predicting the elastic response of organic rich shale using nanoscale measurements and homogenisation methods determination of the mechanical response of shales through experimental procedures is a practical challenge due to their heterogeneity and the practical difficulties of retrieving good quality core samples here we investigate the possibility of using multiscale homogenisation techniques to predict the macroscopic mechanical response of shales based on quantitative mineralogical descriptions we use the novel peakforce quantitative nanomechanical mapping technique to generate high resolution mechanical images of shales allowing the response of porous clay organic matter and mineral inclusions to be measured at the nanoscale these observations support some of the assumptions previously made in the use of homogenisation methods to estimate the elastic properties of shale and also earlier estimates of the mechanical properties of organic matter we evaluate the applicability of homogenisation techniques against measured elastic responses of organic rich shales partly from published data and also from new indentation tests carried out in this work comparison of experimental values of the elastic constants of shale samples with those predicted by homogenisation methods showed that almost all predictions were within the standard deviation of experimental data this suggests that the homogenisation approach is a useful way of estimating the elastic and mechanical properties of shales in situations where conventional rock mechanics test data cannot be measured

magnetic susceptibility anisotropy strain and progressive deformation in permian sediments from the maritime alps france abstract the relationships among magnetic susceptibility anisotropy finite strain and progressive deformation have been studied in permian red shales and slates of the maritime alps southeastern france these rocks contain deformed reduction spots which serve as finite strain indicators the magnetic fabric of undeformed regions is modified during deformation to yield characteristic magnetic susceptibility anisotropy patterns and a magnetic equivalent of the deformation path derived from strain measurements the magnetic fabric changes progressively from oblate to prolate and back to oblate as deformation increases the quantitative relationships between natural strain and magnetic anisotropy in these rocks have been determined they differ between the less and more deformed areas perhaps due to a change in deformation mechanism accompanying an increase in metamorphism the relationships provide a rapid means of strain determination using magnetic measurements but their variation emphasizes the need for local structural control

modelling fluid induced seismicity on a nearby active fault summary we present a numerical investigation of the effect that static stress perturbations due to fluid injection have on a nearby active fault where the fluid does not come in physical contact with the fault our modelling employs a lattice boltzmann pore diffusion model coupled with a quasi dynamic earthquake rupture model as diffusivities and frictional parameters can be defined independently at individual nodes cells this allows us to replicate complex 3 d geological media in our simulations we demonstrate the effect an injection can have on an active nearby fault compared with our control catalogue identical to the original simulation but without the injection the injection not only altered the timing of the next earthquake sequence it also changed its size producing a mw 6 7 event the largest observed earthquake on the fault this large event pushes the fault into a subcritical state from which it took roughly 200yr of continuous tectonic loading for the fault to return to a critical state

structure of lower carboniferous basins of nw ireland and its implications for structural inheritance and cenozoic faulting abstract the geometry of lower carboniferous basins across the northern part of ireland is characterised by a gradual change in polarity of structure from a predominantly southward dipping fault system in northern ireland through the lough allen basin and into a mainly northward dipping fault system of connemara in the southwest this polarity change is reflected in the emergence of northward facing uplifted footwall blocks such as at croagh patrick and the ox mountains in the southwest with southward facing blocks in the northeast despite these structural changes there is a great degree of along strike continuity of lower carboniferous structure strain and displacement with the system forming a link between comparable sized and coeval basins in northern england and scotland to the east and the canadian maritime provinces to the west the spatial distribution of carboniferous normal faults suggests that caledonian structure such as the fair head clew bay line plays an important role in the localisation of post caledonian strain although the reactivation of individual caledonian structures may be less common the terrain of the west of ireland is strongly influenced by major carboniferous faults offsetting the base carboniferous unconformity as opposed to previously suggested cenozoic normal faulting of a tertiary peneplain

response of remanent magnetization to deformation in geological processes using 3d printed structures abstract palaeogeographic reconstructions and construction of apparent polar wander paths are dependent on having reliable palaeomagnetic directions the importance of inclination flattening in biasing the palaeomagnetic record has been debated for over 60 yrs correction for this effect often assumes that the palaeomagnetic vector deforms as a passive line in a novel approach using 3d printed analogue rocks we revisit the question of how a palaeomagnetic vector responds to deformation specifically compaction maghaemite nanoparticles were mixed in the printing resin with a concentration of 0 15 wt and five series of cylinders with 1 cm height and diameter were printed with porosities between 0 and 20 samples were given an anhysteretic remanent magnetization and were subjected subsequently to incremental compaction the magnetic fabric shows an initially weak compaction in the printing plane that becomes larger with increased compaction the palaeomagnetic inclination changes according to the strain that the sample undergoes and the amount of deflection is less than predicted by a passive line model of deformation our results demonstrate that using a single correction factor for inclination flattening is questionable and show the need for a method that considers how the rock deforms further we demonstrate the usefulness of 3d printed analogue rock which can inspire more realistic methods to correct for inclination flattening

application of paleomagnetism to accretionary tectonics and structural geology the use of paleomagnetism to determine translations and rotations of geologic terranes has contributed to a better understanding of deformation within accretionary complexes shear zones and rifts in this report to the i u g g we review the progress resulting from paleomagnetic studies during the last four years in the cordillera of north america the appalachian orogen and the circum pacific region

paleomagnetism and magnetic fabric of the deformed redbeds of the cap de la chevre formation brittany france abstract we present the results of a palaeomagnetism and magnetic fabric study in six sampling sites of the lower ordovician redbeds of the cap de la chevre formation crozon peninsula france measurement of anisotropy of magnetic susceptibility ams shows that these beds are affected by a weak but significant tectonic strain during the hercynian folding of the armorican palaeozoic cover samples containing small elliptical reduction spots allowed us to check the control of strain on ams site mean ams data were then used to provide strain estimates at each site analysis of remanent magnetization through thermal and chemical stepwise demagnetization procedures allowed us to isolate three magnetic components component a defined in the lower unblocking temperature range is a recent overprinting which clusters around the present day dipole field direction components b and c defined respectively in the intermediate and higher unblocking temperature ranges appear to be pretectonic in age the fold test however is inconclusive for both because of the significant dispersion still remaining after the simple tilt correction assuming that at least a part of this dispersion is due to strain we have applied the strain removal technique using ams derived strains at each site this results in a clustering of component c from four sites allowing recovery of the primary ordovician paleomagnetic direction d 261 i 71 95 11 in contrast unstrained component b shows a small circle distribution containing the formation mean of unstrained component c the hypothesis that component b and c distributions arise from beds being tilted during a long term magnetization acquisition in the ordovician field is discussed taking into account the extensional geodynamic context during deposition of lower palaeozoic redbeds in brittany

anisotropy of experimentally compressed kaolinite illite quartz mixtures the anisotropy of physical properties is a well known characteristic of many clay bearing rocks this anisotropy has important implications for elastic properties of rocks and must be considered in seismic modeling preferred orientation of clay minerals is an important factor causing anisotropy in clay bearing rocks such as shales and mudstones that are the main cap rocksofoilreservoirs thepreferredorientationofclaysdepends mostly on the amount of clays and the degree of compaction to studytheeffectoftheseparameters wepreparedseveralsamples compressingattwoeffectiveverticalstressesamixtureofclays illite and kaolinite and quartz silt with different clay quartz ratios the preferred orientation of the phases was quantified with rietveld analysis on synchrotron hard x ray images pole figures for kaolinite and illite display a preferred orientation of clay platelets perpendicular to the compaction direction increasing in strength with clay content and compaction pressure quartz particles have a random orientation distribution aggregate elastic properties can be estimated by averaging the singlecrystal properties over the orientation distribution obtained from the diffraction data analysis calculated p wave velocity anisotropy ranges from 0 pure quartz sample to 44 pure clay sample highly compacted but calculated velocities are much higher than measured velocities this is attributed to uncertainties about single crystal elastic properties and oriented micropores and limited grain contacts that are not accounted for in themodel inthiswork wepresentaneffectivemethodtoobtain quantitative data helping to evaluate the role of clay percentage and compaction pressure on the anisotropy of elastic properties ofclay bearingrocks

multidisciplinary approach to constrain kinematics of fault zones at shallow depths a case study from the cameros demanda thrust north spain thrusting at shallow depths often precludes analysis by means of structural indicators effective in other geological contexts e g mylonites sheath folds shear bands in this paper a combination of techniques including structural analysis magnetic methods as anisotropy of magnetic susceptibility and paleomagnetism and paleothermometry is used to define thrusting conditions deformation and transport directions in the cameros demanda thrust north spain three outcrops were analyzed along this intraplate large scale major structure having 150 km of outcropping length 30 km of maximum horizontal displacement and 5 km of vertical throw results obtained by means of the different techniques are compared with data derived from cross sections and stratigraphic analysis mixed layer illite smectite and vitrinite reflectance indicating deep diagenetic conditions and mature stage of hydrocarbon generation suggests shallow depths during deformation thus confirming that the protolith for most of the fault rocks is the footwall of the main thrust kinematic indicators foliation s c structures and slickenside striations indicate altogether a dominant nnw movement of the hanging wall in the western zone and ne in the eastern zone of the thrust thus implying strain partitioning between different branches of the main thrust the study of ams in fault rocks nearly 400 samples of fault gouge breccia and microbreccia indicates that the strike of magnetic foliation is oblique to the transport direction and that the magnetic lineation parallelizes the projection of the transport direction onto the k max k int plane in sites with strong shear deformation paleomagnetism applied to fault rocks indicates the existence of remagnetizations linked to thrusting in spite of the shallow depth for deformation and a strong deformation or scattering of the magnetic remanence vectors in the fault zone the application of the described techniques and consistency of results indicate that the proposed multidisciplinary approach is useful when dealing with thrusts at shallow crustal levels

