an early history of pure shear in the upper plate of the raft river metamorphic core complex black pine mountains southern idaho abstract although commonly obscured by simple shear pure shear fabrics occur locally within many metamorphic core complexes the cover rocks to the raft river metamorphic core complex exposed within the black pine mountains display an early coaxial strain history which developed prior to the formation of low angle fault bounded allochthons at higher structural levels this is documented by pressure shadows with straight sutures and oppositely rotated antitaxial calcite veins these rocks were sub horizontally extended parallel to bedding by up to 160 in an e w direction the marbles of the lowest exposed levels exhibit well developed crystallographic preferred orientation fabrics calcite c and a axis pole figures are symmetric to slightly asymmetric with obliquities ranging from 0 to 8 which indicate that the percent pure shear ranges between 100 and 76 initial constraints on the timing of this layer parallel extension suggest that extension was occurring locally within the sevier belt hinterland during the late cretaceous these data coupled with previous studies suggest that pure shear might be a much more common component of strain in rocks deformed in extensional environments than is generally acknowledged

early devonian eurypterids from the northwest territories of arctic canada a new eurypterid fauna from the lower bear rock formation early devonian emsian of anderson river in the northwest territories of the canadian arctic is described the material comprises an almost complete specimen and five isolated carapaces of erieopterus microphthalmus an incomplete carapace and telson referred to drepanopterus sp and an isolated prosomal appendage of carcinosoma sp associations include actinopterygian sarcopterygian and acanthodian fish as well as lingulids conchostracans ostracodes coprolites and plant material a nearshore marine environment is inferred this assemblage provides the first canadian record of drepanopterus and the youngest canadian occurrences of erieopterid and carcinosomatid eurypterids

numerical modelling of finite amplitude similar folds developing under general deformation histories abstract a simple model of similar folds in homogeneous materials with anisotropic properties is proposed and it is shown that the model is applicable to a wide range of natural examples of folds a mathematical derivation is given which allows the instantaneous deformation of the folds to be calculated for any imposed bulk deformation a means of projecting the effects of the deformation over a finite time interval is described the modelling is illustrated by simulations of folds observed in the morcles nappe western switzerland based on the assumption that the folds formed under simple shear bulk deformation folds with geometrical features remarkably similar to those natural folds are produced in the modelling by varying the parameters

vorticity analysis in calcite tectonites an example from the attico cycladic massif attica greece abstract although calcite tectonites are widespread in nature their use to quantify flow vorticity is limited we use new micro structural petrofabric and vorticity data to analyse the kinematics of flow in outcrop scale calcite mylonite zones these zones are genetically related to a crustal scale ne directed ductile thrust basal thrust that emplaced the blueschist over the basal unit during the exhumation of the attico cycladic massif calcite microstructures reveal that the last stage of deformation occurred at temperatures 200 300 c achieved by mild heating which is possibly related with the reburial of the basal thrust s footwall vorticity analyses were based on the degree of asymmetry of calcite c axis fabrics as well as on the assumption that the orientation of the long axes of calcite neoblasts within an oblique foliation delineates the direction of instantaneous stretching axis both methodological approaches provide consistent estimates with a simple shear component between 55 and 82 w n 0 76 0 96 the use of the stress axis 1 orientation recorded by twin c axis pairs to quantify vorticity generally gives significantly lower simple shear component comparison of our vorticity estimates with previous estimates inferred from quartz fabrics and rigid porphyroclasts reveals that exhumation related deformation in the nappe pile was steady state

combining natural microstructures with composite flow laws an improved approach for the extrapolation of lab data to nature so far rheological predictions that are based on experimentally derived flow laws appear inconsistent with microstructural observations in natural calcite mylonites because grain size sensitive gss diffusion creep is predicted while microstructural criteria suggest grain size insensitive gsi dislocation creep as the dominant deformation mechanism a way out of this discrepancy between experiment and nature can be found by combining full grain size distributions with composite gsscgsi flow laws we applied this approach to natural carbonate mylonites from the helvetic alps switzerland for steady state microstructures our calculations indicate an increasing gss component with increasing temperature at geologically constrained strain rates of 10 k10 10 k11 s k1 the modeling results are consistent with microstructural observations of the natural mylonites in these rocks dynamically steady state microfabrics show a systematic change in grain size grain size distribution and grain aspect ratio with temperature while crystallographic preferred orientations remain of similar strength when single value mean grain sizes are used rather than grain size distributions rheological modeling fails to give reasonable results further paleostresses estimated from conventional recrystallized grain size piezometers were found to be unrealistically high but were reduced to 80 100 mpa when results of the composite modeling were used hence combining composite flow laws with microstructural data from natural mylonites forms a promising approach for a better extrapolation of experimental data to nature and therefore provides new insight into changes in rheology during large scale geodynamic processes q 2005 elsevier ltd all rights reserved

methods for determining deformation history for chocolate tablet boudinage with fibrous crystals abstract chocolate tablet boudinage with fibrous crystal growths between the boudinaged plates from two localities were studied in one from leytron valais switzerland the deformation history was found to be a succession of plane strain increments with the shortening direction perpendicular to the boudinaged sheet and the extension direction showing a progressive change in orientation within the sheet the incremental and finite strains were evaluated the other specimen from parys mountain anglesey great britain was found to have a more complex history with diachronous break up of the competent layer and flattening strain increments it was found that under these circumstances the direct graphical methods of determining finite and incremental strains gave inconsistent results a numerical model was developed which allowed the simulation of chocolate tablet structure with a complex deformation history the model was applied to the anglesey specimen and three possible strain histories for this structure were tried

