structural anatomy of the kushtagi schist belt dharwar craton south india an example of archaean transpression abstract the nw se trending kushtagi schist belt in the eastern dharwar craton records the tectonic consequences of a transpression regime it was formed due to late archaean oblique convergence in the dharwar craton this relatively unknown schist belt is characterized by two major episodes of deformation the dominant d 1 deformation forming tight to isoclinal non cylindrical folds and a regional axial planar schistosity is a consequence of transpression the transpression related sinistral shear regime is followed by development of a conjugate set of f 2 folds and s 2 cleavages that are superimposed on the d 1 structures the maximum shortening direction associated with the f 2 folds is nw se which suggests longitudinal compression subsequent to transpression riedel shear fractures were developed in the sinistral shear regime and dolerite dykes were emplaced along the conjugate riedel shear fractures intrusive granite bodies within and outside the schist belt were emplaced in the late phase of d 1 transpression

instantaneous end moraine and sediment wedge formation during the 1890 glacier surge of bruarjokull iceland abstract contemporary understanding of the behaviour of surging glaciers and ice streams is hampered by the lack of data on landsystem evolution and sedimentary environments this study concerns the ice marginal environment of the surge type bruarjokull in iceland the sediment distribution in the glacier forefield as well as the morphology sedimentology and tectonic architecture of the 1890 end moraine is investigated for highlighting the interaction between very dynamic ice and sediment landform associations as a result of substrate bedrock decoupling during the 1890 surge subglacial sediment was dislocated across the bedrock surface and deformed compressively leading to gradual substrate thickening and the formation of a sediment wedge in the marginal zone a drop in subglacial porewater pressure at the very end of the surge led to substrate bedrock coupling and a stress transfer up into the sediment sequence causing brittle deformation of the substrate simultaneously the glacier toe ploughed into the topmost part of the marginal sediment wedge initiating the moraine ridge construction fine grained and incompetent sediment deformed in ductile manner resulting in a narrow moraine dominated by rooted folds while coarse grained and competent sediment deformed in brittle fashion resulting in a wider moraine dominated by thrust blocks a new sequential model of subglacial and ice marginal processes operating during a glacier surge is proposed illustrating the stepwise formation of a marginal sediment wedge and an end moraine a twofold inseparable marginal end product that formed during the last days of the 1890 surge

contrasting modes of reactivation in the outer hebrides fault zone northern barra scotland the gently ese dipping outer hebrides fault zone may be traced for c 170 km through the lewisian gneisses of northwestern scotland and defines broad arcuate swings in strike from ne sw to almost north south this study evaluates the kinematic evolution of the fault zone within the hinge region of such a strike swing exposed in the small isles north of barra new structural mapping of this area has concentrated on the geometry kinematic significance and the relative age of different structural elements of the fault zone and provides a comparison with the better known tectonics of the northern portions of the outer hebrides fault zone hitherto unrecognized top to the nw ductile thrusting in this southern portion of the fault zone coupled with abundant evidence of extensional reactivation and lack of sinistral strike slip in phyllonites as reported further north suggests that the present segmented and arcuate geometry is a primary feature of the fault system the original thrust template governed the subsequent reactivation and relative importance of both late caledonian strike slip and extensional collapse components inherited structural architectures may thus provide geometric constraints on the nature and patterns of subsequent reactivation and tectonism

synchronous development of type 2 and type 3 fold interference patterns evidence for recumbent sheath folds in the allendale area broken hill nsw australia abstract synchronous development of type 2 and type 3 fold interference patterns occur in the poly deformed broken hill inlier the interference patterns have resulted from the superposition of recumbent f2 folds and n s oriented upright f3 folds the synchronous development of type 2 and type 3 fold interference patterns is attributed to variation in the hinge of f2 folds by as much as 90 suggesting the development of a regional scale sheath like fold geometry during d2 overprinting relationships along the eastern limb of the pap synform suggests that it formed part of a recumbent fold hinge that was flattened during horizontal crustal shortening this has resulted in the development of a modified type 3 fold interference pattern the results of this study show that the type 2 and type 3 fold interference patterns can develop during the same deformation event in an evolving orogen

kilometre scale sheath folds in the western betics south of spain the three dimensional exposures of the sierra blanca marbles and neighbouring rocks in the western betic cordillera of the south of spain show tight to isoclinal f3 folds re folded by a larger sheath like structure the relationships between both structures are as follows 1 the mean axial surface for sierra blanca sheath fold can be defined as the great circle of best fit through the s axes of sub cylindrical f3 folding domains 2 the asymmetric f3 folds indicate a single mainly eastward normal sense of shear 3 the mean attitude of the axial plane crenulation foliation sc is also sub parallel to the mean axial surface for sierra blanca sheath fold the regional context of the sierra blanca sheath fold is finally discussed and a model of a heterogeneous high strain extensional zone proposed

early curvilinear fold structures and strain in the moine of the glen garry region inverness shire four regional deformation episodes d1 d4 are recognized in the moine rocks loch eil and glenfinnan divisions of the glen garry area inverness shire nw scotland these episodes fall into two groups early sub recumbent d1 d2 structures and later upright d3 d4 structures d2 structures occur on all scales and appear in a regional sense to be more important than the d1 structures the d1 d2 structures are only preserved in their original disposition where subsequent upright d3 reworking was minimal i e in the region to the e of the loch quoich line and in low strain augen developed in d3 hinge zones to the w of this structure the f2 folds are intensely curvilinear and at many localities f2 hinges lie approximately parallel to the n s trending extension lineation examination of these structures in three dimensions and application of the sanderson 1973 model provides information on the nature and orientation of the regional d2 strain ellipsoid in this part of the moine the d2 xy plane is sub horizontal with the x direction trending approximately n s this is markedly different in orientation from the d3 ellipsoid in this region which being of post glen dessary syenite 456 ma age is demonstrably caledonian the d2 linear fabrics suggest that the direction of tectonic transport was s to n or n to s markedly different from the generally accepted wnw directed movement attributed to caledonian orogenesis these findings are consistent with the proposal that the loch quoich line represents an eastern limit of intense caledonian d3 reworking and that the d1 d2 structures are of precambrian age

understanding kinematic data from the moine thrust zone in terms of a kinematics based mathematical model of deforming thrust wedges abstract a salient recess pair along the moine thrust zone exhibits significant lateral kinematic variations specifically the more steeply dipping portion of the thrust surface associated with the recess shows evidence for greater strain magnitudes and more out of the plane motion than the adjoining salient additionally a subset of the grain shape data has grain shape approximations with their long axes oriented perpendicular to the regional transport direction to more fully understand these kinematic features we developed a kinematics based mathematical model of thrust wedge deformation our numerical model predicts an increase in strain magnitude as well as non plane strain flattening in the region of a thrust ramp consistent with the strain patterns observed within the recess along the moine thrust zone furthermore in the absence of a lateral confining boundary condition much of the model thrust wedge experiences transport perpendicular maximum extension except for the region closest to the thrust where simple shear deformation is dominant using our numerical model we predict the amount of lateral confining strain needed to induce plane strain deformation ps as well as the amount of lateral confining strain necessary to make the long axis of the strain ellipsoid parallel with the transport direction parallel we have found that the incremental parallel can be as much as 42 of the incremental strain magnitude thus we consider transport perpendicular lineations like those seen along the moine thrust zone to be the result of lateral confining boundary conditions that are weak enough to allow transport perpendicular maximum extension

