sheath fold morphology in simple shear abstract sheath folds are highly non cylindrical structures often associated with shear zones we investigate the formation of sheath folds around a weak inclusion acting as a slip surface in simple shear by means of an analytical model we present results for different slip surface orientations and shapes cross sections perpendicular to the shear direction through the sheath fold display closed contours so called eye structures the aspect ratio of the outermost closed contour is strongly dependent on the initial slip surface configuration the center of the eye structure is subject to change in height with respect to the upper edge of the outermost closed contour for different cross sections perpendicular to the shear direction this results in a large variability in layer thickness across the sheath fold length questioning the usefulness of eye structures as shear sense indicators the location of the center of the eye structure is largely invariant to the initial configurations of the slip surface as well as to strain the values of the aspect ratios of the closed contours within the eye pattern are dependent on the strain and the cross section location the ratio r of the aspect ratios of the outermost closed contour ryz and the innermost closed contour ry z shows values above and below 1 r shows dependence on the slip surface shape and orientation but not on the number of involved contours using r measurements to deduce the bulk strain type may be erroneous

shear zone structures in the oygarden area west norway abstract investigation of shear related sheath like folds from blastomylonitic gneisses in the oygarden complex norway shows that the folds have not initiated with fold axes systematically normal to or with another fixed angle to the lineation their short limbs do however pass from a period of thickening to final thinning as the folds close and the short limbs become inverted the formation of the folds appears to be controlled by local geometries within the shear zone and we suggest three fold generating mechanisms in one the mylonitic foliation curves ahead of a tectonic lens and starts to fold and in another mesoscopic shear bands with normal fault geometries cut subplanar mylonites and folds are generated by the accretion of material in the evolving gap between the hangingwall and the footwall the third mechanism involves perturbation of the main foliation by slip along an inclined s foliation so that shear folds may develop all the mechanisms involve a rotation of the mylonitic foliation into the compressional field of the instantaneous strain ellipse

ductile and brittle deformations in the northern snake range nevada abstract the northern snake range east central nevada is one of the metamorphic core complexes of the sevier hinterland within the range a major decollement separates an upper plate composed of brittlely deformed paleozoic sedimentary rocks mostly carbonates from a lower plate composed of metamorphic upper precambrian lower cambrian rocks intruded by gneissic granites a study of the geometry and kinematics of structures and fabrics at outcrop scale and in thin sections indicates that the northern snake range decollement has been a zone of intense non coaxial e vergent shear and transport in a n115 e direction outstanding asymmetric boudinage within the marble sheet capping the lower plate testifies for late ductile shear strains of at least 20 the interface between brittlely and ductilely deformed rocks seems too sharp to represent a regional rheological transition but might result from two distinct phases of deformation ductile deformation in and below the decollement could have occurred before brittle deformation in the upper plate brittle faulting in the upper plate related to basin and range extension reactivated the upper surface of the ductile shear zone high topographic relief on the normal faults bounding the range triggered the slide of olistolites from the upper plate into the adjacent oligo miocene basins

problem of folding in ductile shear zones a theoretical and experimental investigation the paper describes micro scale folds within a narrow ductile shear zone of the peninsular gneissic complex south india the characteristics of the folds indicate that they have formed by buckling on the mylonitic foliation parallel to the c fabric this raises the question of how a buckling instability could develop on the mylonitic foliation as there can be no overall shortening along the shear direction on the basis of natural observations we show that mylonitization involves sericitization locally along the c fabrics forming discrete mechanically weak zones which perturb the homogeneous shear stress field in their neighborhood and induce shortening along the bulk shear direction leading to buckling instabilities on the mylonitic foliation the weak zone model is supported with results obtained from physical experiments theoretical analysis shows that the weak bodies with large length to thickness ratios perturb the shear stress field in the shear zone significantly developing compressive stresses along the bulk shear direction required for buckle instabilities to occur on the mylonitic foliation

progressive development of structures in a ductile shear zone in the singhbhum shear zone of eastern india successive generations of folds grew in response to a progressive ductile shearing during this deformation a mylonitic foliation was initiated and was repeatedly transposed the majority of fold hinges were formed in an arcuate manner at low angles to the y axis in an e w trending subhorizontal position and major segments of the fold hinges were then rotated towards the down dip northerly plunging x axis the striping and intersection lineations were rotated in the same manner the down dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations the consistent asymmetry of the folds the angular relations between c and s surfaces and the evidence of two dimensional boudinage indicate that the deformation was non coaxial but with a flattening type of strain with 1 2 the degree of non coaxiality varied both in space and time from the progressive development of mesoscopic structures it is concluded that the 2 3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale when a new set of shear lenses was superimposed on already sheared rocks the preexisting foliation generally lay at a low angle to the lenses no new folds developed where the acute angle was sympathetic to the sense of shear displacements where the acute angle was counter to the sense of shear the pre existing foliation lying in the instantaneous shortening field was deformed into a set of asymmetric folds