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

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

microstructural evolution and rheological behaviour of marbles deformed at different crustal levels abstract microstructures from naturally deformed marbles were investigated from a nappe pile with inverted barrovian metamorphic zones detailed microstructural and textural work combined with existing pt data allows us to correlate microstructural types with tectonic events type 1 microstructure related to d1 continental underthrusting shows highly asymmetrical grain boundaries increasing grain size and increasing intensity of lattice preferred orientation with increasing metamorphic grade these characteristics suggest that dislocation creep with concommitant grain boundary migration operated during the simple shear dominated underthrusting regime paleopiezometric and strain rate estimates show a decrease of flow stress with increasing metamorphic temperature and a strain rate of around 10 14 s 1 thrust related microstructure type 2 is developed in upper and lower nappes and is characterised by grain size reduction increase of grain boundary symmetry and weaker textures microstructural and textural studies indicate dislocation creep with possible contribution of grain boundary sliding sub grain rotation paleopiezometry and strain rate estimates show a decrease in flow stress with respect to the type 1 microstructure and strain rate increase to around 10 12 to 10 13 s 1 type 3 microstructure is developed in thrust related shear zones in the parautochthon and syn convergent extensional zones in the top of the nappe pile this microstructure shows a high contribution of diffusion creep indicating low values of flow stress and significant weakening of marble layers the general feature of studied area is the localisation of deformation into marbles during thrusting in low metamorphic grades while in higher grades marbles are not exploited as lubricating layers

report on the tectonic studies group a g m 18 19 december 1980 university of keele the shape and internal structure of deformed pebbles are compared with theoretical predictions about the deformation of a viscous circular or dliptical inclusion in a matrix of lower viscosity eshelby 1957 ghosh sengupta 1973 bilby et al 1975 a geologically important prediction supported by the finite element calculations of shimamoto 1975 is that the finite strain within the pebble is homogeneous and remains so during continued deformation the deformed inclusion retains an elliptical shape

anisotropy of magnetic susceptibility in diamagnetic limestones reveals deflection of the strain field near the dead sea fault northern israel abstract to exploit the potential of anisotropy of magnetic susceptibility ams as a tool to estimate the strain field around major faults we measured the ams of calcite bearing diamagnetic rocks that crop out next to the dead sea fault dsf in northern israel through integrated magnetic and geochemical methods we found that the rocks are almost pure calcite rocks and therefore the magnetic fabric is primarily controlled by preferred crystallographic orientation pco with the minimum principal ams axes k 3 parallel to calcite c axes we applied a separation procedure in several samples with high fe content in order to calculate the ams anisotropy parameters and compare them to pure diamagnetic rocks aarm thermo susceptibility curves and irm were used to characterize the magnetic phases we found that for fe content below 500 ppm the ams is mostly controlled by the diamagnetic phase and showed that differences in the degree of anisotropy p up to 3 p 1 005 to 1 023 and in anisotropy difference k up to 0 25 10 6 si in diamagnetic rocks are related to differences of strain magnitudes the spatial distribution of the magnetic fabrics indicates n s maximum shortening parallel to the strike of the hula western border fault hwbf one of the main strands of the dsf in northern israel the anisotropy parameters suggest that the strain magnitudes increase eastward with the proximity to the hwbf these results suggest that the strain field near the hwbf is locally deflected as a consequence of the dsf activity in light of the fault weakness model and geological setting of the study area we suggest that the area accommodates dominant transtension during the pleistocene the present study demonstrates the useful application of ams measurements in iron free limestones as recorders of the strain field near plate boundaries

build up and dismembering of the eastern northern calcareous alps the structural geometry and kinematics of a segment of the classical fold and thrust belt of the northern calcareous alps nca is analyzed by map mesoscale fault slip and calcite twin data the most distinct feature of the eastern nca is the miocene semp salzachtal ennstal mariazell puchberg lineament which extends 400 km wsw ene from the northern tauern window in the west to the vienna basin in the east it forms an orogen parallel strike slip zone constitutes the major left lateral wrench corridor accommodating eastward lateral extrusion of crustal wedges along the central eastern alps and crosses the nca as an anastomosing zone of distributed shear the semp line and related structures dismember a fold and thrust belt formed during early transpressional and subsequent orthogonal contraction most early miocene structures are reversed in the late miocene accommodating little strain distinct strain fields are calculated based on fault slip and calcite deformation twin data they are regionally consistent in orientation and relative age and correlate with the following deformation stages 1 late cretaceous to early tertiary top to nw nappe stacking and right lateral wrenching along nw trending tear transfer faults as an expression of the build up of the austroalpine orogenic wedge e3 317 17 e1 058 28 e1 e2 e3 principal strains 2 early tertiary top to n stacking and conjugate strike slip faulting reflecting the change from transpressional to frontal contraction within the nca e3 356 12 e1 088 10 3 early to middle miocene large scale left lateral wrenching including early transpression and late transtension as expression of eastward displacement of the southern part of the nca e3 019 22 e1 109 27 4 post middle miocene e w contraction reactivating strike slip and normal faults e3 092 10 e1 004 12 the distributed nature of the lateral extrusion deformation and the anastomosing faulting along the semp line reflect the rheological heterogeneity inherited by the lithology and multiple deformations of the nca a combined extension and stress deflection model possibly accounts best for the pattern of northward concave faults branching off the semp line

