fracture and separation histories of stretched belemnites and other rigid brittle inclusions in tectonites abstract apparent extension of a stretched rigid inclusion e r l f l 0 1 where l 0 and l f are original and final lengths respectively is always less than strain reversal extension e f calculated by sequentially closing inter fragment gaps until the inclusion is restored to its original length differences between the two measures are strongly influenced by the number of fractures n and their timing within the total stretch history and weakly influenced by the fracture location several fracture and separation models are established to explore these differences using monte carlo simulations all normalized to e r 0 5 when fragment and gap lengths are random and uncorrelated e f values fall in the range 0 5 e f e f values the expected mean value is still much greater than 0 5 when n is small but approaches e r with increasing n and with decreasing time interval between successive fractures model predictions are compared with stretched belemnites in the external french alps results indicate that belemnite fracturing continued throughout a substantial proportion of total stretch history the proportion varying from locality to locality and correlating strongly with maximum stretch recorded this probably reflects variations in stress and suggests that if constant rheology is assumed stretched belemnites may be used to estimate relative strain rates as well as absolute strains

pinch and swell structure and shear zones in viscoplastic layers abstract two dimensional finite element simulations are presented for the extension of stiff viscoplastic layers embedded in a weaker viscous matrix layers and matrix exhibit power law flow laws and the layers exhibit additionally a von mises yield stress the power law flow law applies to rock deformation in the diffusion and dislocation creep regime and the von mises plasticity to the low temperature plasticity regime e g dislocation glide simulations show that pinch and swell structure forms for small viscosity ratio i e 10 20 and typical power law stress exponents i e 1 5 the pinches in layers with initial random geometrical perturbation form consecutively i e not simultaneous in multilayers pinches on both the single layer and the multilayer scale develop furthermore shear zones develop due to the oblique linkage of pinches across the multilayer these shear zones have a stable position cause a normal drag geometry and exhibit significant displacement the numerical results and the importance of low temperature plasticity are supported by field observations microstructural observations and ebsd orientation maps for pinch and swell structure in calcite veins the presented models can explain strain localization by shear zone formation during bulk pure shear extension of viscoplastic multilayers without any material softening or feed back mechanism e g shear heating

strain rate dependent calcite microfabric evolution an experiment carried out by nature abstract a flanking structure developed along a secondary shear zone in calcite marbles on syros cyclades greece provides a natural laboratory for directly studying the effects of strain rate variations on calcite deformation at identical pressure and temperature conditions the presence and rotation of a fracture during progressive deformation caused extreme variations in finite strain and strain rate forming a localized ductile shear zone that shows different microstructures and textures textures and the degree of intracrystalline deformation were measured by electron backscattered diffraction marbles from the host rocks and the shear zone which deformed at various strain rates display crystal preferred orientation suggesting that the calcite preferentially deformed by intracrystalline plastic deformation increasing strain rate results in a switch from subgrain rotation to bulging recrystallization in the dislocation creep regime with increasing strain rate we observe in fine grained 3 m ultramylonitic zones a change in deformation regime from grain size insensitive to grain size sensitive paleowattmeter and the paleopiezometer suggest strain rates for the localized shear zone around 10 10 s 1 and for the marble host rock around 10 12 s 1 we conclude that varying natural strain rates can have a first order effect on the microstructures and textures that developed under the same metamorphic conditions

domino boudinage under layer parallel simple shear abstract the boudin segments of a torn competent layer experience synthetic rotation in layer parallel simple shear as long as the individual segments in a boudin train are constrained by their neighbors even a highly viscous boudin deforms internally to create the necessary space for rotation the rotation rate is then much smaller compared to the case of an isolated segment hence a small tilt of boudin segments is not indicative of low strain the rotation rate at this stage largely depends on the aspect ratio of the boudin segments and the scaled gap width once the tilted boudins are no longer constrained by their neighbors the rotation rate greatly accelerates in the case of a low viscosity ratio between the boudins and the host the boudin segments develop complex shapes which may give an impression of shear band boudins forming under the opposite shear sense we furthermore investigate the behavior of boudin trains of finite length the terminal segments are displaced out of the shear plane deforming into isoclinal folds and separate into groups of boudin segments that rotate into the shear direction and eventually lead to an overall chaotic appearance of the structure natural examples of domino boudinage from a high shear strain detachment zone in the western cyclades greece show many similarities with the modeled structures suggesting that under simple shear deformation the rotation and separation of boudin segments is an indicator for high shear strain