kinematics of syneclogite deformation in the bergen arcs norway implications for exhumation mechanisms the northwestern part of holsnoy island in the bergen arcs norway consists of a granulite facies protolith partially transformed at depth in eclogite 700 c 19 kbars and amphibolite 650 c 8 10 kbars facies during the caledonian orogenesis eclogitized zones are mainly planar objects fractures with parallel reaction bands and cm to 100 m scale shear zones eclogitic zones are distributed in two sets of orientations and the associated deformation can be described as bookshelf tectonics the major shear zones strike around n120 and dip to the north and show consistent top to the ne shear sense throughout the area in the large scale kinematic frame of caledonian nw dipping slab eclogitic shear zones are interpreted as the way to detach crustal units from the subducting slab and to prevent their further sinking as the retrograde amphibolitic deformation pattern is similar to the eclogitic one the detached crustal units started their way up along these eclogitic shear zones radiometric ages of eclogitic and amphibolitic metamorphism and their comparison with the chronology of caledonian orogenesis show that the deformation recorded on holsnoy occurred in a convergent context the mechanism we propose can thus account for the first steps of exhumation during collision

structural analysis of sheath folds in the sylacauga marble group talladega slate belt southern appalachians remnant blocks of marble from the moretti harrah dimension stone quarry provide excellent exposure of meter scale sheath folds tubular structures with elliptical cross sections 4 ryz 5 are the most common expression of the folds the tubes are elongate subparallel to stretching lineation and are defined by centimeter scale layers of schist eccentrically nested elliptical patterns and opposing asymmetry of folds s and z are consistent with the sheath fold interpretation sheath folds are locally numerous in the moretti harrah quarry but are not widely distributed in the sylacauga marble group reconnaissance in neighboring quarries provided no additional observations the presence of sheath folds in part of the talladega slate belt indicates a local history of plastic non coaxial deformation such a history of deformation is substantiated by petrographic study of an extracted hinge from the moretti harrah quarry the sheath folds are modeled as due to passive amplification of initial structures during simple shear using both analytic geometry and graphic simulation as indicated by these models relatively large shear strains y 9 and longitudinal initial structures are required the shear strain presumably relates to nw directed displacement of overlying crystalline rocks during late paleozoic orogeny

the development of sheath folds in viscously stratified materials in simple shear conditions an analogue approach sheath folds are highly non cylindrical folds occurring in a variety of geological settings and have been studied using different approaches with the present work we provide a quantitative analysis of the generation and development of sheath folds in a viscously layered system in simple shear conditions the sheath folds develop from an initial non cylindrical deflection located on the highly viscous layer the analogue experiments investigated the influence of 1 variations in the viscosity ratio between the high viscosity layer and the matrix hvl m 2 variations in the ratio between the amplitude of the initial deflection and the thickness of the high viscosity layer af thvl and 3 progressive simple shear the results show that increases in hvl m will produce progressively less elongated sheath folds while increases in af thvl will result in more elongated sheath folds we present regime diagrams with hvl m and af thvl for different shear strains illustrating under which conditions sheath folds form in case the original deflection amplitude and layer thickness as well as can be retrieved for sheath folds in nature then their geometry can be used to quantify the effective hvl m

3d seismic reflection amplitude and instantaneous frequency attributes in mapping thin hydrocarbon reservoir lithofacies morrison ne field and morrison field clark county ks thin hydrocarbon reservoir facies pose resolution challenges and waveform signature opportunities in seismic reservoir characterization and prospect identification in this study we present a case study where instantaneous frequency variation in response to a thin hydrocarbon pay zone is analyzed and integrated with other independent information to explain drilling results and optimize future drilling decisions in morrison ne field some wells with poor economics have resulted from well placement incognizant of reservoir heterogeneities the study area in clark county kanas usa has been covered by a surface 3d seismic reflection survey in 2010 the target horizon is the viola limestone which continues to produce from 7 of the 12 wells drilled within the survey area seismic attributes extraction and analyses were conducted with emphasis on instantaneous attributes and amplitude anomalies to better understand and predict reservoir heterogeneities and their control on hydrocarbon entrapment settings we have identified a higher instantaneous frequency lower amplitude seismic facies that is in good agreement with distinct lithofacies that exhibit better higher porosity reservoir properties as inferred from well log analysis and petrographic inspection of well cuttings this study presents a pre drilling data driven approach of identifying sub resolution reservoir seismic facies in a carbonate formation this workflow will assist in placing new development wells in other locations within the area our low amplitude high instantaneous frequency seismic reservoir facies have been corroborated by findings based on well logs petrographic analysis data and drilling results