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

structural data constraints for implicit modeling of folds a recent method for modeling folds uses a fold frame with coordinates based on the structural geology of folds fold axis direction fold axial surface and extension direction the fold geometry can be characterised by rotating the fold frame by the pitch of the fold axis in the axial surface and the angle between the folded foliation and the axial surface these rotation angles can be expressed as 1d functions of the fold frame coordinates in this contribution we present methods for extracting and automatically modeling the fold geometries from structural data the fold rotation angles used for characterising the fold geometry can be calculated locally from structural observations the fold rotation angles incorporate the structural geology of the fold and allow for individual structural measurements to be viewed in the context of the folded structure to filter out the effects of later folding the fold rotation angles are plotted against the coordinates of the fold frame using these plots the geometry of the folds can be interpolated directly from structural data where we use a combination of radial basis function and harmonic analysis to interpolate and extrapolate the fold geometry this contribution addresses a major limitation in existing methods where the fold geometry was not constrained from structural data we present two case studies a proof of concept synthetic model of a non cylindrical fold and an outcrop of an asymmetrical fold within the lachlan fold belt at cape conran victoria australia

structural analysis of sheath folds with horizontal x axes northeast canada abstract early proterozoic supracrustal rocks occur below a thick nappe of archaean basement gneiss in the melville peninsula where sheath folds are exposed in a wide zone of middle proterozoic dynamothermal metamorphism outcrop patterns of truncated isoclinal sheath folds resemble cylindrical folds except in relatively small areas around the paraboloidal caps bulk extension axes are parallel to strike in the belt as shown by isoclinal sheath folds with horizontal central axes x axes as well as similarly aligned mullion structure and rotated scapolite prisms extension axes converge from northeast to southwest in the apparent flow direction

the geology and structural evolution of a caledonian fold and ductile thrust zone kyle of tongue region sutherland northern scotland the middle to upper proterozoic metasedimentary rocks of the northern highland moine in sutherland are disposed in a series of major caledonian c 470 430 ma metamorphic thrust nappes produced by nw wnw directed shortening and reworking of parts of a pre existing precambrian c 1000 ma orogen lewisian inliers occur throughout sutherland and represent tectonically interleaved units of the original basement gneisses upon which the moine cover sequence was deposited in the western moine nappe of the kyle of tongue region the rocks are deformed by two main episodes d1 and d2 which are thought to represent the effects of precambrian and caledonian orogenesis respectively little basement cover interleaving or large scale folding occurred during the d1 event in the moine nappe but the associated metamorphism to ut leust garnet grade produced peak metamorphic mineral assemblages subsequent caledonian reworking d2 produced a foreland propagating system of wnw directed ductile thrusts and related major f2 sheath folds both of which resulted in much of the observed moine lewisian interleaving metamorphism associated with d2 decreases in grade westwards occurring at similar or slightly lower grades than the earlier d1 event this retrogressive overprint is most obvious at the w margin of the moine nappe the d2 folds and ductile thrusts are geometrically and kinematically similar to the caledonian structures recognized within the moine thrust zone to the west the very low 5 ramp angles suggesting that the thrusts must have very shallow dipping trajectories through the crust f2 fold formation is attributed to an early phase of wnw directed overthrusting compression which was then progressively superseded by a regional ductile extensional strain contemporaneous with continued wnw directed thrusting the occurrence of internal extension in the orogenic wedge during active thrust related foreshortening implies that the shear strength of the material close to the basal detatchment s was continuously reduced i e wedge motion became supercritical in this context strain softening processes within the ductile thrust mylonites were probably of major significance

3d form line construction by structural field interpolation sfi of geologic strike and dip observations abstract interpreting and modelling geometries of complex geologic structures from strike dip measurements using manually drafted structural form lines is labour intensive irreproducible and inherently limited to two dimensions herein the structural field interpolation sfi algorithm is presented that overcomes these limitations by constructing 3d structural form lines from the vector components of strike dip measurements the sfi interpolation algorithm employs an anisotropic inverse distance weighting scheme derived from eigen analysis of the poles to strike dip measurements within a neighbourhood of user defined dimension and shape ellipsoidal to spherical and honours younging directions when available the eigen analysis also provides local estimates of the plunge vector and associated woodcock distribution properties to assure plunge normal structural form line reconstruction with unidirectional propagation of form lines across fold and fan structures the method is advantageous for modelling geometries of geologic structures from a wide range of structurally anisotropic data modelled vector fields from three case studies are presented that reproduce the expected bedding foliation geometry and provide reasonable representation of complex folds from local to regional scales results illustrate the potential for using vector fields to support geologic interpretation through the direct visualization of geometric trends of structural features in 3d

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 geometry and kinematics of flow perturbation folds abstract minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow using structures associated with wnw directed caledonian thrusting in n scotland we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction these define a series of larger scale fold culminations reflecting surging flow or depressions reflecting slackening flow that are bisected by transport parallel culmination and depression surfaces the fold patterns suggest a dominance of layer normal differential shearing due to gradients in shear strain normal to transport culmination surfaces are marked by along strike reversals in the polarity of structural facing and vergence of minor folds which contrary to classic fold patterns define reverse asymmetric relationships culmination surfaces separate folding in to clockwise z folds and anticlockwise s folds domains relative to the transport lineation the dip of fold axial planes systematically increases as their strike becomes sub parallel to transport resulting in a 3d statistical fanning arrangement centred about the transport direction thus mean s and z fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved culminations display convergent fold patterns with fold hinges becoming sub parallel to transport towards the culmination surface and underlying detachment whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface thus around culminations and depressions there are ordered scale independent relationships between transport direction shear sense fold facing vergence and hinge axial plane orientations the techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow

a modern structural regime in the paleoarchean 3 64 ga isua greenstone belt southern west greenland abstract the footwall gneisses beneath the western part of the paleoarchean 3 8 ga isua greenstone belt southern west greenland are interpreted here in terms of a 3 64 ga stack of mylonitic crystalline thrust nappes the oldest example known on earth in present coordinates the kinematic history of the thrust nappe stack is couched in terms of initial longitudinal strike parallel thrusting towards the southwest followed by transverse thrusting to the northwest and subsequent extensional collapse of the thickened crust toward the southeast diorite and tonalite that form the western margin of granitoids structurally overlying the western part of the isua greenstone belt and its footwall contain 3 5 ga mafic dykes some of which are deformed and or truncated at fault contacts within the granitoids accordingly a component of the deformation structurally above the isua greenstone belt occurred after 3 5 ga significantly later than the formation of the underlying mylonitic nappes probably during the neoarchean the structural regime of mylonitic thrust nappe stacking is very similar to that in modern mountain belts it would appear that the deformational behaviour rheological constitution and overall strength of paleoarchean and modern continental crust were similar

extensional tectonics of the outer hebrides fault zone south uist northwest scotland the outer hebrides fault zone is a major ese dipping reactivated structure within lewisian basement gneisses of the laurentian craton northwest scotland detailed mapping in south uist reveals important new evidence that contributes to a better understanding of the kinematic evolution of the fault zone large quantities of pseudotachylite which characterize the fault zone on south uist may in part be lithologically controlled and therefore of little value in determining areas of greatest deformation and displacement only limited evidence is preserved for ductile and brittle thrust sense movements along this portion of the fault zone the tectonics of the fault zone on south uist are dominated by structures associated with several episodes of pervasive top down to the se to ene brittle extensional deformation which are progressively overprinted by protophyllonitic and phyllonitic fabrics associated with top down to the e to ene extension a series of late stage high angle normal faults record top down to the ese to ene extension and cut the phyllonites fluid inclusion studies from syntectonic quartz veins constrain the conditions of phyllonite formation at 370 20 c field evidence suggests that this section of the outer hebrides fault zone may have been largely unaffected by sinistral strike slip reactivation as reported along strike to the north suggesting both a varied and compartmentalized tectonic and evolutionary history along the length of the outer hebrides fault zone

