regional flow perturbation folding within an exhumation channel a case study from the cycladic blueschists abstract kilometre scale cylindrical folds and associated parasitic folds that trend at small angles to the transport lineation are analysed along a 100 km long transport normal segment of the cycladic blueschists in an attempt to reconstruct the 3d structural architecture within an exhumation channel reversals in the polarity of both fold vergence and the hinge lineation obliquity occur in a flow normal direction defining transport parallel culmination and depression surfaces that root downwards onto an underlying detachment fold patterns generated around culmination and depression surfaces support models of flow perturbation folding where folds initiate at small angles or sub parallel to transport in response to wrench dominated differential shearing successive culmination and depression surfaces are separated from one another by along strike distances of 20 km although atypical fold geometries developed in the flanks of major culmination and depressions follow their own patterns revealing that smaller perturbations occur within the larger scheme major culminations are interpreted to reflect regions of surging flow marked by increased velocity during exhumation whilst the opposite is true for depressions this behaviour implies that on a regional scale differential shear varies laterally in an irregular sinusoidal manner defining areas of relative high and relative low displacement within the exhumation channel

sheath fold development with viscosity contrast analogue experiments in bulk simple shear it has been shown experimentally that sheath folds can develop passively by kinematic amplification of precursor non cylindrical deflections however sheath folds are most commonly multilayers where rheological contrast can be expected non passive sheath folding therefore we studied a natural occurrence of sheath folds where layers show rheological contrast and used analogue modelling to investigate sheath fold development with viscosity contrast the initial deflection in the experiments was always hemispherical and planar layering parallel to the shear plane under the chosen experimental conditions the results show that 1 the viscosity contrast between layers should be lower than one order of magnitude 10 to generate a sheath fold in both higher and lower viscosity layers otherwise the higher viscosity layer behaves as effectively rigid 2 the lower viscosity layer develops into a tubular fold dragged by the higher viscosity layer that undergoes mixed strain low and shear plane parallel translation high 3 the ratio between ellipticities of external and internal ellipses r re ri significantly departs from 1 as the viscosity contrast increases e g r 0 6 for 7 5 in the case where the higher viscosity layer caps the lower viscosity layer this is in great contrast with passively developed sheath folds for which r 1 when the precursor deflection is hemispherical from the present experimental study and previous work investigating other variables we conclude that r should be cautiously used and that extraction of kinematical information from sheath fold geometry alone can be erroneous

key ring structure gradients and sheath folds in the goantagab domain of nw namibia abstract the concept of deformation phases is one of the corner stones of structural geology but despite its simplicity there are situations where the concept breaks down in the goantagab domain of nw namibia structures in an area of complex deformation can be subdivided into at least four sets attributed to four deformation phases on the basis of overprinting relations three of these sets of structures however formed during the same tectonic event under similar metamorphic circumstances but slightly different flow conditions these sets of structures show gradational transitions in space that can be understood by a concept of key ring structure gradients where older d a structures are reoriented and overprinted by new structures d a 1 that have similar orientation and seem to grade into d a structures outside the overprinted area this kind of behaviour may be common in inhomogeneous non coaxial flow in the core of the goantagab domain d 2 structures are thus reoriented and overprinted by local d 2b folds and foliations that have the same orientation and style as d 2 structures outside the domain core