relative timing of calcite twinning strain and fold thrust belt development hudson valley fold thrust belt new york u s a coarse grained limestone samples were collected across the hudson valley fold thrust belt with the aim of determining the temporal and spatial relationship between calcite twinning strain and fold thrust belt development the majority of the samples have well defined oblate strain ellipsoids with the maximum shortening axes e3 perpendicular to bedding strike and e3 magnitudes that range from 1 5 to 6 5 whereas twinning magnitude does not vary systematically through the fold thrust belt twinning fabrics fall into two populations based on the presence of outcrop scale deformational features calcite twinning strain preserved in samples from limestone layers absent of any small scale penetrative deformation such as cleavage and or microfaults are defined as component a strains component a strains generally have maximum shortening axes e3 inclined consistently less than but in the same e w direction as bedding dip the second population of calcite twinning strain population b occurs in limestones pervasively cut by cleavage and or microfaults component b strains has maximum shortening axes e3 that are horizontal irrespective of bedding dip strain population a initiated as a pre folding layer parallel shortening strain that was subsequently modified to its present orientation by active grain scale rotation during flexural folding strain population b reflects post folding superimposed homogeneous strain and is restricted to limestone samples with relatively high clay content 0 1998 elsevier science ltd

a large arthropod trackway from the gaspe sandstone group middle devonian of eastern canada a large arthropod trackway from the cap aux os member of the battery point formation gaspe sandstone group middle devonian from the baie de gaspe eastern canada is described and assigned to the ichnotaxon palmichnium paleohelcura antarcticum gevers et al 1971 a large stylonurid eurypterid or scorpion is considered the most likely producer a shallow water marginal fluvial environment is inferred as the setting the animal making a transition from walking to swimming along the course of the trackway resume la trace d une piste d un grand arthropode dans le membre de cap aux os de la formation de battery point groupe gres de gaspe devonien moyen dans la baie de gaspe dans l est du canada est decrite et assignee a l ichnotaxon palmichnium paleohelcura antarcticum gevers et al 1971 il semble que la trace ait ete produite par un grand eurypteride stylonuride ou un scorpion on presume que c est dans un environnement fluviatile marginal sous une mince tranche d eau que l animal a creuse cette empreinte en marchant vers un lieu de nage traduit par la redaction braddy and milner 1122 a large arthropod trackway was discovered in 1988 milner 1992 along the northern shore of the baie de gaspe grid reference 20u lk 993112 approximately 5 km east of the town of gaspe near the village of cap aux os on the forillon peninsula in the province of quebec eastern can ada fig 1 the locality is within the cap aux os member of the battery point formation gaspe sandstone group approximately 750 m above the base of the formation in the lower part of unit 4 lawrence and rust 1988 fig 7 the battery point formation comprises a coarsening upwards clastic sequence ranging in age from early emsian to early eifelian middle devonian based on abundant spores oc curring throughout the formation mcgregor 1977 the cap aux os member approximately 640 m thick consists mainly of interbedded sandstones and mudstones which were deposited in a meandering fluvial system the prevail ing palaeocurrent direction varied from the north northwest to the west northwest lawrence and rust 1988 sand was deposited in the river channels and finer grained silts and muds on the extensive vegetated floodplain lawrence 1986

microfabric study on the deformational and thermal history of the alpi apuane marbles carrara marbles italy marbles from different geometrical and structural positions within the alpi apuane metamorphic complex show a large variability in microfabric types as indicated by microstructure c axis orientation and temperature analysis statically recrystallized samples showing a granoblastic microstructure and polygonal grain boundaries are characterized by a grain size variation from east to west from 80 100 m to 250 300 m this is correlated with an equilibration calcite dolomite temperature from 360 380 c to 420 430 c two kinds of dynamically recrystallized microstructures have been investigated a first one exhibiting coarse grains 150 200 m with lobate grain boundaries and a strong shape preferred orientation and a second one characterized by a smaller grain size 40 50 m and predominantly straight grain boundaries these microstructural types associated with localized post thermal peak shear zones and meter to kilometer scale folds are interpreted as related to high strain and high temperature crystal plastic deformation mechanisms dislocation creep associated with predominant grain boundary migration type b1 or subgrain rotation recrystallization type b2 these differences in dynamically recrystallized microstructures are related to equilibration temperatures higher in type b1 390 c than in type b2 370 340 c we have been able to relate the development of the different microfabric types to the successive stages of deformation of the alpi apuane metamorphic complex

texture of solnhofen limestone deformed to high strains in torsion solnhofen limestone was deformed in torsion to shear strains ranging from 1 to 12 at a temperature of 750 c 300 mpa confining pressure and a maximum strain rate of 10 3 s 1 these deformation conditions correspond to the intracrystalline power law dislocation creep field close to the boundary to the grain size sensitive superplastic creep field the grain shape microstructure was observed using orientation contrast by backscattered electrons in the sem the grains remain sub equant with an average grain size of around 4 m even to the highest strains lattice preferred orientation was determined using both x ray texture goniometry and automated electron back scatter diffraction the c axis preferred orientation develops from two main maxima with a weak sub maximum through two maxima to a single maximum perpendicular to the shear plane the rate of increase of the intensity of the single maximum with increasing strain diminishes and it appears that there is a tendency to a steady state texture the final single c axis maximum is displaced slightly towards the shortening direction of the applied simple shear the a axes tend to a girdle perpendicular to the c axis maximum it is proposed that the partitioning of deformation between intra and inter crystalline mechanisms results in a pulsating strain state in the grains contributing to the maintenance of sub equant grains it is argued that the lattices of constituent grains rotate continuously with no stable end orientation and that this can lead to a steady state texture the experimental preferred orientation compares well with that of natural calcite mylonites in the position of the c axis maximum and the a axis girdle

