significance of fracture sets associated with stylolite zones geologic notes stylolite zones consist of an irregular seam and often a variety of associated fracture sets the geometry of this zone can be used to determine paleostress directions of the fracture sets commonly observed in close proximity to stylolites one is generally induced by the coring procedure unloading fractures and another is natural and possibly associated with stylolites of tectonic origin so called tension gashes within a unit possessing abundant stylolites the stylolites are most common in zones of relatively low porosity in addition because of the relatively compact poorly sorted fine grained nature of the seam itself stylolite zones commonly have much reduced permeability compared to the undeformed rock above and below laboratory measured permeability associated with stylolite zones can however be misleadingly large owing to the presence of unloading fractures and or tension gashes these fractures may dramatically increase laboratory permeability but commonly are either not present in the reservoir or are relevant only to the size of the laboratory measurement sample it is critical therefore to understand the origin and scale of associated fracture development in interpreting su h data

recycling of foliations during folding bedding on the limbs of early formed regional folds generally lies oblique to the major directions of bulk shortening in the crust that is the horizontal and vertical during subsequent deformations matrix foliations on at least one limb of these folds may start to form but then be destroyed by reactivation of the bedding causing decrenulation and rotation into parallelism with the compositional layering consequently the schistosity parallel to bedding s0 s1 in multiply deformed rocks contains the relics of many deformation events and the 2 or 3 oblique foliations record only the very youngest history this lengthy early history is preserved as inclusion trails within porphyroblasts recycling of foliations depends on the shear sense acting on any newly developing foliation and the orientation of this foliation relative to s0 s1 for some orientations and combinations of shear senses both limbs of a pre existing fold can be reactivated from the commencement of a new deformation event this can result in the decrenulation and obliteration of a new foliation that is beginning to form before it shows any significant degree of development for other combinations one limb of a pre existing fold will behave in this manner whereas a new foliation does develop fully on the other limb however subsequent phases of deformation switch which limb shears versus develops an oblique new cleavage rotating the earlier formed oblique foliation into parallelism with s0 s1

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

significance of composite lineations in the mid to deep crust a case study from the north cascades washington abstract the origin of broadly orogen parallel nw se mineral lineations in cretaceous and paleogene arc plutons and amphibolite facies metamorphic rocks paleodepths of

planar non planar and refolded sheath folds in the phulad shear zone rajasthan india abstract progressive ductile shearing in the phulad shear zone of rajasthan india has produced a complex history of folding with development of planar non planar and refolded sheath folds there are three generations of reclined folds f 1 f 2 and f 3 with a striping lineation l 1 parallel to the hinge lines of f 1 the planar sheath folds of f 1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices l 1 swerves harmoniously with the curving of f 1 hinge line there is a strong down dip mineral lineation parallel to the striping lineation in most places but intersecting it at apices of first generation sheath folds both the striping and the mineral lineation are deformed in u patterns over the hinges of reclined f 2 and f 3 folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non planar sheaths the reclined folds of all the three generations were deformed by a group of subhorizontal folds each generation of fold initially grew with the hinge line at a very low angle with the y axis of bulk non coaxial strain and was subsequently rotated towards the down dip direction of maximum stretching the patterns of deformed lineations indicate that the stretching along the x direction was extremely large much in excess of 6000 percent

are open fractures necessarily aligned with maximum horizontal stress abstract fluid flow in fractured rock is an increasingly central issue in recovering water and hydrocarbon supplies and geothermal energy in predicting flow of pollutants underground in engineering structures and in understanding large scale crustal behaviour conventional wisdom assumes that fluids prefer to flow along fractures oriented parallel or nearly parallel to modern day maximum horizontal compressive stress or s hmax the reasoning is that these fractures have the lowest normal stresses across them and therefore provide the least resistance to flow for example this view governs how geophysicists design and interpret seismic experiments to probe fracture fluid pathways in the deep subsurface contrary to these widely held views here we use core stress measurement and fluid flow data to show that s hmax does not necessarily coincide with the direction of open natural fractures in the subsurface 3 km depth consequently in situ stress direction cannot be considered to predict or control the direction of maximum permeability in rock where effective stress is compressive and fractures are expected to be closed chemical alteration dictates location of open conduits either preserving or destroying fracture flow pathways no matter their orientation

damaged beyond repair characterising the damage zone of a fault late in its interseismic cycle the alpine fault new zealand dfdp 1 was funded by gns science victoria university of wellington the university of otago the university of auckland the university of canterbury deutsche forschungsgemeinschaft and the university of bremen natural environment research council grants ne j024449 1 ne g524160 1 and ne h012486 1 and the university of liverpool and the marsden fund of the royal society of new zealand