strontium carbon and oxygen isotope compositions of marbles from the cycladic blueschist belt greece the cycladic blueschist belt greece is mostly submerged below sea level and regional correlations are difficult to establish marbles are widespread within the belt and locally used as marker horizons to subdivide monotonous schist sequences however owing to the lack of distinctive petrographic characteristics the marbles have not been used for island to island correlations this study aims to investigate the potential of sr c and o isotope compositions of marbles as a tool for unravelling the litho and or tectonostratigraphic relationships across the cycladic islands and as a proxy for the time of sediment formation for this purpose we have studied metamorphic carbonate rocks from the islands of tinos andros syros sifnos and naxos identical 87sr 86sr values for certain marble horizons occurring on tinos andros and sifnos are interpreted to document coeval regional carbonate precipitation the 87sr 86sr values of the apparently least altered samples intersect the seawater curve multiple times within the most likely time interval of original carbonate precipitation 240 ma as indicated by previously published ion probe u pb zircon data and thus an unequivocal age assignment is not possible very broad temporal correlations are possible but more subtle distinctions are not feasible on andros the overlapping sr isotope values of marbles representing the lowest and highest parts of the metamorphic succession are in accordance with a model suggesting isoclinal folding or thrusting of a single horizon or very fast sedimentation in contrast distinct 87sr 86sr values for samples from tinos representing different levels of the metamorphic succession suggest that these rocks represent a temporal succession and not the tectonic repetition of a single horizon based on sr o and c isotope characteristics alone the time equivalence of marbles occurring on different islands could not be documented unambiguously however by using various combinations of these parameters some occurrences can be discriminated from the overall sample population the new data further accentuate the general potential of coupled sr c and o isotope characteristics for identification of archaeological provenance and complement existing datasets for aegean marbles

eye to eye with a mega sheath fold a case study from wadi mayh northern oman mountains sheath folds are highly curvilinear folds typically considered to develop by hinge rotation toward the transport direction during intense deformation most sheath folds are recognized on the centi meter to meter scale with very few detailed studies of larger kilometer scale structures we here present evidence for a megascale sheath fold superbly exposed in the northern oman mountains the saih hatat culmination consists of an exhumed highpressure subduction zone with deep level eclogites and garnet blueschists as sifah unit epidote blueschists hulw unit and carpholite bearing metasediments part of the upper plate sheath folds are present at a variety of scales throughout the as sifah hulw and upper plate tectonic packages curvilinear fold hinges along a 15 km long profi le outline the wadi mayh mega sheath fold which closes to the ssw and faces to the nne it consists of at least four secondary sheath folds with cross sectional y z eye folds measur ing 200 m stacked on top of one another within permian and triassic shelf car bonates these eye folds display highly elliptical geometries ryz 5 and cat s eye fold patterns consistent with simple general shear the sheath fold is enveloped by ordovician and older rocks that do not show equal amounts of internal deformation the upper bounding envelope is a detachment that has been subsequently sheath folded the lower boundary of the sheath fold is the upper plate lower plate detachment which forms a major high strain shear zone showing little vertical displacement but signifi cant horizontal displacement the sheath fold displays a ssw elongation x axis of at least 15 km and possibly up to 25 km coupled with signifi cant amounts of vertical fl attening and wnw ese shortening

scale invariant sheath folds in salt sediments and shear zones abstract sheath folds are developed in a broad spectrum of geological environments in which material flow occurs including gravity driven surficial deformation in ignimbrites unconsolidated sediments and salt together with deeper level ductile shear zones in metamorphic rocks this study represents the first geometric comparison of sheath folds in these different settings across a wide range of scales elliptical closures defining eye folds represent y z cross sections through highly curvilinear sheath folds our analysis of the published literature coupled with field observations reveals remarkably similar ellipticities r yz for sheath folds in metamorphic shear zones r yz 4 23 salt r yz 4 29 sediment slumps r yz 4 34 glaciotectonites r yz 4 48 and ignimbrites r yz 4 34 nested eye folds across this range of materials n 1800 reveal distinct and consistent differences in ellipticity from the outer r yz to the inner most r y z elliptical rings of individual sheath folds the variation in ratios from outer to inner rings r r yz r y z in gravity driven surficial flows typically displays a relative increase in ellipticity to define cats eye folds r r yz that are remarkably constant r 2 0 99 across 9 orders of magnitude sheath y axes range from 0 1 mm to 75 km our findings lead us to conclude that the geometric properties of sheath folds are scale invariant and primarily controlled by the type and amount of strain with r also reflecting the rheological significance of layering associated with original buckle fold mechanisms the scaling pattern of sheath folds reflects the length scales of the precursor buckle folds and width of deformation zones across a broad range of materials and environments with continued deformation the layering marking the original folds may become increasingly passive to define sheath folds these empirical relationships suggest sheath folding is a fundamental mode of viscous response across a broad spectrum of materials strain rates and scales encompassing a variety of deformation settings