early devonian stylonurine eurypterids from arctic canada two new stylonurine eurypterids are described from the peel sound formation early devonian lochkovian of the northern coast of prince of wales island nunavut arctic canada associations including pteraspids and ostracodes indicate a fluvial depositional environment an almost complete stylonurid pagea plotnicki sp nov is recognized by its large size and lack of vaulting on the carapace and it provides evidence that stylonurus and pagea are sister taxa also a smaller incomplete rhenopterid assigned to leiopterella tetliei gen et sp nov is characterized by its broad turbinate carapace and lack of cuticular sculpture this assemblage provides the first canadian record of pagea and the youngest occurrence of a rhenopterid outside the rheno hercynian terrane indicating that these taxa were more geographically widespread than previously supposed

experimental calcite fabrics in a synthetic weaker aggregate by coaxial and non coaxial deformation abstract calcite grain aggregates supported by a weaker macroscopically ductile matrix of set portland cement were deformed triaxially at room temperature with a confining pressure of 150 mpa and at rates corresponding to a pure shear natural strain rate of 10 5 s 1 extensive grain rotation accompanied the twinning of calcite stress orientations inside the specimens determined from calcite twins agree well with the stresses imposed upon the specimens thus calcite twins are sound kinematic indicators in this study various sample configurations were used to simulate pure shear transpression and simple shear in pure shear or coaxial strain preferred dimensional orientations pdo of calcite are produced more efficiently in the presence of high pore fluid pressure those grain alignments are stronger than the bulk strain would predict assuming homogeneous strain strain is overestimated by methods assuming continuum behaviour because they fail to take into account the extensive interparticle motions that occur in the presence of high pore fluid pressure fluid pressure suppresses the twinning of calcite by reducing the intergranular stresses during deformation small rapid variations in pore fluid pressure are believed to represent ephemeral dilatations accompanying particulate flow as groups of grains slide past one another in dry specimens pure shear is more effective than simple shear in producing a pdo of the calcite grains calcite grains are rotated rigidly and somewhat strained to produce l at comparable strains in dry tests e twinning is best developed in transpression next best developed in coaxial strain and least well shown by simple shear high fluid pressure considerably reduces the incidence of twinning

change of direction of overthrust shear in the helvetic nappes of western switzerland abstract the lowest of the helvetic nappes in western switzerland the morcles and the doldenhorn nappes are large scale recumbent folds in equivalent tectonic positions in the alpine chain a comparison of the orientations of stretching lineations in both nappes shows that the lineations in the two stratigraphically normal sequences trend n s whereas those in the two stratigraphically inverted sequences trend nw se a similar geometrical relationship has been found from an analysis of the calcite c axis directions of these limestones the two normal sequences show nw se directed c axes whereas the two inverted sequences show w e directed c axes these observations lead to the following conclusions 1 regionally consistent shear movements occurred over a strike distance of more than 60 km and these gave rise to similar deformation patterns in both nappes 2 a progressive rotation of these shear movements from a northerly direction in the earlier formed limbs to northwesterly in the later formed inverted limbs can be deduced the finite strain stretching lineations lag behind the crystallographic fabric axes formed in the later part of the deformation history

microfabrics of calcite ultramylonites as records of coaxial and non coaxial deformation kinematics examples from the rocher de l yret shear zone western alps microfabrics were analysed in calcite mylonites from the rim of the pelvoux massif western alps france wnw directed emplacement of the internal penninic units onto the dauphinois domain produced intense deformation of an eocene age nummulitic limestone under lower anchizone metamorphic conditions slightly below 300 c two types of microfabrics developed primarily by dislocation creep accompanied by dynamic recrystallisation in the absence of twinning coaxial kinematics are inferred for samples exhibiting grain shape fabrics and textures with orthorhombic symmetry their texture crystallographic preferred orientation cpo is characterised by two c axis maxima symmetrically oriented at 15 from the normal to the macroscopic foliation non coaxial deformation is evident in samples with monoclinic shape fabrics and textures characterised by a single oblique c axis maximum tilted with the sense of shear by about 15 from the analysis of suitably oriented slip systems for the main texture components under given kinematics it is inferred that the orthorhombic textures which developed in coaxial kinematics favour activity of b10 11n and b02 21n slip along the f and r planes respectively with minor contributions of basalban slip in contrast the monoclinic textures which developed during simple shear are most suited for duplex ban slip along the basal plane the transition between the dominating slip systems for the orthorhombic and monoclinic microfabrics is interpreted to be due to the effects of dynamic recrystallisation upon texture development since oblique c axis maxima documented in the literature are most often rotated not with but against the shear sense calcite textures alone should not be used as unequivocal shear sense indicators but need to be complemented by microstructural criteria such as shape preferred orientations grain size estimates and amount of twinning