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

magnetic fabrics and paleomagnetism of the storsjon edsbyn deformation zone central sweden abstract a regional deformation zone separating tectonomagmatic blocks in the central part of the fennoscandian shield has been studied using anisotropy of magnetic susceptibility and paleomagnetism in order to define the tectonic character and age of the zone and to put the deformation into a plate tectonic context the deformation zone the storsjon edsbyn deformation zone sedz separates the ratan granite of the trans scandinavian igneous belt tib and the southern svecofennian subprovince in the southwest from older svecofennian granitoids of the central svecofennian subprovince in the northeast the sedz is the most prominent deformation zone in a regional pattern of nw se trending shear zones on basis of ams data the width of the zone which cuts through 1 7 1 9 ga old rocks is estimated at 10 12 km in its central southern part and 20 25 km in its northern part the orientations of the susceptibility axes vary systematically across the zone in a way that suggests that the sedz is a pure shear dominated transpression zone caused by a compressive stress from the present ssw paleomagnetic data from a dolerite sill plat 10 plon 166 and from a dyke plat 25 plon 190 that cut the sedz indicate that no plastic deformation has occurred along the zone during the past 1 25 ga and probably not for the past 1 5 1 6 ga ams data do not point to any deformation of the 1 70 ga old ratan granite which further limits the time span of deformation the main phase of deformation most likely took place between 1 80 and 1 70 ga ago the geological evolution of this part of the fennoscandian shield at the time between 1 80 and 1 70 ga ago is very complex and in part still enigmatic this time period involves magmatism of the tib which is related to eastward subduction it also comprises the beginning of the gothian orogeny and the development of a regional system of transpressive deformation zones the compressive stress field from the present ssw indicated in this study is interpreted to be related to an initial stage of plate collision between the southwestern scandinavian subprovince and older parts of the fennoscandian shield this collision resulted in a system of regional nw se trending intra plate deformation zones one of which is the sedz

comparison of strain and magnetic fabrics in dalradian rocks from the southwest highlands of scotland abstract the magnetic fabrics of 235 samples from 31 localities in argyllshire scotland were determined to study the development of the caledonian tectonic fabric in the southwest highlands of scotland the regional fabric indicates a strong ne sw compressional foliation due to the primary deformational phases which in parts has been overprinted by secondary deformations a detailed comparison of the anisotropy data and the available strain data shows that the two fabric ellipsoids are co axial and that their axial mean ratios seem to be related by an empirical power relationship of the type xi xj li lj a for i 1 2 3 j 1 2 3 and i j where i and j are orthogonal principal axes and li and lj are the corresponding orthogonal principal strain axes the exponent a for the sites from scotland is 0 088 0 017 compared with 0 142 0 001 and 0 145 0 005 found in the caledonian slates of the english lake district and the welsh slate belt

deformation of remanent magnetism in a synthetic aggregate with hematite a hematite calcite aggregate bonded with portland cement is deformed triaxially at 200 mpa confining pressure and at a constant strain rate of 10 5 s 1 in some cases with a pore fluid pressure of 120 mpa this aggregate deforms in a ductile fashion at room temperature and is a good textural analog of naturally penetratively strained hematite bearing rocks the changes in inclination of an isothermal remanent magnetisation are indistinguishable from the behaviour expected of a homogeneously strained non material line rotating according to wettstein s equation this is encouraging for paleomagnetists who might wish to restore remanence in tectonically strained hematite bearing rocks an unknown combination of rigid grain rotation and intracrystalline stress must be responsible for rotation of the grains that carry remanence and thus for the changes in inclination of remanence the reason that wettstein s equation is approximately valid may be because the large aspect ratios of rigid hematite grains cause them to rotate nearly as quickly as a passive line p hydrostatic compaction wet deformation and dry deformation in that order are progressively more effective in reducing the intensity of remanence however wet deformation is more effective than dry deformation in reducing the intensity of the most coercive components presumably by spinning the finest grains in the cement matrix and scattering their individual moments p where multiple direction components of remanence are present deformation reduces the components with higher coercivities 70 mt more than those with lower coercivities in previous experiments on magnetite bearing samples the lower coercivity components were most reduced thus the natural tectonic deformation of an nrm may be expected to preserve selectively low coercivity components in hematite and high coercivity components in magnetite this may be used to predict the success of preservation of a characteristic nrm and conversely the success of overprinting by syntectonic remagnetisation

tectonic strain and paleomagnetism experimental investigation a porous clay rich sandstone with a natural remanent magnetisation shows complex changes in the orientation of the remanence vector when the sandstone is subjected to experimental deformation at 100 mpa room temperature and constant natural strain rates of 1 10 5 s 1 the paleomagnetic vector does not simply rotate away from the axis of compression when the rock is shortened perpendicular to bedding the remanence vector rotates toward the bedding as expected but the remanence also rotates toward bedding when the rock is shortened parallel to bedding moreover in some cases the remanence vector changes azimuth within the specimen during experimental deformation thus bedding anisotropy and the accompanying mineral fabric anisotropy are more influential than strain in controlling deflections of the paleomagnetic vector in this study this anisotropy also influences the post deformational behaviour of remanence 4 months after deformation some cores shortened perpendicular to bedding relaxed their remanence vectors back towards the pre deformational attitude whereas some other cores continued to show progressive changes intragranular recoverable elastic strains in the magnetic grains may be partly responsible for the experimentally induced deflections of remanence

ultrasonic velocity and mechanical anisotropy of synthetic shale with different types of clay minerals clay minerals are the most abundant materials in shale their presence significantly influences the elastic behavior of reservoir rocks as a function of mineral type volume and distribution and their orientation controls the shale s intrinsic anisotropic behaviors thus knowing the elastic properties of shale with different types of clay minerals is imperative for fully understanding the seismic properties of the reservoir however it is extremely difficult to measure the elastic properties of natural shale by means of a single variable in this case the type of clay due to the influences of multiple factors including water toc content complex minerals composition and so on thus we use quartz clay kaolinite illite and smectite carbonate and kerogen extract as the primary materials to construct synthetic shale with different types of clay ultrasonic experiments were conducted to study the anisotropy of velocity and mechanical properties young s modulus and poisson s ratio in dry synth

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

experimental deformation of synthetic magnetite bearing calcite sandstones effects on remanence bulk magnetic properties and magnetic anisotropy we have quantified effects of experimental deformation on the magnetic properties of a set of synthetic calcite sandstone samples containing magnetite the deformation was carried out in a microcomputer controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10 5 s 1 most samples were deformed under dry conditions but a few were deformed with a pore fluid present the samples deformed under dry conditions required substantially higher differential stresses macroscopically ductile shortening strains of up to 25 produced the following irreversible changes in magnetic properties 1 increased bulk coercivity remanence coercivity and mean anhysteretic remanence susceptibility 2 decreased mean low field susceptibility 3 decreases in the component of remanence parallel to shortening 4 smaller decreases for most samples in the component normal to shortening resulting in a net rotation of the remanence away from the shortening axis 5 larger decreases in the normal component in a few samples resulting in a net rotation of the remanence towards the shortening axis 6 increased magnetic anisotropy and 7 increased deformation of initial magnetic ellipsoids a comparison of data for samples deformed under dry and wet conditions higher and lower differential stresses respectively indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain i e rotation and displacement of particles whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress

apparent polar wander paths and the close of late archean crustal transpression northern ontario 1 lamprophyre dikes of the southern superior province of the canadian shield crosscut archean structures including a late archean unconformity and its overlying younger archean timiskaming sedimentary and volcanic rocks the dikes also cut the single schistosity that formed in these rocks nevertheless the lamprophyre dikes show a weak internal tectonic fabric that is approximately parallel to the schistosity in the bedrock and oblique to dike walls the consistent orientation of this internal tectonic fabric has been recognized in every lamprophyre dike using anisotropy of magnetic susceptibility ams thus the lamprophyre dikes provide a useful chronological marker trapping the last pulse of archean tectonism unfortunately our previous attempts to compare the lamprophyre paleopoles with those of the age calibrated apparent polar wander path apwp failed because it was not possible to isolate separate young and old components of magnetization that failure occurred due to the overlap of blocking temperatures or coercivities for the different characteristic vector components borradaile et al 1994a using new cores from the same specimens here we distinguish successfully between consistently oriented vector components by using low temperature demagnetization ltd before thermal demagnetization although the paleopoles appear well defined comparisons with published apwps for the superior province from late archean irving 1979 dunlop 1983 symons et al 1994 to early proterozoic buchan et al 2000 are difficult moreover the earlier parts of the paths are progressively less precise and complete nevertheless a component remanences for the dikes give a paleopole near the path at 2650 ma for a timiskaming volcanic breccia with a u pb age of 2696 ma the a component paleopole may lie between 2600 and 2650 ma b component remanences yield paleopoles in the nw pacific region incompatible with any proposed archean path but perhaps representing some much younger proterozoic remagnetization event e g keweenawan 1100 ma

simulating hydraulic fracturing failure in soft versus hard rocks in this contribution we discuss the dynamic development of hydraulic fractures their evolution and the resulting seismicity during fluid injection in a coupled numerical model the model describes coupling between a solid that can fracture dynamically and a compressible fluid that can push back at the rock and open fractures with a series of numerical simulations we show how the fracture pattern and seismicity change depending on changes in depth injection rate young s modulus and breaking strength our simulations indicate that the young s modulus has the largest influence on the fracture dynamics and the related seismicity simulations of rocks with a young s modulus smaller than 10 gpa show dominant mode i failure and a growth of fracture aperture with a decrease in young s modulus simulations of rocks with a young s modulus higher than 10 gpa show fractures with a constant aperture and fracture growth that is mainly governed by a growth in crack length and an increasing amount of mode ii failure these results are very important for the prediction of fracture dynamics and seismicity during fluid injection especially since we see a transition from one failure regime to another at around 10 gpa a young s modulus that lies in the middle of possible values for natural shale rocks