garnet sm nd data from the saualpe and the koralpe eastern alps austria chronological and p t constraints on the thermal and tectonic history abstract garnets from recrystallized staurolite and kyanite bearing mica schists from the central saualpe base ment representing the host rocks of the type locality eclogites give concordant sm nd garnet whole rockisochronagesbetween88 5 1 7and90 9 0 7ma themillimetre sized mostly inclusion freegrainsshowfairlyhomogeneouselement pro les withpyropecontentsof25 27 narrowrims withanincreasein fe and mn and a decrease in mg document minor local re equilibration during cooling accordingto phengite geothermobarometry peak metamorphic conditions at 90 ma were close to 20kbar and680 c and similar to those recorded by the eclogites the garnet rims record about 575 c 7kbar forthe nal stages of metamorphism a phengitic garnet mica schist sampled at the immediate contact withthe gertrusk eclogite gave a garnet whole rock sm nd age of 94 0 2 7 ma garnet porphyroclasts separated from a pegmatite mylonite of the koralpe plattengneiss near stainzare unzoned and show spessartine contents of 15 composition and sm nd ages of close to 260 mapoint to a magmatic origin for these garnets the garnet data from the saualpe document an intense alpine metamorphism for this part of theaustroalpine basement the mica schists recrystallized during decompression and rapid exhumation atthe nal stages of and immediately following a high p event the koralpe data show that high alpinetemperatures did not reopen the sm nd isotope system implying a closure temperature in excess of c 600 c for this isotopic system in garnet key words alpine orogenesis closure temperature eastern alps high p metamorphism metamorphicand magmatic garnets p t t paths saualpe koralpe basement sm nd geochronology

a back of the envelope approach to boudinage mechanics abstract smith has shown from linear hydrodynamic stability that pinch and swell boudinage is possible only in a non newtonian material by assuming uniaxial stress and strain and large viscosity contrast between the competent layer and the incompetent matrix smith s result can be derived in a very simple way the one dimensional force balance also predicts growth rates although not dominant wavelengths however we question the ability of even linear stability analysis to accurately predict boudin widths

boudinage in multilayered rocks under layer normal compression a theoretical analysis abstract this paper presents a dynamic analysis of boudinage in multilayers of alternate brittle and ductile layers under layer normal compression based on the mode of fracturing of individual brittle layers boudinage is classified into three types tensile fracture boudinage type 1 shear fracture boudinage type 2a and extensional shear fracture boudinage type 2b the layer thickness ratio t r t b t d and the strength ratio f t 2 e between the brittle and the ductile units are the principal physical factors determining the type of boudinage type 1 boudinage develops rectangular boudins and occurs when t r is low f is high 0 8 in contrast type 2a boudinage takes place when t r is high 8 5 or f is low a r of all the types of boudins is inversely proportional to layer thickness ratio t r however type 1 and type 2 boudins have contrasting aspect ratios which are generally greater and less than 1 respectively

progress in microboudin method for palaeo stress analysis of metamorphic tectonites application of mathematically refined expression abstract the microboudin method for palaeo stress analysis is mathematically refined by incorporating the recently proposed shear lag model which describes the relationship between stress distribution along a fibre embedded within an elastic matrix and the far field differential stress applied to the matrix the refined probability density function of fractured fibres g with respect to the aspect ratio of the fibre r and the stress parameter is given by g r 1 exp m 1 m r m e f e q m 1 1 e q e f 1 cosh ar m where eq and ef are the elastic constants of the matrix and fibre m is the weibull modulus of the fibre material and a is a constant the stress parameter is defined as 0 s where 0 is the far field differential stress applied to the matrix and s is the modal fracture strength of the fibre material at r 1 this method is applied to tourmaline and piemontite boudins in quartzose rocks from six areas and it is revealed that the probability density function adequately models the proportion of boudinaged tourmaline and piemontite grains the value of is determined for each sample allowing 0 to be determined by this method when s is known

