mechanisms of flexural flow folding of competent single layers as evidenced by folded fibrous dolomite veins abstract flexural flow is thought unlikely to occur in naturally deformed competent isotropic single layers in this study we discuss a particular case of folded bedding parallel fibrous dolomite veins in shale in which the internal strain pattern and microstructural deformation features provide new insights in the mechanisms enabling flexural flow folding strain in the pre folding veins is accommodated by two main mechanisms intracrystalline deformation by bending and intergranular deformation with bookshelf rotation of dolomite fibres the initially orthogonal dolomite fibres allowed a reconstruction of the strain distribution across the folded veins this analysis shows that the planar mechanical anisotropy created by the fibres causes the veins to approximate flexural flow during folding synkinematic veins overgrow the pre folding fibrous dolomite veins microstructures and dolomite growth morphologies reflect growth during progressive fold evolution with evidence for flexural slip at fold lock up homogeneous flattening as evidenced by disjunctive axial planar cleavage subsequently modified these folds from class 1b to 1c folds our study shows that the internal vein fabric has a first order influence on folding kinematics moreover the fibrous dolomite veins show high viscosity contrasts with the shale matrix essential in creating transient permeability for subsequent mineralising stages in the later synkinematic veins during progressive folding

comment on first records of syn diagenetic non tectonic folding in quaternary thermogene travertines caused by hydrothermal incremental veining by billi et al tectonophysics 700 701 2017 60 79 abstract billi et al 2017 proposed a new interpretation for the origin and internal structure of thermogene travertine deposits on the basis of evidence from two quarries located in southern tuscany italy they interpreted some travertine beds as calcite veins and argued that undulating travertine beds formed by syn diagenetic i e non tectonic folding that was caused by laterally confined volume expansion caused by incremental veining they assumed that such a process causes changes to the rock properties including porosity reduction rock strengthening and age rejuvenation the interpretations by billi et al 2017 challenge and question the current understanding and interpretation of thermogene travertine deposits this understanding based on numerous studies since the 1980s is that these deposits form from thermal water flowing downslope and precipitating calcium carbonate here we explain how the comparison with active depositional systems is essential for the understanding the origin of structures in older inactive travertine deposits such as those studied by billi et al 2017 we further argue that the three dimensional setting of travertine deposits should be taken into account in order to discuss the possible development of secondary structures indeed travertine deposition on slopes typically leads to the formation of terraced morphologies with pools bordered by rounded rims and separated from each other by steep walls the resulting three dimensional structures can be misinterpreted as asymmetric folds in two dimensional views i e in saw cut walls of quarry in this paper we debate the interpretations offered by billi et al 2017 and their criteria to recognise syn diagenetic non tectonic folds in travertine deposits and explain why many of their ideas are questionable

parasitic folds with wrong vergence how pre existing geometrical asymmetries can be inherited during multilayer buckle folding abstract parasitic folds are typical structures in geological multilayer folds they are characterized by a small wavelength and are situated within folds with larger wavelength parasitic folds exhibit a characteristic asymmetry or vergence reflecting their structural relationship to the larger scale fold here we investigate if a pre existing geometrical asymmetry e g from sedimentary structures or folds from a previous tectonic event can be inherited during buckle folding to form parasitic folds with wrong vergence we conduct 2d finite element simulations of multilayer folding using newtonian materials the applied model setup comprises a thin layer exhibiting the pre existing geometrical asymmetry sandwiched between two thicker layers all intercalated with a lower viscosity matrix and subjected to layer parallel shortening when the two outer thick layers buckle and amplify two processes work against the asymmetry layer perpendicular flattening between the two thick layers and the rotational component of flexural flow folding both processes promote de amplification and unfolding of the pre existing asymmetry we discuss how the efficiency of de amplification is controlled by the larger scale fold amplification and conclude that pre existing asymmetries that are open and or exhibit low amplitude are prone to de amplification and may disappear during buckling of the multilayer system large amplitude and or tight to isoclinal folds may be inherited and develop type 3 fold interference patterns

rheological controls on the shapes of single layer folds abstract information about rheology can potentially be gained from analyzing the shapes of folds in isolated buckled layers we employ two dimensional finite element models of incompressible flow in power law viscous fluids to investigate this we first show that the shape of the initial perturbation has relatively little effect on the final shape of the folds when the buckling instability is high we find that the most significant factor affecting the shapes of single layer folds is the stress exponent n l in the flow law of the layer the hinges of outer arcs become sharper as n l increases and the limbs become relatively longer and straighter these differences can be expressed quantitatively by defining a curvature index ki which has a value of 0 for a fold formed by circular arcs and 1 for a chevron fold results show a dependence of ki on n l that is well defined for l h 10 with ki increasing with n l data for experimentally produced folds are consistent with the numerical results limb dip ki and l h can all be readily measured on natural folds and provide a basis for comparing the shapes of natural and computer simulated folds the data for small folds in siltstone layers in shales in the central appalachians are consistent with highly non linear flow of the stiff siltstone layers during buckling

folding with thermal mechanical feedback discussion abstract a recent paper in this journal by bruce hobbs klaus regenauer lieb and alison ord hobbs b regenauer lieb k ord a 2008 folding with thermal mechanical feedback journal of structural geology 30 1572 1592 presents an alternative theory to the traditional biot ramberg theory for folding of viscous rocks that involves non equilibrium thermodynamics and thermal mechanical feedback the authors convey a strong message throughout their paper that the folds produced by this theoretical and numerical modelling are geologically realistic and provide a better explanation for many natural folds than the traditional theory they promise the same approach for boudinage and present this folding paper as part of a unified framework for rock deformation processes readers of the journal of structural geology might be led to conclude that this paper provides a good alternative model for folding of rocks our discussion will disagree on four counts

numerical simulations of parasitic folding in multilayers we use the finite element method to simulate slow viscous newtonian flow in two dimensions without gravity and to model asymmetric sand z shaped and symmetric m shaped parasitic folds during multilayer folding during multilayer folding the matrix between stiffer layers shows a deformation close to pure shear in the hinge area and a combination of pure and simple shear in the limb areas thinner layers placed between thicker layers develop symmetric parasitic folds in the hinge and eventually asymmetric parasitic folds in the limbs of the larger fold our results verify numerically the theory that asymmetric parasitic folds develop from symmetric buckle folds that are sheared by the hingeward relative displacement of the thick layers in the limbs of the first order fold to develop asymmetric shapes the amplitudes of the parasitic folds must exceed a critical value before the first order fold begins to amplify otherwise the parasitic folds are unfolded during flattening that takes place in the limb area between the thick layers more than five thin layers are necessary to generate distinct asymmetric parasitic folds for the applied model setting more layers generate higher amplification rates in the thin layers and hence higher amplitudes 2006 elsevier ltd all rights reserved