the elastic anisotrophy of shales shales constitute about 75 of the clastic fill sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure the elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients w sub lmn in an expansion of the clay particle orientation distribution function in generalized legendre functions application is made to the determination of the anellipticity of shales for transverse isotrophy the anellipticity quantifies the deviation of the p wave slowness curve from an ellipse and is shown to depend on a single coefficient w sub 400 in the expansion of the clay particle orientation distribution function if w sub 400 is small the anellipticity may be neglected as is apparently the case for a near surface late tertiary shale studied by winterstein and paulson strongly aligned clay particles result in a positive value of w sub 400 and a positive anellipticity in agreement with the majority of the field measurements however less well ordered shales could have a significantly positive second moment w sub 200 but only a small positive or even negative value of w sub 400 for such shales the anellipticity would be small or negative despite amore preferred alignment of clay particles in the bedding plane numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given less

differences in paleomagnetic interpretations due to the choice of statistical demagnetization and correction techniques kapuskasing structural zone northern ontario canada abstract the kapuskasing structural zone ksz reveals a section through the archean lower crustal granoblastic gneisses our new paleomagnetic data largely agree with previous work but we show that interpretations vary according to the choices of statistical demagnetization and field correction techniques first where the orientation distribution of characteristic remanence directions on the sphere is not symmetrically circular the commonly used statistical model is invalid fisher r a proc r soc a217 1953 295 any tendency to form an elliptical distribution indicates that the sample is drawn from a bingham type population bingham c 1964 distributions on the sphere and on the projective plane phd thesis yale university fisher and bingham statistics produce different confidence estimates from the same data and the traditionally defined mean vector may differ from the maximum eigenvector of an orthorhombic bingham distribution it seems prudent to apply both models wherever a non fisher population is suspected and that may be appropriate in any tectonized rocks non fisher populations require larger sample sizes so that focussing on individual sites may not be the most effective policy in tectonized rocks more dispersed sampling across tectonic structures may be more productive second from the same specimens mean vectors isolated by thermal and alternating field af demagnetization differ which treatment gives more meaningful results is difficult to decipher especially in metamorphic rocks where the history of the magnetic minerals is not easily related to the ages of tectonic and petrological events in this study thermal demagnetization gave lower inclinations for paleomagnetic vectors and thus more distant paleopoles third of more parochial significance tilt corrections may be unnecessary in the ksz because magnetic fabrics and thrust ramp are constant in orientation to the depth at which they level off at approximately 15 km depth with archean geothermal gradients primary remanences were blocked after the foliation was tilted to rise on the thrust ramp therefore the rocks were probably magnetized in their present orientation tilting largely or entirely predates magnetization

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

correlation of strain and the anisotropy of magnetic susceptibility in the onaping formation evidence for a near circular origin of the sudbury basin abstract measurements of finite strain the anisotropy of magnetic susceptibility the magnetic carrier of the susceptibility and fieldwork have been carried out in the onaping formation of the sudbury basin ontario the magnetic fabric indicates that changes in the magnitude and type of magnetic susceptibility ellipsoids closely mirror the changing pattern of deformation from northwest to southeast across the onaping formation outcrops a quantitative correlation has been established between finite strain measurements and the magnetic susceptibility results and this allows the number of available strain measurements to be extended by more than 500 samples the finite strain results suggest a deformation path in which the onaping formation was first affected by nw se directed horizontal layer parallel shortening which produced an upright tectonic cleavage subsequently components of overthrust nw directed simple shear were superimposed upon the already deformed rocks of the onaping formation the result of removing the strain according to the sequence of superimposed deformations revealed by the deformation path suggests that the sudbury basin was nearly circular in shape prior to its deformation consequently a major objection to the meteorite impact theory for origin of the sudbury structure is eliminated

remagnetisation of a rock analogue during experimental triaxial deformation abstract synthetic specimens containing pseudo single domain magnetite were given a univectorial anhysteretic remanent magnetisation they were deformed in a ductile manner at room temperature under a confining pressure of 150 mpa 1 5 kbar the strain rate was 10 5 s 1 and the maximum strain achieved was 23 shortening a weak 0 035 mt magnetic field was present in the pressure vessel during deformation hydrostatic compaction alone produced pressure demagnetisation of low coercivity components of remanence but did not change remanence directions macroscopic differential stress was needed to deform the specimens and this also permitted syndeformational remagnetisation for values of more than 20 mpa 200 bars this added new components of piezo remanent magnetisation prm approximately parallel to the direction of the magnetic field in the pressure vessel they were acquired in all coercivity intervals but the largest components were added below 35 mt the final remanence orientation and its spatial distribution of vector components were dictated by the relative orientations of the remagnetising field and the initial remanence prm contributions almost completely masked the changes related to strain induced grain rotation at these low strains the directions of the externally imposed stress strain axes may not significantly influence the direction of new remanence components prm acquisition occurs when differential stress exceeds a threshold value that probably depends on the previous stress history of the magnetite grains in part this may explain the origin of syntectonic remagnetisation in some naturally deformed rocks where the remanence is not reset by recrystallisation or thermal or chemical processes

experimental simple shear deformation of magnetic remanence abstract a series of shear box experiments were run on magnetic rock analogues to determine how the remanence vector behaves during simple shear strain samples of silicone putty and silicone putty magnetite and kaolinite magnetite mixtures were given an anhysteretic remanence at high moderate and low angles to the shear plane in order to determine if the magnetization deformed as a passive line marker or as if carried by actively rotating rigid grains out of a total of twenty experiments twelve unequivocally contradict passive line behavior with an additional two experiments in strong disagreement with passive rotation of the magnetic particles when compared to numerical modeling results nine experiments clearly support active rotation of rigid magnetic particles thus in settings where simple shear may have been a dominant component of deformation history active rotation of remanence carrying grains rather than passive line behavior may be an appropriate description of the deformation of the remanence vector

magnetic susceptibility of rocks in the nutbush creek ductile shear zone north carolina abstract applicability of anisotropy of magnetic susceptibility ams as a strain indicator is investigated for the falls lineated gneiss a strongly lineated weakly foliated tectonite lying within the nutbush creek fault zone ncfz the ncfz is a major alleghanian ductile shear zone within the southern appalachian piedmont and is characterized along most of its length by a pronounced linear trend on aeromagnetic maps sampling was conducted along a traverse near raleigh n c that extended through the lineated gneiss across the eastern boundary of the ncfz and into adjacent supposedly unsheared raleigh belt gneiss petrofabric element orientations are similar within the falls lineated gneiss and the raleigh belt gneiss but foliation is better developed than lineation in the raleigh belt gneiss mineral lineation parallels the strike of the ncfz results of the ams analysis are remarkably consistent along the entire traverse mean susceptibilty ellipsoids are uniformly prolate and display little variation in spatial orientation in both the falls lineated gneiss and the raleigh belt gneiss mean maximum susceptibility axes are oriented along the trend of mineral lineation and mean minimum susceptibility axes coincide with poles to foliation magnetic fabric in both lithologies is dominated by the alignment of magnetite grains in the direction of silicate mineral lineation magnetic lineation is better developed than magnetic foliation even in raleigh belt rocks similarity of the magnetic fabrics in the lineated gneiss and the raleigh belt gneiss suggests that both fabrics were produced by the same deformational process the possibility that the raleigh belt rocks are actually part of the ncfz is suggested by alignment of petrofabric elements along the strike of the fault zone the presently mapped eastern boundary of the ncfz which was defined on the basis of lithologie and mesoscopic rock fabric differences may separate two distinct units within the fault zone ams results for raleigh belt rocks located north of the study area suggest that deformation associated with the fault zone may extend several kilometers east of its presently mapped boundary at certain locations ams results provide insight into the size of the strain field associated with the ncfz but cannot be used to estimate either the strain history or the magnitude of strain experienced by rocks within the fault zone

moment tensors and micromechanical models abstract a numerical modelling approach that simulates cracking and failure in rock and the associated seismicity is presented and a technique is described for quantifying the seismic source mechanisms of the modelled events the modelling approach represents rock as an assemblage of circular particles bonded together at points of contact the connecting bonds can break under applied stress forming cracks and fractures in the modelled rock if numerical damping is set to reproduce realistic levels of attenuation then energy is released when the bonds break and seismic source information can be obtained as damage occurs a technique is described by which moment tensors and moment magnitudes can be calculated for these simulated seismic events the technique basically involves integrating around the source and summing the components of force change at the surrounding particle contacts to obtain the elements of the moment tensor matrix the moment magnitude is then calculated from the eigenvalues of the moment tensor the modelling approach is tested by simulating a well controlled experiment in which a tunnel is excavated in highly stressed granite while microseismicity is recorded the seismicity produced by the model is compared to the actual recorded seismicity underground the model reproduces the spatial and temporal distribution of seismicity observed around the tunnel and also the magnitudes of the events a direct comparison between the actual and simulated moment tensors is not performed due to the two dimensional nature of the model however qualitative comparisons are presented and it is shown that the model produces intuitively realistic source mechanisms the ability to obtain seismic source information from the models provides a unique means for model validation through comparison with actual recorded seismicity once it is established that the model is performing in a realistic manner it can then be used to examine the micromechanics of cracking failure and the associated seismicity and to help resolve the non uniqueness of the geophysical interpretation this is demonstrated by examining in detail the mechanics of one of the modelled seismic events by observation of the time dependence of the moment tensor and by direct examination of the particle motions at the simulated source

