the rotation of magnetic grains abstract it has been questioned whether magnetic remanence rotates as a rigid marker or as a passive marker with no material properties during tectonic strain the remanence of a rock is actually the sum of the moments of individual grains so we must first understand their rotation simple shear provides a simple strain history which may be used to distinguish between the two extreme possibilities a passive marker cannot rotate through the shear plane but a rigid marker can this is a useful criterion to distinguish between the two extreme models however for reasonable strains 10 the passive model is successful in explaining the rotation of these grains even though their behaviour is mechanistically closer to that of a rigid marker this explains the success of field studies in which the remanence of redbeds has been de strained using the hypothesis of passive behaviour notwithstanding the reality that the natural iron oxide grains do not rotate in that manner

paleomagnetic direction obtained by strain removal in the pyrenean permian redbeds at the col du somport france abstract in order to investigate the possibility and limitations of paleomagnetic works within strained regions a paleomagnetic study related with strain analysis has been conducted in the deformed pyrenean permian redbeds in the col du somport area paleomagnetic sampling together with strain estimates have been conducted in 6 sites through a fold the results obtained by measuring the orientation and axial ratios of elliptical reduction spots show that 1 the shale beds have undergone a penetrative strain 2 the sandy beds can be regarded as tectonically unstrained with reduction spots flattened in the bedding showing that they recorded the compaction it is shown that the total strain recorded in the slaty beds probably results from the superimposition of tectonic strain upon the compaction fabric the paleomagnetic study shows that the primary pretectonic magnetization is widely overprinted by a secondary syn or post tectonic magnetic component as both components appear to be carried by hematite pigment their separation using classical demagnetization procedures has been difficult a characteristic remanent magnetization chrm has however been determined when possible as the hardest component in demagnetization curves then the chrm direction distributions are represented in stereographic density plots although these chrm directions exhibit a clear tendency towards se declinations and shallow inclinations characteristic of permian paleomagnetic field direction for the iberian plate the tilt correction does not induce a clustering of these directions strain is inferred to be responsible for this situation assuming that both pretectonic magnetization directions and bedding planes closely follow the material plane and line strain response model of march 1 an attempt has been made to remove the effect of strain upon the remanent magnetization it is shown that when using a reconstructed tectonic strain tensor i e the total strain tensor as measured in the field corrected for an estimated compaction we obtain a significant clustering of chrm directions the computation of the relevant vgp gives a pole position 210 5 e 42 0 n compatible with the reference apwp for the iberian plate it is therefore inferred that the strain removal technique is a usable tool in order to obtain paleomagnetic results within such strained rocks

simple shear experiments on magnetized wax hematite samples c a e s s laboratoire de geophysique interne uniuersite de rennes 1 f 35042 rennes francereceived july 1 1991 revision accepted may 28 1992abstractwe present the results of a series of simple shear experiments on hematite bearing paraffin wax samples homogeneousand continuous deformation was obtained up to a shear strain value of y 1 4 by deforming the samples in atemperature controlled bath and at a constant strain rate of 9x10 5 s r during deformation hematite plateletsprogressively rotate which has been checked by measuring the anisotropy of magnetic susceptibility ams principalsusceptibility directions tend to parallel the corresponding principal strain directions and ams intensity increases withincreasing strain this supports the idea that the hematite population develops a preferred orientation by progressive rigidrotation within the paraffin matrix before each deformation step an isothermal remanent magnetization irm was given to the samples within ara finite strain plane the containing the shear direction and normal to the shear plane with various initial angles to theshear direction ranging from 0 to 180 after each deformation step the irm was measured it showed no deflectiontowards the a finite strain direction in contrast it did show a systematic rotation within the ara plane always in the samesense as the rotation of strain axes e g counterclockwise in sinistral shear furthermore irm deviation is of the sameorder of magnitude for each initial direction from 0 to 180 this contradicts the passive behaviour model that haspreviously been shown to hold for irm in coaxially deformed samples finally we show that in the shear strain range of ourexperiments y 0 to 1 4 the deviation of irm depends only on the shear value and not on its initial direction and thatthis deviation is equal to the rigid rotation angle of the shear strain tensor