symbiotic interactions in the ordovician of baltica abstract eleven symbiotic associations between taxa occur in the ordovician of baltica most involving colonial animals bryozoans formed the most numerous associations including possible polychaete bioclaustrations possible polychaete borings conulariids and crinoids and had the highest number of symbiosis cases in baltica the late ordovician symbiotic associations differ from those in the middle ordovician by a remarkable increase in the numbers of recorded cases this is most likely explained by the great ordovician biodiversification event gobe the higher faunal diversity may have created more opportunities for symbiotic relationships these changes in the symbiotic interactions in the ordovician of baltica were probably not related to climate change but had an evolutionary cause

physical experiments of transpressional folding abstract in order to understand the process of folding in obliquely convergent settings we formed folds within a shear box capable of creating homogeneous transpressional deformations folds were created in a single layer of stiff mixed plasticine and silicone that overlay a newtonian silicone for a variety of plate convergence angles as small amplitude folds became visible they were parallel to the long axis of the horizontal finite strain ellipse with increasing deformation the fold hinges rotated parallel with the long axis of the horizontal finite strain ellipse for all angles of convergence this parallelism indicates that fold hinges once formed rotate with the horizontal strain ellipse rather than as material lines the experiments highlight several interesting effects of transpression dynamics the fold hinges initiate parallel to either 1 or 2 and are parallel to either s 1 or s 2 with increasing deformation neither infinitesimal strain stress nor finite strain is resolvable solely from fold geometry further the net amount of contraction determined by folding across the zone was overestimated in all cases except pure contraction this effect is obvious for the case of wrenching where folding implies that the zone contracts if elongation parallel to the fold hinge is not considered therefore attempts to balance cross sections in transpressional zones will tend to overestimate contraction unless the wrench component of deformation is addressed this result is validated by applying the modeling results in folding in central california adjacent to the san andreas fault where cross section balancing indicates higher amounts of contraction than predicted by plate motion

a new type of numerical experiment on the spatial and temporal patterns of localization of deformation in a material with a coupling of grain size and rheology abstract the patterns of localization of deformation in recrystallising materials are investigated via a series of two dimensional grain scale numerical simulations these simulations couple a grain size and strain dependant viscous rheology with grain size reduction and grain growth processes the simulations are able to predict the mechanical microstructural and strain evolution of the polycrystals to moderate strain and allow us to examine the nature of the time dependent feedback between mechanical and microstructural behavior it was found that significant strain localization occurred only when the grain size dependence of the viscosity was non linear and was greatly enhanced by the activity of the grain size modifying processes the intensity and location of the zone of strain localization varied spatially and temporally and displayed two length and time scales with the result that the finite strain state showed a much broader and hence less intense zone of localized deformation than the instantaneous state

nucleation of ductile shear zones on dilatant fractures small shear zones in granitic rocks of the sierra nevada california and near roses rosas northeast spain display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation many zones have sharp nearly planar boundaries between their highly deformed interiors and the undeformed wall rock the roses shear zones narrow continuously to hairline fractures at their ends mineralized microcracks oriented parallel to the shear zones cluster near the zone boundaries and are interpreted as relicts of the earlier fracturing episode dynamic recrystallization of minerals filling these microcracks demonstrates that fracturing predated the ductile deformation gradients in ductile strain suggest that deformation spread laterally into the wall rocks after nucleation of the shear zones we suggest that cracks enhance the wall rock ductility by increasing local fluid fluxes thereby promoting chemical alteration and or hydrolytic weakening

outcrop scale tomography insights into the 3d structure of multiphase boudins abstract we analyze the 3d geomorphology of a boudinaged amphibolite layer encased in marble using meter scale tomography by serial sectioning high resolution imaging and 3d reconstruction of a 2 m block from a quarry in the high grade core of the naxos core complex local uncertainties in the model are resolved by dissolution of marble in selected slabs yielding a large highly accurate 3d model results show five generations of brittle extension of the amphibolite in ductile marble although examination of the boudinaged layer on the outside of the block only suggested one generation most prominent are the rotated domino boudins where part of the dilatancy between domino boudins can be shown to be due to later reactivation shear fractures between domino boudins have relatively large throw gradients indicating ductile rupture layer parallel stretch as measured by reconstruction of domino boudins decreases with amphibolite layer thickness although the layer has undergone the same bulk stretch everywhere this suggests that with decreasing layer thickness deformation becomes delocalized offering tantalizing glimpses of rheology under metamorphic conditions

coordinated biotic and abiotic change during the great ordovician biodiversification event darriwilian assembly of early paleozoic building blocks abstract the ordovician period records an extraordinary biodiversity increase known as the great ordovician biodiversification event gobe which coincided with a series of environmental changes to the earth system notably a cooling global ocean increased oxygenation and increased nutrient supply from volcanism and continental weathering the co evolution of earth and its biota during this interval has been studied in various contexts on multiple paleocontinents emerging patterns depend on the lens of investigation here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records recent paleontological studies mainly focused on rhynchonelliform articulated brachiopods bryozoa cephalopods trilobites graptolites echinoderms and reef organisms have documented details of diversification body size increase development of ecosystem complexity and intensification of inter continental dispersal from the late cambrian through late ordovician biomass increased markedly between the early and middle ordovician furthermore diversification rates increase statistically during the mid darriwilian age both globally and regionally coincident with these biotic changes geochemical proxies record significant changes to earth s physical system oceanic temperatures decreased and atmospheric oxygen levels increased to near modern levels in concert with the middle ordovician diversification of shelly fauna anoxic pulses ceased and evidence for deep ocean ventilation prevails in middle ordovician strata furthermore a major middle late ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings this multi proxy dataset records near simultaneous changes in fossil rich shallow marine environments during exactly the interval of greatest diversification by integrating biotic and geochemical datasets a clear picture of the co evolution of earth and its biota emerges indicating that the darriwilian was the critical interval facilitating increased capacity of ecosystems we therefore recommend restricting the term gobe to indicate this short interval of rapid diversification and ecosystem change and using ordovician radiation when referencing the sum of diversifications that occurred throughout the ordovician period

strain paths of three small folds from the appalachian valley and ridge maryland structural analysis indicates that for a given set of conditions subtle differences in layer configuration and rheology can result in major differences in fold kinematics we studied three hand sample scale folds from the maryland valley and ridge province in an effort to understand the accommodation of folding on meso and microscopic scales and to constrain the deformation histories of the folds sampled two single layer folds and one multilayer fold were taken from two nearby outcrops of the wills mountain anticline and are therefore interpreted to have developed under virtually identical pressure and temperature conditions although all three folds are dominated by carbonate material structural fabrics are unique for each fold indicating rheological contributions to and local structural control of fabric development both single layer folds have asymmetric vein distributions that are consistent with strain directions expected from asymmetric flexural flow slickenlines on one sample and cleavage in an adjacent layer in the other sample support this interpretation the first of these folds appears to have undergone late stage hinge tightening as evidenced by the development of crosscutting bed normal stylolites in contrast veins stylolites and the intracrystalline deformation in the multilayer fold are suggestive of symmetric tangential longitudinal strain followed by heterogeneous sub horizontal flattening the three folds are interpreted to be buckle folds with differing mechanisms accommodating strain within the competent layers