origin of calcite in syntectonic veins as determined from carbon isotope ratios a particular feature of many alpine tectonic thrust sheets composed of carbonates is the development of distinct generations of calcite filled veins in the doldenhorn nappe one of the helvetic nappes of switzerland we were able to distinguish five different generations of calcite filled veins determination of carbon isotope ratios in these calcite veins and their adjacent limestone matrix indicates that the earlier vein systems contain carbonate material not in isotopic equilibrium with the adjacent matrix in the case of the later vein systems the carbon 13 content approaches that of the nearby matrix the isotope data indicate a transition from a chemically open system to a closed system this trend is related to an overall modification of the deformation mechanism of the nappe resulting in the increasing importance of pressure solution with time

determination of deviatoric stress tensors based on inversion of calcite twin data from experimentally deformed monophase samples preliminary results abstract following previous successful paleo stress determinations from numerically generated aggregates and naturally deformed samples preliminary results about stress reconstructions based on inversion of calcite twin data from experimentally deformed monophase samples are reported this new study carried out on samples of carrara marble and crinoidic limestone from eastern france provides a test of the validity of stress reconstructions using inverse methods which had not been properly calibrated before the percentages of correlated twins are discussed for each stress tesnor and the range of uncertainties on the determination of stress orientations and differential stress magnitudes is evaluated

constraints on geological strain rates arguments from finite strain states of naturally deformed rocks a summary of known finite strain states is presented longitudinal strains 1 e as measured in many rocks often range from 1 to 40 and 1 to 0 025 the time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m y possibly less than 1 m y which constrains conventional strain rates into the range of 10 sup 13 s sup 1 to 10 sup 15 s sup 1 for both pure and simple shear the most efficient way and much less efficient way to accumulate incremental strains respectively the ellipticity of the finite strain ellipse increases in a nonlinear manner finite strain variations in adjacent layers which give rise to features such as cleavage refraction arise with only slight differences in the strain rates within these layers

using neutron diffraction to examine the onset of mechanical twinning in calcite rocks experimental calibration of the calcite twin piezometer is complicated by the difficulty of establishing the stresses at which the twins observed in the final deformation microstructures actually formed in principle this difficulty may be circumvented if the deformation experiments are performed in a polychromatic neutron beam line because this allows the elastic strain and hence stress in differently oriented grains to be simultaneously monitored from diffraction patterns collected as the experiment is proceeding to test this idea small strain 0 3 uniaxial compression experiments have been performed on carrara marble grain size 150 m and solnhofen limestone 5 m at temperatures of 20 600 c using the engin x instrument at the isis neutron facility uk at the lowest temperatures 25 c carrara 200 c solnhofen the deformation response was purely elastic up to the greatest stresses applied 60 mpa carrara 175 mpa solnhofen the sign of the calcite elastic stiffness component c14 is confirmed to be positive when the obverse setting of the calcite rhombohedral lattice in hexagonal axes is used in the carrara marble samples deformed at higher temperatures elastic twinning was initiated at small stresses 15 mpa in grains oriented such that the schmid factor for twinning was positive on more than one e twin system at greater stresses 65 mpa at 200 c decreasing to 41 mpa at 500 c there was an abrupt onset of permanent twinning in grains with large schmid factors for twinning on any one e twin system no twinning was observed in the solnhofen limestone samples deformed at 200 or 400 c at applied stresses of 180 mpa these results highlight the potential of this approach for detecting the onset of twinning and provide through experiments on samples with different microstructures a strategy for systematically investigating the effects of microstructural variables on crystallographically controlled inelastic processes

calcite twinning and determination of paleostress orientations three methods compared the classic turner 1953 method of determining 1 and 3 from an analysis of calcite twins is compared with a graphical method modified after dietrich and song 1984 and a newly devised method which treats calcite twin glide directions and twin planes in an analogous way to slickensides on fault planes all three methods provide similar positions for 1 but none gives a reliable position for 3 if a c axes preferred orientation exists 3 can most successfully be identified as the axis of a girdle of turner compression axes computer assisted adaptations of the turner method are recommended as the most rapid and internally consistent means of determining 1 and 3 the manual graphical method provides a useful check on turner determinations of 1 and a means of cleaning data

a study of the depth of weathering and its relationship to the mechanical properties of near surface rocks in the mojave desert weathered granite extends 70 m deep at hi vista in the arid central mojave desert of southern california the low strength of this granite is due to the alteration of biotite and chlorite montmorillonite deep weathering probably occurs in most granites although we cannot rule out some anomalous mechanisms at hi vista geophysical instruments set in these slightly altered rocks are limited by the unstable behavior of the rocks thus tectonic signals from instruments placed in shallow boreholes give vague results geophysical measurements of these weathered rocks resemble measurements of granitic rocks near major faults the rheology of the rocks in which instruments are placed limits the useful sensitivity of the instruments

role of shear in development of the helvetic fold thrust belt of switzerland the geometric features of the helvetic nappes of the alps show intense internal deformation set up predominantly by simple shear during nappe transport the nappes are bounded by thrusts that parallel incompetent layers and ramp upward stratigraphically across competent units in the direction of tectonic transport stratal layering is oblique to nappe boundaries thrust surfaces formed after ductile shear zones developed in underlying pre triassic gneissic basement the broad zone of shearing narrowed down as deformation increased to form zones of intense shear and ultimately to form thrust faults contraction along competent layers obliquely inclined to shear surfaces produced buckle folds the geometry of these folds was modified by further shearing as their normal limbs became aligned in the extension field of the strain ellipse for simple shear producing normal faults