mixing sedimentation rates and age dating for sediment cores abstract analytic solutions to the advection diffusion equation useful as a guide for the interpretation of mixing sedimentation rate and age dating effects in sediment cores have been derived various simplified mixing models appropriate to 14 c and 210 pb measurements have been considered the emphasis has been on a discussion of the various interrelations and dependencies among the mixing coefficient sedimentation rate and radioactive decay constant parameters in determining the 14 c and 210 pb concentrations as a function of depth porosity variations within the sediment column are also considered and the transformed advection diffusion equation including porosity effects is derived

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

charge contrast imaging of fine scale microstructure and compositional variation in garnet using the environmental scanning electron microscope gaseous secondary electron gse imaging of eclogite garnets under the environmental scanning electron microscope esem at low chamber gas pressures reveals detailed image contrast patterns charge contrast images cci that are not present in back scattered or secondary electron images image intensity is a function of the amount of surface charge accumulation successful acquisition of cci depends on frame size and beam scan rate at a given chamber gas pressure and beam current images are obtained in a few seconds and are stable and reproducible cci patterns do not correlate with cracks or grain boundaries but do correspond closely to variations in major element composition both in the form of concentric growth zoning and branching linear features interpreted as cracks that have been healed by new garnet growth causes of cci are not yet well understood but may be related to variations in lattice defect density and their influence on charge trapping and dissipation these in turn influence the rate of charge build up at or very close to the specimen surface one interesting possibility is that cci images detect vacancies related to non homovalent coupled substitutions involving for example ree and hydroxyl so the method offers a way of imaging the distribution of these trace species in garnets the cci images are rich in microstructural detail and offer the potential for rapid high resolution low noise reconnaissance mapping of intragranular microstructure and compositional variation in both natural and synthetic garnets

correcting distorted paleosecular variation in late glacial lacustrine clay abstract an undisturbed horizontal chronostratigraphic marker horizon of laminated red glacio lacustrine clay crops out over 25 000 km 2 in northern ontario canada the primary clastic hematite laminae possess two stable vector components of magnetization we sampled a 1 6 m vertical section at overlapping 2 cm intervals in cubic specimens 8 cm 3 n 106 precisely oriented in geographic coordinates which permitted measurement of inclination and declination alternating field demagnetization 12 17 steps per specimen isolated a characteristic chrm primary component coercivity 40 mt approximately 30 shallower than the mean geomagnetic field inclination at this latitude with distorted paleosecular secular variation psv a lower coercivity overprint 20 40 mt is 6 shallower than the present geomagnetic field and similarly inclined to the depositional remanence deflection when the clay was re sedimented in the laboratory we believe this angle to be representative of the inclination of the deflected primary remanence inclination during deposition caused by the magnetic anisotropy of the clay using this as a proxy for the initial inclination the chrm hard component inclinations were restored to their original values assuming vertical compaction which averages to 51 the restored inclinations are compatible with site latitude and the restored secular variation loops centre reasonably on the geographic north pole the structural correction based on homogeneous vertical shortening over simplifies the reality of heterogeneous particulate flow in which grain rotations depend on shape size and packing and effectively combines influences of compaction and depositional settlement nevertheless this correction makes the psv data more useful and interpretable in terms of paleopole migration the more logical correction technique using anisotropy of anhysteretic remanence aarm jackson m j banerjee s k marvin j a lu r gruber w 1991 detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results geophys j int 104 95 103 is foiled here due to the high coercivity of the remanence bearing grains and their unknown orientation distribution and shape distribution