structural development and petrofabrics of eclogite facies shear zones bergen arcs western norway implications for deep crustal deformational processes caledonian eclogite facies shear zones developed from grenvillian garnet granulite facies anorthosites and gabbros in the bergen arcs of western norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation cpo in lower continental crust field relations on the island of holsnoy show that the eclogites formed locally from granulite facies rocks by progressive development of 1 eclogite adjacent to fractures 2 eclogite in discrete shear zones 2 m thick 3 eclogite breccia consisting of 80 well foliated eclogite that wraps around rotated granulite blocks and 4 anastomosing subparallel eclogite facies shear zones 30 100 m thick continuous over distances 1 km within the granulite terrane these shear zones deformed under eclogite facies conditions at an estimated temperature of 670 50 c and a minimum pressure of 1460 mpa which corresponds to depths of 55 km in the continental crust detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite garnet rich and kyanite zoisite rich layers a consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite the cpo of omphacite determined from five axis universal stage measurements shows a strong b axis maximum normal to foliation and a c axis girdle within the foliation plane with weak maxima parallel to the lineation direction these patterns are consistent with deformation of omphacite by slip parallel to 001 and suggest glide along 010 the lineation and cpo data reveal a consistent sense of shear zone movement although the displacement was small localized faulting of high grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth deformation of granulite and partial conversion to eclogite such as is exposed on holsnoy island may be an orogenic scale process in the lowermost crust of collisional orogens

strain and shape fabric variations associated with ductile shear zones abstract the foliated and compositionally banded granitic orthogneisses in the central core of the maggia nappe a lower pennine basement nappe of the central swiss alps are shown to be the sheared equivalent of late hercynian age granitic intrusions these ductile shear zones show mineral assemblages in amphibolite facies are alpine in age and form an anastomosing network enclosing remnant lozenge shaped pods of relatively undeformed rock the foliation developed within the shear zones concomitantly with a change in shape of quartz grain aggregates from initially equidimensional through tear drop shapes to ribbon like aggregates these shape changes occurred by intracrystalline glide together with intercrystalline slip on deformation induced planar surfaces

northwest verging folds and the northwestward movement of the caledonian jotun nappe norway abstract a family of asymmetric northwest verging back folds and crenulations occur on a regional scale in the caledonian decollement zone in southwestern norway they are the result of reversal of the emplacement movement of the jotun and other nappes in early devonian time the back folds may have formed progressively by two different mechanisms actively by buckling of the layering and more passively by transfer of slip across the layering reverse slip crenulations to balance the thinning of the zone caused by foliation parallel slip and shear band formation the former mechanism is considered to be most important during the initial stages of fold development gradually giving way to the latter mechanism the hinge lines of these folds appear to have remained stationary during deformation this is explained by the observation that the hinge lines lie within or close to the shear plane i e the plane of no rotation in simple shear deformation integration of strain across the deformation zone indicates a magnitude of back movement in the order of ca 20 36 km and it is argued that the deformation was close to simple shear

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

shear zone evolution during core complex exhumation implications for continental detachments abstract the formation of low angle detachments involves exhumation of previously ductile material and fault zone weakening to better understand this relationship we studied a deeply eroded metamorphic core complex which formed in the core of the bergen arcs w norway during caledonian post orogenic collapse multi scale structural mapping in the oygarden complex constrains three structural levels characterized by localized shear upper unit distributed deformation middle unit and a migmatite double dome lower unit all levels show retrogressive e w stretching accompanied by extension parallel recumbent folding albeit with opposing shear senses at upper and middle lower levels the systematic comparison of 23 shear zones constrains the ductile to brittle structural evolution initially high temperatures and partial melting controlled pervasive deep crustal flow and ductile doming during retrogressive shearing lithological heterogeneity controlled strain localization and channelized fluid flow causing retrograde phyllosilicate growth this established a feedback loop of fluid flow fabric weakening and progressive shear localization the interconnection of inherited and newly formed weak phyllosilicate rich layers promoted the formation of bivergent detachments that rapidly exhumed a dome of previously ductile crust retrogressive weakening in a kilometer wide ductile to brittle processing zone may be essential for the formation of continental detachments

