringes fluid inclusions have low salinities 2 4 nacl equivalent in all parts of strain fringes and we propose that fluids were derived from clay dehydra tion variations in fluid temperature and pressure derived from stable isotope and fluid inclusion analysis mimic changes in the orientations of fibers indicating a strong corre lation between tectonics and fluids earliest fluids had temperatures of 240 8c temperatures declined over time to 200 8c and rose again to 260 8c fluid pressures began at 1 6 kbar 160 mpa and fell to 0 8 kbar 80 mpa before rising to 2 kbar 200 mpa we interpret these results in the context of thrust faulting and the dynamics of thrust wedges our model begins with thrusting along the bird mountain fault placing rocks from depth on shallower ones and raising the temperature of the footwall increasing temperatures would have stimulated clay dehydration raising fluid pressures and weak ening the rocks within the orogen reduced strength of the taconic accretionary wedge would have required lower critical taper angles stopping motion on the bird mountain fault and stimulating horizontal elongation within the taconic thrust wedge as fluid pressures and temperatures declined increasing strength of the rocks would have de manded higher critical taper angles stimulating renewed bird mountain thrusting and increases in temperature and fluid pressure we cannot uniquely determine the depth of burial during strain fringe growth but we can constrain the depth for different geother mal gradients and evaluate how close fluid pressures were to lithostatic the highest fluid pressures were 70 of lithostatic and could have been higher similar mechanisms tying fluid conditions to tectonics in a feedback loop may be active in accretionary prisms and thrust belts worldwide

40ar 39ar age of shear zones in the southwest meguma zone between yarmouth and meteghan nova scotia the cape st marys cranberry point and chebogue point shear zones lie within a zone of d2 deformation that overprinted d1 fabrics and reactivated f1 folds between the shear zones d2 was accompanied by greenschist facies metamorphism that variably overprinted earlier metamorphism of varying grade a collection of single muscovite grains from pressure shadows defining l2 in a sample from the cape st marys shear zone yields an average apparent age of 316 5 ma the age obtained for these grains from an isotope correlation diagram is 320 5 ma ages for muscovite separates for two samples from the cranberry point shear zone are 325 4 and 319 4 ma an amphibolite from this zone yields a discordant spectrum interpreted to reflect the combined effects of acadian metamorphism and excess argon the muscovite ages approximate the time of syntectonic deformation because muscovite closure temperatures approximate the temperature of metamorphism during d2 thus together with published data from the chebogue

flexural slip folding in the meguma group nova scotia canada abstract flexural slip folding has been established for two anticlines in the meguma group nova scotia flexural slip postdates the main acadian episode of flexural flow fold growth and represents a late brittle ductile reactivation of the fold belt flexural slip structures are dominated by bedding parallel movement horizons but include a linked system of frontal and lateral ramps and conjugate movement horizons slip amount calculated from displacement of discordant quartz veins combined with vein spacing indicates an average local shear strain of approximately 0 5 however because flexural slip occurred when the fold limbs were steep the shear strain accounts for a small change in limb dip 4 8 variation of slip amount with spacing of movement horizons suggests progressive flexural slip was accommodated by continuous initiation of new movement horizons as well as continuing slip on existing movement horizons the presence of quartz veins along flexural slip structures together with evidence for ductile deformation coeval with flexural slip suggests that slip episodes were fluid assisted and intermittent separated by periods of ductile deformation and build up of fluid pressure flexural slip veins in the ovens anticline are auriferous and a flexural slip model may be appropriate for other similar meguma gold deposits thrust sheets up to 10 m thick occurring on a fold limb are interpreted to have originated in flat box fold hinges and to have been placed onto the steep limbs of the chevron folds during the flexural slip episode thrusting may have assisted the transformation of a box fold into a chevron

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

buckle folding and deep crustal shearing of high grade gneisses at the junction of two major high strain zones central gneiss belt grenville province ontario low plunging transport parallel f3 folds are common at all scales in the central gneiss belt of the grenville province but few of these folds are sheath folds where the d1 d2 parry sound shear zone intersects the d3 shawanaga shear zone ssz at a high angle f3 folds formed at several scales centimetre to greater than outcrop scale in layered d1 d2 straight gneisses at the start of their evolution the f3 folds formed just beyond the ssz with hinges near orthogonal to the d3 shear direction and with typical buckle features e g wavelengths vary with layer thickness and hinges are discontinuous and bifurcate the buckle folds evolved within the ssz by rotation of hinges towards the shear direction even though hinges initiated at a high angle to the shear direction sheath folds were not produced in addition to tightening the buckles the ductile reorientation produced thin thick extended shortened limb pairs and very straight ridge like fold hinges and removed small folds from the extended

shear zone related folds folds in ductile shear zones are common structures that have a variety of origins these can be pre existing folds that become modified or folds developed during the shearing event among the syn shearing folds a second subdivision is based on the relative age of the folded surface which can be pre existing or newly formed during the shearing event in each of the three categories final fold geometry and orientation show complex relationships with the kinematic frame the final fold geometry depends on the vorticity within the shear zone the rheology and the initial orientation of the folded surface relative to the kinematic framework it follows that folds are complex structures difficult to use as kinematic indicators however in shear zones where undeformed wall rocks with pre shear structures are accessible and where kinematics can be well established folds can provide a valuable natural means to understand the initiation and evolution of structures under non coaxial regimes we point to the need of discriminating among different plausible categories based on the nature of the folded surface and on the inherent structural features of each category q 2004 elsevier ltd all rights reserved