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

experimental and numerical modeling of irm rotation in deformed synthetic samples abstract the effect of strain upon isothermal remanent magnetization irm carried by hematite particles embedded in a plasticine matrix has been investigated magnetized artificial hand samples were deformed by simple coaxial shortening after deformation the hand samples were cut into 8 15 specimens and each specimen s irm measured hence a mean irm direction was obtained for each shortening step the main results are 1 when the initial direction of irm is parallel to the shortening axis 3 the hand sample mean irm stays parallel to 3 during deformation however individual magnetization directions are deflected towards the flattening plane and thus the specimen directions are scattered by strain this is accompanied by a significant decrease of magnetization intensity 2 in contrast when the initial direction of irm is perpendicular to 3 the shortening results in a clustering of specimen magnetization directions the mean direction being unchanged this is accompanied by a slight increase of magnetization intensity 3 in all other cases where initial irm direction makes an intermediate angle with shortening direction deformation induces a deviation of each specimen irm toward the flattening plane this results in an overall deflection of hand sample mean irm the amount of rotation increases with strain intensity changes of grouping and intensity depend on the initial angle between irm and 3 defining a continuum in behavior between irm directions being either parallel to 3 or perpendicular to 3 finally it is shown that all the experimental changes in irm due to deformation direction grouping intensity can be modeled by a passive marker rotation of hematite planar particles

mathematical model relationship between the paramagnetic anisotropy and strain in slates discussion discussion sur l article de hrouda f mathematical model relationship between the paramagnetic anisotropy and strain in slates tectonophysics p 323 327

passive rotation of hematite during deformation a comparison of simulated and natural redbeds fabrics abstract a simulation model is used to generate pole figures assuming strain induced passive rotation of hematite crystals comparison of simulated and natural hematite fabrics allows us to distinguish between dominant passive rotation and other grain scale deformation mechanisms two natural examples are discussed redbeds from the french maritime alps and pyrenean redbeds fabrics indicate that passive rotation is the dominant reorientation mechanism of primary hematite grains during progressive deformation of redbeds

evaluation of paleomagnetic and finite strain relationships due to the alleghanian orogeny in the mississippian mauch chunk formation pennsylvania 1 three localities of the mississippian mauch chunk formation around the pennsylvania salient broad top synclinorium minersville syncline and a sublocality in the minersville syncline were sampled for detailed paleomagnetic and strain analysis paleomagnetic analysis showed three magnetization components an intermediate temperature b component t b 620 c and an sw component unblocked over a wide range of temperatures and observed only on the northern segment of the salient normalized fry strain and r f strain were measured in three dimensions and correlated to the paleomagnetic results at one site jtsf locality a clear correlation between the amount of strain e s and remanence direction indicates that axial planar cleavage formation has caused remanence rotation the correlations at the minersville syncline and broad top synclinorium are not definitive but suggest that different mechanisms are operating the observed heterogeneity in paleomagnetic response to strain indicates that the effects will be strongly dependent on the strain history at a particular site

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

interpretation procedures for the determination of sediment parameters from time dependent flux inputs abstract a formulation for the effects of a time dependent input flux to a sedimentation system consisting of a diffusive layer underlain by a non diffusive medium has been developed the intent is to provide interpretation procedures for the determination of sedimentation rate diffusion coefficient and mixed layer thickness from observed tracer concentrations they have been applied for illustrative purposes to published137cs and239 240pu measurements