linking the northern hemisphere sea ice reduction trend and the quasi decadal arctic sea ice oscillation the nature of the reduction trend and quasi decadal oscillation in northern hemisphere sea ice extent is investigated the trend and oscillation that seem to be two separate phenomena have been found in data this study examines a hypothesis that the arctic sea ice reduction trend in the last three decades amplified the quasi decadal arctic sea ice oscillation asio due to a positive ice ocean albedo feedback based on data analysis and a conceptual model proposed by ikeda et al the theoretical conceptual model predicts that the quasi decadal oscillation is amplified by the thinning sea ice leading to the asio which is driven by the strong positive feedback between the atmosphere and ice ocean systems such oscillation is predicted to be out of phase between the arctic basin and the nordic seas with a phase difference of 3 4 with the nordic seas leading the arctic the wavelet analysis of the sea ice data reveals that the quasi decadal asio occurred actively since the 1970s following the trend starting in the 1960s i e as sea ice became thinner and thinner as the atmosphere experienced quasi decadal oscillations during the last century the wavelet analysis also confirms the prediction of such out of phase feature between these two basins which varied from 0 62 in 1960 to 0 25 in 1995 furthermore a coupled ice ocean general circulation model gcm was used to simulate two scenarios one without the greenhouse gas warming and the other having realistic atmospheric forcing along with the warming that leads to sea ice reduction trend the quasi decadal asio is excited in the latter case compared to the no warming case the wavelet analyses of the simulated ice volume were also conducted to derive decadal asio and similar phase relationship between the arctic ocean and the nordic seas an independent data source was used to confirm such decadal oscillation in the upper layer or freshwater thickness which is consistent with the model simulation a modified feedback loop for the sea ice trend and asio was proposed based on the previous one by mysak and venegas and the ice albedo and cloud albedo feedabcks which are responsible for the sea ice reduction trend

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

large scale foliation boudinage in gneisses abstract large scale extensional structures in gneisses symmetric and asymmetric foliation boudinage c type shears are common in crustal scale shear zones and may constitute shear criteria as well as being indicators of high strain

complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones abstract many of the complex strain patterns seen in shear zones and thrust zones such as variable fabric orientations refolded folds and fabrics together with folds with hinges almost parallel to the main transport direction can be explained in terms of differential movement within the shear zones these strains are developed at the frontal and lateral tips of the zones as they propagate examples are taken from the moine thrust zone of scotland which show variations in strains particularly at the lateral tips the form of differential movement described here may lead to complex strain paths and non plane strain ellipsoids and the spatial variations in finite strain may be used to delineate zones of extensional and compressional flow and differential movement in the shear zones or thrusts

the origin of s c mylonites and a new fault zone model abstract this paper presents a synthesis of experimental data on mechanical behaviour and deformation textures of simulated halite shear zones with special regard to the internal structures of s c mylonites and their mechanical implications halite is the only mineral so far for which a complete transition between brittle shearing deformation to fully ductile shearing flow in the dislocation glide regime has been studied experimentally under large shear strains that may be encountered along faults or plate boundaries moreover deformation textures in the shear zones are similar to those of mylonites and cataclasites thus the experimental results provide an insight into the petrogenesis of fault rocks mylonites in particular and the establishment of a realistic fault model between the brittle regime and the fully ductile regime i e pressure insensitive inelastic regime there exists a wide and distinct regime called semiductile in which deformation textures are nearly identical to those developed in the ductile regime yet the shear resistance is pressure dependent and potentially unstable fault motion can occur in the low pressure part in cooking salt shear zone experiments the characteristic texture in the semiductile and ductile regimes is foliation resembling mylonitic foliation which formed in the direction of maximum elongation in the semiductile regime nearly homogeneous shearing deformation characterized by the formation of uniform and pervasive foliation changes into heterogeneous deformation after a critical shear strain and this strain localization induces potentially unstable fault motion the primary mode of internal slip after the strain localization is slip along y riedel shears parallel to the shear zone boundary interconnected with r 1 and some p riedel shears these shear surfaces of concentrated deformation are very similar to the internal structures of so called s c mylonites hence the semiductile regime is the primary candidate for the formation of s c mylonites and at least some s c mylonites must have formed at seismogenic depths this interpretation is consistent with recently reported interlaced pseudotachylytes and mylonites it is also shown that the size of recrystallized halite grains presumably mostly postdeformational decreases with increasing shear strains but not with the shear stress and this is consistent with the grain size reduction with strain concentration which has been confirmed for many mylonites in the world thus estimation of flow stress from recrystallized grains needs to be done with great care and might often be erroneous existing fault models are critically examined from the viewpoint of the halite experiments and a new fault model is proposed whereby fault zones are divided into brittle semibrittle semiductile and ductile regimes with increasing depth the shear resistance of faults is highest in the semiductile regime and the seismogenic depth is likely to extend down to the upper part of this regime the semiductile regime thus emerges as an area of primary interest with regard to the petrogenesis of many s c mylonites modelling of large and great shallow earthquakes along pre existing faults and plate boundaries and quantitative evaluation of the mechanical interactions of plates across their boundaries