a theory of stress and strain variations in viscous layers and its geological implications abstract a system of contrasting viscous layers in oblique deformation is considered a method of deriving the attitudes and relative values of the principal deviatoric stresses in a layer of particular attitude and viscosity is presented for most oblique layers the method is numerical and graphical but some algebraic solutions are given examples demonstrate that stresses refract from layer to layer and vary in size the results are more useful to the geologist as infinitesimal strains it is shown that strain is not homogeneous in layered viscous systems and strong variations in orientation size and shape of strain ellipsoids may occur some tentative geological implications are suggested

single layer buckle folding in non linear materials i experimental study of fold development from an isolated initial perturbation abstract scale model experiments involving pure shear compression of a single layer embedded in a weaker matrix were carried out to study the influence of a pre existing isolated perturbation in the shape of an otherwise planar layer on the location and shape of folds different grades of paraffin wax were used as analogues for rocks with power law rheology the wax used for the matrix flows in steady state with a stress exponent of about 3 8 the layer has a stress exponent of about 3 and strain softens viscosity ratios considered at yield stress for the layer and matrix of 30 1 and 8 1 were used bell shaped perturbations were introduced into the layer with initial average wavelengths of 8 16 and 32 times layer thickness and corresponding initial limb dips of 12 5 and 3 respectively the fold packet develops at the site of the introduced perturbation and spreads slowly along the layer with increasing shortening the shape of the initial perturbation strongly influences the fold geometry broader initial perturbations develop broader folds undergo more layer parallel shortening and develop a wider zone of contact strain the influence of the introduced perturbation shape is most marked when its average wavelength component is larger than the dominant wavelength i e the wavelength of maximum growth rate the degree of bonding between layer and matrix also influences fold development folds with welded layer matrix boundaries amplify much more slowly with a greater proportion of layer parallel shortening than folds where slip between layer and matrix is promoted

symbiosis of conulariids with trepostome bryozoans in the upper ordovician of estonia baltica abstract the trepostome bryozoans diplotrypa abnormis d bicornis d petropolitana esthoniopora communis e subsphaerica mesotrypa excentrica m expressa m raritabulata and monotrypa jewensis have symbiotic associations with the conulariid climacoconus bottnicus in the upper ordovician of estonia all bryoimmured conulariids are very small and oriented perpendicular or nearly so to the growth surface of the host trepostome colony muddy seafloors may have promoted this symbiosis between conulariids and bryozoans because the former required a hard substrate for attachment it is possible that the numerous smaller specimens among endobiotic conulariids usually died as juveniles together with their bryozoan host or alternatively the smaller endobiotic conulariids may have been lilliput forms of free living conulariids that died at a mature age conulariid trepostome associations were likely not a result of accidental intergrowth of two organisms additional protection against predators provided by the calcitic bryozoan skeleton may have been among the benefits for the conulariid symbionts usually trepostomes with conulariid symbionts do not contain other invertebrates but in the katian in some cases they also hosted anoigmaichnus bioclaustrations the exact type of symbiosis between trepostomes and conulariids remains unresolved but most likely the associations were slightly parasitic or commensal the available data suggest that bryozoans preferred cnidarians over the other invertebrates as symbionts

finite element models of buckle folds in non linear materials abstract buckle fold development in single layers has been simulated by means of a two dimensional finite element model incorporating incompressible power law viscous fluids we have produced folds with the stiff layer initially parallel to and oblique to the bulk maximum shortening direction and investigated how fold growth rates and geometry vary as functions of viscosity ratio and power law exponent results for single layer buckling and the related instabilities of boudinage mullions and inverse folding closely match theoretical predictions in terms of initial growth rates which depend strongly on the power law exponent the results further show that the fold shape is sensitive to changes in the power law exponent of the stiff layer the most striking feature is that the fold hinges become sharper and the limbs become relatively longer and straighter as the exponent increases asymmetry of folding can arise in bulk pure shear models of asymmetric folds with inclinations of the stiff layer of up to 30 have been made the results indicate that initial growth rates are independent of the inclination of the stiff layer results of this work suggest that a number of geometrical properties of folds may be useful in providing information on the rheological behavior of rocks deformed under very slow natural conditions

folding with thermal mechanical feedback folding of viscous layered rocks is traditionally viewed as an instability arising from viscosity differences between layers this approach derives from biot and for purely viscous materials predicts the growth of single wavelength systems the dominant wavelength is sensitive to layer thickness the viscosity ratio between the layers the amount of shortening and the boundary conditions this paper presents one alternative theory of folding to that of biot and addresses the deformation of elastic viscous materials where the viscosity ratio between layers is small and biot theory predicts folds will not form such ratios are consistent with situations in the mid to lower crust as indicated by experimental data folding results from the coupling between temperature dependent viscosity and heat generated by deformation the result is weakening in the hinges of embryonic folds and subsequent buckling this process is distinct from the biot buckling process the structures that develop resemble natural structures in that folds develop at a range of length scales hinges undergo strong thickening and axial plane crenulations form this approach is grounded in non equilibrium thermodynamics the coupling of deformation to fluid flow and chemical reactions is explored as part of a unified framework for rock deformation processes

information from fold shapes abstract folds contain information about the deformation rocks have undergone and the condition of the rocks during deformation how much of this can we decipher the geometrical characteristics of folds and the strain distribution within them are perhaps the key to unlocking this information if the folded layers were originally planar they reflect inhomogeneous deformation but without additional originally planar or linear markers of other orientations we cannot determine the state of strain the strain in a parallel fold for example can be accommodated either by flexural slip or by tangential longitudinal strain two quite different but geologically realistic strain distributions among an infinite number of possible ones there are some constraints imposed by fold shapes on possible strain distributions fold asymmetry reflects in most circumstances sense of shear strain parallel to the general orientation of the folded layering problems may arise in shear zones with large strains and in foliated rocks in which kink bands develop at small strains information on the orientations of principal stresses cannot be obtained from folds unless the strain is small and the mechanism of folding understood as for kink bands or unless the bulk flow is of constant vorticity which is difficult to demonstrate the distribution of measured values of wavelength thickness in a population of single layer folds together with measures of strain and estimates of amplification can be used to estimate the viscosity ratio of the stiff layer to its matrix and the degree of non linearity in the flow law if effective power law flow is assumed information on the rheological state of rocks at the time of folding can also be obtained from the pattern of curvature variation in individual single layer folds as demonstrated by the use of computer simulations of folding where applied such methods indicate that the slow natural flow of rock involved in folding is mostly consistent with non linear power law rheology as expected from the results of experimental rock deformation involving crystal plastic deformation mechanisms

numerical modelling of porphyroclast and porphyroblast rotation in anisotropic rocks abstract the rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field experimental and numerical modelling studies often centred on the question whether objects rotate or not in non coaxial deformation with numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations originally shape and orientation were investigated while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century due to improved numerical techniques anisotropic rheology has become the focus of most recent studies indicating that it is a primary factor in the rotation behaviour of objects we present new simulations investigating the role of anisotropy on different scales relative to the object and show how this influences the rotation rate as well as the inclusion patterns in case of syntectonically growing porphyroblasts these simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects the variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field