the relationship of strain and preferred orientation of phyllosilicate grains in rocks a review abstract the calculation of strain from the preferred orientation of platy phyllosilicate grains taking strain to be homogeneous is surprisingly often successful from a pragmatic point of view although numerous theoretical objections to this procedure can be and have been raised for the calculation to succeed the original distribution of mica or clay mineral grains must be known or assumed the usual assumption that the starting distribution was random and therefore nearly uniform implies that the strain history of pelitic sediments is taken to start when the watery sediment was last stirred by waves or bioturbation or if it was laid down in still anoxic waters from the moment of first deposition it therefore includes compaction as part of the strain history which is usually neglected by structural geologists originally random distribution of clay flake orientations has been observed in recently deposited sediments in many instances and has been theoretically explained in various ways exceptions probably exist but their frequency has not been estimated biologically disturbed sediments may be brought nearer to a state of random orientation of clay flakes than they were at deposition with the exception of whole beds or concretions in which pores are early filled with cement mudrocks are almost always compacted by the weight of later deposits the strain so induced is axially symmetric about the vertical and thus about the pole of the bedding planes compaction acts on soft unconsolidated rocks but may continue with increasing overburden on rocks strengthened by cementation and by increasing interlocking of grains tectonic deformation may start early before the rock is fully compacted or it may begin only much later and the mechanisms of deformation increase in complexity with the increasing temperature and hydrostatic stress of deeper burial up to a certain level of mild metamorphism however phyllosilicate grains seem to be rather permanent if not as mineral species then at least by preservation of their constituent sheets of silica tetrahedra given this permanence the mechanisms of rock deformation whether intergranular slip cataclasis or dissolution and reprecipitation and various intragranular processes in the non phyllosilicate matrix hardly matter as far as the recording of strain is concerned even a certain amount of stress solution at especially heavily loaded points of the phyllosilicate grains themselves does not destroy their value as strain markers as long as the dissolved material is disposed of mainly by crystallographically continuous overgrowth on existing grains the same holds for local dissolution of phyllosilicates in portions of old grains that have become thermodynamically unstable by bending widespread irregular or random nucleation and growth of newly nucleated phyllosilicate grains at intermediate or high grades of metamorphism can reduce the reliability of the strain record and so can domainal structure of the deformed rock if both the volume fraction of phyllosilicate grains and the intensity of total strain differ sharply from one domain to another if distinct domains are not equant and have themselves a strong preferred orientation of shape nevertheless rocks containing an anastomosing network of zones rich in phyllosilicate grains as slates and other deformed mudrocks commonly do have in many cases been found to have average preferred orientations of mica or chlorite which if used to estimate the strain give results similar to those obtained from independent strain markers in the same rocks thus the most significant verification of the usefulness of strain estimates by measurement of preferred orientation is the mutual confirmation with independently determined strain the limit to such verification is the rarity of strain markers so that the coexistence of two independent ones is exceptionally rare for this reason however it is important to make full use of the orientation of phyllosilicate grains where they are available the alternative to have no strain measurements at all causes structural geologists to miss potential information

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

mathematical model relationship between the paramagnetic anisotropy and strain in slates abstract a simple mathematical model is used to investigate the relationship between the paramagnetic anisotropy and strain in slates the relationship found is similar to the relationship of whole rock magnetic anisotropy to strain in weakly magnetic slates the anisotropy is probably mostly due to paramagnetic phyllosilicates and the whole rock relationship reflects the paramagnetic fraction

strain magnetic fabric and paleomagnetism of the deformed red beds of the pont rean formation brittany france a joint study of strain anisotropy of magnetic susceptibility ams and paleomagnetism has been conducted in the deformed red beds of the ordovician pont rean formation brittany france analysis of results from 13 sampling sites shows that homogeneous strain estimated from the shape of elliptical reduction spots in each site controls the development of a magnetic fabric during the hercynian folding of this formation the good correlation between strain and ams tensors is interpreted as mainly arising from a strain induced progressive rotation of planar hematite that tends to align within cleavage plane this rotation of magnetic carriers in turn induces a rotation of net measurable pretectonic remanent magnetization toward cleavage because of the high angle between the initially subvertical ordovician remanent magnetization and the regionally subhorizontal shortening direction this deviation of the pretectonic magnetization is achieved without any scattering but with a probable clustering effect upon within site initial dispersions for the same reason the overall distribution of mean site pretectonic magnetization vectors is clustered at the in situ deformed state this results in an anomalous behavior of the classical fold test that could erroneously lead to assignment of a syntectonic to post tectonic age to the magnetizations finally the strain removal technique based upon the passive behavior hypothesis is applied to the deviated vectors it allows recovery of the initial direction of these pretectonic ordovician magnetizations d 235 i 75 95 6 5