ultrasonic velocities in cretaceous shales from the williston basin compressional and shear wave velocities were measured in the laboratory from 1 bar to 4 kbar confining pressure for wet undrained samples of cretaceous shales from depths of 3200 and 5000 ft in the williston basin north dakota these shales behave as transversely isotropic elastic media the plane of circular symmetry coinciding with the bedding plane for compressional waves the velocity is higher for propagation in the bedding plane than at right angles to it and the anisotropy is greater for the 5000 ft shale for shear waves the sh wave perpendicular to bedding and the sv wave parallel to bedding propagate with the same speed which is about 25 percent lower than that for the sh wave parallel to bedding in general compressional and shear velocities are higher for the indurated 5000 ft shale than for the friable 3200 ft shale all velocities increase with in increasing confining pressure to 4 kbar the 3200 ft shale exhibits velocity hysteresis as a function of pressure whereas this effect is a

elastic anisotropy of source rocks implications for hydrocarbon generation and primary migration we combined acoustic velocities and anisotropy measurements with x ray diffraction mineralogy rock eval organic geochemical analyses vitrinite reflectance and scanning electron microscope observations on 69 cores of hydrocarbon source rocks collected from eight sedimentary basins the velocity data indicate the lamellar distribution of solid organic matter kerogen in compacted black shales and show a strong correlation between their elastic anisotropy and organic richness the experimental results are consistent with the two component anisotropic model of black shales with a continuous kerogen network in both bedding parallel and bedding normal directions beginning from relatively low toc values the strong elastic anisotropy of tight thermally mature source rocks i further enhanced by overpressure induced bedding subparallel microcrack development during the main stage of hydrocarbon hc generation and primary migration at r0 greater than 0 75 in agreement with published petrographic observations these microcracks significantly aid primary hc migration particularly horizontally elastic anisotropy of medium to high porosity illite smectite shales is weaker than in lower porosity predominantly illite shales however the overall positive trend of elastic anisotropy with kerogen content remains unchanged

the effect of anisotropy on the young s moduli and poisson s ratios of shales young s modulus and poisson s ratio are required for geomechanics applications such as hydraulic fracture design analysis of wellbore stability and rock failure determination of in situ stress and assessment of the response of reservoirs and surrounding rocks to changes in pore pressure and stress shales are usually anisotropic and models that neglect shale anisotropy may fail to describe geomechanical behaviour correctly anisotropy in shales results from a partial alignment of anisotropic clay particles kerogen inclusions microcracks low aspect ratio pores and layering for shales the young s modulus measured parallel to bedding e1 is usually greater than the young s modulus measured perpendicular to bedding e3 however the poisson s ratio 31 corresponding to stress applied perpendicular to bedding and strain measured parallel to bedding can be greater than equal to or less than the poisson s ratio 12 for stress applied parallel to bedding and strain measured parallel to bedding for transverse isotropy the elastic anisotropy resulting from a partial alignment of clay particles can be written in terms of the coefficients w200 and w400 which describe the impact of clay particle orientation on the anisotropy of shales disorder in the orientation of clay particles acts to reduce w400 faster than w200 since w400 is a higher order moment of the clay particle orientation distribution function than w200 this is confirmed by analysis of measured anisotropy parameters for shales a partial alignment of clay particles is consistent with the measured young s moduli for shales and with values of poisson s ratio 31 12 but not with values 31 12 these values can be explained if there exist kerogen inclusions microcracks or low aspect ratio pores aligned parallel to the bedding plane

re computing palaeopoles for the effects of tectonic finite strain abstract the pre messinian limestone cover 58 8 ma to the troodos ophiolite 88 ma of southern cyprus is penetratively strained as shown by ubiquitous magnetic fabrics and in many sites stylolitic cleavage these define a gently n dipping foliation and an n plunging extension south vergent folding and thrusting is well known in very localized whereas the bulk of the strained limestone cover dips gently south disturbed by faulting the magnetic fabrics and stylolitic cleavage define the axes of finite strain in all sites studied and the calcite matrix was suitably ductile to permit the original palaeomagnetic directions to be de strained assuming continuum behaviour the optimum de straining 30 40 shortening in a flattening strain is compatible with the stylolitic cleavage development restores bedding to the near horizontal and restores the characteristic remanent magnetization vectors chrms to concentrated symmetrical fisherian distributions the strain corrected chrms yield more reasonable palaeopole locations for the lefkara and pakhna limestone and more uniform micro plate rotation rates corrected palaeopoles reveal a relatively uniform anticlockwise rotation of the troodos plate since the creation of the late cretaceous 88 ma ocean lithosphere it did not accelerate during the deposition of the limestone cover as required by palaeopoles calculated from data not corrected for finite strain but turned at 1 5 ma 1 since 58 ma

oroclinal bending in the alborz mountains northern iran new constraints on the age of south caspian subduction and extrusion tectonics abstract in this study we report an extensive paleomagnetic study 76 sites carried out in the alborz mts northern iran with the aim of reconstructing the rotation history and the origin of curvature of this orogenic chain the analyzed deposits are the sedimentary successions of the upper red formation miocene lower red formation oligocene and eocene clastic units paleomagnetic results indicate that the alborz mts can be considered a secondary arc that originated as a linear mountain belt that progressively acquired its present day curvature through opposite vertical axis rotations along its strike the curvature of the arc was entirely acquired after the middle late miocene which is the age of the youngest investigated sediments upper red formation overall our paleomagnetic data indicate that the alborz mts can be considered an orocline our results define for the first time the rotational history of the entire alborz curved mountain belt and enable us to reconstruct the paleogeographic and tectonic evolution of northern iran in the framework of arabia eurasia continental deformation the kinematics inferred by the pattern of paleomagnetic rotations is at odds with the present day kinematics of northern iran characterized by the westward extrusion of the south caspian block and by a left lateral shear between central iran and the central and western sectors of the alborz mts by integrating paleomagnetic data with stratigraphic thermochronological structural and gps information we propose that the initiation of south caspian subduction and the activation of westward extrusion of south caspian block occurred diachronously and that the initiation of the present day kinematics of northern iran was quite recent lower pleistocene

tracking holocene palaeostratification and productivity changes in the western irish sea a multi proxy record abstract the western irish sea preserves an exceptionally thick ca 40 m holocene succession that is ideally suited to understanding the pattern of palaeostratification and water mass productivity changes in the region and their relationship with sea level sedimentation and biota additionally the presence of shallow buried methane provides an opportunity to explore its potential impact on the local pattern of holocene marine environmental change multi proxy investigation of a cored borehole succession through the holocene interval tracks changes from mixed to seasonally stratified conditions in the earliest holocene 11 2 10 ka high productivity mixed water conditions prevailed with abundant and diverse foraminifera and dominant heterotrophic dinoflagellate cysts productivity was probably driven by high nutrient fluxes related to high rates of sedimentation 1600 cm kyr in turn influenced by relatively low sea level and restricted sediment accommodation space across shelf areas to the east of the borehole site eastern irish sea basin with rising sea level in the later part of the early holocene the region evolved into a relatively lower productivity mixed water mass system with significant changes in ecology revealed by dinoflagellate cysts and foraminifera in the latest early holocene and earliest mid holocene ca 8 4 8 2 ka a return to higher productivity is signalled by dinoflagellate cyst data a result of seasonal stratification becoming established evidenced by sharply increased summer sea surface temperature estimates typically 16 17 c that contrast with an opposite more positive trend in 18o values for benthic foraminifera reductions in turbulent mixing associated with stratification might have exacerbated the palaeoecological impact of shallow buried methane associated with the borehole site potentially evidenced by a significant change in dominant benthic foraminifera and strong localised excursions in the benthic 13c 18o record

natural fractures in shale a review and new observations natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low porosity and low permeability shale host rock many shale outcrops cores and image logs contain fractures or fracture traces and microseismic event patterns associated with hydraulic fracture stimulation have been ascribed to natural fracture reactivation here we review previous work and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization orientation and size patterns a wide range of shales have a common suite of types and configurations of fractures those at high angle to bedding faults bed parallel fractures early compacted fractures and fractures associated with concretions these fractures differ markedly in their prevalence and arrangement within each shale play however constituting different fracture stratigraphies differences that depend on interface and mechanical properties governed by depositional diagenetic and structural setting several mechanisms may act independently or in combination to cause fracture growth including differential compaction local and regional stress changes associated with tectonic events strain accommodation around large structures catagenesis and uplift fracture systems in shales are heterogeneous they can enhance or detract from producibility augment or reduce rock strength and the propensity to interact with hydraulic fracture stimulation burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated the role of microfractures in production from shale is currently poorly understood yet potentially critical we identify a need for further work in this field and on the role of natural fractures generally

experimental deformation of two component irm in magnetite bearing limestone a model for the behaviour of nrm during natural deformation abstract experimental room temperature triaxial deformation at 200 mpa 2 kbar and at a controlled strain rate of 10 5 s 1 is performed on three limestones with magnetic behaviour dominated by single domain magnetite permanent deformation is achieved mainly by twinning of calcite isothermal remanent magnetisation irm in the samples is reduced in magnitude especially when parallel to compressive stress the decrease in magnitude correlates slightly better with peak stress than with strain in accord with previous work in uniaxial tests indicating the role of stress in deformational demagnetisation remanences have their vector plots condensed after deformation particularly in the range 0 30 mt a two component irm is used as a simple model of a multi component natural remanent magnetisation nrm the resultant vector is deflected by the action of stress towards whichever components are least reduced by deformation in the case of magnetite bearing rocks this appears to be the most coercive components in the case of a natural deformation it is predicted that the effects of stress at any stage should be to remove progressively any low coercivity components added to an nrm by remagnetisation during tectonism especially those parallel to compressive stress