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

deformation fields around a fault embedded in a non linear ductile medium summary the geological record of deformation is often characterized by a combination of discontinuous deformation in which strain is concentrated in faults and continuous deformation in which strain is distributed through the material where slip occurs on a fault that terminates the surrounding material is deformed in the lower crust and in cases where large strains occur over long geological time scales it is appropriate to model the deformation using a viscous probably non linear viscous rheology we describe a method for practical finite element solution of this problem using a dynamically self consistent formulation for stress and displacement on a fault of arbitrary geometry the accuracy of the method is tested by comparison with an analytical solution for the linear rheology we describe here the instantaneous deformation fields around a mode ii fault under both plane strain and plane stress conditions and a range of rheological exponents n where strain rate is proportional to deviatoric stress to the nth power the distributions of stress and strain rate around the fault tip are controlled primarily by the rheological exponent n a localized zone of high strain rate projects beyond the end of the fault if n is about 3 or greater and the degree of localization of deformation increases with the value of n the zone of high shear strain rate can be defined in practical terms by considering 1 the region in which the creep velocity differs by more than 20 per cent from the velocity on the nearby external boundary and 2 the region in which the maximum shear strain rate is greater than about twice the externally imposed shear strain rate for n 1 the volumes so defined differ considerably but for large values of n the two definitions both describe the same narrow zone of deformation beyond the end of the fault evaluation of the navier coulomb criterion for brittle failure of the medium surrounding the fault tip shows first that brittle failure is much more likely on the extensional side of the fault than the compressional side it also shows that the volume of material subject to brittle failure decreases rapidly with increasing n because of the relatively weaker stress singularity we analyse previously published displacement versus distance data for faults terminating in sedimentary rocks at 0 1 to 100 m length scales under different tectonic conditions in order to determine the rheological exponent n these analyses result in n values between approximately 0 85 and 5 for the different faults with error bounds on n typically pl 1 the variation in n values may result from differences in pressure temperature and fluid conditions at the time of faulting more importantly the analysis demonstrates a new method for the determination of the effective rheological exponent under in situ geological conditions

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

single layer folding in simple shear abstract despite the common occurrence of simple shear deformation laboratory and numerical simulations of folding have so far been almost exclusively in pure shear here we present a series of finite element simulations of single layer folding in simple shear up to high shear strains 4 and up to 75 shortening of the folding layer in the simulations we vary the viscosity contrast between layer and its surroundings 25 100 the stress exponent 1 or 3 and the kinematics of deformation pure versus simple shear in simple shear fold trains do not show a clear asymmetry axial planes form perpendicular to the developing fold train and rotate along with the fold train differences in geometries between folds formed in simple and pure shear folds are thus difficult to distinguish visually with simple shear folds slightly more irregular and with more variable axial plane orientation than in pure shear asymmetric refraction of an axial planar cleavage is a clearer indication of folding in simple shear the main effect of an increase in stress exponent is an increase in effective viscosity contrast with only a secondary effect on fold geometry naturally folded aplite dykes in a granodiorite are found in a shear zone in roses ne spain comparison of the folded dykes with our numerical simulations indicates a viscosity contrast of around 25 and a stress exponent of 3 the natural folds confirm that at this moderate viscosity contrast a significant amount of shortening 20 30 is achieved by layer thickening instead of folding

dynamic recrystallisation of ice aggregates during co axial viscoplastic deformation a numerical approach results of numerical simulations of co axial deformation of pure ice up to high strain combining full field modelling with recrystallisation are presented grain size and lattice preferred orientation analysis and comparisons between simulations at different strain rates show how recrystallisation has a major effect on the microstructure developing larger and equi dimensional grains but a relatively minor effect on the development of a preferred orientation of c axes although c axis distributions do not vary much recrystallisation appears to have a distinct effect on the relative activities of slip systems activating the pyramidal slip system and affecting the distribution of a axes the simulations reveal that the survival probability of individual grains is strongly related to the initial grain size but only weakly dependent on hard or soft orientations with respect to the flow field dynamic recrystallisation reduces initial hardening which is followed by a steady state characteristic of pure shear deformation

self consistent generation of tectonic plates in three dimensional mantle convection abstract despite the fundamental importance of plates in the earth s mantle convection plates have not generally been included in numerical convection models or analog laboratory experiments mainly because the physical properties which lead to plate tectonic behavior are not well understood strongly temperature dependent viscosity results in an immobile rigid lid so that plates where included at all in 3 d models have always been imposed by hand an important challenge is thus to develop a physically reasonable material description which allows plates to develop self consistently this paper focuses on the role of ductile shear localization in two dimensional geometry it is well established that strain rate softening non newtonian rheologies e g power law visco plastic cause weak zones and strain rate localization above up and down wellings resulting in a rudimentary approximation of plates three dimensional geometry however is fundamentally different due to the presence of transform plate boundaries with associated toroidal motion since power law and visco plastic rheologies do not have the property of producing shear localization it is not surprising that they do not produce good plate like behavior in three dimensional calculations here it is argued that a strain rate weakening rheology previously shown to produce plate like behavior in a two dimensional sheet representing the lithosphere is a reasonable generic description of various weakening processes observed in nature one and two dimensional models are used to show how this leads to shear localization and the formation of faults this rheology is then applied to the high viscosity lithosphere of 3 d mantle convection calculations and the velocity pressure viscosity solution for the entire 3 d domain lid and underlying mantle is solved self consistently it is found that the lithosphere divides into a number of very high viscosity plates separated by narrow sharply defined weak zones with a viscosity many orders of magnitude less than the plate interiors broad weak zones with dominant convergent divergent motion above up and down wellings are interconnected by a network of narrow weak zones with dominant strike slip motion passive spreading centers are formed in internally heated cases while the resulting plates are not fully realistic these results show that self consistent plate generation is a realizable goal in three dimensional mantle convection and provide a promising avenue for future research