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

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

the effects of grain scale deformation on the bloomsburg formation pole previous paleomagnetic results from the silurian bloomsburg formation in the central appalachian valley and ridge province suggest that the bloomsburg s characteristic magnetization is a synfolding magnetization acquired during the acadian orogeny however there is little geological evidence to support extensive acadian folding in the central appalachians a more favorable explanation is that the characteristic magnetization originated as a prefolding silurian magnetization and has been altered by grain scale deformation to mimic a synfolding devonian remagnetization in order to investigate whether penetrative deformation has altered the bloomsburg formation s characteristic signal the relationship between strain and remanence was examined around three central appalachian valley and ridge folds structural analysis of mesoscopic and microscopic features along with center to center finite strain analysis at these three folds indicates a deformation sequence that includes prefolding layer parallel shortening overprinted by top to the foreland bedding parallel shear and flexural slip flow folding inclination of the characteristic magnetization varies systematically with the magnitude of finite strains this relationship between strain and remanence suggests that the characteristic magnetization has been progressively rotated toward shallower inclinations by penetrative deformation analogous to rigid particle rotation in a viscously deforming matrix using directions from sites with relatively low finite strain values a new bloomsburg formation pole is calculated at 18 7 n 107 4 e k 58 4 a95 8 2 this new pole falls on the silurian track of the north american apparent polar wander path between the silurian wabash reef pole and the silurian devonian andreas pole in addition these results suggest that previously reported 20 30 difference in declination of site mean directions north arid south of the pennsylvania salient may also be the result of grain scale reorientation of the remanence carrying grains and not oroclinal bending

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

modeling reactive transport in sediments subject to bioturbation and compaction current bioturbation models are marked by confusion in their treatment of porosity different equations appear to be needed for different biodiffusion mechanisms i e interphase mixing where biological activity causes bulk mixing of sediment affecting both tracer and porosity profiles versus intraphase mixing where the solid components are intermixed but the porosity is left unchanged another issue is whether the model depends upon the particle type with which tracers are associated e g 137 cs on small clay particles versus 210 pb on larger grains this uncertainty has lead to conflicting conservation equations for radiotracers and in particular to the question whether the porosity should be placed inside or outside of the differential term that governs the biodiffusive flux we have reexamined this situation in the context of multiphase multicomponent continuum theory most importantly we prove that under the assumption of steady state porosity there exists only one correct form of the steady state conservation equation for a radiotracer regardless of biodiffusion mechanism and particle type i e

flow directions of carboniferous ignimbrites southern new england orogen australia using anisotropy of magnetic susceptibility abstract anisotropy of magnetic susceptibility ams is used to define regional flow patterns and to locate possible source vents of carboniferous ignimbrites in the rocky creek region northern tamworth belt southern new england orogen the major axes of ams fabrics from 115 sites are oriented in mainly eastward or northeastward directions and the fabrics can be classified into three groups normal inverse and intermediate comparisons between flow directions indicated by ams fabrics and the alignment of feldspar laths in oriented thin sections from major ignimbrite units indicate a moderate to good level of agreement the contoured intersections of flow lineations indicate a number of possible source regions such as paleo valleys which controlled the dispersal of particular flows when all trends from contoured intersections of flow lineations are combined two major trends emerge ne sw and e w these trends converge within an area approximately 20 km west of the mooki thrust at the overthrust margin of the orogen this convergent area is proposed as a possible major volcanic source region that may have existed during the late carboniferous because of westward overthrusting with a magnitude greater than 20 km during the late permian to early triassic this region is now probably located beneath carboniferous forearc basin rocks of the tamworth belt