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

the effect of stylolite spacing on quartz cementation in the lower jurassic sto formation southern barents sea abstract the shallow marine lower jurassic quartz arenites of the sto formation in the southern barents sea comprise 1 intervals where dispersed detrital clay is absent and where the spacing between clay rich laminae that evolved into stylolites upon burial is exceptionally large up to several meters and 2 intervals where minor detrital clay matrix occurs clay laminae are very common and stylolite spacing is typically less than a centimeter point counting of thin sections and cathodoluminescence micrographs shows that quartz cement contents are far lower in the intervals where stylolite spacing is exceptionally large 4 11 versus 10 20 outside these intervals there is also a correlation between distance to nearest stylolite and volume of quartz cement samples located a centimeter or less from a stylolite contain 10 20 quartz overgrowths for distances of 3 20 cm quartz cement content is 4 10 and only 3 8 when the closest stylolite is more than 20 cm distant modeling of quartz cementation with the exemplartm diagenetic modeling program indicates that the observed trend of decreasing quartz cement abundance outwards from stylolites is not caused by variations in grain size degree of grain coating or content of quartz grains i e the trend is not due to more quartz surface area being available for overgrowth formation close to stylolites on the contrary the modeling suggests that the samples situated more than 20 cm from stylolites contain 5 8 less quartz cement than what would have been the case given a more normal stylolite abundance this study indicates that sandstones with exceptionally few clay rich or micaceous laminae and without clay or mica at individual grain contacts will be significantly less quartz cemented and more porous than other sandstones with similar temperature histories however such sandstones seem to be highly unusual on the norwegian continental shelf this suggests that exceptionally low abundance of stylolite precursors may be of only local importance for preserving reservoir quality at elevated temperatures and that it is normally not necessary to include stylolite spacing and distance to the nearest stylolite as variables in quantitative models of quartz cementation

the problem of shearing along axial plane foliations abstract the structural significance of axial plane foliations cannot be understood unless we make a distinction between rotation of the material plane foliation and rotation of the geometrically defined x y plane of the strain ellipsoid if the foliation rotates as a material plane at any stage of deformation then its final orientation will be different from that of the x y plane it is suggested that reorientation of foliation takes place by some combination of the formation of foliation e g recrystallization along the x y plane and passive rotation of the material plane foliation in the same continuous deformation the deviation between the foliation and the x y plane is then much less than 5 degrees however because of this deviation a considerable amount of shear strain may develop along the foliation the analysis thus explains how a foliation can be approximately parallel to the x y plane and yet be a plane of shearing

patterns of deformed early lineations over later folds formed by buckling and flattening abstract the patterns of deformed early lineations l1 over later folds f2 can be classified into several morphological types depending on the nature of variation of l1 f2 over the folds the field relations indicate that the folds under consideration are neither shear folds nor parallel folds modified by flattening the lineation patterns are therefore interpreted in terms of an empirical model of simultaneous buckling and flattening in which it is assumed that i the central surface of the folded layer remains a sine curve in transverse profile ii the ratio of rates of buckle shortening to homogeneous strain is proportional to sin 2a with a as the dip angle and iii the progressive deformation is coaxial with the z axis of bulk strain parallel to the planar segments of the early folds the model gives an insight into the relative importance of different physical factors which control the development of dissimilar lineation patterns not all lineation patterns are explicable by this simplified model thus complex patterns with variable l1 f2 along the fold axis may develop by a progressive rotation of the geometrically defined fold hinge through successive material lines the theoretical results have been applied to interpret the lineation patterns in central rajasthan india it is concluded that l1 was initially very close to the e ese trending subhorizontal z axis of bulk deformation during f2 folding and that the x axis was subhorizontal or gently plunging with a n nne trend