mullions in the high ardenne slate belt belgium numerical model and parameter sensitivity analysis in the high ardenne slate belt belgium pre existing layer perpendicular quartz veins in psammite layers acted as mechanical boundaries due to the difference in competence between vein quartz and psammite this caused the formation of mullions during layer parallel shortening in this paper the process of mullion formation is modelled using finite element techniques a parameter sensitivity analysis of the mullion model demonstrates that the stress exponent of psammite the total horizontal shortening of the psammite layers the initial aspect ratio of the psammite segments between the veins and the competence contrast between psammite and vein quartz are the controlling parameters for the shape of the mullions our results suggest that the morphology of the mullions can be used to constrain the rheology of psammite deforming in the middle crust moreover the parameter sensitivity analysis illustrates the range of layer parallel shortening extension structures associated with layer perpendicular quartz veins that can be expected in nature e g dogbones inverted mullions and creates future perspective for the use of the model as a paleorheological gauge for these structures

modelling of segment structures boudins bone boudins mullions and related single and multiphase deformation features finite element modelling has been used to simulate the development of segment structures deformed layer segments separated by veins such as boudins mullions and bone boudins a parameter sensitivity analysis is used to compare the influence of the nature of the flow the relative viscosities of veins in necks and the host rock and the initial geometry of the layer segments parameter fields have been determined for the relative viscosity of veins and layers and the kinematic vorticity number of flow reworked segment structures can have several shapes such as bone bulging shortened bone boudins and their asymmetric equivalents such as domino and shearband boudin geometry the model for asymmetric reworked segment structures is applied to such features from the lower ugab metaturbidites in nw namibia the model suggests that these structures form where the neck veins are stronger than the boudinaged layer with a significant simple shear component of the bulk flow the quartz filled necks in the lower ugab are therefore stronger than the quartz rich wall rock in greenschist facies where the progressive deformation occurred bone boudins are usually interpreted to form in transpressional flow but simulations of the rotation of tension gashes show that they can also form in simple shear or slightly transtensional shear flow

use of boudinaged rigid objects as a strain gauge insights from analogue and numerical models abstract using analogue experiments we investigate how far the extensional strain estimated from an array of rigid boudins tracks the bulk extension of the host rock the quadratic elongations of boudinaged objects b were compared with the corresponding bulk quadratic elongations in physical models m both for pure and simple shear a noticeable difference occurs between the two which depends on the aspect ratios of initial object r o and boudins r b boudin types and the nature of bulk deformation for a given r o the difference is proportional to r b and tends to diminish with increasing r o when r b is constant the difference is normalized with the quadratic elongation measured from boudins b to define a departure factor m b b approaches a stable value with increasing b in pure shear the stable for torn symmetric boudins is larger 10 1 than that for asymmetric boudins among which the planar type shows the departure close to 0 boudins in pure shear show values smaller than that in simple shear in case of simple shear antithetic slip boudinage involves lower values compared to synthetic slip boudinage finite element models with elasto viscous rheology also corroborate the experimental findings

tension fractures and boudinage oblique to the maximum extension direction an analogy with luders bands a study has been made of the orientation of boudin necklines and extension fractures with respect to the maximum extension direction x determined by infilling fibre growth several localities within the variscan belt of western europe the pyrenees and the alps have been investigated in detail it has been generally accepted that boudinage and extension fracturing occur perpendicular to x in the rock at the time of their formation however we have shown that is not the case boudin necklines and extension fractures occur at an angle between 45 90 to x with the most frequent orientations between 65 70 and 80 85 angles of exactly 90 are rare in order to explain this obliquity an analogy is made with the phenomenon of luders bands localised regions of plastic deformation developed in thin metal plates under tensile testing because the thickness of a layer is negligible compared with its lateral extent we will only be concerned here with the case of thin metal plates