deformation and magnetic fabrics in ductile shear zones a review abstract the anisotropy of magnetic susceptibility ams is a well established petrofabric tool for indicating relative strain and microstructural character and has been validated on various rock types and different structural settings the magnetic susceptibility of a rock k depends primarily on the nature and abundance of magnetic minerals the physical arrangement and lattice preferred orientation of these magnetic minerals give rise to magnetic anisotropy the ams scalar parameters most commonly used to constrain strain include the corrected degree of anisotropy p 1 a proxy for fabric intensity and the shape factor 1 t 1 an indicator of the magnetic fabric symmetry prolate vs oblate a number of studies have shown that a positive correlation generally exists between p and strain thus the ams shows a great potential as a tool for examining deformation in geologic structures characterized by large strain gradients such as shear zones however a number of caveats exist i the increase of p with strain cannot be solely attributed to deformation because p also increases with k regardless of deformation ii strain across shear zones is typically heterogeneous and is often localized in units of different lithology thus making the separation of the lithological and strain controls on ams difficult also deformation is commonly accompanied by mineral segregation or fluid rock interaction that induces changes in magnetic mineralogy iii even if the undeformed lithology was uniform across a shear zone variations in strain rate or temperature may result in different deformation mechanisms hence the relationship between p and strain depends strongly on both the mineral carriers of ams and on deformation mechanisms and iv the ams is unable to resolve composite fabrics such as those resulting from s c structures where minerals on the c and s planes respectively contribute to ams

morphological analysis of archetypal calcite cement archetypal cements from their deposition to the time they are sampled preserve a record of subsurface fluid evolution that can cover many millions of years their morphological development after nucleating on fissure walls is simulated by graphical models showing maturation from isolated to competitive to parallel growth graphical models are not available for the cementation of pores that are more complex and diverse the reconstruction of cement growth in pores is accomplished here using multiple cathodoluminescent cl growth zones sediment pores are filled in two stages separated by a cementation threshold associated with a rapid drop in permeability pre threshold cement is centripetal ordered regularly distributed and nurtured by a megapore network post threshold cement is disordered sporadically distributed and nurtured by labile micropore connections the crystallographic form of euhedral crystals at cement s growing front has environmental significance while the anhedral shape of aggregate crystals it leaves behind is controlled by geometrical selection growth of seeded cement is divided into epitaxial and mantle stages syntaxial when these two are not identified the epitaxial stage begins with a seed at multiple points then coalesces simplifies and morphs towards the wulff or equilibrium surface the extent of epitaxial growth is correlated with the size of crystals forming pore walls increments of mantle growth can be initiated over the whole surface of a pre existing crystal or can be restricted to its edges and or corners restricted seeding is often linked with changes of crystal habit resulting in the reorientation of the crystal s fastest growth direction and the direction of maturation some impingement intercrystalline boundaries have offset growth zones explained by dissolution such a self cannibalization process is new the existing explanation that some enfacial junctions are due to pauses in growth is confirmed by the arrangement of growth zones around triple points but a new type is recorded formed from continuously growing crystals whose intercrystalline boundaries become modified the presence of the cement s internal growth surfaces allow its development to be reconstructed providing the foundation for further cement studies however one example is presented where the disproportionate filling of pores in adjacent millimeter thick layers is obscure because its crystals lack internal growth surfaces

how to overcome imaging problems associated with carbonate minerals on sem based cathodoluminescence systems on sem based cathodoluminescence systems scanned cl methods are needed to overcome image quality problems caused by persistent luminescence of carbonate minerals an effective solution to the persistence problem is to acquire images using only the shorter wavelengths most easily done by using a broadband short wavelength uv blue range filter the filter used provides 80 to 90 transmissivity in the range of 385 to 495 nm and some transmissivity as low as 350 nm the filter allows transmission of the relatively nonpersistent uv violet luminescence present in most carbonates in the range of 350 to 425 nm but blocks the common orange red wavelength luminescence found in carbonates the lack of imaging problems in subsequent images shows that persistent luminescence in carbonates is primarily in the orange red wavelengths cathodoluminescence images produced using a short wavelength filter are comparable in detail to those obtained from conventional light microscope based cathodoluminescence systems in almost all examples features visible in the orange red wavelengths show corresponding variations in luminescence in the shorter wavelengths