baltica a synopsis of vendian permian palaeomagnetic data and their palaeotectonic implications torsvlk t h smethurst m a van der voo r trench a abrahamsen n and halvorsen e 1992 baltlca a synopsis of vendlan permmn palaeomagnetic data and their palaeotectonlc implications earth sci rev 33 133 152 in light of recent additions to the palaeozoic palaeo magnetlc data base particularly for the ordovlclan era a revised apparent polar wander apw path for baltica has been constructed following a rigorous synthesis of all late precambrlan perm lan data the apw path is characterized by two prominent loops firstly a late precambrlan camb rlan loop probably relating to a rifting event and secondly a younger loop relating to a mid silurian scandlan colhsion event these features imply major change in plate tectonic reconfiguratlon baltlca probably represented an individual continental unit m early palaeozoic times and was positioned m high southerly latitudes in an inverted geographic orientation in such a reconstruction baltlca was separated from the northern margin of gondwana by the tornqulst sea and from laurentla by the iapetus ocean the tornqulst zone is thus interpreted as a passive or dextral transform margin during the early palaeozoic while undergoing counter clockwise rotations up to 1 6 ma baltica drifted northward through most ol the palaeozoic except for a short period of southerly movement in late silurian early devonian times after colhslon with laurentla rapid movements in latitude up to 9 cm yr are noted m late precambrlan earl y palaeozoic times and slgmficant decrease in velocities throughout palaeozoic time probably reflect the progressive amalgamation of a larger continent by early devonian euramerlca and permian pangea times the tornqulst sea had a principal component of palaeo east west orientation hence it is difficult to be precise in the timing of when micro continents such as eastern avalonla and the european masslfs ultimately collided along the southwestern margin of baltica these micro continents are considered to have been peripheral to gondwana m high southerly latitudes during the early ordovlclan eastern avalonla clearly had rifted off gondwana by llanvlrn llanded o times and may have collided with baltlca during late ordovlclan times although the present available sdunan palaeomagnetlc data from eastern avalonia may suggest colhslon in late silurian times across the lapetus facing margin of baltica laurentla was s tuated in equatorml to southerly latitudes during most of the lower palaeozoic these continents collided in mid silurian times l e a first collision between southwestern norway and greenland scotla nd which gave rise to the early scandlan orogeny 425 ma in southwestern norway possible followed by a later but less dramatic scandlan event in northern norway at around 410 ma since baltlca was geographically reverted m early palaeozoic times the colhsional margin could not have been a margin that once rifted off laurentm as assumed in a number of plate tectonic models

localised folding in general deformations abstract one control on the buckling of a layer or layers embedded in a weaker matrix is the reaction force exerted by the deforming matrix on the layer if the system is linear and this force is a linear function of the layer deflection as for linear elastic and viscous materials the resulting buckles can be sinusoidal or periodic however if the system is geometrically nonlinear as in general non coaxial deformations or the matrix material is nonlinear as for nonlinear elastic non newtonian viscous and plastic materials the buckling response may be localised so that individual packets of folds form the resulting fold profile is not sinusoidal these folds are called localised folds most natural folds are localised one view is that irregularity derives solely from initial geometrical perturbations we explore a different view where the irregular geometry results from a softening material or geometrical nonlinearity without initial perturbations localised folds form in a fundamentally different way than the biot wavelength selection process the concept of a dominant wavelength does not exist folds grow and collapse sequentially rather than grow simultaneously we discuss the formation of localised folds with recent considerations of constitutive behaviour at geological strain rates for general three dimensional deformations

initiation of folding and boudinage in wrench shear and transpression abstract a linear stability analysis is performed on a deforming layered three dimensional linear viscous system the system consists of a single layer of viscosity embedded in a medium of viscosity the layer is oriented normal to the z axis and both the layer and medium are subjected to uniform simple shearing parallel to the y axis wrench shear with superposed uniform shortening parallel to x and extension parallel to z transpression the stability of cylindrical perturbations of the form a t cos ax y is examined it is found that fold type perturbations are unstable and pinch and swell disturbances are unstable in some cases for the case of wrench shear alone the fastest growing buckling disturbances are oriented at 45 to the positive y axis while the fastest growing pinch and swell disturbances have positive growth rates and are oriented 90 to the fold axes additional shortening parallel to the x axis transpression causes the fold axes to initiate at lower angles to the y axis pinch and swell disturbances may or may not be unstable in transpression depending on the magnitude of stretching parallel to the z axis

recumbent folds key structural elements in orogenic belts abstract this review has two main parts the first of them presents existing ideas and data related to recumbent folds reviewing aspects such as the physical conditions of the development of these folds the strain inside the folded layers the kinematic mechanisms of their formation the role of gravitational forces the tectonic context of their development and the structures associated with them in the second part the above ideas are discussed and possible mechanisms for the development of these folds are presented it is proposed that initial perturbations of the layers are essential to give rise to the asymmetry of recumbent folds these perturbations may be non planarities of the layering or may be linked to the existence of a core or basement of competent rock that hinders the normal propagation of the deformation this could explain why many large recumbent folds have a root zone deformation with an important component of simple shear is a general condition for the formation of recumbent folds in areas with very low grade metamorphism competent layers often play an active role during the deformation and undergo buckling with the development of an overturned fold limb which can be stretched and thinned to finally produce a pair of recumbent folds separated by a thrust in areas with low or medium metamorphism buckling under a simple shear regime is probably the most important mechanism for producing large folds with gentle or moderately dipping axial surfaces subsequent kinematic amplification by coaxial strain components with vertical maximum shortening is important for the formation of recumbent folds these components involve a sub horizontal stretching that can cause a problem of strain compatibility and give rise to a basal thrust in areas deformed under high p and t conditions recumbent folds can develop by flow perturbations and kinematic amplification of folds this is probably a common mechanism in ductile shear zones

when do folds unfold during progressive shear folded layers in rocks can be stretched again potentially unfolding the folds back to straight layers little is known however about how to recognize partly or even entirely unfolded layers when folded layers can unfold what determines their mechanical behavior and how can we recognize them in the field in order to address these questions we present a series of numerical simulations of the stretching of previously folded single layers and multilayers in simple shear layers do not completely unfold when they undergo softening before or during the stretching process or when adjacent competent layers prevent them from unfolding intrafolial folds and cusp like folds adjacent to straight layers as well as variations in fold amplitudes and limb lengths of irregular folds are indicative of stretching of a fold train

a rugose coral bryozoan association from the lower devonian of nw spain abstract a new rugose coral cystoporate bryozoan association is here described from the devonian of nw spain this is the first evidence of intergrowths between devonian rugose corals and bryozoans in this case bryozoans provided a suitable substrate for the settlement of corals which were subsequently encrusted by the bryozoans the hypothesis of intergrowth between living organisms is supported by the absence of encrustation of the rugose coral calices by the cystoporates we suggest that the association was specific and developed through chemical mediation this symbiosis was facultative for the bryozoans but likely not for the corals the association provided the bryozoan host with additional substrate for encrustation as well as protection from various predators and it allowed the rugose corals to grow in a muddy environment and benefit from the feeding currents of the bryozoans