coaxial deformed and magnetic fabrics without simply correlated magnitudes of principal values abstract results of a detailed strain analysis of accretionary lapilli in the borrowdale volcanic sequence of england indicate a bedding memory component to the rock s fabric a simple new strain analysis method uses variation in thickness of lapilli rims and appears not to be influenced by the bedding fabric the orientations of the principal magnetic susceptibilities show a strong correlation with the principal strain orientations as in other studies but we can detect no correlation between strain magnitudes and principal susceptibility magnitudes in the same small specimens

the remagnetization of devono carboniferous sediments from the ardenno rhenish massif abstract a paleomagnetic study was conducted on the paleozoic sedimentary rocks of the ardenno rhenish massif and the brabant massif lower carboniferous and middle devonian carbonates in the dinant synclinorium and inde syncline carry a thermoviscous remagnetization residing in magnetite lower devonian red beds in the dinant synclinorium carry a chemoremanent remagnetization residing in hematite the sediments were remagnetized by burial metamorphism during the asturian phase of the variscan orogeny with a few exceptions in all formations the magnetizations are of inverse polarity corresponding to the kiaman paleofield in the permo carboniferous the virtual geomagnetic poles vgps of the inde syncline agree with the permo carboniferous pole position of europe vgps of the dinant synclinorium which deviate from the permo carboniferous reference direction are due to syntectonic remagnetizations and subsequent 10 20 maximum 40 dextral shear deformations these deformations can be correlated with the northwards overthrusting of the dinant nappe for the inde syncline post asturian rotational deformations can be excluded should a n s transport of this part of the dinant nappe not have occurred the declination pattern indicates that the western part of the dinant nappe was transported further north than the eastern part

effect of progressive deformation on remanent magnetization of permian redbeds from the alpes maritimes france abstract the relationships among the anisotropy of magnetic susceptibility the finite strain and the characteristic remanent magnetization chrm have been investigated in deformed red sediments from the alpes maritimes in southeast france the magnetic properties of these rocks are due to hematite the finite strain is known from field studies of the shapes and orientations of green reduction spots and from optical microscopy and x ray texture goniometry studies of chlorite and illite the strain data permit a classification of the magnetic results according to four main stages of progressive deformation in the compaction stage magnetic anisotropy is flattened in the bedding due to compactional loading the chrm directions approximate to a fisherian distribution about a mean value close to the known permian direction for the region as deformation proceeds pencil structure first develops and the anisotropy is characterized by a prolate pattern with maximum axis parallel to the subhorizontal azimuth of the pencil structure and fold axes with increasing deformation the magnetic susceptibility ellipsoid becomes oblate and finally triaxial at these stages of high deformation the anisotropy patterns show that the hematite grain orientations are strongly influenced by the cleavage the remanence directions in deformed rocks do not correspond to a fisher distribution and eventually deviate strongly from permian directions these results show that paleomagnetic vectors are affected by the distortional strains which accomodate shape change in the rock simple tilt correction without compensation for the strain would yield incorrect paleopole positions an attempt has been made to compensate for the effects of deformation by regarding the remanent vector as a passive marker march 1932 equations for strain removal were applied to the remanence in two ways 1 a using strain data directly obtained in the field for each site and 2 b by converting the magnetic susceptibility anisotropy of each sample to a corresponding finite strain value correcting for the finite strain by both methods improves the within site grouping of paleomagnetic vectors in strongly deformed rocks strain removal assuming the march model results in overcorrection of the remanent magnetization data in low strain regions distortion may produce mechanical rotation of the hematite particles but recrystallization may alter the deformation mechanism in regions of high strain the limits of applicability of the passive marker treatment could not be tested adequately in the deformed red sediments of the alpes maritimes

the effect of magnetic anisotropy on paleomagnetic directions in high grade metamorphic rocks from the juiz de fora complex se brazil the juiz de fora complex is mainly composed of granulites and granodioritic migmatite gneisses and is a cratonic basement of the ribeira fold belt which was formed during the last stage of brasiliano orogeny samples widely distributed over the studied region se brazil rio de janeiro state were collected for paleomagnetic and magnetic anisotropy determinations after measurement of anisotropy of low field magnetic susceptibility ams the same specimens were submitted to both alternating field for anisotropy of remanent magnetization arm determinations and thermal demagnetization for paleomagnetic analyses demagnetization processes allowed isolating an eastern steep positive inclination characteristic remanent magnetization chrm direction from the rocks the studied specimens are strongly heterogeneous at sample scale regarding the amount of magnetic minerals even for specimens from the same core rock magnetism indicates that there is a complex mixture of magnetic minerals with both low and high coercivity which are associated with coarse grained titano magnetite and either fine grained titano magnetite or titanohematite grains respectively these magnetic minerals are responsible for both remanent direction and magnetic anisotropies even though for some specimens paramagnetic minerals are the carriers of ams fabric the chrm direction found for the rocks should represent the cooling phase of the brasiliano event in the area and it was acquired after the magnetic fabric was formed the ams and arm fabrics are coaxial and tectonic in origin and compare favorably with the mesoscopic scale fabrics in the adjacent areas however arm measurements have reached only low coercivity grains and it could be determined in few specimens then only an estimate of arm effect on chrms correction is shown the rocks are strongly magnetically anisotropic 50 and foliated the paleomagnetic directions from each specimen within a site tend to approach its ams or arm foliation kmax kint or armmax armmin plane these directions were corrected specimen by specimen from the sites taking into account the pole of ams or arm foliation kmin or armmin and the value of the degree of anisotropy p results show that even for specimens with high p the angular difference between corrected and uncorrected magnetization direction may be insignificant if the geomagnetic field at the time of acquired magnetization was approximately parallel to the magnetic foliation of the specimens our data set suggests that ams is a powerful tool to correct the chrm deviation from paleofields in rocks with complex magnetic mineralogy such as those from the juiz de fora complex

magnetic fabric study across the ailao shan red river shear zone the anisotropy of magnetic susceptibility ams of 351 specimens from 51 sites across the ailao shan red river shear zone asrr was measured to determine its magnetic fabric rocks range westward from core schistose gneiss through lowgrade schist to triassic sediment magnetic ellipticity analysis shows that 41 of 51 sites have an oblate compressional fabric and the other 10 sites have a prolate fabric p j value drops by 22 4 in the low grade schist and by 27 4 in the triassic sediment on average with respect to the gneiss suggesting a rapid decrease of deformational intensity the directions of principal susceptibilities are closely related to the deformation of the ailao shan red river shear zone the susceptibility plane always coincides with the schistosity or cleavage plane most of the maximum susceptibility axes trend nw se in the shear zone the maximum susceptibility axes k max are parallel to the lineation within the foliation plane with increasing distance from the shear zone there is a trend that they become parallel to the down dip of reverse faults or cleavage this indicates changes in deformation mode inside and outside the shear zone within the shear zone horizontal movement is dominant outside shortening prevails the overall minimum magnetic axes align ne sw with subhorizontal to low dip angles suggesting that the dominant shortening is ne sw directed caution should be exercised when ams is used to determine shear sense in strong shear zones because the angle between the minimum susceptibility axis k min and pole of foliation is small and also because the attitude of foliation varies from place to place they result in unreliable or even wrong shear sense another important result is the axial ratio of magnetic susceptibility ellipsoid along the study section with these data it is possible to establish an axial ratio relationship between the finite strain ellipsoid and magnetic susceptibility ellipsoid for quantitative calculation of offset c 2002 elsevier science b v all rights reserved

remanent magnetism and ductile deformation in an experimentally deformed magnetite bearing limestone abstract a limestone with pseudo single domain psd pure magnetite is given an isothermal remanent magnetization irm samples are deformed triaxially at 200 mpa 2 kbar confining pressure at room temperature and at a strain rate of 10 5 s 1 the limestone deforms in a ductile manner by twinning to produce sufficiently homogeneously strained specimens up to about 20 shortening the uniform saturation irm rotates away from the shortening axis approximately by an amount expected for the homogeneous strain of a non material line marker hydrostatic compaction alone shows that this is time dependent and compatible with progressive damage of the ferrimagnetic grains strain produces an increase in coercivity of the magnetite an increase in its saturation remanence and an increase in its anisotropy of magnetization with respect to irm these observations are compatible with reduction in effective grain size and change of shape of magnetite experimental deformation reduces the intensity of magnetisation chiefly by the removal of low coercivity components of remanence when there is a pure irm vector

eocene 46 44 ma onset of australia pacific plate motion in the southwest pacific inferred from stratigraphy in new caledonia and new zealand

variability in spatial distribution of mineral phases in the lower bowland shale uk from the mm to μm scale quantitative characterization and modelling the microstructure of a highly laminated lower bowland shale sample is characterized at the micron to millimeter scale to investigate how such characterization can be utilized for microstructure based modelling of the shale s geomechanical behavior a mosaic of scanning electron microscope sem back scattered electron bse images was studied mineral and organic content and their anisotropy vary between laminae with a high variability in fracturing and multi micrometer aggregates of feldspars carbonates quartz and organics the different microstructural interface types and heterogeneities were located and quantified demonstrating the microstructural complexity of the bowland shale and defining possible pathways for fracture propagation a combination of counting box dispersion covariance and 2d mapping approaches were used to determine that the total surface of each lamina is 3 to 11 times larger than the scale of heterogeneities relative to mineral proportion and size the dispersion approach seems to be the preferential technique for determining the representative elementary area rea of phase area fraction for these highly heterogeneous large samples supported by 2d quantitative mapping of the same parameter representative microstructural models were developed using voronoi tessellation using these characteristic scales these models encapsulate the microstructural features required to simulate fluid flow through these porous bowland shales at the mesoscale