identifying biogenic silica mudrock micro fabric explored through charge contrast imaging visual inspection optical microscope point counting and silica abundance show that laminated shale from the late cretaceous of colombia contains high levels of detrital quartz silt and sand particles closer examination using the charge contrast imaging cci technique however illustrates that much of the quartz is authigenic micro quartz and thus not exclusively of detrital origin in addition many sand grains that otherwise appear to represent simple detrital quartz particles are actually of biogenic origin representing the tests of agglutinated foraminifera formed from cemented silt sized quartz particles finally cci shows that original detrital grains have undergone authigenic modification with both syntaxial overgrowths and micro quartz without recognition of these features the relative proportion of detrital quartz sand would otherwise be greatly overestimated with important implications for environmental interpretation furthermore the recognition of biogenic structures including agglutinated foraminifera provides additional environmental information that otherwise could be easily overlooked

pencil structure and strain in weakly deformed mudstone and siltstone abstract weakly deformed mudstone and siltstone middle ordovician knobs formation of the appalachian valley and ridge province south western virginia u s a show strain dependent transitions between bedding fissility pencil structure and cleavage pencil structures are associated with a bulk inhomogeneous shortening deformation where minimum principal strain z ranges between 9 and 26 shortening assuming a plane strain and constant volume where strains are less bedding fissility dominates pencil fragments are defined by intersecting fracture sets subparallel to the pre existing bedding fissility and cleavage their long axes are both parallel to the bedding cleavage intersection and to the inferred y axis of the tectonic strain ellipsoid pencil development is considered to result from fracturing along both fabric anisotropies during weathering and post tectonic stress relaxation pencils show variations in size and shape depending on lithology grain size and composition degree of initial clay preferred orientation degree of cleavage development type of cleavage total bulk strain and degree of strain homogeneity their shape l w however is a direct measure of total y z strain since strain determinations from chlorite pressure fringes on framboidal pyrite within the pencil fragments give y z 0 913 0 019 l w pencil structure is therefore a potential strain marker in weakly deformed rocks

imaging of cathodoluminescence zoning in calcite by scanning electron microscopy and hyperspectral mapping abstract imaging of cathodoluminescence cl emission from carbonate minerals by scanning electron microscopy sem is problematic owing to the slow rate at which cl decays from each point as the electron beam is scanned over an area the phenomenon of phosphorescence using sem cl and a newly developed electron probe based technique of hyperspectral mapping we have evaluated methods that have been proposed to overcome phosphorescence with the dwell time technique the duration of time that the electron beam is static is increased such that cl emission from a given point makes a negligible contribution to the total signal from subsequent points the dwell time required to form sharp sem cl images of calcite that has a high intensity of luminescence at orange wavelengths is equal to or greater than 6 4 milliseconds whereas calcite that luminesces predominantly at ultraviolet to blue wavelengths can be imaged using submillisecond dwell times by using longer dwell times 1000 ms cl and x ray spectra can also be acquired from each point to form hyperspectral maps the limited wavelength imaging technique employs optical filters to excise slowly decaying long wavelength emission so that the image is formed only using the more rapidly decaying ultraviolet to blue wavelengths allowing shorter dwell times to be used both techniques have some disadvantages the main drawback of the dwell time technique is the long period of time required to acquire high resolution images whereas the success of limited wavelength imaging may depend on the sensitivity of the cl detector being used cl images and emission spectra acquired by hyperspectral mapping also show that there is a nonlinear relationship between luminescence intensity variations at ultraviolet to blue wavelengths and intensity variations at orange wavelengths indicating that short wavelength emission is an imperfect proxy for zoning at longer wavelengths additionally we have been unable to identify the controls on spatial variations in the intensity of ultraviolet to blue luminescence although our data discount fe2 concentrations as being the sole determinant by using hyperspectral mapping in combination with electron probe microanalysis to obtain cl and x ray spectra from the same micrometer sized volume of a material it is now possible to understand and quantify the controls on activation and quenching of cl in calcite and other minerals such as apatite and zircon