an ibexian early ordovician trilobite faunule from the type section of the rabbitkettle formation southern mackenzie mountains northwest territories the rabbitkettle formation is a widespread lower paleozoic unit in the northern cordillera a trilobite faunule collected from the middle part of the type section and described here documents an early ibexian early ordovician age for this interval and provides the first biostratigraphic reference point for the type section this faunule is dominated by the agnostoid micragnostus the leiostegiid leiostegium and the remopleuridid kainella these genera and rarer taxa have north american south american and austral asian affinities a new species kainella eleutherolfi is established

computed tomography scan imaging of natural open fractures in a porous rock geometry and fluid flow computed tomography scan imaging techniques have been used on core samples to investigate the effect of natural open fractures on reservoir flow in the snohvit gas condensate field firstly computed tomography ct scanning was used to describe the 3d geometrical properties of the fracture network including orientation and fracture density two types of fractures were observed f1 fractures are short and stylolite related and f2 fractures are longer cross cutting the core and without any obvious relationship to stylolites secondly monitoring of single and two phase flow experiments on samples containing these two types of natural open fractures was performed under 10 and 80 bar net confining pressure while using ct scanning 1 phase miscible flooding experiment shows approximately 3 times higher flooding velocity in an open f2 fracture than in the matrix 2 phase flooding by gas injection into a 100 water saturated core gave early gas breakthrough due to flow in the fracture and thereafter very little water production the flow experiments showed that the presence of open fractures has a significant local effect on fluid flow even in a case with relatively high matrix porosity 200 300 md the sample containing f1 fractures showed a complex flow pattern influenced both by open fractures and stylolites the ct scan data enables an exact representation of the fracture network in core scale simulation models and therefore improves the understanding of fracture influence on flow in a fractured porous medium ct scanning of core samples provides an effective tool for integrating geology and fluid flow properties of a porous fractured medium

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

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

visualizing tephra deposits and sedimentary processes in the marine environment the potential of x ray microtomography localized tephra deposition in marine sequences is the product of many complex primary and secondary depositional processes these can significantly influence the potential applicability of tephra deposits as isochronous marker horizons and current techniques used in isolation may be insufficient to fully unravel these processes here we demonstrate the innovative application of x ray microtomography ct to successfully identify tephra deposits preserved within marine sediments and use these parameters to reconstruct their internal three dimensional structure three dimensional visualizations and animations of tephra dispersal in the sediment permit a more thorough assessment of postdepositional processes revealing a number of complex microsedimentological features that are not revealed by conventional methods these features include bioturbation burrows and horizontally discontinuous tephra packages which have important ramifications for the stratigraphic placement of the isochron in a sedimentary sequence our results demonstrate the potential for utilizing rigorous two and three dimensional microsedimentological analysis of the ichnofabric to enhance and support the use of tephra deposits as isochronous marker horizons and to identify the stratigraphic position that best reflects the primary fallout of ash the application also provides an exceptional insight into the style and rate of sedimentation processes and permits an assessment of the stratigraphic integrity of a tephra deposit we discuss the possibility of applying these ct methods to the identification of cryptotephras within various paleoclimatic sequences and to enhance our understanding of marine sedimentation processes

multiply deformed terrains problems of correlation abstract the best tools available for carrying out a structural analysis in a complex area are overprinting style and patterns of orientation unfortunately all have limitations which are reviewed briefly and discussed in areas of continuous unfaulted outcrop the deformational history can be established purely on the basis of overprinting but even in such areas there can be problems since not all overprinting relationships are unambiguous most fold interference patterns are reliable overprinting relationships based on foliations however are commonly unreliable and various examples are cited and discussed even in areas where the sequence of deformational events can be established their temporal significance is unclear since the methods applied only determine local relative timing and absolute relationships must vary in space and time across an orogen

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

3d quantification of mineral components and porosity distribution in westphalian c sandstone by microfocus x ray computed tomography abstract microfocus x ray computed tomography ct measurements were performed on deltaic sandstone samples from the diagenetically altered westphalian c strata campine basin north east belgium in order to collect three dimensional data on mineral quartz kaolinite ankerite etc and porosity distribution the acquired ct data were compared with point counting results of stained impregnated thin sections to qualify the sensitivity of ct comparison between techniques shows positive results with regard to the quantification of main mineral phases such as quartz and ankerite and porosity distribution however major discrepancies exist when studying the clay minerals this can be explained by the influence of microporosity associated with these clay minerals resulting in partial volume effects

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

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

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