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

d 2 asymmetric folds and their vergence meaning in the montagnola senese metamorphic rocks inner northern apennines central italy abstract this study describes asymmetric folds whose vergence is derived from refolding of a pre existing inclined foliation the study area is in the montagnola senese area where mesozoic rocks in the greenschist facies crop out these rocks were affected by deformation during the collisional stage late oligocene early miocene of the northern apennines d 1 event and during the post collisional extensional tectonics d 2 event that affected the inner zone of the northern apennines since the early middle miocene during the d 1 event sw dipping axial plane schistosity and non cylindrical folds indicative of a highly heterogeneous strain developed during the second event westward verging folds developed these folds are characterised by thick steep limbs the s 2 foliation which is well developed in narrow and localised zones is mainly a pressure solution cleavage with ne plunging stylolitic teeth micro and meso structural observations are used to discuss the relation between cleavage distribution and strain during the d 2 event the evolution of d 2 folds was mainly controlled by a dissolution process and by a component of partitioned shear strain in the relatively high strain domains deformation took place by the combined effects of volume loss and shear displacement in low strain domains deformation took place by a veining and flattening process in this way the pre existing foliation can be deformed in folds with vergence opposite to the sense of shear

the anatomy of shallow crustal transpressional structures insights from the archaean carajas fault zone amazon brazil abstract the carajas fault zone cfz is the most prominent structure in the e w trending carajas strike slip system casss an ancient upper crustal fault network of probable late archaean age that cuts across the precambrian amazonian craton in brazil the subvertical faults reactivated pre existing high grade basement shear zone fabrics itacaiunas shear zone and display a complex long lived history of movement 1 ga dominated by oblique and strike slip displacements a postulated early regional phase of dextral movement downfaulted units of low grade to unmetamorphosed basement rocks into dilational bends and offsets in the casss subsequent sinistral transpression ca 2 6 ga led to faulting and folding during partial inversion of the cover rocks in these dilational sites with much of the associated deformation focused in the immediate vicinity

planar dispersion of folds in ductile shear zones and kinematic interpretation of fold hinge girdles abstract as indicated by theory and multiple case studies hinge lines of minor folds in ductile shear zones are dispersed in a plane that is incompletely rotated toward mylonitic foliation this yields a geometric relationship between the fold hinge girdle and mylonitic foliation that can be used to determine sense of shear statistical study of published data from various shear zones and of new data from the blue ridge thrust complex reveals a small to moderate angle between the best fit plane to hinge lines and mean mylonitic foliation this angle is 4 9 in eight of 12 test cases moderate angles 22 34 are associated with nappes kinematic analyses of 12 fold hinge girdles indicate the sense of shear otherwise determined for host shear zones a theoretical model of contemporary fold rotation in simple shear closely simulates the geometric relationships described above for natural folds using this model large shear strains 10 are implied by the 4 9 angle between the fold hinge girdle and mylonitic foliation

progressive deformation structures associated with ductile thrusts in the moine nappe sutherland n scotland in the deeper parts of mountain belts polyphase structural sequences observed at outcrop scale can arise due either to overprinting of regionally distinct deformation phases e g reworking changes in orogenic boundary conditions or to localized controls that bring about transient changes in the patterns of ductile flow these are unlikely to be mutually exclusive processes and examples from scotland demonstrate that once regionally separate events are delimited using radiometric evidence it is possible to isolate complex deformation sequences arising due to local controls in the western moine nappe of sutherland the dominant structures were formed during caledonian ductile thrusting towards the wnw whilst earlier precambrian phases are relatively minor in importance two groups of caledonian folds and fabrics are recognized in many exposures main phase d2 structures which are broadly contemporaneous with ductile thrust fabrics and later secondary phase f3 folds the latter can be divided into two geometric groups sheath fold types which formed initially as wnw overturned buckles subsequently modified by ductile shearing and asymmetric types which are commonly open folds apparently formed with axes close or sub parallel to the thrust transport direction secondary structures show a close spatial association with high strain zones along ductile thrusts and can be shown to have formed during the later stages of thrusting in certain critical exposures i propose that they may form due to strain perturbations resulting from variations in the relative rates of ductile flow within the mylonites where differential shearing occurs due to flow normal perturbations wrench related asymmetric fold types may form in contrast secondary sheath fold structures may result from localized compression phases caused by flow parallel perturbations local flow perturbation models may be appropriate in situations where the distribution of later structures is apparently related to spatial variations in strain intensity and or there is a strong geometric and kinematic similarity between sets of folds and fabrics if such features are not observed a regional polyphase sequence is more likely and it is therefore necessary to obtain radiometric dates or some other independent source of data to demonstrate a time separation

the distribution geometry and kinematic significance of caledonian buckle folds in the western moine nappe northwestern scotland within internal regions of orogenic belts systems of ductile thrusts and folds may form which are geometrically and kinematically analogous to those observed in foreland thrust belts the moine nappe of sutherland northwestern scotland developed during wnw directed caledonian displacements under amphibolite facies conditions broad zones of mylonite associated with the ductile ben hope and moine thrusts both contain numerous small scale folds which can be attributed to transient flow perturbations it is possible to isolate and differentiate generations of such folds basedon their association with particular major thrust zones and or their geometry in regions of lower strain between these thrusts there are a series ofkilometre scale buckle folds the axial traces of which define arcuate trends those with hinges orthogonal to transport are northwestvergent whilst those plunging subparallel to the mineral lineation display variable vergence the spatial distribution of these geometries suggests a causal relationship with displacements on specific underlying thrusts we propose that they reflect larger kilometre scale and possibly longer lived flow perturbations the transport parallel folds are persistent features that often centre on earlier structural culminations and are thought to reflect components of differential shear the most prominent set of regional buckle folds root downwards into the ductile moine thrust and openly refold the structurally overlying ben hope thrust and its associated buckle folds thus supporting a foreland propagating sequence of ductile displacements

structural assemblage of high pressure mantle and crustal rocks in a subduction channel cabo ortegal nw spain 1 high pressure metamorphic fold nappes and ductile thrusts occur in the cabo ortegal upper allochthons they involve arc root lithospheric mantle peridotites high pressure volcanic arc granulites normal mid oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high pressure gneiss units confluence of units from such disparate tectonic settings occurred at an eo hercynian convergent plate margin their internal deformation juxtaposition and high pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations fold nappes exhibit a remarkable sheath like form and contain mineral and stretching lineations defined by high pressure mineral assemblages subparallel to fold axes we argue that these structures outline the internal organization of high pressure sheets at deep tectonic realms of an oblique subduction collision orogenic channel