strain development and kinematic significance of the alpine folding on andros western cyclades greece abstract the general trend of both fold axes and stretching lineation in the cycladic blueschist unit is ne sw to nne ssw this orientation forms a large angle almost perpendicular with respect to the hellenic trend that is inferred from the main thrusts on mainland greece thus the kinematic significance of the stretching parallel folding in the cycladic blueschist unit is non trivial since within the western cyclades the ne trending folds are best exposed on the island of andros it is a key locality for understanding the timing style and kinematic significance of folding here we show that the ne trending folds on andros formed within the stability field of glaucophane after the peak high pressure metamorphism and simultaneously with the early stage of retrogression the axes parallel stretching was non rotational it started during the ne folding at blueschist facies conditions and continued long afterward and well into the retrograde greenschist overprint furthermore we present the result of a finite strain calculation which shows that the large ne folds could not have been reoriented at 90 as previously thought instead it is suggested that these folds formed under constrictional strain regime during regional ne sw extension and represent coeval transverse nw se shortening and vertical thinning this implies that ne extension and southwest directed rollback of the active margin prevailed in the western aegean between the eocene and early miocene

particle velocity fields and depositional processes in laboratory ash flows with implications for the sedimentation of dense pyroclastic flows we conducted laboratory experiments on dam break flows of sub 250 m volcanic ash generated by the release of gas fluidized and variably non expanded to expanded up to 35 beds in order to gain insights into the internal kinematics of pyroclastic flows the flows were typically several cm thick and had frontal speeds of up to 2 m s 1 high speed videos taken through the transparent sidewall of the 3 m long channel were analyzed with a particle tracking algorithm providing a spatial and temporal description of transport and sedimentation the flows deposited progressively as they traveled down the flume being consumed by sedimentation until they ran out of volume deposition commenced 5 20 cm rearward of the flow front and for a given expansion proceeded at a rate independent of distance from the lock gate deposit aggradation velocities were equal to those inferred beneath quasi static bed collapse tests of the same ash at the same initial expansions implying that shear rates of up to 300 s 1 have no measurable effect on aggradation rate the initially non expanded and just fluidized flow deposited progressively at a rate indicative of an expansion of a few percent perhaps due to shear induced reynolds dilation during initial slumping the fronts of the flows slid across the flume floor on very thin basal shear layers but once deposition commenced a no slip condition was established at the depositional interface within the flows the trajectory of the constituent particles was linear and sub horizontal the velocities of the particles increased with height above the depositional interface reached a maximum then declined slightly towards the flow surface perhaps due to air drag at a given location the velocity profiles were translated upwards as the deposit aggraded the results show that even cm thin poorly expanded flows of ash deposit progressively as inferred for many pyroclastic flows the change from frontal slip to rearward no slip conditions at the bases of the laboratory flows are qualitatively consistent with some textural features of pyroclastic flow deposits

3d visualization of sheath folds in ancient roman marble wall coverings from ephesos turkey abstract archaeological excavations and restoration of a palatial roman housing complex in ephesos turkey yielded 40 wall decorating plates of folded mylonitic marble cipollino verde derived from the internal hellenides near karystos greece cipollino verde was commonly used for decoration purposes in roman buildings the plates were serial sectioned from a single quarried block of 1 25 m3 and provided a research opportunity for detailed reconstruction of the 3d geometry of meterscale folds in mylonitized marble a gocad model is used to visualize the internal fold structures of the marble comprising curtain folds and multilayered sheath folds the sheath folds are unusual in that they have their intermediate axis normal to the parent layering this agrees with regional tectonic studies which suggest that cipollino verde structures formed by local constrictional non coaxial flow sheath fold cross section geometry exposed on the surface of a plate or outcrop is found to be independent of the intersection angle of the fold structure with the studied plane consequently a single surface cannot be used as an indicator of the three dimensional geometry of transected sheath folds