an early devonian emsian acanthodian from the bear rock formation anderson river northwest territories canada a new genus and species of an early devonian emsian mesacanthid acanthodiform from outcrops of the bear rock formation along the anderson river northwest territories canada is described the new taxon is distinct from other mesacanthid genera and or species in having mesiolaterally expanded skull roof plates and paired nasal bones mesacanthids have fin spines that are similar in size number position and proportions this new genus is the fourth in the family mesacanthidae the wider distribution and apparent taxonomic diversity represented by the discovery of the new genus are indications that the latest surviving and arguably most advanced acanthodian group the acanthodiformes was already well established in the early devonian

microstructural and textural development of calcite marbles during polyphase deformation of penninic units within the tauern window eastern alps abstract the evolution of calcite microstructures and crystallographic preferred orientations cpos is well understood due to well constrained experimental studies however the interpretation of naturally deformed calcite marbles is more difficult because of less constrained strain paths a multiphase deformation history and variable p t conditions the penninic units within the tauern window eastern alps have been affected by several deformation events and metamorphic overprint generally three major deformational events can be distinguished d 1 is related to underthrusting of penninic units beneath the austroalpine nappe complex and top to the n nappe stacking within the penninic continental units deformation stage d 2 is interpreted as reflecting the subsequent continent collision between the penninic continental units and the european foreland d 3 is related to the formation of the dome structure of the tauern window this polyphase deformation history can be partly reconstructed by the evolution of calcite microfabrics and cpos three types of calcite fabrics are distinguished within the penninic units of the tauern window and the lower austroalpine unit d 1 fabrics are characterized by equilibrated microstructures and lt cpos the cpos are generally strong and symmetric with one well developed cluster near the z axis of the finite strain ellipsoid these fabrics have locally been overprinted by subsequent amphibolite to greenschist facies metamorphism generally the occurrence of lt fabrics coincides with the occurrence of amphibolite facies metamorphic mineral assemblages in the central part of the tauern window while ht fabrics have been observed outside this area fabrics from the central part of the tauern window have likely been strengthened during subsequent thermal equilibration while the fabrics that have been observed at the peripheral parts have been less affected by subsequent metamorphic overprint therefore d 1 fabrics do not reflect d 1 conditions but subsequent thermal equilibration similar observations have been made for the evolution of d 2 fabrics lt fabrics dominate inside the amphibolite facies isograde ht fabrics occur outside greenschist facies metamorphic conditions the fabrics are characterized by high finite strains near the margins of the tauern window the intensity of which decreases towards the central parts where microstructures are characterized by recrystallization and thermal equilibration due to amphibolite facies metamorphic overprint the strong cpos document this influence the ht fabrics and microstructures within peripheral areas indicate that they have less been affected by this thermal event and therefore are more indicative for the deformational conditions during d 2 d 3 is restricted to distinct shear zones along the tectonic boundaries of the tauern window from the central parts to the shear zone boundaries a clear evolution of microfabrics can be observed in internal parts microstructures are characteristic for intracrystalline plasticity with a dominating activity of r glide twinning was less important during the final phases of deformation on approaching the shear zone boundaries the grain size decreases due to dynamic recrystallization and secondary grain size reduction until ultramylonites are formed within these domains grain boundary sliding seems to have been dominant in conclusion calcite cpos from polyphase areas do not only include information on the deformation conditions but also bear information about the thermal overprint subsequent to the main deformational event

inverse magnetic fabric in deformed limestones of the morcles nappe switzerland anisotropy of magnetic susceptibility ams was measured at 15 sites in deformed helvetic limestones of the morcles nappe in western switzerland inverse magnetic fabrics characterized by axes of maximum susceptibility subperpendicular to bedding or cleavage were observed at four sites in the root zone of the nappe acquisition and thermal demagnetization of isothermal remanent magnetization show that the ferromagnetic mineralogy is dominated by pyrrhotite with various proportions of magnetite hematite and goethite the matrix minerals were studied in thin section with a staining technique to distinguish different carbonates by x ray texture goniometry and by microprobe analysis observations of ams at room and liquid nitrogen temperatures and of the temperature dependence of bulk susceptibility lead to the conclusion that the inverse fabrics result from a paramagnetic component due to fe2 ions in the calcite matrix

a new technique to automatically quantify microstructures of fine grained carbonate mylonites two step etching combined with sem imaging and image analysis a two step etching technique for fine grained calcite mylonites using 0 37 hydrochloric and 0 1 acetic acid produces a topographic relief which reflects the grain boundary geometry with this technique calcite grain boundaries become more intensely dissolved than their grain interiors but second phase minerals like dolomite quartz feldspars apatite hematite and pyrite are not affected by the acid and therefore form topographic peaks based on digital backscatter electron images and element distribution maps acquired on a scanning electron microscope the geometry of calcite and the second phase minerals can be automatically quantified using image analysis software for research on fine grained carbonate rocks e g dolomite calcite mixtures this low cost approach is an attractive alternative to the generation of manual grain boundary maps based on photographs from ultra thin sections or orientation contrast images