from xy tracking to buckling axial plane cleavage fanning and folding during progressive deformation folding of axial plane cleavage can occur during progressive deformation without a change in the overall background flow two field examples of upright lachlan fold belt se australia and recumbent naukluft nappe complex central namibia folds are presented in which strongly refracted pressure solution cleavage in competent layers on the fold limbs is buckled as a result of ongoing fold amplification finite element modelling confirms that cleavage refraction on limbs can be sufficient for cleavage planes to be subsequently shortened and therefore folded cleavage refraction is unequally developed on opposite limbs of asymmetric folds formed by oblique shortening of a layer in coaxial flow or by folding in a more general shear environment the differences in finite strain on opposite limbs can be quite marked even when the fold shapes themselves are not obviously asymmetric for folding in simple shear flow as specifically modelled here refraction is only strong on the fold limb that rotates against the imposed sense of shear in known shear environments this provides a potential kinematic indicator in folded units at relatively low strain e g in simple shear of around one where other higher strain indicators typical of mylonites are not yet sufficiently developed or are equivocal

information from folds a review abstract folds are spectacular geological structures that are seen in layered rock on many different scales to mark 30 years of the journal of structural geology we review the information that can be gained from studies of folds in theory experiment and nature we first review theoretical considerations and modeling from classical approaches to current developments the subject is dominated by single layer fold theory with the assumption of perfect layer parallel shortening but we also review multilayer fold theory and modeling and folding of layers that are oblique to principal stresses and strains this work demonstrates that viscosity ratio degree of non linearity of the flow law anisotropy and the thickness and spacing distribution of layers of different competence are all important in determining the nature and strength of the folding instability theory and modeling provide the basis for obtaining rheological information from natural folds through analysis of wavelength thickness ratios of single layer folds and fold shapes they also provide a basis for estimating the bulk strain from folded layers information about folding mechanisms can be obtained by analysis of cleavage and fabric patterns in folded rocks and the history of deformation can be revealed by understanding how asymmetry can develop in folds by how folds develop in shear zones and how folds develop in more complex three dimensional deformations

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

rheology of fine grained siliciclastic rocks in the middle crust evidence from structural and numerical analysis constitutive models to describe the rheology of the crust are based on a combination of indirect methods and many aspects are still unknown and controversial here we present a new method to quantify the rheology of fine grained siliciclastic rocks which are common in the middle crust and deform by solution precipitation processes we use a combination of structural analysis at different scales and a geomechanical model to develop a parameter estimation scheme to calculate rheological parameters at 350 400 8c and geologic strain rates the results of this study demonstrate that fine grained siliciclastic rocks in the middle crust have a newtonian viscous rheology approximately 10 times weaker than wet quartz this is in agreement with observed microstructures our results imply that the strength of polyphase quartz rich rocks located in the middle crust is much lower than predicted by conventional models based on flow laws for dislocation creep because fine grained siliciclastic rocks control the rheology of the middle crust in many sedimentary basins our results provide new quantitative parameters for geodynamic modelling in settings where dissolution precipitation creep is important d 2005 elsevier b v all rights reserved

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

structural softening of the lithosphere structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant structural softening is fundamentally different than material softening where the decrease in stress is generated by a change in rheological properties with progressive deformation such as grain size reduction resulting from large shearing strain we study structural softening generated by folding of the crust mantle boundary which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere for ductile rheologies the stress decrease represents a decrease of the effective lithospheric viscosity which is proportional to the ratio of stress to lithospheric shortening strain rate we present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate the decrease in effective viscosity can be up to 10 fold

the development of δ clasts in non linear viscous materials a numerical approach abstract winged porphyroclasts are often used as kinematic or strain indicators in shear zones in particular it has been suggested that the geometry of clasts in mylonites can give information about the rheological behaviour of the matrix around the clast passchier et al 1993 we tested this hypothesis with a numerical finite element simulation of the flow field of a viscous matrix around a relatively rigid clast power law viscous flow properties of the matrix with stress exponents of 1 linear viscous 3 and 5 were tested as well as three different sets of approximately simple shear boundary conditions which are relevant to experimental and or natural situations the simulations show that the flow field is primarily determined by the boundary conditions the stress exponent is of secondary importance in general the different boundary conditions produce two types of flow fields about the rigid clast an eye shaped flow field and a bow tie shaped flow field the former is produced by boundary conditions that approximate simple shear applied at an infinite distance from the rigid clast and the latter is produced by boundary conditions which are equivalent to a shear box or ring shear apparatus although both the shear box and ring shear boundary conditions produce bow tie flow patterns the wing geometries which result from the flow patterns for these two cases can be significantly different the geometry of the wings that develop on a clast is a direct function of the flow field around the clast for linear and non linear rheologies only the ring shear boundary conditions produce wings that show stair stepping i e wings that are parallel but not in the same plane in natural shear zones clasts both with and without stair stepping are observed in comparing geologic observations with experimental or numerical modelling it is essential to understand which boundary conditions are appropriate

3d numerical modeling of forward folding and reverse unfolding of a viscous single layer implications for the formation of folds and fold patterns abstract the main aims of this study are to show i that non cylindrical three dimensional 3d fold shapes and patterns can form during a single unidirectional shortening event and ii that numerical reverse modeling of 3d folding is a feasible method to reconstruct the formation of 3d buckle folds 3d viscous newtonian single layer folding is numerically simulated with the finite element method to investigate the formation of fold shapes during one shortening event an initially flat layer rests on a matrix with smaller viscosity and is shortened in one direction parallel to the layering forward modeling with different initial geometrical perturbations on the flat layer and different lateral boundary conditions generates non cylindrical 3d fold shapes and patterns the simulations show that in reality the initial layer geometry and the boundary conditions strongly control the final fold geometry fold geometries produced from the forward folding models are used as initial setting in numerical reverse folding models with parameters identical to those of forward models these reverse models accurately reconstruct the initial geometry of forward models with also only one extension event parallel to the previous shortening direction the starting geometry of the forward models is inaccurately reconstructed by the reverse models if a significantly different viscosity ratio than in the forward models is used this work demonstrates that reverse modeling has a high potential for reconstructing the deformation history of folded regions and rheological constraints such as viscosity ratio reverse models may be applied to natural 3d fold shapes and patterns in order to determine if they formed i during a single or multiple deformation events and ii as active buckle folds with a viscosity ratio 1 or as passive kinematic folds without buckling this approach may find much application to fold interference patterns in particular

deformation of granitic rocks experimental studies and natural examples crustal deformation is inherently complex because the crust is inhomogeneous and anisotropic on a wide variety of scales advances in our understanding have come from a combination of field observations laboratory experiments and theoretical models what necessarily links all these investigations is a focus on process crystal plasticity dislocation creep is one of several grain scale deformation processes or mechanisms each process is dominant over a certain range of conditions in the crust has a particular form of the flow law and produces a characteristic set of microstructures characterizing the deformation mechanisms for different minerals over a range of conditions allows one to link the mechanical behavior or flow law associated with that process to the microstructures it produces the information on microstructures can be used to help interpret the thermomechanical history of naturally deformed rocks and flow laws can be used to create more realistic models predicting crustal behavior in different tectonic settings iterations among field theoretical and experimental approaches are critical for making progress in our understanding the deformation process of crystal plasticity is the main focus of this volume crystal plastic deformation of the crust is in many ways more complex than that of the mantle because most crustal rocks are polyphase aggregates and the strengths of common crustal silicates such as quartz feldspar and mica are very different at a given set of conditions yield strength envelopes for the crust as shown schematically in figure 1 are commonly plotted using the dislocation creep flow law for quartz which inherently assumes that quartz is the controlling phase and undergoes steady state flow but is the flow law for dislocation creep of quartz a good approximation for the deformation of a granite let alone a crustal plum pudding of regions and layers of different lithologies the crust