numerical modelling of boudinage under pure shear implications for estimating viscosity ratios and finite strain from natural examples abstract numerical experiments of torn boudinage reworking under layer parallel stretching are reported in our simulations we have reproduced the whole spectrum of previously reported boudin shapes due to reworking and we also report some new structures including field examples the results show that boudin shape depends on both the boudin to matrix viscosity ratio and the initial gap width normalized upon the layer thickness central bulges develop within the interior boudin faces for intermediate gap widths in the range of between 0 05 and 0 15 of the initial layer thickness convex interiors are reported for initially very narrow separations of about 0 01 the initial layer thickness the initial normalized gap width together with the viscosity ratio control the internal deformation and separation of the boudins lowering the viscosity of the gap infill strongly influences boudin shape evolution and it suppresses the bulging of the interior faces boudin aspect ratio has only minor influence on the developing boudin shape the non linear viscous reworking of boudins is not significantly different to the one observed in linear viscous materials we show a natural example of how the geometry of the developing bow tie vein combined with boudin shape and separation can be used to infer strain and rheological properties rhomboidal boudins under layer parallel stretching exhibit rotation and they tend to produce monoclinic structures which are similar to structures forming due to layer parallel shear we caution the use of asymmetric boudins as shear sense indicators

tephra stratigraphy and eruptive volume of the may 2008 chaiten eruption chile on may 1st 2008 mount chaiten southern chile interrupted a long period of quiescence generating a sequence of explosive eruptions and causing the evacuation of chaiten town located a few kilometers south of the volcano the activity was characterized by several explosive events each associated with plumes which reached up to about 19 km above sea level the products were dispersed across a wide area with the finest ash reaching the atlantic coast of argentina our field observations in the proximal medial area 3 25 km from the vent indicate that the may 2008 tephra deposit consists of numerous layers most of which can be correlated with individual eruptive events these layers vary from extremely fine grained ash to layers of lapilli and blocks composed of both juvenile and lithic material here we describe the stratigraphy and physical characteristics of the may 2008 deposits and propose a reconstruction of the timing of the may 2008 events the deposits are mainly associated with the three main explosive phases which occurred on 1st 2nd may 3rd 5th may and 6th may with an estimated bulk tephra volume of 0 5 1 0 km3 integration of both exponential and power law fitting for the 6th may event represented by a layer composed mainly of lithic lapilli and blocks 2 mm an isopleth map was compiled from which a 19 km plume height was determined which is in good agreement with satellite observations

discrete element modeling of boudinage insights on rock rheology matrix flow and evolution of geometry 1 we use discrete element model simulations to model the full boudinage process from initial fracturing of intact material to post fracture flow of material into gaps between fragments and to investigate the role which the material properties of the weak and strong layers play in this process the models are deformed in 2d plane strain under a range of confining stresses in coaxial bulk flow results show that the material properties i e mohr coulomb or quasi viscous for the matrix and elastic brittle for the competent layer lead to the development of natural looking boudin morphologies and deformation patterns in the matrix the details of the matrix rheology only have a minor influence on the morphology of the boudins by varying the material properties of the competent layer between fully brittle and semi ductile we obtain a wide range of deformation patterns ranging from pinch and swell structures to a variety of boudin types including drawn shear band and straight sided torn boudins in a number of models we observe rotation of the boudin blocks despite the applied deformation being purely coaxial these rotations are generally related to asymmetrical rhombic boudin shapes some features observed in natural boudins such as concave block faces or the formation of veins between fragments are not modeled because pore fluids are not yet included in our model

the role of solution in the formation of boudinage and transverse veins in carbonate rocks at rheems pennsylvania the interaction of pressure solution with extensional strain is the most important factor influencing boudinage and vein structures in a mesoscopically folded sequence of subgreenschist grade carbonate rocks from the overturned limb of the lebanon valley fold nappe at rheems pennsylvania the formation of stylolites and slickolites in association with boudins and veins indicates that extreme bed normal contraction is intimately connected with bed parallel extension a variety of boudin shapes in dolomitic beds from rectilinear to pinch and swell and fish mouth structures are formed by a combination of cataclastic flow and heterogeneous pressure solution many veins record catastrophic failure and collapse of wallrock prior to or during crystallization of the vein fill at the earliest stage of boudinage veins in boudin necks are straight and perpendicular to bedding with continued extension they adopt a bow tie form in cross section as a result of inward flow of the surrounding matrix and pressure solution of the fragment corners and vein margins with continued stylolitization and separation of boudins bow tie veins are converted into transverse veins of extraordinary dimension failure to recognize the role of pressure solution in the formation of boudinage structure may result in inaccurate estimates of extension erroneous assumptions regarding volume change and incorrect interpretations of the changes in stress regime with time