kinematic analysis of an en echelon continuous vein complex an array of sigmoidal tension gashes from the idaho wyoming thrust belt changes laterally into a continuous vein detailed mechanical twin analysis was used to determine the strain variation in the optically and chemically homogeneous blocky calcite filling in the continuous portion of the vein complex the shortening axes are parallel to the vein boundary however the orientation of the shortening axes in the tip areas of the sigmoidal gashes are at an angle of approximately 35 to the vein boundary and are parallel to the trend of the tips twinning patterns in the central portions of the gashes record two principal strain axes of shortening of nearly equal magnitude with the maximum perpendicular to the vein trend everywhere in the vein complex the orientation of the maximum extension axis is parallel to the twist axis of the gashes the petrofabric strain results show that the vein filling has largely recorded local strains the pattern of variation in orientation of the principal strains in the vein complex is in close agreement with the theoretically determined stress distribution in similar structures our results show that the sigmoidal gashes were formed at the leading edge of a propagating vein and that the sigmoidal shape reflects changes in the local strain field rather than a remote shear the orientation of these local strains closely corresponds to the orientation of the local stresses

calcite fabrics in a natural shear environment the helvetic nappes of western switzerland the aim of this contribution is to describe and interpret the crystallographic orientations and microstructures of calcite in the upper jurassic limestones of the helvetic nappes between the mont blanc and aar massifs over a distance along strike of about 180 km the c axis preferred orientation pattern which could be inferred is regionally very constant the c axis point maxima are everywhere subperpendicular to the regional strike of the chain there is an obliquity between the orientation of the macroscopically dominant cleavage and the microscopically visible long axes of individual grains the c axis maxima in most specimens are parallel to the axis of shortening of the microscopic grain shape fabric this fabric geometry indicates either continuous shear deformation or alternatively a two stage deformation history the obliquity of the grain shape fabric may be due to the fact that the flattening plane of the individual grains reflects only the last increment of a continuous shear process or it may represent a later finite deformation which has led to the formation of a regionally developed second cleavage there is a systematic relationship between the direction of these late shear movements and the main overthrust direction as given by the finite strain pattern this suggests that the microfabric of the rocks was acquired during a progressive rotation with time of the main overthrust direction the observed pattern of preferred orientation has been correlated by several authors with intracrystalline deformation mechanisms a combination of slip on r and twinning on e however the e twinning observed in the specimens investigated is younger than the development of the crystallographic fabric a new method for stress determination from e twinning is proposed

direct observation of fault zone structure at the brittle ductile transition along the salzach ennstal mariazell puchberg fault system austrian alps 1 structural analysis of two key exposures reveals the architecture of the brittle ductile transition bdt of the subvertical strike slip salzachtal fault at lichtensteinklamm the fault zone is dominantly brittle with a 70 m wide high strain fault core highlighted by a 50 m thick highly foliated gouge zone in contrast at kitzlochklamm deformation is dominantly ductile albeit with relatively low strain indicated by weak lattice preferred orientations lpos the marked contrast in structural style indicates that these sites span the bdt the close proximity of the outcrops coupled with raman spectroscopy indicating similar maximum temperatures of 400 c suggests that the difference in exhumation depth is small with a commensurately small difference in total downdip width of the bdt the small strains indicated by weak lpos at kitzlochklamm coupled with evidence for brittle slip at the main fault contact and along the sides of a 5 m wide fault bounded sliver of klammkalk exposed 30 m into the grauwacken zone rocks suggest the possibility that this exposure may record hybrid behavior at different times during the earthquake cycle with ductile deformation occurring during slow interseismic slip and brittle deformation occurring during earthquakes as dynamic coseismic stresses induced a strain rate dependent shift to brittle fault behavior within the nominally ductile regime in the lower part of the bdt a key aspect of both outcrops is evidence of a high degree of strain localization through the bdt with high strain fault cores no wider than a few tens of meters

calcite fabrics around folds as indicators of deformation history abstract in the morcles nappe lowermost helvetic nappe in western switzerland two phases of folding have been established in this article the relationship between the calcite fabrics present in folded limestones and the folding history is analysed calcite fabrics around the first and second phase folds are related to the second phase of deformation the following fabric patterns have been found 1 the fabric geometry around a second phase fold from the most internal part of the inverted limb of the nappe locality saillon can be related to an overall simple shear deformation sequence the sense of shear being related to the last advance of the morcles nappe over the underlying autochthonous sedimentary cover of the aiguilles rouges massif 2 in more external parts of the inverted limb locality petit pre the fabrics around a second phase fold are interpreted as indicating a change in the deformation history from simple shear to a strain regime which can account for shortening along the first phase cleavage by the formation of buckle folds this change in the local strain regime could be related to a locking of the frontal folds of the nappe during the last overthrust shear movements 3 in the normal limb of the nappe the fabrics around a late second phase kink fold locality neimia are earlier than the folding event and appear to be rotated passively by the fold 4 the fabrics around a first phase fold locality la routia are later than the folding event and overprint the fold

calcite texture development in experimentally induced ductile shear zones specimens of fine grained micritic limestone were deformed in plane strain geometry in pure shear a combination of simple and pure shear and in simple shear temperatures were 400 c and 500 c confining pressure was 100 mpa in the experiments with a simple shear component strain is concentrated and approximately homogeneous in a 2 3 mm wide shear zone shear displacement is documented by marker lines and circles shear strain varies between 0 84 and 1 56 strain is recorded by flattening of individual grains defining a foliation normal to the axis of principal finite shortening e1 no twinning is observed on a macroscopic scale