secondary cleavages in ductile shear zones abstract secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone these are the orientation of a single secondary cleavage oblique to the shear zone boundaries conjugate sets with opposite senses of shear and multiple sets with the same sense of shear these features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation s coaxial stretching along the foliation and spin

mechanisms for folding of high grade rocks in extensional tectonic settings abstract this review of structures developed in extensional high grade terrains combined with results of centrifuge analogue modelling illustrates the range of fold styles and mechanisms for folding of amphibolite to granulite facies rocks during rifting or the collapse of a thrust thickened orogen several extensional fold mechanisms such as folding within detachment shear zones are similar to those in contractional settings the metamorphic p t t path and not fold style or mode of formation is therefore required to determine the tectonic setting in which some folds developed other mechanisms such as rollover above and folding between listric normal shear zones and folding due to isostatic adjustments during crustal thinning are unique to extensional tectonic settings several mechanisms for folding during crustal extension produce structures that could easily be misinterpreted as implying regional contraction and hence lead to errors in their tectonic interpretation it is shown that isoclinal recumbent folds refolded by open upright folds may develop during regional extension in the deep crust folds with a thrust sense of asymmetry can develop due to high shear strains within an extensional detachment or from enhanced back rotation of layers between normal shear zones during back rotation folding layers rotated into the shortening field undergo further buckle folding and all may rotate towards orthogonality to the maximum shortening direction this mechanism explains the presence of many transposed folds folds with axial planar pegmatites and folds with opposite vergence in extensional terrains examples of folds in high grade rocks interpreted as forming during regional extension included in this paper are from the grenville province of canada norwegian caledonides albany mobile belt and leeuwin complex of western australia ruby mountains in the basin and range province of nevada the at sund area of greenland the napier complex of enderby land in east antarctica and the kigluaik mountains in western alaska

structural analysis of sheath folds and geochronology in the cuonadong dome southern tibet china new constraints on the timing of the south tibetan detachment system and its relationship to north himalayan gneiss domes

the interaction between oblique and layer parallel shear in high strain zones observations and experiments abstract the scandinavian caledonides consist of a rigid basement a weak decollement zone and a series of crystalline nappes the basement is cut by oblique west dipping extensional shear zones associated with west verging folds in the overlying decollement zone the oblique shear zones may theoretically have formed after prior to or during post contractional top to the west shear movements physical modelling of this geometrical mechanical situation shows that each of the three deformation histories results in a characteristic structure if the oblique extensional shear zones postdate the layer parallel decollement shear a normal type oblique shear zone structure occurs if the layer parallel shear occurs after the development of oblique shear zones the resulting structure is characterized by a low strain region above the oblique shear zone some folds typically develop above this protected region however simultaneous oblique and layer parallel shear provides abundant asymmetric folds with gently dipping axial planes and sub horizontal shear zone parallel fold axes in the region above the oblique shear zone a neutral surface may be encountered above and parallel to the oblique shear zone separating a lower thin zone of layer parallel extension from the main contractional region the differences between the three types can be used to interpret field observations of this type of structures the large scale post contractional structures in the scandinavian caledonides are consistent with layer parallel shear related to back movement of nappes synchronous with oblique shear zones in the basement similar geometries are found as meso and micro scale structures in ductile high strain zones where oblique shear bands or shear zones affect more competent layers within the tectonites also these structures compare well with those produced experimentally by simultaneous layer parallel and oblique shear and are consistent with the general assumption that oblique shear bands form contemporaneous with the general shear in shear zones

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

hot versus cold orogenic behavior comparing the aracuai west congo and the caledonian orogens observations and modeling show that temperature controls crustal rheology and therefore also the orogenic evolution of continent continent collision zones and the associated tectonic style in order to explore the effect of temperature in a natural environment we compare eroded sections through the unusually cold lower paleozoic north atlantic caledonian scandian collision zone and the very hot brasiliano pan african aracuai west congo orogen a cold and stiff subducting caledonian continental margin was able to subduct as a rather coherent unit to ultrahigh pressure conditions twice as deep as the pan african brasiliano crust that was quickly heated and softened and got involved in partial melting furthermore the caledonian collision developed large coherent thrust sheets that were transported hundreds of kilometers toward the foreland this was never achieved in the hot aracuai west congo orogen where much of the tectonic stress was absorbed by the partially molten central part of the orogen through magmatic state deformation major mylonite zones thrusts such as those seen in the caledonides are therefore less common in the aracuai west congo orogen further the deep continental subduction in the caledonides developed a strongly asymmetric collision zone with rapid variations in pressure and temperature in contrast the aracuai west congo orogen soon developed into a more symmetric geometry due to its easily flowing hot crust with a relatively flat base and a corresponding plateau in its upper part deformation of the cold caledonian crust was controlled by plate tectonic stress while gravitational forces more strongly influenced the hot brasiliano pan african example