inversion of geological knowledge for fold geometry abstract the process of building three dimensional 3d geological models can be framed as an inverse problem where a model describing the 3d distribution of rock units is non uniquely derived from geological observations the inverse problem theory provides a powerful framework for inferring these parameters from all geological observations in a similar way to how a geologist can iteratively update their structural interpretation while mapping existing geological knowledge is usually indirectly incorporated into 3d models using the geologist s non unique interpretation as form lines cross sections and level maps these approaches treat constraints derived from geological knowledge in the same way as direct observations diluting and confusing both information provided by geological knowledge and hard data resulting in significant subjectivity we present a geological inversion using bayesian inference where geological knowledge can be incorporated directly into the interpolation scheme with likelihood functions and informative prior distributions we demonstrate these approaches on a series of synthetic fold shapes as a proof of concept and a case study from the proterozoic davenport province in the northern territory australia the combined inversion of geological data and knowledge significantly reduces the uncertainty in possible fold geometries where data is sparse or highly ambiguous this could be used by geologists while mapping to propagate information about uncertainties throughout the mapping model building process and would allow for different structural interpretations to be rapidly tested for targeted data collection

late brittle deformation in a caledonian ductile thrust wedge new evidence for gravitational collapse in the moine thrust sheet sutherland scotland abstract in the ductile regions of orogenic wedges late brittle structures formed at shallow crustal depths have received little attention however they are important as they of ten record the final stages in deformational history during uplift and erosion of the orogen in the moine rocks of sutherland late caledonian brittle folds and low angle faults post date ductile structures formed during wnw nw directed thrust movements earlier in the same orogenic event in the western moine nappe the late structures display geometries which indicate that they formed during regionally ese directed extensional movements down the dip of the pre existing regional foliation these late structures are correlated with similar wnw directed extensional low angle faults recognised in the moine thrust zone south of the assynt culmination both are thought to have formed in response to late caledonian gravity induced thinning of the moine thrust wedge and both pre date devonian extension associated with the formation of the west orkney basin along strike changes in the location and sense of extensional movements north and south of the assynt culmination are explained by changes in the pre existing structural geometry of the caledonian ductile thrust nappes within the moine hence earlier deformation patterns are thought to exert important controls over later events in an evolving orogenic wedge

anatomy of a 10 km scale sheath fold mount hay ridge arunta region central australia the structural record of deep crustal flow 1 mount hay ridge arunta region central australia exposes a 10 km scale antiformal sheath fold formed in the deep crust p 0 6 0 9 gpa in proterozoic time tilting and exhumation during the devonian alice springs orogeny resulted in an oblique cross section such that the once subhorizontal sheath fold now has a moderately dipping gently folded axial plane and overturned southern limb fabric types e g s l l s were mapped in the field and finite strain data were collected from a widely distributed gabbroic granulite unit fabric and finite strain data sets exhibit remarkable correspondence and confirm the common assumption that fabric type is directly related to finite strain type for deep crustal rocks both data sets reveal a distinct pattern of oblate strain on fold limbs grading into prolate strain in the fold hinge there is no field evidence for significant strain localization and measured strain magnitudes are consistent across the fold thus we infer that different structural domains record different though coeval strain paths and that strain compatibility was maintained across domain boundaries the uniformity of foliation lineation and mesoscale fold orientations across mount hay and the consistent magnitude of maximum stretch across structural domains indicate that the sheath fold and subsidiary structures formed during a single progressive deformational event based on its geometry the inferred scale of the enclosing shear zone 8 km in thickness and regional tectonic correlations we infer that the mount hay sheath fold formed in a subhorizontal crustal scale flow zone during the paleoproterozoic strangways orogeny 1740 1690 ma the record of deformation is consistent with fold formation either on one side of a channel or through detachment flow

a model for stratigraphic and metamorphic inversions at sulitjelma central scandes the relationship between disposition of metamorphic isograds and early strain history is examined for the sulitjelma fold nappe of the central scandes evidence for stratigraphic inversion and the widespread inversion of the garnet isograd is reviewed and new evidence for the nature of early scandian deformation is presented a two stage model is presented which explains the inversions by i an initial phase of horizontal compression resulting in crustal thickening by vertical stretching during closure of a marginal basin followed by ii continued horizontal compression and crustal thickening by eastward directed nappe emplacement the emplacement of the sulitjelma fold nappe during the latter phase of progressive simple shear dominated deformation was accompanied by the development of sheath fold geometries and the overturning of isotherms to produce the widespread inversion of the garnet isograd

vergence an evaluation abstract fold asymmetry is an independent concept distinct from facing fold vergence is a direction relating to the sense of asymmetry of folds the complementary concept of cleavage vergence is defined allowing asymmetry to be used to locate major fold traces where no minor folds exist cleavage vergence is a valuable tool in complex structural terrains both concepts can be used singly or in combination to identify refolding vergence can define structural way up where sedimentological or stratigraphic way up cannot be recognised

transport parallel cross folds within a mid crustal caledonian thrust stack northern scotland abstract cross folds are typically associated with zones of anomalous foliation trend that lie at high angles to orogenic strike this case study concentrates on a region of transport parallel cross folding developed by buckling during caledonian ductile thrusting within the moine and naver nappes of northern scotland detailed structural analysis reveals a systematic angular relationship between the trend of tectonic transport fold axes and the vergence of minor folds large scale cross folding is considered here to be related to wrench dominated differential shearing during thrust sense displacements along an important possibly out of sequence structure here termed the ben blandy shear zone this suggests that patterns of folding within the internal parts of the caledonian orogen in scotland are principally controlled by the kinematic constraints imposed by low angle thrusting thus early main phase folding is associated with the initiation and propagation of ductile thrusts whilst later secondary structures including cross folds can be related to the development of flow perturbations during displacement along well established regionally important detachments

discussion on evidence for non plane strain flattening along the moine thrust loch strath nan aisinnin north west scotland by mathew strine and steven f wojtal

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

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

shear zone versus fold geometries at the cannington ag pb zn deposit implications for the genesis of bht deposits existing structural analysis of drill core and sulphide zonation were taken as evidence for a large scale synform that dominates the geometry of the cannington ag pb zn deposit a broken hill type bht deposit underground mapping in this study has found that northwesterly directed thrusting took place during peak temperature d1 and that s1 gneissosity is continuous around an amphibolite mega boudin d2 sillimanite biotite shear zones contain asymmetric folds previously interpreted to be parasitic folds but in this study they are interpreted as due to easterly directed extension of s1 s1 and s2 indicate that shear zones were continuous and not folded into the current deposit geometry d2 is associated with garnet pyroxene alteration that proved to be a favourable host for low temperature sulphide precipitation retrograde deformation post d2 controlled lode geometry such a concept of high temperature shear zone formation can be applied to other bht deposits such as the broken hill main lode and the aggeneys gamsberg base metal deposits in south africa in the shear zone model cannington and other bht deposits are situated within high temperature thrust shear zones that formed in conjunction with regional large scale folding