heterogeneous constrictional deformation in a ductile shear zone resulting from the transposition of a lineation parallel fold abstract we use new micro structural petrofabric strain and vorticity data to analyze the deformation path in a mesoscopic quartz mylonite zone the mylonite zone resulted from the complete transposition of a stretching lineation parallel isoclinal fold symmetric cleft girdle quartz c axis fabrics were recorded in the middle domain which occupies the inner limbs of the precursor isoclinal fold while asymmetric cleft and crossed girdle fabrics were observed in the upper and lower domains that represent the outer limbs constrictional strain with increasing k values towards the middle domain is inferred from petrofabric and 3d strain data oblique grain shape fabrics yield vorticity estimates of 0 72 0 90 in the zone however in the middle domain pure shear dominated deformation is suggested by orthorhombic crystallographic fabrics strain rate is constant throughout the zone a strain decrease towards the zone center implies that deformation ceased earlier in the middle domain the data indicates that fold transposition and subsequent mylonitization started as pure shear dominated constrictional deformation and progressively changed to simple shear dominated plane strain during this flow path the asymmetric quartz c axis fabrics likely developed by depopulation of cleft girdle maxima rather than from the synthetic rotation of fabric maxima itself

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

subduction and exhumation related structures in the cycladic blueschists insights from south evia island aegean region greece 1 detailed geological mapping structural investigation and amphibole chemistry analyses in southern evia aegean sea greece allow us to place new constraints on the internal structural architecture and tectonic evolution of the cycladic blueschists we show that the early deformation history was related to ese directed thrusting resulting in the stacking of the styra and ochi nappes which constitute the cycladic blueschist unit in evia these early thrust movements initiated just before and proceeded at peak conditions of the eocene high pressure metamorphism subsequent constrictional deformation gave rise to e w trending upright folding accomplished at the early exhumation stage the main ductile stage exhumation occurred during a single deformation phase associated with the decompression of blueschist rocks from the stability field of crossite to that of actinolite this phase was characterized by localization of ductile deformation into a series of major tens of meters thick ene directed shear zones which cut up section in their transport direction and restack the early thrust and fold sequence locally bringing the structurally lower styra nappe over the higher ochi nappe it is suggested that these zones operated as thrusts rather than normal sense shear zones as has been previously argued and were possibly formed during the oligocene ene ward extrusion of the blueschists brittle ductile ne dipping normal faulting of post early miocene age was probably responsible for the final exhumation of rocks

kilometre scale sheath fold at mattmark and implications for transport direction in the alps folds were the first tectonic features of rocks to be studied and as such are taken as a reference for the interpretation of other structures intuitively fold axes were assumed to be subperpendicular to the direction of transport nevertheless many studies of metamorphic belts showing intense ductile deformation describe folds with axes close to the transport direction in particular sheath folds1 9 are highly curvilinear folds elongated parallel to the direction of transport we discuss here a spectacular large sheath fold situated in a crustal shear zone in the monte rosa nappe swiss alps our analysis implies that both the geometry and significance of folds in the internal alps must be re examined we further demonstrate the existence of crustal ductile shear zones in an area of the alps where previously emphasis was generally placed on fold nappes the movements on these shear zones is often tens of kilometres

upper cretaceous palynostratigraphy of the dry island areathis article is one of a series of papers published in this special issue on the theme albertosaurus the overall palynology of the upper horseshoe canyon formation to lower scollard formation within the edmonton group is reviewed providing new insights into biostratigraphic relationships for the interval including the recognition of unconformities in the stratigraphic package the palynomorph recovery from the horseshoe canyon formation and overlying units documents a number of significant events that allow the subdivision of the section the albertosaurus bonebed occurs within the lower maastrichtian portion of the edmonton group the bonebed is situated just below the base of the scollardia trapaformis palynomorph zone an important marker assemblage that has been documented from areas of the yukon northwest territories alberta and saskatchewan this indicates that the bonebed lies at the top of the mancicorpus vancampoi zone the transition between the zones is marked by a colour change in the sediments the appearance of coals and organic rich sediments above the transition a decrease in numbers