the three dimensional shape and localisation of deformation within multilayer sheath folds abstract sheath folds are widely believed to develop by the passive geometric amplification of folds in which layering has no mechanical influence during non coaxial deformation where layering becomes rheologically significant then active sheath folds may form in which the inner nose of the sheath decouples along detachments and undergoes a translation relative to the outer folded layers that surround and envelope the nose we present a systematic 3 d analysis of multiple folded layer geometries in a serially sectioned natural sheath fold weak layers which are reactivated as detachments define different shaped sheath folds relative to other layers with the sense of cut off along detachments reversing across the axial surface as the inner fold nose has protruded into the surrounding sheath envelope detachment layers are more tightly folded meaning that such active sheath folds are non similar shapes the obliquely oriented bifurcating en echelon nature of the fold hinges developed in adjacent layers suggest that pre cursor folds formed with pronounced 3 d obliquity relative to the subsequent shear plane mineral lineations folded around sheath closures display asymmetric star burst patterns consistent with recrystallisation during active folding and hinge rotation we show that the eye fold shapes exposed in any 2 d y z slice can be used to predict the geometry of marker horizons back along the x axis in the third dimension this self similarity may be of value when tracing stratiform mineralised horizons in large scale sheath folds

parallel stretching lineations and fold axes oblique to a shear displacement direction a model and observations abstract spatial variations in shear strain rate are expected in ductile shear zones where the variation is a change in shear strain perpendicular to the displacement direction the effect is to rotate the shear slip planes this is a mechanism for giving a rotation of fold axes into parallelism with the slip and extension direction in a rock if such a variation in shear strain affects rocks with a strong planar anisotropy it is possible to produce a fabric with an apparent stretching lineation parallel to fold axes but both significantly oblique to the slip direction a possible example of this is seen in strongly deformed quartz mica schists from syros greece where a stretching lineation is seen parallel to fold hinges over a range of fold axes orientations of at least 40

shear structures in anhydrite at the base of thrust sheets antalya southern turkey abstract minor scale structures in anhydrite layers and pods within one thrust contact of the antalya thrust system are described folds whose size range from some centimeters to several meters display a wide variety of attitudes their axes are curvilinear and vary from orthogonal to subparallel to the stretching lineation leading to sheath folds and eyed type sections shear bands are frequent in alternating white pure and brown clay rich anhydrite regularly spaced extensional shear bands form typical tilted block patterns in homogeneously foliated anhydrite they form isolated meter long extensional shear bands with brecciation along their central planes reverse fault type shear bands are associated with meter scale reclined folds with axes subperpendicular to the stretching lineation normal and reverse fault type shear bands are self exclusive and both indicate a sense of shear toward the south at the scale of thrust system a model of progressive deformation of the anhydrite layers is presented anhydrite layers which have undergone a strong unstable shearing deformation along a thrust have been boudinaged and now form elongate asymmetric pods in thinned layers normal extensional shear bands developed and recumbent fold axes are strongly reoriented toward the direction of the stretching lineation in the thickened layers reverse fault type shear bands developed particularly within the inverted limbs of asymmetric reclined folds whose axes are at a high angle to the stretching lineation

structural pattern and emplacement mechanism of the neka valley nappe complex eastern alborz iran the neka valley nappe complex is exposed in the south of gorgan county in the eastern alborz fold and thrust belt we use the results of a regional survey of the structural data and their patterns to interpret the mechanisms that emplaced the unmetamorphosed nappes in the foreland fold and thrust belt of the alborz mountains most of the strain magnitudes are low in the study area but increase slightly towards what are probably their proximal ends strain ellipsoid is dominantly oblate with xy aligned along and across the belt or the nappe complex the average kinematic vorticity number w k 0 6 which indicates most of the strain partitioning resulted in a general shear most of flinn s k values and the stretch along the shear plane values are lower than 1 structural indicators such as orthogonal extensional joints pinch and swell structures anastomosing cleavages and listric normal and growth faults developed by push from the rear large scale thrust complexes with opposed dips such as triangle zones as well as k and values 1 are compatible with the shear flow diverging distally and streamlines expected of the rear compression emplacement mechanism together with a later minor brittle deformation these major ductile strains appears to provide a general model suitable for the emplacement of the nappes studied in a thin skinned fold and thrust belt where the sedimentary cover strata shortened and imbricated in the upper crust