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

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

the stereographic analysis of facing abstract facing directions represent an accurate means by which tectonic structure and sedimentary way up may be related a simple method is presented for determining facing directions accurately using a stereonet once determined larger amounts of facing data can be quickly collated and analysed using similar techniques to those employed for other lineations an example is given for some sheath fold structures in the moine rocks of sutherland n scotland

origin of striping lineation and transposition of linear structures in shear zones abstract striping lineation occurs as fine colour stripes on foliation surfaces of strongly sheared rocks that have a mylonitic colour banding although described as intersection lineation striping lineation in mylonites does not show two different intersecting s surfaces unlike ordinary bedding cleavage intersection striping lineation is a typical product of ductile shearing the colour stripes may have formed at a very early stage of deformation as intersections of a bedding or a colour banding on the foliation surface at a later stage the bedding or the colour banding ceased to be reoriented by external rotation the foliation became the only active s surface and the deformation of the colour stripes was controlled entirely by the nature of the strain after the appearance of a layer trace on the axial plane cleavage in an early stage of deformation the later history of its changing width and orientation becomes independent of changes in the thickness and orientation of the layer in strongly sheared rocks the nature of the original layering is greatly modified hence a colour striping can remain with very small width even when the foliation and bedding have become parallel the striping lineations in the phulad shear zone in western india and singhbhum shear zone in eastern india are often transposed and retransposed in response to folding sheath folding and strong stretching the process is very similar to the development of transposed foliation

structure of the isan orogeny under cover to the east of the mount isa inlier revealed by multiscale edge analysis and forward and inverse modelling of aeromagnetic data aeromagnetic data reveals the presence of three north south orientated tens of kilometre scale elliptical structures under cover to the east of the mount isa inlier in queensland australia the holy joe creek boot creek and nora creek structures are typically defined by three narrow continuous magnetic ridges highly magnetically susceptible amphibolite bodies similar to those exposed in the inlier are probably the source of most of these magnetic anomalies multiscale edge analysis 2d forward modelling and 3d inverse modelling methods have been used to resolve the geometry and near surface dip of the structures these techniques indicate that the structures dip moderately to steeply around their edges the holy joe creek structure exhibits sub vertical to steeply west dipping sides and a moderate plunge around the northern closure the boot creek and nora creek structures are more complex magnetically but their eastern edges also have sub vertical to steep westerly dips diapirism and domes in an egg box style type 1 fold interference pattern are considered as possible origins for the elliptical structures however disparity between the shape of the structures and possible diapiric bodies and the lack of complementary basin structures to the domes makes these two explanations unlikely a more plausible alternative is that the structures are the result of deformation in a single phase producing folds with a variable hinge line plunge these results indicate that east west shortening during the d2 event of the isan orogeny was the major deformation in the buried rocks of the study area suggesting a shared structural history with the mount isa inlier to the west

vergence and facing patterns in large scale sheath folds the careful geometric analysis of minor structural detail elucidates the relationships and evolution of associated large scale curvilinear hinge geometries developed during wnw directed caledonian thrusting exposed in neoproterozoic moine psammites of the moine nappe reversals in the polarity of structural facing associated with minor folding mark the position of major sheath folds which parallel transport upwardly convex sheaths closing in the direction of thrust transport cored by older gneissose basement inliers are termed culminations whilst those opening in the transport direction and cored by moine psammites are termed depressions sheath folds are bisected by transport parallel and foliation normal culmination depression surfaces which separate not only the reversals in facing but also delineate zones of minor fold hinge obliquity into clockwise and anticlockwise domains relative to the transport direction the sense of obliquity of minor z and s folds is thus dependent on position with respect to the surfaces of culmination and depression and not the fold axial surfaces surfaces of culmination and depression may be superimposed on original overturned antiformal and synformal folds to produce a variety of dome culmination on antiform saddle depression on antiform inverted saddle culmination on synform and basin depression on synform configurations the curvilinear hinges of minor folds may also be asymmetrical about the transport direction and within the plane of the regional foliation to define patterns of fold hinge line vergence classical concepts of fold limb vergence may thus relate to larger antiformal and synformal hinges whilst the fold hinge line vergence defines major curvilinear hinges associated with culminations and depressions major sheath folds may therefore be interpreted in terms of both minor fold hinge line and limb vergence coupled with fold axis obliquity and reversals in the polarity of structural facing the ability to recognise consistent and reliable structural relationships between facing and hinge obliquity at small scales indicates that the regional deformation process forms a linked and coherent system through several orders of magnitude 1999 elsevier science ltd all rights reserved

structural evolution of the accretional continental margin of the paleoproterozoic svecofennian orogen in southern sweden abstract volcanic and sedimentary rocks of the vastervik and valdemarsvik groups along the southern margin of the 1 90 1 85 ga south svecofennian crustal province in sweden have been migmatised and intruded by granitoids and gabbros during extension deformation phase d1 at a depth of at least 10 km peak metamorphic conditions were reached around 1825 ma extension switched to compression before cooling the partially molten middle crust facilitated shortening and thickening by isoclinal and sheath folds on scales from a decimetre to 10 km d2 a pervasive mylonitic s2 foliation developed these structures were refolded into eastward plunging crustal scale upright folds with e se w nw trending axial planes d3 d2 d3 deformation structures are colinear and developed during top to the wnw slip along s2 locally zones of high ductile strain in garnet bearing migmatite assemblages record considerable vertical displacement from 850 to 300 mpa possibly in nappe roots some major d2 sheath folds are embraced by d3 like but nnw trending megafolds d4 apparently resulting from shortening around the former d2 d4 deformations are interpreted to be broadly coeval and related to transpressive crustal shortening against the northern svecofennian continental back stop after cooling to high greenschist facies conditions early structures underwent renewed transpression in north south direction expressed in localized retrogressive shear zones phase d5 much d5 strain was accommodated by the 10 km wide nw se striking right lateral loftahammar linkoping deformation zone lldz operative around 1800 1780 ma regional strain partitioning caused transverse shortening in 15 20 km wide border zones of the lldz later overprints d6 involve narrow subparallel nw se striking and steeply ne dipping low greenschist facies mylonite zones some of which merge into the regional polyphase asbro norrkoping deformation zone d6 was transpressive oblique left lateral ne block up slip suggests a w sw e ne orientation of 1 peak metamorphism and partial melting in the supracrustal rocks was approximately coeval with 1800 1835 ma old subduction related magmatism in the oskarshamn jonkoping belt ojb 100 km to the south west seismic reflection profiling off the baltic coast babel line b suggests northward polarity of subduction beneath the ojb and a back arc environment of the vastervik and valdemarsvik groups between the ojb arc and the margin of the 1850 ma old svecofennian orogen we suggest that d1 d5 deformation was caused by intracontinental back arc extension and subsequent closure of the back arc basin by oblique accretion of the ojb marginal arc onto the northern continent accretion involved northward indentation by mid crustal wedges one of which coincides with and possibly caused the nucleation of the d5 stage lldz 40ar 39ar data from hornblende and white mica identify a prolonged cooling history of the area 1810 1490 ma influenced by the nearby late and postorogenic transscandinavian igneous belt and possibly by largely hidden c 1530 ma old anorogenic rapakivi type intrusions

progressive refolding in ductile shear zones abstract the foliation formed in a ductile shear zone can become folded by continuing shear in the zone without the foliation having to enter the shortening sector of the flow field if the foliation lies parallel to the shear plane infinitesimal variations in the rate of shear strain cause compensating rotations that may amplify into folds small increases in the rate cause backward rotations counter to the sense of shear these do not amplify small decreases cause forward rotations that do amplify