boudinage in nature and experiment abstract deformation of rocks produces structures at all scales that are in many cases periodic folding or boudinage with variable amplitude and wavelength here we focus on boudinage a process of primordial importance for tectonics in the present study we carried out measurements of natural boudins and experimentally tested the effects of two variables on boudinage layer thickness and compression rate the models were made of a competent layer mostly brittle as in nature of either elastic soft paper or viscoelastoplastic clay material embedded in a ductile matrix of linear viscous silicone putty the competent layer lied with its greatest surface normal to the principal shortening axis and greatest length parallel to the principal stretching axis the model was then subjected to pure shear at constant piston velocity and variable competent layer thickness model 1 or at different piston velocity and constant layer thickness model 2 the results of model 1 show an exponential dependence of boudin width on competent layer thickness in disagreement with data from the studied natural occurrence this indicates that variables other than competent layer thickness are hidden in the linear relationship obtained for the natural boudinage the results of model 2 show that the higher the velocity the smaller the boudin width following a power law with exponent very similar to that of analytical predictions the studied natural boudinage occasionally occurs in two orthogonal directions this chocolate tablet boudinage can be the result of two successive stages of deformation buckling followed by stretching of competent sandstone layers or buckling followed by rotation of reverse limbs into the extensional field of simple shear

correlation of fluvial terraces within the hikurangi margin new zealand implications for climate and baselevel controls abstract a correlation of fluvial terraces is presented for eight non glacial catchments of the eastern north island new zealand within the actively uplifting hikurangi margin using a combination of loess and tephra coverbed stratigraphy and radiocarbon and osl dating of fluvial deposits and loess coverbeds we demonstrate correlation of four fill terraces t1 t4 the available age constraints suggest t1 15 30 ka t2 31 50 ka t3 50 70 ka and t4 115 ka but the association and temporal link with loess deposits suggest correlation with cold periods and thus the refinement of t2 late mis 3 31 40 ka t3 mis 4 55 70 ka t4 mi substage 5b 90 ka 5d 110 ka or mis 6 140 160 ka the ability to correlate terraces between catchments plus the lack of independent evidence for tectonic triggering events indicates terraces have probably formed in response to either climate terrestrial or baselevel sea level control climate control is indicated by the temporal link of post glacial incision with re establishment of forest cover and of lgm aggradation with limited grass and shrub cover and periglacial processes aggradation due to increased sediment supply under reduced vegetation is dramatically demonstrated by formation of the taupo pumice alluvium terrace in response to inundation by volcanic deposits unwelded ignimbrite following the 1 8 ka taupo eruption and the response to post settlement

a strain reversal method for estimating extension from fragmented rigid inclusions abstract in the standard method of strain analysis using fragmented rigid inclusions such as stretched belemnites extension is estimated by comparing the initial length of the inclusion with its final stretched length this approach fails to recognise that not all of the total extension is recorded by separation of the fragments even when cross fractures are present at the onset of deformation this paper presents an alternative approach in which the extensional strain is reversed the inter fragment gaps being incrementally closed until the inclusion is restored to its original state this allows an estimate of the unrecorded extension accommodated by inhomogeneous matrix flow near the inclusion as well as that recorded by fragment separation model experiments in which the fragmented inclusion is represented by wooden blocks set in a ductile matrix allow the strain reversal method to be compared with the standard method the mean percentage relative error is much smaller in the former 10 2 than in the latter 37 2 the main disadvantage of the strain reversal method is that it requires the collection of more data fragment lengths and gap lengths in correct sequence which must be processed by computer a short basic program is provided although there is no one to one correspondence between strain estimates derived from the two methods an approximate numerical transformation from one to the other may be possible one such transformation based on a study of natural boudinage has been applied to the experimental data and reduces the mean percentage relative error in the standard method estimates to 11 5