studies of strain and rheology of conglomerates abstract conglomerates can show significant strain variations among their component rock types previous studies have exploited this feature as a method of quantifying the effective viscosity ratios among different rock types using inclusion matrix models that we review here we further investigate the rheology of conglomerates via models of multiphase mixtures of spherical clasts with varying phase fractions using idealised models with two to four phases we can quantify the influence of phase viscosities and volume fractions on the bulk viscosity and reveal how this controls the strain partitioning among the different phases we consider practical methods of analysing conglomerates and choose r f analysis for measuring clast strain and fry analysis for whole rock strain the methods are illustrated through case studies of two very different conglomerates the port askaig tillite scotland and cesson conglomerate brittany we define the important rock phases in each and assess their strain variations although providing strain measurements of regional interest our main purpose is to quantify the viscosity ratios among the component rocks we find that among the most common rock types semipelite quartzite psammite volcanics granite the viscosity ratios span about one order of magnitude agreeing with previous studies these small numerical values for viscosity ratios among many rock types in many conglomerates lead us to conclude that these rocks each deformed at approximately constant viscosity i e as linearly viscous rather than according to a power law

preparation and deformation of synthetic aggregates of quartz synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder impure silica gel and high purity silicic acid by hydrothermal isostatic pressing at 300 mpa the resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests the flow stress at 1200 1300 k was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity it is questionable that the natural quartz origin specimens were fully equilibrated in respect to the activity of the water but equilibration has probably been achieved in the amorphous silica origin specimens which crystallized under the experimental conditions in the deformation of the latter materials the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kj mol 1 for both however the impure gel origin material gave a stress exponent of about 2 3 whereas the high purity silicic acid origin material gave a stress exponent of about 4 in spite of the grain sizes being about 90 m and 20 m respectively it is concluded that while the impure specimens have higher intragranular strengths there is also a significant contribution of grain boundary processes to the strain in them which is absent in the high purity specimens

a tale of two viscosities abstract we describe a series of 12 large strain two dimensional simple shear numerical experiments on two phase linear and non linear viscous materials the modal percentage of the two phases their relative viscosities and the stress exponent of the dependence of viscosity on stress were systematically varied allowing us to observe the effects of these parameters on the flow behaviour of the system all experiments exhibited localisation but the extent and type of localisation varied considerably and the range of parameters allows both classical sc as well as sc structures to form the inter granular finite strain geometries also varied systematically although there is a tradeoff between non linearity of viscosity and the viscosity contrast that results in similar geometries for quite different mechanical behaviour the finite rotation of the harder phases was similar within any one experiment originally parallel passive marker lines thus remain sub parallel after deformation and rotation which may be mistaken for non rotation the internal deformability and interaction between grains allows meta stable grain elongation orientations to form without any additional consideration of external slip surfaces the slight variability of initial grain geometry and subsequent grain interactions leads to a wide variability in final grain shapes for the same initial viscosities

life in a living substrate modular endosymbionts of bryozoan hosts from the devonian of spain abstract a new bioclaustration structure is described from the emsian lower devonian aguion formation of asturias nw spain it is the result of intergrowth of two bryozoan species leioclema elegans and loxophragma cf leptum hosting an unknown soft bodied modular symbiont these structures point to the development of host specific symbiotic associations in which both partners grew simultaneously the association is tentatively assessed as commensalism no evidence suggesting parasitism or mutualism has been found a revision of the existing literature on devonian bryozoans reveals that tubular structures similar to those described herein have been observed in different regions of the world only a few trepostome genera developed this kind of association during the devonian and leioclema seems to have been a suitable partner in intergrowths with a range of soft bodied and skeletonized organisms it is suggested that the association described herein was relatively common during the devonian and that molecular factors such as chemical mediation may have played a role in conducting the development of these specific intergrowths

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

diverse early endobiotic coral symbiont assemblage from the katian late ordovician of baltica abstract a diverse early endobiotic coral symbiont assemblage has been detected within heliolitid tabulate corals of katian age from northwestern estonia this assemblage indicates that the earliest endobiotic coral symbiont communities were not restricted to north america the symbiotic endobiont assemblage comprises abundant cornulites aff celatus and rare conchicolites hosholmensis and chaetosalpinx sp the cornulitids are the earliest known symbiotic endobionts of this group the symbiotic endobionts presumably occurred in certain hosts only and preferred protoheliolites dubius over propora speciosa this record indicates that early endobiotic cornulitids were more diverse than previously thought and appeared in large numbers in the katian it is possible that the appearance of abundant skeletal symbiotic coral endobionts may coincide with the global ordovician biodiversification event supporting the hypothesis of escape from increased predation

rock rheology and sharpness of folds in single layers abstract many folds in rocks display angular profiles with sharp hinges and straight limbs previous studies by many authors have demonstrated that such fold shapes in multilayers result from an intrinsic anisotropy possessed by layered or foliated rocks our results of two dimensional finite element modeling show that folds with sharp hinges and straight limbs may also develop in isolated competent layers under suitable rheological conditions two basic material properties that affect fold shape are non linearity and anisotropy viscous plastic flow power law flow strain rate softening and strain softening are three types of non linear behavior all lead to folds with sharp hinges and straight limbs anisotropy has a similar influence on fold geometry angularity of folds in isolated competent layers increases with increasing non linearity of the layer or increasing anisotropy of the layer and a quantitative relationship between fold angularity and degree of non linearity or anisotropy may be established virtually identical angular fold shapes may be produced by either non linear or anisotropic layer behavior the strain distribution associated with these shapes is very different however for non linear behavior strain is focused in the fold hinges and minimal on the limbs and for anisotropic behavior the reverse is the case with shear in the limbs being dominant these differences suggest that it should be possible to distinguish non linear from anisotropic rheological behavior using layer shape and strain pattern