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

dome and basin refolding and transpressive inversion along the karatau fault system southern kazakstan the karatau fault system of southern kazakstan forms a crustal scale zone of strike slip dominated transpressional tectonics which has undergone multiple phases and styles of deformation during a protracted history of reactivation from the neoproterozoic to the cenozoic ductile fabrics associated with dextral kinematic indicators are present in amphibolites along the main karatau fault the ages of the possibly ophiolitic protoliths and the ductile fabrics are not well constrained but they are plausibly pre late riphean late ordovician granites stitch thrusts and folds which deform late riphean to llanvirn clastics preserved beneath a regional late devonian angular unconformity this deformation may be related to a contemporary continental collision ne of the karatau two phases of late palaeozoic deformation affected the upper devonian and carboniferous carbonate succession the first phase is related to sinistral transpression along the karatau fault system the second to dextral transpressional reactivation the combination of these events produced a regional scale dome and basin fold interference pattern similar polyphase deformation appears to have affected large areas of central asia and was possibly caused by the late palaeozoic orogenies at asia s margins such as the accretion of tarim and the east european craton to asia and the closure of the kazakstan orocline normal faults in the karatau are related to the formation of the leontiev graben south turgay basin and the yarkand fergana basin in the early to mid jurassic during renewed dextral slip along the karatau talas fergana fault late cenozoic deformation is minor and resulted in the uplift and incision of a cretaceous palaeogene peneplain without a major tectonic overprint reversals in the sense of strike slip dominated transpressive deformation across major fault systems results in transpressive inversion this represents an ideal process with which to generate overprinting orthogonal fold systems resulting in classic dome and basin interference patterns on a regional scale

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

geometry of sheath folds and related fabrics at the luikonlahti mine svecokarelides eastern finland the luikonlahti cu co zn sulphide ore deposit is hosted by metasediments associated with serpentinites in the 1 97 ga old outokumpu assemblage in the svecokarelides of eastern finland polyphase deformation of the host rocks a history shared by the ore body includes a phase of sheath fold propagation a modified vergence rule utilizing only the intersection geometry of planar fabric elements permits recognition of these extremely curvilinear folds in poorly exposed terrain the detailed geometry of these rocks is independently resolved from borehole and underground stope records sheath fold propagation occurred during d2 the second phase of regional deformation in the kaavi district d2 major structures are either thrusts or thrust related the luikonlahti sheaths are located in a steeply dipping shear zone formed during this deformation episode

relationships between distributed and localized shear in the tectonic evolution of a caledonian fold and thrust zone northwest ireland broad zones of distributed shear operating through mid crustal regions of orogenic belts may incorporate narrow horizons of intense localized deformation culminating in discrete large magnitude displacements the relative importance and relationship between distributed and localized shear are influenced by a variety of factors including lithological variation pre existing structural anisotropy strain rate and migration of fluids rigorous structural analysis of lower amphibolite facies dalradian metasediments in northwestern ireland demonstrates that an early d 1 discrete ductile detachment was subsequently reactivated during distributed non coaxial d 2 deformation operating in a broad zone through the structural pile regional shear was directed towards the southeast and resulted in the generation and translation of kilometre scale isoclinal recumbent sheath folds which close and face towards the transport direction the d 1 detachment is clearly folded around the hinges of these major folds whilst on fold limbs it was reactivated and acted as a local decollement within the zone of distributed shear shear criteria along the detachment indicate a southeast directed translation of the major folds in sympathy with regional shear a broad zone of d 3 translation operating through the nappe pile resulted in coaxial refolding of large scale f 2 folds by the d 3 ballybofey nappe producing a complex fold interference pattern non coaxial d 3 deformation resulted in continued reactivation of local decollements together with the initiation of east southeast directed oblique thrusts and partial dismemberment of d 2 folds detailed structural investigation allows concepts of distributed and localized shear to be evaluated and models of crustal deformation to be assessed

tectonic and kinematic evolution within mid crustal orogenic root zones a case study from the caledonides of northwestern ireland the slieve league peninsula of northwest ireland lies on the western limb of a major orogenic strike swing in which regional foliation trends have deviated from the northeast southwest trends typical of much of scotland to west east orientations across strike coastal exposures on the western tip of the peninsula through neoproterozoic dalradian metasediments enable a detailed examination and analysis of the structural evolution of a caledonian orogenic root zone which has been previously correlated with the loch awe syncline of southwest scotland minor structural development may be evaluated in terms of regional strain profiles and overprinting relationships over much of the area a composite steep northeast southwest trending s2 s3 foliation containing a gently southwest plunging quartz mineral elongation lineation is the dominant fabric at outcrop and is associated with mp2 almandine amphibolite facies metamorphism f2 folds are isoclinal with curvilinear hinges and similar geometry they typically plunge steeply towards the southwest and display variable dextral or north directed vergence whilst minor f3 fold hinges plunge moderately towards the southwest and typically verge sinistrally towards the south major composite d1 d3 tectonic slides are developed in the argyll group structural and stratigraphic relationships indicate that d1 induced large scale reversals in younging across tectonic slides resulting in reversals in subsequent f2 and f3 facing patterns tectonic sliding is associated with an intensification of strain demonstrated by increasingly intrafolial and curvilinear folding together with extensional crenulations sheared quartz pods and metre scale asymmetric boudinage of metadolerites all of which indicate dextral d2 and sinistral d3 shear after unfolding subsequent folds f4 this corresponds to top to the north d2 and top to the south d3 translations d4 results in regionally northwest verging structures with minor crenulations and the s4 cleavage transecting fold hinges in an anticlockwise sense suggesting a dextral component of deformation the detailed kinematic data indicate that the overall geometry of this western deep level arm of the root zone is not a product of the classic mushrooming fountain of nappes model but rather major interference between consistent northerly directed d2 thrusting and a later phase of southeast directed d3 retrocharriage back folding which intensifies towards the south

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

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

the northern inner piedmont southern appalachians usa kinematics of transpression and sw directed mid crustal flow abstract analysis of new and existing field structural petrologic and geochronologic data in the northern inner piedmont ip southern appalachians provide insight into the processes and kinematics of neoacadian late devonian mississippian crustal flow the ip is characterized by 360 345 ma middle to upper amphibolite facies metamorphism gentle dip of the dominant s2 foliation and curved l2 mineral lineation pattern recognized map scale sheath folds are cored by henderson gneiss or outlined by devonian granitoids and record sw directed axial elongations of 120 486 net neoacadian fault displacement exceeds 400 km with over 200 km associated with sw directed orogen parallel flow map scale sheath folds and mineral lineation patterns suggest that nw and w directed flow rooted to the east was deflected sw by an oblique buttress in the footwall beneath the neoacadian brevard fault zone kinematic vorticity estimates remain in the realm of sub simple shear across the ip and approaches simple shear in the neoacadian brevard fault zone curved ip flow paths provide a model for middle to lower crustal deformation and flow in ancient to active orogens like the himalayas including the possibility that the ip is an exhumed orogenic channel

earth science rain on the parade an analysis of landforms in the bolivian andes suggests that surface uplift has shaped the climate and landscape this contrasts with previous work suggesting that climate controls topography and deformation along the mountain range