tilting and transpression of an archaean anorthosite in northern ontario abstract an archaean anorthosite complex crops out over more than 300 km 2 at the junction between an archaean metasedimentary accretionary prism quetico belt and a volcanic greenstone terrain to the north wabigoon belt the anorthosite is feebly deformed and weakly foliated however anisotropies of low field susceptibility ams and anhysteretic remanence aarm define consistent magnetic fabric directions extending over subareas 25 km 2 in extent whereas magnetic fabrics cannot define strain magnitudes they normally provide reliable indications of finite strain orientations even where the fabrics developed by stress controlled nucleation during a progressive noncoaxial strain history the field schistosity s 1 being the oldest foliation rotated closest to the shear plane ams foliation due to preferred crystallographic orientation of silicates and pyrrhotite as well as preferred dimensional orientation of magnetite lies at a greater angle to the shear plane because it formed late then s 1 had less time to rotate the few measurements of the youngest aarm foliation caused by magnetite and pyrrhotite suggest that it may lie at a still greater angle to the shear plane thus s 1 and succeeding magnetic foliations provide a cryptic shear sense indicator they reveal relative motions along the terrain boundary that include transpression due to nw se shortening causing dextral e w shear with a north side up component palaeomagnetic data confirm a progressive northwards steepening of the anorthosite sills that is consistent with northwards subduction of the accretionary prism to the south

a paleomagnetic study of tectonically deformed red beds of the lower glarus nappe complex eastern switzerland a paleomagnetic study at 15 sites 315 samples in permo triassic red beds of the helvetic nappes of eastern switzerland isolated stable characteristic and secondary remanent directions thermal demagnetization and isothermal remanent magnetization experiments suggest pigmentary hematite carries the characteristic direction secondary directions are scattered but may have been acquired during the alpine deformation in the oligocene and miocene the characteristic directions also show large between site scatter anisotropy of magnetic susceptibility indicates that the hematite grains are flattened within the slaty cleavage and that there is a north south lineation this anisotropy pattern implies that the magnetic grains were realigned by the alpine deformation it is not possible to explain the characteristic remanent magnetization deviations on a site by site basis but the site mean directions are streaked along a great circle the down dip direction of which is approximately parallel to the regional fold axes 60 20 this spread could be explained by a penetrative simple shear deformation associated with the nappe internal deformation

the tectono magnetic signature of the old red sandstone and pre devonian strata in the hasteinen area western norway and implications for the later stages of the caledonian orogeny the structural fabrics recorded from devonian rocks and substrate in the hasteinen area western norway show near e w trends in the devonian rocks these fabrics are a consequence of post depositional regional folding at a considerable depth presumably attended by the aquisition of an thermoviscous tvrm or thermochemical tcrm remanence group a post or possibly late syntectonic group a remanences declination 210 inclination 18 95 9 constitute the principal remanence in both devonian rocks and their substrate whereas a younger group b magnetization declination 047 inclination 59 95 7 relates to early mesozoic shearing the palaeomagnetic group a and structural data from the hasteinen and kvamshesten old red sandstone massifs favor a regional compressive n s shortening event svalbardian solundian which probably occurred during latest devonian times some recent extensional basin models appealing to listric normal faulting to account for both the sedimentation and the subsequent deformation of the western norwegian devonian rocks are rejected on the basis of inadequate structural information

seismic anisotropy of shales the seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure x ray diffraction techniques and electron microprobe backscatter bse imaging the velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding anisotropy at elevated pressures caused mainly by preferred orientation of clays illite parallel to bedding ranges from 20 vp and 19 vs for a sample of new albany shale to 30 vp and 35 vs for a sample of chattanooga shale the degree of clay mineral alignment in the shales is constrained by orientation indices produced using simple x ray diffraction techniques a strong positive correlation is found between the degree of preferred orientation as expressed in the orientation indices and seismic anisotropy bse images of the shale fabrics confirm in a qualitative manner the results of the x ray study to investigate wave propagation in the shales elastic constants of each sample are calculated and used to produce phase and group wave velocity surfaces which describe variation in velocity as a function of angle to the bedding normal the calculated velocity surfaces constrained by independent velocity measurements display a lack of shear wave splitting at near normal incidence in even the most anisotropic shales for the highly anisotropic chattanooga shales group velocity surfaces differ significantly from corresponding phase velocity surfaces

experimental strain of isothermal remanent magnetization in ductile sandstone abstract a macroscopically ductile sandstone to which a homogeneous isothermal remanent magnetization irm was applied has been deformed at 150 mpa confining pressure and constant strain rate of 10 5 s 1 hydrostatic compaction does not produce a deflection of the irm vector although it is reduced in intensity pure shear producing shortening in the range 2 to 35 steadily reduces the intensity of magnetization but also homogeneously rotates the remanence vector toward the plane of flattening the amount of rotation is slightly less than that expected for a non material line undergoing homogeneous strain deformation selectively removes weakly coercive components of remanence as revealed by alternating field af demagnetization during deformation a weak deformational viscous remanent magnetization dvrm is acquired from the pressure vessel this is different from a conventional viscous remanent magnetization vrm in that it is not acquired when the specimen is subject to hydrostatic confining pressure alone even for periods three times longer than the longest deformation test

the relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks several hundred microearthquakes with local magnitudes ranging between 4 and 2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site fenton hill new mexico these events are probably caused by shear failure induced by high pore fluid pressures since the event locations seem to cluster in a narrow band near the hydraulic fracture we were able to use micro seismic techniques to locate the hydraulic fracture and monitor its growth we calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures by comparing a pore pressure distribution calculated using a one dimensional diffusion model with the distribution of event foci we demonstrate that high pore pressures are a probable cause for these microearthquakes stress drops for these microearthquakes calculated from spectral parameters increased by a factor of 3 during the course of the experiment the stress drop increase correlates with a migration of seismic activity away from the injection well bore which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressurizations

palaeogene alpine tectonics and icelandic plume related magmatism and deformation in northern ireland the cenozoic tectonic history of nw europe is generally attributed to some combination of three principal controlling factors north atlantic opening alpine collision and formation of the icelandic mantle plume using constraints from the high resolution tellus aeromagnetic survey of northern ireland we show that palaeogene tectonics can be attributed to approximately north south alpine related compression forming nnw sse trending dextral and ene wsw trending sinistral conjugate strike slip faults with the latter defined by kilometre scale displacements along reactivated caledonian or carboniferous faults this tectonism was however punctuated by pulsed magmatic intrusive and extrusive events including four distinct dyke swarms that are attributed to ne sw to east west directed plume related extension although this evidence shows for the first time that north south alpine compression was periodically overwhelmed by the dynamic stresses and uplift associated with pulsed mantle plume related deformation associated strike slip faulting may have controlled the locus of volcanic activity and central igneous complexes and the location of sedimentary depocentres

finite strain patterns and their significance in permian rocks of the alpes maritimes france abstract more finite strain data has been obtained from autochthonous permian mudstones of the alpes maritimes s e france these new data were computed from field measurements of green spots on all available sections deformed mudcracks and from the quantitative correlation between magnetic susceptibility anisotropy and finite strain in these rocks previously published finite strain data and the new results are presented on a series of structural maps and cross sections for the dome de barrot the tinee and vionene region and the roya region as in previous studies difficulties arise in explaining the apparently variable extension parallel with the 100 subhorizontal bedding cleavage intersection either this is real or there were large volume changes during the tectonic deformation study of quartz fibres developed in deformed mudcracks in the tinee valley suggest that early in the tectonic history incremental stretching directions were parallel with the bedding cleavage intersection while later they were down dip in the 100 trending cleavage since these permian rocks have remained stuck to the argentera basement they also record displacements and deformations in the basement the early 100 subhorizontal stretching is consistent with nw se dextral strike slip basement faulting while later down dip stretching in the cleavage is consistent with contraction faults in the basement this information and new palaeomagnetic data on the same samples are combined with recent geophysical evidence and regional tectonic studies to provide a new precision to the tectonic history of this part of the western alpine external zone

strain removal applied to paleomagnetic directions in an orogenic belt the permian red slates of the alpes maritimes france abstract detailed investigations of relationships between strain and directions of characteristic remanent magnetization chrm have been conducted on the permian red series of the alpes maritimes france the analysis of the results obtained by structural and paleomagnetic studies on 14 sites leads to the following observations in most sites the mean direction of the strain shortening axis is close to the expected permian paleomagnetic direction in this case we observe a large scatter of the chrm directions within each site which is interpreted as an increase of an initial dispersion due to strain moreover this dispersion more or less increases with strain intensity in other cases where the angle between the shortening axis and the paleomagnetic direction is high the whole population is deflected towards the cleavage xy plane these observations lead us to assume a material line like behaviour of the chrm vectors and consequently an attempt has been made to recalculate these directions by strain removal these calculations give positive results at two levels 1 a statistically significant improvement in the clustering of the within site distributions and 2 after tilt correction using unstrained bedding planes a significant improvement in the clustering of the between site distribution the formation mean direction after strain removal is in good agreement with known permian results for stable europe

paleoremanence dispersal across a transpressed archean terrain deflection by anisotropy or by late compression the quetico belt is an archean metasedimentary terrain sandwiched between greenstone subprovinces a single regional schistosity is subparallel to magnetic foliation defined by anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanence aarm aligned micas are the primary cause of ams aligned pyrrhotite is the sole cause of aarm each magnetic subfabric is due to minerals that grew in overlapping intervals in the strain history their slight obliquity may be due to crystallization during noncoaxial strain magnetic fabric ellipsoids suggest varying severity of flattening very strong flattening on the northern boundary less severe on the southern boundary and least severe in the interior the interior and the southern boundary of the belt stretched horizontally and the shortening direction varies gradually from ns in the north to nnw in the south the paleomagnetic record carried by pyrrhotite shows characteristic remanences dispersed around a great circle subparallel to the regional schistosity and magnetic foliations late stress effects rotated paleoremanences into this pattern we discount deflection of the paleofield by anisotropy the pattern of dispersal indicates uniaxial oblate flattening of the region with nnw sse shortening late in the metamorphic cooling history 300 c this postdates schistosity ams and aarm fabrics but it is controlled by the same kinematic pattern