numerical modelling of asymmetric boudinage abstract asymmetric boudinage structures are commonly used as shear sense indicators but their development is incompletely understood this paper describes the influence of initial shape and kinematic parameters on the evolution of boudin trains using a numerical approach based on the finite difference code flac boudin trains are simulated as a series of competent objects embedded in a soft matrix subjected to general monoclinic ductile flow deformation of boudin trains includes heterogeneous stretching rotation of boudins and offset along the neck regions the sense of relative boudin offset is mainly influenced by the initial orientation of the interboudin plane in the boudinaging layer while kinematic vorticity number of the flow and the orientation of the boudin train with respect to the flow extensional eigenvector usually the shear zone boundary also play a role viscosity ratio and aspect ratio influence the magnitude of offset along the neck regions and the amount of rotation of boudins but not the sense of slip and rotation on the interboudin plane knowledge of the orientation of the interboudin plane itself is insufficient to use asymmetric boudins as independent shear sense indicators details of the boudin geometry such as the sense of deflection of marker horizons along the interboudin plane must be used for this purpose

asymmetrical pull aparts and foliation fish as kinematic indicators abstract from field observations asymmetrical boudins pinch and swell structures and ellipsoidal volumes of oblique foliation segments in anisotropic rocks lacking competent layers are potentially useful kinematic indicators in shear zones they are derived by the modification of initially symmetrical pull aparts and or layering by superposed layer parallel shearing the response of boudins to such shear is a synthetic flow of those corner volumes lying within the extensional quadrants of the kinematic framework of the deformation into the inter boudin gaps type 1 asymmetrical pull aparts the response of pinch and swell structure is a back rotation of the swells to form type 2 asymmetrical pull aparts discrete extensional shears in the pinches may be either absent type 2a or present type 2b analogues of types 2a and 2b asymmetrical pull aparts occur in anisotropic materials with no compositionally distinct competent members where ellipsoidal volumes contain back rotated foliation segments foliation fish the field based kinematic interpretation was successfully tested by laboratory experiment wherein initially symmetrical trains of pre formed competent internally anisotropic boudins and pinch and swell structures were deformed in bulk simple shear a mechanical model is presented in terms of two material properties of the inclusions competence viscosity and internal rheological anisotropy in an attempt to account for the observed flow in the experiments and the deduced flow and kinematic significance of the natural examples in the presence of angular corners and associated geometrical stress concentrations competence viscosity controls the response to shear in the boudins in the absence of angular corners internal anisotropy controls the response to shear in the pinch and swell structures and in the foliation fish

bone shaped boudins in progressive shearing abstract some peculiar bone shaped boudins are observed in two areas of progressive shearing they are characterized by large quartz filled gashes that separate thinner boudins these gashes often have a consistent obliquity suggesting a rotation synthetic to the shear sense these may be formed by rigid material that crystallizes in tension gashes between the boudins during further ongoing deformation these behaved as rigid inclusions in a more ductile matrix and show systematic rotations the sense of rotation of the gashes constitutes a shear criterion

the development of bone shaped structures in initially segmented layers during layer parallel extension numerical modelling and parameter sensitivity analysis in orogenic zones the geometric occurrence of specific tectonic structures can often be related to contrasting rheological properties of rock materials one of such tectonic structures is layer perpendicular naturally fractured layers with vein infill that underwent subsequent deformation internal deformation by coaxial extension of these segmented layers with host rock material less competent than the vein material gives rise to the development of bone shaped structures i e dogbones in this paper the formation of such dogbones is modelled using finite element techniques a parameter sensitivity analysis of the model demonstrates that the degree of extension of the host rock segments their initial aspect ratio and the competence contrast between vein material and host rock are the controlling parameters for the final shape of the dogbones during coaxial extension the results moreover suggest that the specific geometry of the dogbones can serve to quantify competence contrasts between the host rock and vein material during flow at geological strain rates