a model for radionuclides in sediments influenced by mixing and compaction a model for the activity of radionuclides in sediments has been set up based on the one dimensional advection diffusion equation the model considers sedimentation mixing and compaction observed distributions of deposit feeding organisms and the nature of physical disturbances indicate that a gaussian diffusion coefficient may be appropriate two layer models with constant and gaussian mixing respectively in the upper layer are compared with measured pb 210 profiles for two cores from green bay lake michigan and it is found that the maximal diffusion coefficient d0 in the gaussian model is considerably greater than the constant d value e g d0 4 versus d 0 2 cm2 year if compaction effects are neglected the solution for the gaussian model is based on an approximate power series method compaction can be taken into account by using an exponential sedimentation rate or bulk sediment density by comparing with measured data for a lake ontario core it is seen that compaction alone can give an appreciable flattening of the activity curves near the top layers

charge contrast imaging cci revealing enhanced diagenetic features of a coquina limestone abstract charge contrast imaging cci is a low vacuum scanning electron microscope lv sem technique that can be induced through partial surface charge suppression of uncoated nonconductive samples imaged with a suitable detector such as a gaseous secondary electron detector gsed the technique commonly produces results similar in style to that of sem cathodoluminescence sem cl providing information on zoning twinning annealed fractures and subtle chemical changes the current work outlines an example from a brazilian lower cretaceous coquina limestone in which both optical and sem cl imaging produces a limited response from much of the sample backscattered electron bse imaging typically suggests only a hint of the cement present whereas cci clearly displays a rich and varied cement stratigraphy the earliest cement displays strong cci but appears mainly dark under cl imaging conditions sem cl and optical cl later stage manganese enriched carbonate cement displays luminescence with both optical and sem cl as well as a cci response therefore cci can provide additional information on cement zonation in an area where cl cannot

flexural flow folding and the paleomagnetic fold test an example of strain reorientation of remanence in the mauch chunk formation the relationship between the remanent magnetization and the detailed strain geometry around a first order fold in the appalachian valley and ridge province was investigated to examine whether penetrative strains associated with folding can generate a apparent synfolding geometry from a prefolding magnetization paleomagnetic results from the mississippian mauch chunk formation on both limbs of the frackville anticline near lavelle pennsylvania yield two magnetic components an intermediate unblocking temperature 300 c 600 c kiaman remagnetization and a two polarity high unblocking temperature 650 c 680 c characteristic magnetization when the magnetic directions are incrementally corrected for bedding tilt the intermediate temperature component is most tightly clustered at 85 unfolding d 176 i 3 and the high temperature component is most tightly clustered at 75 unfolding d 184 i 27 mesoscopic and microscopic structural fabric analyses suggest a strain history that includes a significant component of flexural slip flow folding in the coarser grained sandstone units folding has largely been accommodated by slip on bedding while in the finer grained beds folding has been accommodated by grain scale deformation mechanisms such as pressure solution and low temperature plasticity finite strain measurements determined from center to center distances between quartz grains yield strain ellipsoids consistent with this folding model inclination of the characteristic component varies as a function of the magnitude of the finite strain this variation suggests that the characteristic magnetization has been systematically reoriented with respect to bedding during folding remanence directions on the south dipping limb have been rotated to shallower inclinations while those on the north dipping limb have been rotated to steeper directions causing the prefolding magnetization to appear synfolding these rotations are in agreement with models of rigid particle rotation in deforming viscous media unlike the characteristic magnetization the secondary component appears to be unaffected by the deformation and its synfolding behavior is interpreted as the acquisition of a secondary magnetization during alleghenian folding these results show that it is important to consider penetrative strains when evaluating the significance of apparent synfolding magnetizations

modelling the relationship between magnetic fabric and strain in polymineralic rocks abstract we present a model that is applicable to the relatively frequent case of rocks in which the magnetic fabric is dominated by uniaxial paramagnetic minerals and in which the deformation pure shear corresponds to the march fernandez model borradaile et al s procedure for the isolation of the magnetic fabric of monocrystals of anisotropic components allows us to obtain the magnetic properties equivalent to those of all the component minerals if they should be artificially aligned in the case of real component minerals with similar shape parameters dependent on dimension ratio these properties will correspond to those of a theoretical equivalent mineral therefore finite strain and orientation tensors may be determined from magnetic fabric measurements of such polymineralic rocks