when shearing is a cause of folding abstract numerous publications on shearing as a cause of folding do not refute the idea of buckling being the main process responsible for the formation of fold structures deformations of other types may either produce conditions for layer parallel shortening needed for buckling or modify fold shapes but do not generate new fold structures on any scale the origin of similar folds commonly considered to be a result of inhomogeneous shearing across layering may be explained by buckling of a multilayer where layers have uniform favourable rheological properties tightening and amplification of folds near fault surfaces is not necessarily a result of shearing caused by friction at fault surfaces on the contrary in these cases faulting is a result of strain localization during folding recumbent isoclinal folds so common in metamorphic rocks originate mainly by buckling of subvertical planar structures intrafolial folds are usually very flattened relics of former folds the idea of sheath folds being produced mostly by shearing is not confirmed by the available geological data sheath folds that were not produced by shearing are rather common more likely they are formed by constriction or flattening of folds with curved hinges the latter are a result either of fold interference or of the constriction in the layering plane

ductile and brittle shortening extension parallel folds and maintenance of crustal thickness in the central aegean cyclades greece in contrast to previous studies that concentrated on the two dimensional crustal strain in the central aegean region cyclades it is shown that nne ssw extension via ductile and brittle stretching and low angle detachments was accompanied and or alternated with horizontal shortening perpendicular to the stretching direction since at least the early miocene roughly e w directed ductile shortening produced large scale overturned and upright folds having axes parallel or slightly oblique to the stretching lineation upright folding and arching of low angle normal faults occurred above the brittle ductile transition and brittle e w compression culminated with vertical axis block rotations strike slip faults and minor thrusts since at least the early miocene the structure of the cycladic massif evolved through alternating and or coeval increments of horizontal shortening and vertical thinning associated with an approximately constant nne ssw stretching exact magnitudes of the vertical and horizontal deformational components are difficult to assess nevertheless we note that extensional tectonics that affected the cyclades during the last 15 20 m y have produced no net crustal thinning we suggest that crustal thickness was maintained by extension parallel folds which represent true contractional structures and that crust was fed into the extended region from its margins

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

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

structural and hydrothermal evolution of the shear zone related iron oxide enrichment in metavolcano sedimentary rocks of the intracontinental aracuai orogen the case of the espirito santo iron deposit abstract the rio sao joao metavolcano sedimentary sequence is located in the eastern border of the northern espinhaco fold thrust belt in the intracontinental aracuai west congo orogen mafic schist quartz amphibole and carbonate itabirites as well as a carbonate silicate rock occur in the study area these contain mineral assemblages compatible with the amphibolite facies metamorphic conditions magnetite and hematite rich itabirites form a series of iron deposits in the region near igapora caetite bahia state brazil the espirito santo iron deposit has compact hematite layers truncating quartz itabirite with magnetite varying from 10 to 55 vol of the rock volume siliciclastic and mafic rocks also host iron rich domains variably enriched in magnetite and hematite the deposit is hosted in compressional duplexes with structural top towards the sw these structures have an sn 1 sn foliation represented by compositional banding and parallel schistosity that is observed at all scales other common features are s c c boudins pinch and swell structures as well as sheath syn tectonic folds and a mineral stretching lineation lxn hydrothermal alteration was contemporaneous to the development of duplex and the sn 1 sn foliation a distal alteration zone is dominated by a potassic muscovite and or biotite chlorite assemblage and the intermediate zone by carbonate development the proximal alteration zone constitutes the iron oxide magnetite and hematite domains sulfide minerals as well as quartz veins silicification in association with other phases also formed hydrothermal magnetite is associated with c structures which grows either including silicates and skeletal carbonates or forming corrosion borders in these minerals lamellar hematite forms polycrystalline aggregates characterizing the s c structure in the sn 1 sn foliation as well as the axial planar foliation sn in intrafolial folds and the mineral stretching line lxn data obtained by the la icpms technique show that in general in quartz rich and amphibole itabirites hydrothermal magnetite is richer in ree than an early stage magnetite hydrothermal magnetite ree pattern is close to that of the country rock of the mineralization the formation of hematite and magnetite rich domains is related to the percolation of hydrothermal fluids that leached all components of the original silicates quartz and the carbonates of all host rocks and precipitated new hydrothermal phases taking advantage of the ediacaran shear structures