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

geometrical similarity in successively developed folds and sheath folds in the basement rocks of the northwestern indian shield an intensely deformed gneiss migmatite terrane and a relatively undeformed granulite granitoid terrane constitute the bulk of precambrian basement in the northwestern indian shield this article traces the structural evolution in the gneiss migmatite terrane where traditional methods of structural analysis are difficult to apply and shows how successively developed folds can assume identical geometry and orientation at an advanced stage of progressive ductile shearing the gneiss migmatite terrane exemplifies a regional scale ductile shear zone that preserves the history of polyphase folding and sheath folding geometrical similarity between individual domain scale sheath folds and mesoscopic regional scale folds implies that sheath folding is common at all scales in the gneiss migmatite terrane as the mylonite foliation that traces successive folds is curviplanar the successively initiated hinge lines were curvilinear from their inception in the shear zone at the advanced stage of ductile shearing the hinge line curvatures were accentuated due to their rotation towards subvertically directed maximum stretching x and variably oriented fold axial planes were brought into approximate parallelism with the upright principal plane xy of the bulk strain ellipsoid eventually all the folds irrespective of their relative order of development became strongly non cylindrical extremely tight isoclinal and approximately co planar with respect to each other it is due to the above geometrical modifications during ductile shearing that folds irrespective of their order of development now appear identical with respect to isoclinal geometry axial plane orientation and hinge line curvilinearity evidence from the fold orientations the deformed lineation patterns and the sheath fold geometry suggest that the shearing occurred in a general shear type of bulk strain and nnw sse directed subhorizontal compression resulted in subvertically directed stretching in the gneiss migmatite terrane

analysis of transpressional deformation from geometrical evolution of mesoscopic structures from phulad shear zone rajasthan india abstract transpressional deformation in ductile shear zones may be analyzed by simultaneous wall parallel simple shear with strain rate xy along with three dimensional coaxial deformation with strain rates x y and z several types of transpression may be distinguished on the basis of relative values of strain rate ratios x a y b and z c it is possible to identify the nature of transpressional deformation from a detailed analysis of shear zone structures in the phulad shear zone of rajasthan india evidence of general flattening a thrusting sense of shear occurrence of transport parallel stretching lineation frequent occurrence of sheath folds with apical direction parallel to the stretching lineation and occurrence of u shaped lineation patterns indicate that the deformation was transpressional with b a ratio ranging between 1 and 2 the bulk deformation was not dominated by simple shear but involved both simple and pure shear with extrusion along the transport direction much greater than along the vorticity vector rotation of long tectonic clasts caused reorientation of monoclinic rolling structures so that strongly monoclinic structures with opposite senses of asymmetry in different domains appear on subhorizontal outcrops parallel to the vorticity vector although the component of simple shear is zero on these surfaces

tectonic stratigraphy and structural continuity of the northernmost moine thrust zone and moine nappe scottish caledonides the moine thrust zone and overlying moine nappe represent classic ground in scottish geology in which neoproterozoic moine metasedimentary rocks were translated towards the wnw over the laurentian foreland comprising lewisian basement gneisses and a cambrian cover succession during caledonian orogenesis at c 430 ma systematic mapping of both the moine nappe and underlying moine thrust zone in the loch hope to whiten head region of sutherland extends the tectonic stratigraphy developed in other parts of the thrust zone to the little studied north coast section an apparent continuum of deformation is recognized from the moine nappe down into the underlying thrust zone with a consistent gently ese dipping foliation associated with down dip mineral and extension lineations the polyphase fold history present within the moine nappe and the thrust zone results from progressive deformation during thrusting thrust related fold axes and bedding cleavage intersections are typically subparallel to the transport direction in high strain zones whereas gently north or south plunging attitudes are common in lower strain portions of the thrust zone thrusts are commonly deformed by folds developed within their footwalls consistent with a foreland propagating sequence of deformation the newly defined lochan riabhach thrust is interpreted as an out of sequence structure that sliced at low angles across the thrust zone and toward the wnw after initial thrust stacking comparison with the downfaulted segment of the moine nappe and thrust zone exposed on the foreland at faraid head demonstrates cross strike continuity of the thrust sheet template and structural history for at least 15 km to the nw

axial directions of folds in rocks with linear planar fabrics abstract this paper examines the geological implications of an analysis of constraints on the orientation of fold axes in orthotropic materials we argue that rocks with penetrative linear and planar shape fabrics may have orthotropic anisotropic properties during deformation two forms of anisotropy rheological and structural could be potentially important in the control of fold axial directions we discuss a model of deformation of rocks with linear planar fabrics where in a single deformation event major fold axes need not be parallel to minor fold axes and neither need be perpendicular to the principal compressive stress direction geological and model examples of anistropic control on fold axial directions are given

eclogite facies polyphase deformation of the drosdal eclogite western gneiss complex norway and implications for exhumation abstract exhumation of the deep crust during orogenic extension is accepted as a geological phenomenon but structures formed during burial and the earliest stages of exhumation are often overprinted by deformation events occurring at shallower crustal levels the western gneiss complex wgc of norway comprises variably retrogressed high pressure hp and ultrahigh pressure uhp eclogite bodies enclosed in predominantly felsic amphibolite facies rocks the drosdal body located in the sunnfjord area is one of the largest and best preserved eclogites in the wgc and comprises over ca 3 km2 of exposed eclogite facies rocks it is an excellent example of an eclogite tectonite and displays a wealth of structures formed during deformation at a minimum depth of 50 km and peak temperatures of ca 800 c large volumes of mylonites with the eclogite facies assemblage garnet clinopyroxene quartz amphibole clinozoisite phengite kyanite rutile are preserved structures associated with the eclogite facies metamorphism include e w trending isoclinal folds boudinage hinge parallel lineations and meter scale kyanite dominated veins the drosdal body was tightly folded on the kilometer scale under eclogite facies conditions subsequently the shapes of the drosdal body and other mafic bodies in the sunnfjord area were modified during eclogite to amphibolite facies e w directed stretching to give boudin like lenses the timing of formation of a pervasive eclogite facies lineation eclogite facies folds and kyanite bearing veins overlapped substantially and this portion of the deformation history was dominated by a constrictional strain field partitioning of deformation occurred after a substantial amount of eclogite facies deformation had already taken place and resulted in the relative rotation of linear features in specific zones on the scale of 400 m the eclogite facies lineations and fold hinges within the drosdal body are subparallel to amphibolite and greenschist facies structures throughout the wgc although circumstantial this suggests that structures belonging to each of these three metamorphic facies formed during one progressive deformation event corresponding to extensional exhumation our observations are consistent with models involving inhomogeneous deformation of the lower crust during early orogenic extension we envisage that transtensional noncoaxial deformation dominated the extensional deformation of the wgc but that coaxial deformation must have occurred locally

structure timing and kinematics of the early eocene south fork slide northwest wyoming usa abstractthe south fork slide sfs is exposed over an area of 800 km2 in northwest wyoming and its evolution is connected in space and time to the adjacent and overlying 48 87 ma heart mountain slide hms the youngest rocks deformed as part of the sfs are the eocene willwood formation sands and mudstones which have a u pb detrital zircon youngest depositional age tuffzirc calculation of 51 80 1 04 1 08 ma as well as a spectrum of older zircons that were eroded from the sevier highlands to the west and the archean beartooth uplift to the north n 379 u pb zircon ages the sfs is overlain by allochthonous paleozoic carbonate and eocene volcanic rocks of the hms the sfs deformation involves shallowly plunging thin skinned folds that trend northeast southwest where the slip surface could have been the top of the jurassic gypsum springs formation detailed mapping reveals the absence of any cleavage or joints and a plethora of minor structures unreported in any thin skinned belt as well as the fac