remanence rotation due to rock strain during folding and the stepwise application of the fold test numerical modeling was conducted to determine the effects of simple models of folding strain on prefolding fisherian distributions of magnetization i found that the constant volume simple shear associated with flexural slip flexural flow folding can rotate a prefolding magnetization to have a maximum precision parameter k value at partial unfolding and thus appear to be synfolding in age if the magnetization is treated as a passive line marker this behavior only occurs for steep initial magnetizations the initial homogeneity of the fisherian precision parameters from each limb is degraded by this type of deformation and may be used to detect a strain modified remanence if the magnetization is carried by rigid mechanically active ellipsoidal or spherical particles a maximum k value at partial unfolding will occur for any initial inclination with the special case that rigid spheres will be rotated to cause a maximum k value at 50 unfolding for any initial inclination homogeneity of limb precisions is not affected in this case a discontinuous model of strain during folding in which pressure solution removes material at an axial planar spaced cleavage and causes limb dips steeper than bedding dips could also produce a maximum k value at partial unfolding strain and remanence studies of flexural slip folding in the allentown formation dolomite and the bloomsburg formation red sandstones and mudstones suggest that 1 the rigid grain model may be a more realistic model of magnetic particle behavior during shear strain and 2 about one quarter to one third of the theoretically predicted simple shear strain occurs in these rocks this amount of strain cannot explain the synfolding behavior observed in the allentown formation but could explain the maximum k values at 80 90 unfolding seen in the bloomsburg formation and perhaps other appalachian clastic rocks

effects of tectonic deformation on the remanent magnetization of rocks paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis however the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known anisotropy of magnetic susceptibility ams provides a quantitative basis for strain determination the principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid a linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types if the correlation method used is valid ams observations can be used to establish the magnitudes and directions of the principal strains strain modifies bedding planes and realigns the grains that carry magnetic remanence compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain the effects of strain on bedding can be corrected using passive marker theory attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive during deformation linear and planar elements deflect away from the axis of maximum shortening the passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions the passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix

controls of hydrocarbon generation on the development of expulsion fractures in organic rich shale based on the paleogene shahejie formation in the jiyang depression bohai bay basin east china abstract the development of expulsion fractures in organic rich shale is closely related to hydrocarbon generation and expulsion from kerogen organic rich shales from the upper part of the fourth member and the lower part of the third member of the paleogene shahejie formation in the jiyang depression bohai bay basin east china are used as an example based on thin sections sem and thermal simulation experiments the characteristics of hydrocarbon generation and the conditions supporting the development of expulsion fractures were explored the key factors influencing these fractures include the presence of kerogens their distribution along laminae and around particle boundaries their exposure to heat and the build up in pressure due to confinement by low permeability the development of excess pore fluid pressures and intrinsic low rock fracture strength are the main influencing factors pressurization by rapid generation of hydrocarbon provides impetus for fracture initiation and cause bitumen to migrate quickly the shale laminae results in distinctly lower fracture strength laminae parallel than laminae normal and this directs the formation of new fractures in the direction of weakness when pore fluid pressure increases maximum and minimum principal effective stresses decrease by different proportions with a larger reduction in the maximum principal effective stress this increases the deviatoric stress and reduces the mean stress thus driving the rock towards failure moreover the tabular shape of the kerogen aids the generation of hydrocarbon and the initiation of expulsion fractures from the tip and edge the resulting fractures extend along the laminae when the tensile strength is lower in the vertical direction than in the horizontal direction particle contact boundaries are weak and allow fractures to expand around particles and to curve as the stress strength regime changes when pore fluid pressure fields at different fracture tips overlap fractures will propagate and interconnect forming a network this paper could provide us more detailed understanding of the forming processes of expulsion fractures and better comprehension about hydrocarbon expulsion primary migration in source rocks

ordovician docking of the carolina slate belt paleomagnetic data paleomagnetic results from eight sites in argillaceous metasediments from union county north carolina in the central carolina slate belt yield a consistent high blocking temperature magnetization component the precision of the mean declination and inclination of all site means is maximized at 50 tilt correction and the improvement in grouping relative to 0 and 100 tilt correction is significant at the 95 confidence level this tends to indicate that the high blocking temperature component was acquired during folding the metamorphism and deformation in this area are rather well constrained by recent 40ar 39ar data to the late ordovician 450 ma we therefore interpret the computed pole position 29 6 n 122 1 e 95 5 11 as late ordovician in age and note that it lies close to some of the late ordovician poles for cratonic north america the paleolatitude 22 2 s of the sampling site is certainly unlike those derived from armorica and gondwana for this time interval and is rather similar to that proposed for the iapetus coast of laurentia during the late ordovician indeed the magnetization and deformation may be contemporaneous with the docking of the carolina slate belt onto the laurentian continent

anisotropy of magnetic susceptibility sedimentological igneous and structural tectonic applications as was noted by graham 1954 anisotropy of magnetic susceptibility ams data has many applications to the study of geological processes this type of anisotropy expresses directional variation in the magnetization induced in a rock most notably in its iron oxides and especially in its magnetite and hematite the ams is commonly expressed by an ellipsoid of which the principal axes are k1 k2 and k3 from greatest to least this ellipsoid is generally interpreted in terms of the distribution and shape of grains of magnetic minerals in a rock although the ellipsoid determined depends also on magnetocrystalline anisotropy and on factors of instrumental technique ellwood et al in press 1987 the challenge for the geoscientist in analyzing ams data is first to identify the origin of the ams geometry then second to relate that geometry to the natural processes which produced the inferred or measured distribution of magnetic materials

restoring paleomagnetic data in complex superposed folding settings the boltana anticline southern pyrenees abstract complex kinematic scenarios in fold and thrust belts often produce superposed and non coaxial folding interpretation of primary linear indicators must be based on a careful restoration to the undeformed stage following the reverse order of the deformation events therefore sequential restoration to the ancient coordinate system is of key importance to obtain reliable kinematic interpretations using paleomagnetic data in this paper a new paleomagnetic study in the western flank of the boltana anticline southern pyrenees illustrates a case study of a complex tectonic setting having superposed non coaxial folds the first stage of nw se folding linked to the oblique boltana anticline took place during lutetian times the second stage was linked to the vertical axis rotation and placed the boltana anticline in its present day n s configuration our data support a long lasting lutetian to priabonian period with main rotational activity during the bartonian priabonian other authors support a var coeval with anticlinal growth the third stage resulted in southwards tilting related to the emplacement of the n120e striking guarga basement thrust oligocene early miocene based on this deformational history a sequential restoration was applied and compared with the classic bedding correction at the site scale single bedding correction gives errors ranging between 31 and 31 in the estimation of vertical axis rotations at the locality scale in sites grouped in three folds from w to e arbella planillo and san felizes the bedding corrected data display rotation values in accordance with those found in the ainsa basin by other authors sequential restoration based on the afore mentioned evolution in three steps improves both some locality means and the internal consistency of the data therefore reasonably constrained sequential restoration becomes essential to reconstruct the actual history of superposed folding areas

stress dependent seismic anisotropy of shales a simple theory for the stress dependent seismic anisotropy of shales can be obtained in terms of a second rank tensor and a fourth rank tensor that depend on the orientation distribution of contacts between clay platelets the theory allows the normal and shear stiffness of the contact regions between clay platelets to be obtained as a function of stress from measurements of seismic p and s wave velocities for shales the ratio of the normal to shear compliance bn bt of the contact regions between clay particles is found to be sensitive to the saturation state of the shale inversion of velocity measurements for fully saturated shales shows a low value of bn bt when compared with measurements on air dry shales consistent with the expected reduction in normal compliance in a fluid saturated low permeability rock for all shales considered bn bt is found to be less than unity the contacts between clay particles are therefore more compliant in shear than in compression

late cretaceous early eocene counterclockwise rotation of the fueguian andes and evolution of the patagonia antarctic peninsula system abstract the southernmost andes of patagonia and tierra del fuego present a prominent arc shaped structure the patagonian bend whether the bending is a primary curvature or an orocline is still matter of controversy new paleomagnetic data have been obtained south of the beagle channel in 39 out of 61 sites they have been drilled in late jurassic and early cretaceous sediments and interbedded volcanics and in mid cretaceous to eocene intrusives of the fuegian batholith the anisotropy of magnetic susceptibility was measured at each site and the influence of magnetic fabric on the characteristic remanent magnetizations chrm in plutonic rocks was corrected using inverse tensors of anisotropy of remanent magnetizations normal polarity secondary magnetizations with west directed declination were obtained in the sediments and they did not pass the fold test these characteristic directions are similar to those recorded by mid cretaceous intrusives suggesting a remagnetization event during the normal cretaceous superchron and describe a large 90 counterclockwise rotation late cretaceous to eocene rocks of the fueguian batholith record decreasing counterclockwise rotations of 45 to 30 these paleomagnetic results are interpreted as evidence of a large counterclockwise rotation of the fueguian andes related to the closure of the rocas verdes basin and the formation of the darwin cordillera during the late cretaceous and paleocene the tectonic evolution of the patagonian bend can thus be described as the formation of a progressive arc from an oroclinal stage during the closure of the rocas verdes basin to a mainly primary arc during the final stages of deformation of the magallanes fold and thrust belt plate reconstructions show that the antarctic peninsula would have formed a continuous margin with patagonia between the early cretaceous and the eocene and acted as a non rotational rigid block facilitating the development of the patagonian bend