quartz fabric and strain partitioning in sheath folds an example from the voltri group western alps italy abstract a mesoscopic sheath fold defined by an almost pure quartzitic level in quartz mica schists of the voltri group southeastern border of the western alps was sampled and studied in detail microstructural analysis was carried out on thin sections from different structural domains in the quartzitic layer with the emphasis to analyse quartz shape fabrics and the crystallographic preferred orientation of the c axes of this mineral quartz was dynamically recrystallized but its degree of preferred orientation is not as high as it would be expected in a high shear strain regime required for the development of sheath folds this may suggest strain partitioning between the quartzitic level and the micaceous matrix crystallographic preferred orientation of c axes and grain shape fabric asymmetry indicate the same sense of shear on both fold limbs in the xz plane this is in agreement with a model of sheath fold development by the passive rotation of material lines clustering of the c axis poles shows a configuration indicative of a different strain state in different domains of the sheath fold and suggests an evolution of the fold characterized by combined simple shear and coaxial progressive deformation

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

controls on lineation development in low to medium grade shear zones a study from the cap de creus peninsula ne spain lineations composed of similarly oriented elongate mineral aggregates or grains are a common feature in deformed rocks but it is unclear which factors control the development of such lineations field observations and microstructural analysis of samples which were taken from discrete greenschist to lower amphibolite facies shear zones of the easternmost variscan pyrenees show that strain is only one of several factors that control the strength and type of a lineation dynamic recrystallization metamorphic reactions and rigid body rotation are also important controlling factors for the development of lineations the most important of these is dynamic recrystallization the way in which dynamic recrystallization influences lineation development is largely a function of the initial fabric in a specific rock type different lithologies with different initial fabrics produce distinctly different types and strengths of lineations even if deformed to the same finite strain in the same shear zone an initial fine grain size and a monomineralic composition of the parent rock type commonly hinder the development of lineations in contrast in initially coarse grained polymineralic rocks well developed lineations commonly develop therefore the ratio of initial to dynamically recrystallized grain size determines to a large extend the development of such lineations

the geometry and topology of natural sheath folds a new tool for structural analysis abstract curvilinear sheath folds are classically depicted as displaying symmetrical geometries about two orthogonal mirror planes centred along the x y axial surface and the x z medial culmination depression surface which bisects the fold nose however 10 000 geometric analyses of minor folds and fabrics formed during ductile thrusting in the caledonides of northern scotland reveals that major dome and basin sheath folds can display distinct and predictable asymmetries across both axial and medial surfaces the strain is typically heterogeneous so that structural fabrics and younging evidence are preserved within sheath folds at varying stages of development this allows an analysis of the evolution of such structures from tongue folds to more extreme tubular forms geometric relationships between measured orientations of fold hinges axial planes extension lineations and foliations are compared on fabric topology plots ftps which provide an effective tool for monitoring planar and linear fabric rotations with increasing progressive non coaxial deformation they consistently display systematic variation from regions of lower to higher strain on passing from upper to lower fold limbs across major axial surfaces and on crossing medial surfaces from short to long hinge line segments axial and medial surfaces effectively therefore divide major sheath folds into quadrants with different amounts senses and combinations of planar and linear fabric rotation within each domain such heterogeneous deformation implies that models of intense non coaxial deformation uniformly affecting pre existing folds may overestimate bulk displacement and shear strain variable fold hinge line rotation about medial surfaces also provides an effective mechanism for the closure of major sheaths which may otherwise project for unfeasible distances in the x direction bedding cleavage intersections are developed at greater angles to the transport direction than fold hinges which they transect in a consistent and predictable sense thereby confirming the direction of fold rotation even in areas which lack information on fold facing in cross section asymmetric tear drop eyes indicate the sense of fold hinge line vergence whilst fold limb and hinge line asymmetry may be combined on 3 d vergence ranking and rotation grids which allow location and relative strain states of minor structures to be accurately predicted within the overall sheath framework systematic fabric analysis on ftps may be applied to the investigation of ductile deformation across a broad range of scenarios

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