inferring stress from faulting from early concepts to inverse methods abstract we review the evolution of concepts on and methods of estimating the state of stress from fault movements theories of failure in isotropic materials suggested a simple geometrical construction of optimal principal stress directions from a fault plane and its associated slip these optimal directions align shear stress and slip directions and maximize the difference between shear stress and frictional resistance on the fault plane optimal stress directions for calcite twinning are obtained by a similar construction with the difference that they maximize shear stress force representation of seismic sources independently introduced pressure p and tension t axes at positions that also maximize shear stress on both nodal planes frictional slip theory and the constraint that slip and shear stress directions be parallel allowed to address reactivation of pre existing faults this suggested that stress could also be inverted from reactivated fault and slip data or earthquake focal mechanisms early methods relied on geometrical constructions as a substitute for calculations whereas later methods relied on software as these calculations became tractable with the help of computers similar methods were developed for the inversion of stress from crystal twin gliding with non optimal geometry with a different criterion that relies on a threshold of the component of shear stress along the gliding line even though these methods seek a common stress tensor compatible with fault and slip data their main use is to separate polyphase data into homogeneous subsets and help deciphering complex tectonic histories fault and slip data can also be analyzed to constrain the strain rather than the stress tensor in most cases this involves a summation and yields an average strain for the considered rock volume stress inversion thus appears better suited for differentiating heterogeneous data whereas strain analysis appears better suited for homogenizing them

the role of veining and dissolution in the evolution of fine grained mylonites the mcconnell thrust alberta microstructural observations of the mcconnell thrust fault rocks demonstrate the importance of vein formation and calcite dissolution in the evolution of the mcconnell thrust limestone mylonites the protolith to the mylonite is a micritic limestone the mylonite contains numerous discontinuous bedding parallel calcite veins that range from relatively undeformed except for twinning and undulose extinction to highly sheared the highly sheared veins show evidence for dynamic recrystallization and grain size reduction along grain and twin boundaries that can lead to complete recrystallization to a grain size of approximately 1 3 m the bulk of the calcite matrix within the mcconnell thrust mylonite is interpreted to have formed by dynamical recrystallization of veins the emplacement of relatively coarse grained veins promoted the activation of dislocation creep within the fault rocks of the thrust in the absence of the increase in grain size afforded by vein emplacement it is anticipated that the fine grained protolith would have deformed predominantly by solution transfer with grain boundary sliding and localized brittle shearing the layer parallel calcite veins are found in close proximity to sheared clay seams which are interpreted to represent the insoluble residue left behind after dissolution of calcite these clay seams may have contributed to the build up of high pore fluid pressures which if intermittently reaching lithostatic conditions during displacement would result in transient brittle failure and thrust surface parallel vein emplacement in an otherwise ductilely deforming rock

alternating contraction and extension in the hinterlands of orogenic belts an example from the raft river mountains utah combined macroscopic to microscopic structural analyses detailed geologic mapping and thermochronology were used to study the structural evolution of midcrustal rocks of the sevier orogenic belt hinterland in northwestern utah these results when combined with previous studies provide new insight into the structural history of this region and suggest alternating tectonic contraction and extension during mesozoic to early cenozoic time two allochthons form the upper plate of the miocene raft river detachment fault in the eastern raft river mountains the lower allochthon comprises neoproterozoic ordovician and pennsylvanian strata and is bounded below by the raft river detachment fault and above by the middle detachment fault strata within the lower allochthon were dramatically attenuated by two episodes of ductile deformation the first deformation d1 took place at metamorphic temperatures of 500 c and resulted in penetrative fabrics throughout these rocks that record combined flattening and top to northeast shearing strains the second deformation d2 resulted in significant stratigraphic attenuation along discrete top to the west shear zones that are generally parallel to lithologic contacts separates of synkinematic muscovite from the penetrative fabric yield 40ar 39ar cooling ages that indicate that d1 deformation occurred prior to cooling ca 90 ma both fabrics were subsequently folded about d3 kilometer scale recumbent folds the areally extensive middle allochthon composed chiefly of pennsylvanian and permian rocks metamorphosed in the greenschist facies was emplaced d4 along the low angle middle detachment fault this fault cuts across various structural levels of the recumbently folded lower allochthon in its footwall and juxtaposes greenschist facies over amphibolite facies metamorphic rocks the lower and middle allochthon were subsequently deformed d5 into open folds with north trending axes neogene extension d6 produced an 200 m thick top to east ductile shear zone in precambrian rocks and formed the younger raft river detachment fault which forms the present upper contact of the ductile shear zone northeast vergent d1 fabrics probably record shortening deformation on the basis of fabric correlations with the grouse creek and albion mountains deformation kinematics and synkinematic prograde metamorphism d2 attenuation faults have been interpreted to record crustal extension of late cretaceous age if d3 recumbent folds developed during extension then the deformation sequence records early shortening followed by protracted extensional unroofing with variable structural styles the favored alternative is that d3 recumbent folds developed during shortening in this case d1 through d4 record episodic alternations of contraction and extension such alternations are consistent with observations from analog and theoretical models of contractional mountain belts and suggest that the hinterland of the sevier orogenic belt underwent dynamic adjustments in crustal thickness and deformation kinematics in response to changes in the boundary conditions of the orogenic belt