stress and strain evolution during single layer folding under pure and simple shear abstract folds in rocks are commonly used in geology for strain analysis they contain information about rock rheology and the kinematics and mechanics of deformation the systematic analysis of the evolution of stress and strain during rock folding can provide key information on the mechanical behaviour of rocks and the efficiency of the folding process in order to investigate the evolution of rock rheology during fold development a series of two dimensional simulations of single layer folding are presented here they are run using the finite element method basil integrated within the software platform elle to simulate linear and non linear viscous deformation the kinematics of deformation the competence ratio between the folding layer and surrounding matrix as well as the stress exponent of the power law viscous material are systematically varied the results allow comparing the stages of folding under different deformation kinematic conditions for all simulations the folding amplification process starts when the second invariant of the strain rate tensor in the competent layer deviates from the theoretical strain rate curve for a homogeneous material which corresponds to a viscosity ratio between layer and matrix of m 1 the relative time when the fold amplification starts is determined by the viscosity ratio between the competent layer and its surrounding matrix the initial layer orientation with respect to the shear plane the kinematics of deformation and the stress exponent the folding process is more effective in cases with high viscosity ratio non linear rheology and layers initially oriented at a low angle with respect to the shear plane because the second invariant of the strain rate tensor at the layer deviates earlier from the theoretical curve the results also show differences depending on the boundary conditions where 1 folding a competent layer requires less work in simple shear than in pure shear and 2 the geometrical softening experienced by the competent layer due to fold development is followed by a hardening stage in pure shear and by a major softening phase in simple shear the simulation results suggest that the decrease of stress of a competent layer without decreasing the mechanical strength has a direct influence on the behaviour of a lithospheric layer around the crust mantle boundary which may experience geometrical softening depending on the tectonic settings rather than material softening due metamorphic reactions or grain size reduction

a theory of finite strain variation through contrasting layers and its bearing on cleavage refraction abstract a theory of finite strain variation in contrasting viscous layers is presented the theory is applicable to layers which are oblique to two principal strains but parallel to the third and is not restricted to plane strain results may be obtained algebraically or by use of the mohr diagram for strain solutions are given for various examples of layering attitude and viscosity ratio measured with respect to a reference layer it is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers and in some cases the principal axes may be exchanged low strain is indicated in relatively more viscous layers and high strain with extension at a smaller angle to the layering in less viscous layers the latter may approximate simple shear parallel to layering if the strain is sufficiently high the geological implications of the theory are strong strain variations in layered rock sequences it is suggested that in general strain will be inhomogeneous from layer to layer both in orientation and amount the relationship of strain and cleavage are reviewed in the introduction to establish the validity of predicting cleavage patterns from strain data qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the xy planes of strain with this assumption some three dimensional features of planar and linear fabric refraction in contrasting rocks are predicted

are viscosity ratios of rocks measurable from cleavage refraction abstract in this questions contribution i consider some of the accepted methods of measuring viscosity ratios in rocks and pose a new one for a wide range of strain and orientations it can be shown that the bedding normal is immeasurably close to the xy plane and this has applications to the relationship between cleavage and strain i therefore propose that with some limitations cleavage refraction can provide a measure of effective viscosity ratios in layered rocks examples show that cleavage refraction across competence contrasts yields surprisingly small viscosity ratios this method might also provide a way of distinguishing newtonian from non newtonian behaviour of rocks over time and space

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

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

an apparatus to experimentally model the dynamics of ductile shear zones abstract we present a ring shear apparatus designed to deform soft ductile rock analogues to arbitraily high shear strains in simple shear in experiments attempting to model processes that occur in natural ductile shear zones the sintered specimen is deformed between two unsupported cylinders by two rotating grips under a normal and shear stress up to 5 mpa and at temperatures up to 600 k first results of experiments with octachloropropane ocp c 3 cl 8 and camphor c 10 h 16 o are presented experiments with pure ocp samples show that a steady state strain rate is achieved at a shear strain of around 4 when a steady state grain shape and crystallographic preferred orientation is established mechanical properties compare well with those of cylindrical samples deformed in triaxial compression shear localisation is relatively minor samples of pure camphor on the other hand show very strong shear localisation behaviour and mechanical properties which do not compare well with those of cylindrical samples deformed in triaxial compression experiments with two phase samples ocp containing stronger camphor inclusions show an initial softening of the composite followed by hardening this is associated with the development of a foliation defined by camphor inclusions that stretched to lenses and layers and the subsequent folding of this foliation at shear strains above 50 a steady state strain rate is not reached even at strains of 100 the experiments produce results closely resembling the kinematics observed in natural shear zones and give a first look into the complex changes in mechanical properties which occur at very high shear strains they show how this technique in conjunction with transparent deformation cells can be used to study both microstructural and rheological development during long simple shear deformation histories

flexural flow folding does it occur in nature flexural flow produces a continuous deformation equivalent to that of a flexed deck of cards of vanishing card thickness it is assumed to be a common process by which parallel folds are formed in nature in competent anisotropic rock layers the strain pattern produced by flexural flow contrasts strongly with that produced in parallel folds formed by the bending or buckling of isotropic competent layers but it has not been documented in nature two basic questions are thus 1 if anisotropy is sufficiently large will flexural flow develop and 2 if so are competent rocks sufficiently anisotropic for them to respond to deformation by flexural flow a measure of planar anisotropy a is given by the ratio of viscosity in shortening to viscosity in shear finite element modeling indicates that if the value of this ratio is greater than about 50 the strain pattern in a buckle fold in a competent layer will be almost indistinguishable from one of flexural flow such a high anisotropy however is unlikely in natural competent layers the most texturally anisotropic rocks tend to be those rich in phyllosilicates and these usually behave in an incompetent fashion common competent rocks such as those rich in quartz and feldspar are rarely significantly anisotropic a strong preferred crystallographic orientation as a result of crystal plastic flow can produce anisotropic layer behavior but a is unlikely to exceed 10 which is insufficiently great for folding by flexural flow we thus conclude that flexural flow is unlikely in single competent layers the aggregate response to deformation however of composite layers of alternating competent and incompetent rocks may be highly anisotropic and on average correspond to flexural flow

strain and competence contrast estimation from fold shape a new method to estimate strain and competence contrast from natural fold shapes is developed and verified by analogue and numerical experiments strain is estimated relative to the nucleation amplitude an which is the fold amplitude when the amplification velocities caused by kinematic layer thickening and dynamic folding are identical an is defined as the initial amplitude corresponding to zero strain because folding at amplitudes smaller than an is dominantly by kinematic layer thickening for amplitudes larger than an estimates of strain and competence contrast are contoured in thickness to wavelength h l and amplitude to wavelength a l space these quantities can be measured for any observed fold shape contour maps are constructed using existing linear theories of folding a new nonlinear theory of folding and numerical simulations all for single layer folding the method represents a significant improvement to the arc length method the strain estimation method is applied to folds in viscous newtonian power law non newtonian and viscoelastic layers also strain partitioning in fold trains is investigated strain partitioning refers to the difference in strain accommodated by individual folds in the fold train and by the whole fold train fold trains within layers exhibiting viscous and viscoelastic rheology show different characteristic strain partitioning patterns strain partitioning patterns of natural fold trains can be used to assess the rheological behaviour during fold initiation d 2001 elsevier science b v all rights reserved

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