paleomagnetic results from the lower devonian llandstadwell formation dyfed wales abstract a paleomagnetic investigation of the lower devonian llandstadwell formation in southwest wales has yielded a characteristic direction declination 196 inclination 5 that passes a fold test comparison of the corresponding virtual geomagnetic pole 334 e 39 s to previously published poles for great britain indicates that the magnetization is secondary and likely to be of carboniferous age no evidence for post hercynian rotation of pembrokeshire is indicated by our data

effect of magnetic anisotropy on the natural remanent magnetization in the mcu ive layer of the bjerkreim sokndal layered intrusion rogaland southern norway a strong negative magnetic anomaly caused by an intense natural remanent magnetization nrm approximately opposite today s geomagnetic field is observed above the mcu ive unit of the bjerkreim sokndal layered intrusion the anomaly is strongest in the east close to heskestad and decreases when following the layer toward the north and west this study investigates how the nrm changes along the layer and how its direction and intensity are affected by magnetic fabrics in the intrusion nrm low field anisotropy of magnetic susceptibility and anisotropy of anhysteretic remanence have been measured on 371 specimens from 46 sites the orientation of both the magnetic fabrics and the nrm changes for different locations along the layer and it appears that the nrm is tilted away from the mean paleofield and toward the direction of maximum susceptibility and maximum anhysteretic remanence when nrm directions are corrected for magnetic fabrics the angle between the nrm and mean paleofield direction generally decreases for specimens with a single component nrm no correlation was found between the nrm intensity and the directional relationship between nrm magnetic fabric and mean paleofield

strain and magnetic fabric in the santa catalina and pinaleno mountains metamorphic core complex mylonite zones arizona anisotropy of magnetic susceptibility ams is capable of recording finite strain in weakly magnetized rocks ams was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history ams measurements were compared with finite strains determined from dike rotations and from foliation orientations in one zone the santa catalina mountains the orientations of susceptibility and finite strain ellipsoids are in excellent agreement and there is a logarithmic relationship between susceptibility difference ki ki k k and finite strain magnitude in the second zone the pinaleno mountains minimum susceptibility is perpendicular to the finite flattening plane but the maximum susceptibility does not parallel the maximum extension direction and there is no systematic relationship between susceptibility magnitude and strain magnitude oriented polished thin sections indicate that magnetite in the protolith of the santa catalina mylonite occurs as randomly oriented elongate grains with subsequent deformation the long axes are rotated into the maximum extension direction in the pinaleno mylonites both equant and elongate magnetite grains are present with deformation the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane ams in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric the final orientation of the elongate grains is a function of their initial orientation as well as the finite strain therefore despite similar deformational histories the two zones display different ams patterns due to the differences in occurrence initial orientation and shape of ferromagnetic grains

velocity anisotropy in shales a petrophysical study using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content clay mineralogy and porosity at a wide range of effective pressure we find that elastic anisotropy of shales increases substantially with compaction the effect is attributed to both porosity reduction and smectite to illite transformation with diagenesis a means of kerogen content mapping using velocity versus porosity crossplot for shales is shown matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0 4 at total organic carbon toc 15 20 a strong chemical softening effect was found in shales containing even minor amounts of swelling smectite clay when saturated with aqueous solution this effect results in a significant p wave anisotropy reduction as compared to dry and oil saturated shales since mature black shales are normally oil wet this effect can only have a local significance restricted to the wellbore wall accurate measurements of phase velocities including velocities at a 45 direction to the bedding plane allow us to immediately calculate elastic stiffnesses and anisotropic parameters intrinsic high pressure properties of shales display an e 0 relation introduction of the bedding parallel microcracks in overpressured shales results in a decrease when fully fluid saturated and a increase when partially gas saturated with a characteristic effect on the shape of the p wave velocity surface at small angles of incidence filtering the contribution of the intrinsic anisotropy of shales it is possible to estimate the pore fluid phase microcrack density and aspect ratio parameters using seismic anisotropy measurements

comparison of quartz microfabric with strain in recrystallized quartzite abstract the relationship between quartz c axis microfabric and strain is examined in six specimens of recrystallized quartzite conglomerate in which strain was measured using pebble shapes four rocks subjected to plane strain display a direct relationship between the strength of preferred orientation and the strain intensity the c axis distributions in these rocks as well as a rock subjected to moderate extensional strain are crossed girdles with maxima near the intermediate principal strain axis and connecting girdles at acute angles to the direction of maximum shortening a rock subjected to moderate flattening strain has several maxima clustered near the direction of maximum shortening and a weak connecting girdle through the intermediate principal strain axis these results are generally similar to those of other studies comparing strain and tectonite fabrics and also with experimental and computer simulation studies of fabrics the degree of preferred orientation is related to total strain and therefore microfabrics in quartzites may be cautiously interpreted as qualitative indicators of strain intensity uncertainties are greater however for correlations of fabric patterns with shapes of the strain ellipsoid an observed increase in recrystallized grain sizes with increasing strain suggests that flow stress was lower in the more strained rocks

seismic anisotropy of shales shales are a major component of sedimentary basins and they play a decisive role in fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid lithology and pore pressure from seismic data and to understand time to depth conversion errors and non hyperbolic moveout a single anisotropy parameter thomsen s parameter is sufficient to explain the difference between the small offset normal moveout velocity and vertical velocity and to interpret the small offset avo response the sign of this parameter is poorly understood with both positive and negative values having been reported in the literature is sensitive to the compliance of the contact regions between clay particles and to the degree of disorder in the orientation of clay particles if the ratio of the normal to shear compliance of the contact regions exceeds a critical value the presence of these regions acts to increase and a change in the sign of from the negative values characteristic of clay minerals to the positive values commonly reported for shales may occur misalignment of the clay particles can also lead to a positive value of for transverse isotropy the elastic anisotropy parameters can be written in terms of the coefficients w 200 and w 400 in an expansion of the clay particle orientation distribution function in generalized legendre functions for a given value of w 200 decreasing w 400 leads to an increase in while for fixed w 400 increases with increasing w 200 perfect alignment of clay particles with normals along the symmetry axis corresponds to the maximum values of w 200 and w 400 given by w max 200 10 8 2 and w max 400 3 2 8 2 a comparison of the predictions of the theory with laboratory measurements shows that most shales lie in a region of the w 200 w 400 plane defined by w 400 w 200 w max 400 w max 200

strain and magnetic remanence abstract experimental data may be compatible with the hypothesis that a single direction of magnetic remanence rotates as a rigid marker with strains up to 40 shortening in coaxial perfect flattening x y z detailed agreement with the passive line model is relatively poor for the specimens in which remanance is carried by magnetite however for this range of strains the differences with the passive line model wettstein s equation are so slight that the latter model may be more easily employed to de strain or restore deformed remanance to its original attitude in the case of hematite bearing remanences the differences between the passive line and rigid marker model are even smaller because of the higher aspect ratios of grains of hematite therefore it is suggested that wettstein s equation may be safely used to restore remanence after even higher strains where the remanence is carried by hematite

shallow water methane derived authigenic carbonate mounds at the codling fault zone western irish sea abstract methane derived authigenic carbonate mdac mound features at the codling fault zone cfz located in shallow waters 50 120 m of the western irish sea were investigated and provide a comparison to deep sea mdac settings carbonates consisted of aragonite as the major mineral phase with 13 c depletion to 50 and 18 o enrichment to 2 these isotope signatures together with the co precipitation of framboidal pyrite confirm that anaerobic oxidation of methane aom is an important process mediating methane release to the water column and the atmosphere in this region 18 o enrichment could be a result of mdac precipitation with seawater in colder than present day conditions or precipitation with 18 o enriched water transported from deep petroleum sources the 13 c depletion of bulk carbonate and sampled gas 70 suggests a biogenic source but significant mixing of thermogenic gas and depletion of the original isotope signature cannot be ruled out active seepage was recorded from one mound and together with extensive areas of reduced sediment confirms that seepage is ongoing the mounds appear to be composed of stacked pavements that are largely covered by sand and extensively eroded the cfz mounds are colonized by abundant sabellaria polychaetes and possible nemertesia hydroids which benefit indirectly from available hard substrate in contrast to deep sea mdac settings where seep related macrofauna are commonly reported seep specialist fauna appear to be lacking at the cfz in addition unlike mdac in deep waters where organic carbon input from photosynthesis is limited lipid biomarkers and isotope signatures related to marine planktonic production e g sterols alkanols were most abundant evidence for microbes involved in aom was limited from samples taken possibly due to this dilution effect from organic matter derived from the photic zone and will require further investigation

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity

a comparison of the effectiveness of clast based finite strain analysis techniques to ams in sandstones from the sevier thrust belt wyoming abstract geological strain analysis of sedimentary rocks is commonly carried out using clast based techniques which rely on assumptions regarding clast shapes and orientations as well as distances between clast centres to evaluate the accuracy of these methods in low strain regimes we have compared strain analysis estimates to the anisotropy of magnetic susceptibility ams in sandstone samples from the wyoming salient the cordilleran mountain belt of north america is one of the world s classic foreland fold and thrust belts the sevier belt represents the thin skinned front of this orogenic event characterized by thrust faults and folds that shortened and translated sequences of devonian to cretaceous strata eastward deformation increases westward providing an ideal laboratory and geological setting to explore the potential correlation of anisotropy of magnetic susceptibility ams to clast based strain analyses studies attempting to define the relationship between ams and finite strain have been in vogue since the link between layer parallel shortening and ams was first established this relationship despite proven strong correlations between the ams tensors and tectonic directions is generally complicated by competing sub fabrics as well as the various magnetic anisotropy properties of the minerals contributing to the ams fabric even with these complications we conclude that ams is much more sensitive to incipient tectonic fabrics than clast based methods