a computer model of sheath nappes formed during crustal shear in the western gneiss region central norwegian caledonides abstract the eastern western gneiss region of central norway is part of the deepest exposed norwegian caledonides where basement gneisses and an overlying thrust nappe sequence have been folded into large fold nappes structural analysis of a fold nappe within the central part of the district the grovudal area suggests that it has a strongly sheath like form and that other fold nappes of the western gneiss region may also have sheath like forms the structural history within the grovudal area is dominated by intense east directed subhorizontal shear in an overthrust sense followed by asymmetric refolding with an easterly vergence a computer generated kinematic model was developed to test whether the regional interference patterns could be explained by sheath fold development during this type of deformation the computer model shows that the major regional interference patterns could have been formed by such a kinematic history but does not rule out other possibile histories the proposed kinematic history is however compatible with the regional tectonic history of the main caledonian nappe pile suggesting that the complex nappe interference patterns typical of the region were formed in a kinematically simple but intense ductile deformation associated with caledonian continental imbrication

experimental study of sheath fold development around a weak inclusion in a mechanically layered matrix article i nfo we present quantitative laboratory models investigating the mechanics of sheath fold formation around a weak inclusion in simple shear sheath folds are intriguing highly non cylindrical structures that stimulated extensive field and experimental studies leading to an ongoing debate concerning their formation and evolution through a parametric study we test the influence of a mechanically layered matrix on the devel opment of sheath folds using silicone models our models show how 1 the viscosity ratio between the layers in the matrix and 2 the layer thickness control the shapes of the resulting folds and their visibility all experiments with a weak inclusion resulted in strong deformation of the layers an increase in viscosity ratio however leads to less evolved sheath folds which are shorter and show a larger opening and dip angle in contrast to former studies we show that a mechanically layered matrix does not hinder the forma tion of sheath folds the visibility of the sheath folds in our models strongly depends on the aspect ratio be tween the inclusion height and the layer thickness we observed sheath folds for a ratio larger than 7 5 the experiments reproduced the first order features of natural sheath folds our results challenge previously pub lished studies where sheath folds were considered as purely passive structures we demonstrate that sheath folds readily develop around slip surfaces suggesting that this might be a relevant formation mechanism in nature

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

early paleozoic orogenic collapse tectonic stability and late paleozoic continental rifting revealed through thermochronology of k feldspars southern norway southern norway was subjected to contractional orogenesis and subsequent extensional collapse during the early to middle paleozoic when baltica collided with laurentia resulting in the caledonian orogen the largely unknown post caledonian paleozoic thermal history of the crust in southern norway has been assessed through application of multiple diffusion domain thermal modeling of k feldspar 40ar 39ar data samples were collected along a 250 km e w traverse from the internides of the orogen along the western coast near bergen through a series of middle to upper crustal nappes to the east the k feldspar thermal models which complement published mica and hornblende 40ar 39ar data are consistent with rapid cooling at the end of the caledonian orogenic cycle when early devonian 400 ma extensional collapse resulted in exhumation through mainly midcrustal conditions following extensional collapse a period of relative tectonic and thermal stability may have prevailed until 300 ma the k feldspar modeling suggests that southern norway entered a new phase of relatively rapid cooling a few c ma in the permo carboniferous and that the increase in cooling rate was diachronous across the traverse the onset of this cooling phase occurs at about the time of the earliest faulting and magmatism in the oslo graben to the southeast the increase in cooling rate may also indirectly date the onset of the first important rift phase and associated sedimentation in the north sea graben system the distribution of permian and younger fission track ages in both southern continental norway and its marginal basin sediments is consistent with the permo carboniferous rapid cooling recorded in the k feldspars the simplest explanation for the late paleozoic increase in cooling rate is that rifting around the margin of southern norway has resulted in a decrease in the base level and an increase in the rate of erosional denudation resulting in essentially uniform exhumation of 2 4 km along the traverse

folding in high grade rocks due to back rotation between shear zones abstract folds with a sense of asymmetry opposite to the bulk shear sense may form in high grade rocks due to back rotation between ductile shear zones layers back rotated into the shortening field can undergo additional buckle folding with progressive deformation shear zones and domains of back rotated foliation between shears rotate towards the x y flattening plane of the finite strain ellipsoid in high grade gneiss the migration of melt into shear zones may facilitate displacement along them and accentuate back rotation of material between shear zones folding during back rotation occurs when the spacing between shear zones does not increase sufficiently to accommodate the length of back rotated layers or between non parallel shear zones folds formed by back rotation between shear zones commonly have pegmatites sub parallel to their axial planes thickened overturned limbs and occur in localised packages associated shear zones have the same sense of shear on both attenuated limbs of folds formed by back rotation examples are given for folds between transcurrent and normal ductile shear zones in the albany mobile belt and leeuwin complex western australia and the dharwar craton india