rotation of long tectonic clasts in transpressional shear zones abstract the two dimensional analysis of rotation of rigid ellipsoidal inclusions is not applicable to situations in which either the bulk deformation deviates from plane strain or one of the principal axes of the inclusion is at an angle to the vorticity vector both these situations may occur in certain types of transpressional ductile shear zones the model presented here shows how long cylindrical or ellipsoidal tectonic clasts with the longest axis parallel to the walls of the shear zone but at an angle to the vorticity vector rotate in transpressional shear zones long clasts initially at a low angle to the vorticity vector and at a large angle to the stretching lineation will rotate and tend to become subparallel to the stretching lineation the rotation of the principal axes of the elliptical cross section around the cylinder axis cannot be unlimited for any inclusion including the inclusion with circular cross section r 1 if the deformation is sufficiently large there is always a stable position of orientation the model explains simultaneous occurrence of monoclinic rolling structures in sections parallel and perpendicular to the vorticity vector depending upon the initial orientation the long axes of different clasts in the foliation plane may rotate clockwise and counterclockwise consequently opposite senses of asymmetry of rolling structures may appear in sections parallel to the vorticity vector

3d geometry and kinematic evolution of the wadi mayh sheath fold oman using detailed mapping from high resolution photography abstract the wadi mayh sheath fold in north eastern oman is one of the largest and best exposed sheath folds known and presents a unique opportunity to better understand this somewhat enigmatic style of deformation we undertook high resolution photographic surveying along wadi mayh to document the sheath fold in 61 georeferenced panoramic photomerges here we present ten such images that provide a structural interpretation of the sheath fold and surrounding structure we resolve this structure in a simplified three dimensional model and in two orthogonal cross sections and propose a kinematic evolution to explain the geometry the wadi mayh sheath fold is the most prominent example within what we suggest is a composite sequence of sheath folds which is itself enclosed within a ssw closing recumbent syncline at the base of the major saih hatat nappe sheath folding is accommodated within permian saiq formation limestones showing carpholite assemblages 6 8 kbar 275 375 c a major discontinuity separates this sequence from enveloping older rock units the sequence formed during progressive top to north ductile shearing as the overlying nappe migrated northwards with respect to the underthrusting hulw unit this process occurred during ssw directed exhumation of partially subducted continental crust in ne oman approximately 15 ma after obduction of the oman ophiolite initiated

evidence for non plane strain flattening along the moine thrust loch srath nan aisinnin north west scotland we report quartz c axis patterns grain shape fabrics and microstructures for 11 mylonitic quartzites and quartz phyllosilicate schists from a transect across the moine thrust at loch srath nan aisinnin north west scotland in the footwall samples collected more than 42 m normal distance from the thrust surface quartz c axis textures indicate a general flattening strain i e 0 k 1 samples within 19 m normal distance of the thrust are completely recrystallized and exhibit asymmetric c axis patterns recrystallized hanging wall fault rocks exhibit random c axis patterns on the scale of a standard thin section relict footwall grains provide the closest approximation of finite strain they have octahedral shear strains s between 1 10 and 1 47 and exhibit general flattening k values 0 0524 0 659 the long axis of the mean relict grain shape trends parallel to the regional transport direction and plunges gently to the ese in contrast recrystallized footwall grains have a mean grain shape with the longest axis oriented nearly perpendicular to the transport direction furthermore these samples have grain shape k value ranges from 0 157 to 0 295 recrystallized hanging wall grain shapes exhibit the lowest octahedral shear strains s 0 532 0 733 and largest mean k values 0 351 0 961 of this sample set the long axes of the mean recrystallized hanging wall grain shapes are parallel to transport similar to that of relict footwall grains unrecrystallized quartz overgrowths about opaque mineral grains suggest concurrent elongation in all directions within the mylonitic foliation and support the inference of general flattening deformation the mylonitic foliation and penetrative lineation are consistent with a wnw shearing direction however both were folded during later deformation increments recrystallized grains in footwall quartzites suggest a 305 320 azimuth for the shearing direction the best fit axis of the poles to the foliation is 18 145 close to the mean trend plunge of mesoscopic fold hinges at this site 18 151 both suggest a nnw azimuth for the shearing direction thus structural microstructural and fabric indicators suggest a change in the azimuth of the shearing direction from 286 to 331

intense non coaxial shear and the development of mega scale sheath folds in the arunta block central australia abstract the strangways orogenic belt of 1800 ma granulites in the central arunta block was completely reworked at granulite facies grades during the middle proterozoic 1400 1500 ma the bulk strain regime of the initial episodes of this reworking is presented ductile reworking involved non coaxial deformation on a regional scale and produced map scale sheath folds high bulk shear strains are inferred on microscopic and macroscopic scales and the spatial variation in bulk shear strain throughout the region is discussed the high bulk shear strains were accommodated by east over west ductile ramping and fold repetition resulting in easterly inclined structures crustal shortening and crustal over thickening

ductile nappe extrusion in constrictive strain at the origin of transverse segments of the allochthon boundary thrust in the manicouagan imbricate zone central grenville province quebec abstract separating the parautochthonous and allochthonous belts the allochthon boundary thrust abt is a major lithotectonic boundary that generally runs parallel with the length of the grenville province but with some transverse segments in this contribution we investigate one of these segments the manicouagan imbricate zone miz by adding new field based data supplemented by new u pb deformation ages to a compilation of previous research conducted on both sides of the abt in the region our results reveal striking differences between the footwall and hanging wall of the abt in terms of strain and structures and concludes that the transverse orientation of the abt is not related to folding an updated strain analysis allows us to determine that deformation of the miz was dominated by a constrictive type of strain we build on a previously proposed conceptual model to document ductile extrusion over basement ramps driven by tectonic forcing and lateral density contrast in the upper crust at ca 990 ma the insertion of the hot ductile nappe into partially molten metasediments of the surrounding parautochthonous belt was accommodated by constrictive strain within the former and flattening and transverse folding in the latter

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

thrust sequences and the significance of low angle out of sequence faults in the northernmost moine nappe and moine thrust zone nw scotland in common with most thrust belts the moine thrust zone scotland preserves a foreland propagating sequence of thrusting however significant examples of local to regional overstep sequences are also recognized and their interpretation remains controversial remapping of the northernmost moine thrust zone has defined a thick belt of mylonites in the centre of which lies the ductile wnw directed moine thrust the base of the mylonite belt however is marked by a regionally significant low angle west directed brittle fault here termed the lochan riabhach thrust that truncates thrust related structures in its footwall comparison of the tectonic stratigraphy of the mylonite belt with that preserved in a downfaulted outlier to the west at durness indicates that the lochan riabhach thrust cuts down section in its footwall and therefore appears to be extensional locally it is unequivocally breached by underlying imbricate thrusts we prefer to interpret the lochan riabhach thrust as an extensional fault that formed during a hiatus in thrust stacking we relate this faulting episode to the changes in the stability of the evolving orogenic wedge and reiterate that such extensional faults may be common although possibly unrecognized features in many other thrust belts