composite fabrics in mid crustal gneisses observations from the oygarden complex west norway caledonides subhorizontal caledonian shear zones that transform migmatitic gneisses into layered gneissic tectonites in the western part of the oygarden complex show a number of composite fabrics the associated structures consist of two sets of related planar surfaces showing a consistent top to the west sense of shear the development of these structures depends mainly on the nature and variation of finite strain rheology strain heterogeneities and extent of metamorphic mineral growth and recrystallization in the shear zones the structures have one set of surfaces represented by slip surfaces or narrow high strain zones oblique to this is a complementary set of surfaces representing either a plane of mineral flattening or aligned metamorphic minerals in s c structures or rotated gneissic foliation in contractional and extensional composite structures rotation by folding and subsequent slip along limbs or axial planar cleavage results in contractional composite structures while rotation by downward cutting extensional type slip surfaces form extensional composite structures contractional and extensional composite structures involve slip transfer across the mylonitic layering commonly between terminating weak layers within strongly anisotropic layered tectonites combinations of the different types of composite structures are common but the method of shear sense identification is always the same

on the internal geometry of mylonite zones mylonite zones are generally characterized by abrupt and very large strain transitions which commonly result in excessively anastomosing schistosities on a wide range of scales when compared with nonmylonitic foliations this geometry is very susceptible to remodification during progressive mylonitization resulting in unusual and complex fold lineation and foliation geometries and interrelationships open folds of the mylonitic foliation with axes parallel to the stretching lineation in the surrounding mylonite cannot have formed by the rotation of fold axes through a large angle within their axial planes as has been usually proposed for isoclinal and sheath folds in mylonitic zones open folds initiate with axes parallel or close to the stretching lineation due to the geometric effects of folding a mylonitic foliation which anastomoses around an ellipsoidal pod of less deformed material this initial geometry also allows the generation of fold axes curved within their axial plane through 1800 about the stretching lineation at the time of nucleation successive mylonitic foliations develop during this folding and refolding process with boundaries that truncate and isolate earlier fold hinges and portions of fold limbs as a consequence stretching and intersection lineations can vary from plane to plane through the mylonite zone although careful examination often reveals a weak overprinting stretching lineation parallel to the bulk movement direction for the whole zone fold asymmetry in mylonite zones is a potential indicator of shear sense across a zone if the fold axes lie at an angle to the bulk stretching lineation direction in such circumstances however a single asymmetry projected onto a plane perpendicular to the mylonitic foliation and containing the bulk stretching lineation can indicate either sense of shear depending on a variety of factors these include whether the foliation folded is primary or mylonitic and in the latter case whether the mylonite zone formed with a steep dip and horizontal stretching lineation or in some other orientation the most satisfactory sense of shear indicator is the asymmetry of s and c planes

shear criteria and structural symmetry abstract during the last decade it has been shown that most relevant shear criteria within ductile rocks are asymmetric structures e g pressure shadows shear bands c s structures fabrics tension gashes folds veins the correspondence between coaxial or non coaxial deformation and symmetric or asymmetric particle velocity fields respectively accounts for the use of structural symmetry as an indicator of strain history the application of this symmetry concept to various field examples emphasizes that i the degree of symmetry of a given structural pattern reflects the bulk strain regime irrespective of the size and the mechanical behaviour of the considered system and ii the strain regime can also be inferred from the order of appearance and dominance of structures which contribute to the total deformation pattern even where the progressive deformation results in a complex pattern which cannot be directly interpreted

boudinage and shearband boudins a meso to micro scale tool in structural analysis abstract a classification of boudinage and boudins is made presenting a simplified parametric description of boudin block geometry end members the most important shearband boudins parameters ratios microstructural domains and internal evolutions are described and interpreted shearband boudins are related to a large number of geological meso and microstructures e g pinch and swell drawn or torn boudins mica fish porphyroclasts foliation boudinage tectonic lozenges and c s c several factors can control the asymmetric boudinage process such as the obliquity of a competent layer the orientation of an initial fracture boudin axial ratio the viscosity ratio between matrix and layer flow type and shear strain this review highlights the correlation of shearband boudins with other asymmetrically deformed structures and shows their geological relevance finally this study presents a phenomenological framework for shearband boudins