magnetic mineralogy of pelagic limestones summary due to their extremely small size and low concentration the carriers of magnetization in limestones are usually identified through indirect rock magnetic methods this paper describes a method for extracting tiny ferromagnetic grains from consolidated calcareous rocks and summarizes a scanning electron microscope sem investigation of magnetic phases from several palaeomagnetically important pelagic limestones details of surface characteristics and elemental composition are readily accessible with the sem and complement normal rock magnetic experiments in giving a more complete assessment of the major ferromagnetic phases their origin and the timing and acquisition of magnetization the results confirm many indications of the indirect methods and introduce several important new features in addition to detrital titanomagnetite diagenetic titanohaematite and iron sulphides there is a significant contribution by authigenic microcrystalline haematite detrital cosmic spherules chromian spinels and skeletal titanomaghaemite

magnetic fabrics of the south dabashan belt at the north margin of south china constraint on the origin of the arcuate trace of chengkou fault abstract the south dabashan belt at the northern margin of the south china block is a typical arcuate shaped orogenic belt in western china its formation and evolution are important for understanding the collisional orogeny and intracontinental deformation in the south qinling orogen and its foreland basin anisotropy of magnetic susceptibility ams can provide fabric patterns which can be used to discuss the strain of the south dabashan belt rock magnetism reveals that paramagnetic minerals dominate magnetic susceptibility and some ferromagnetic minerals also exist magnetic fabrics of triassic and jurassic strata are essentially type ii fabric with well developed magnetic lineation further they rarely show sedimentary fabric indicating the early stage of the deformation processes magnetic fabrics show that early horizontal shortening strain is characterised by a radial pattern in the south dabashan belt our results indicate that the chengkou fault initially formed as a normal fault with arcuate shape during the opening of the mianlue ocean and has a controlling effect on the formation of the curved fold and thrust belt then the later shortening strain overprints the previous deformation but barely changes the curvature of the chengkou fault

quantitative correlation of texture and magnetic anisotropy of compacted calcite muscovite aggregates abstract deformation of carbonate rocks leads to crystallographic preferred orientations cpos of calcite which in turn generates an anisotropy of magnetic susceptibility ams the use of the diamagnetic anisotropy which arises from the preferential orientation of calcite for texture characterization is usually hindered by 1 the weakness of the anisotropy and 2 overlapping contributions from paramagnetic and ferromagnetic minerals to the ams in this study the diamagnetic paramagnetic and ferromagnetic sub fabrics of synthetic calcite muscovite samples have been separated using high field torque measurements the cpo of calcite and muscovite has been determined by neutron diffraction and the magnetic sub fabrics are compared to the ams that was re calculated from the cpo the isolated diamagnetic sub fabric shows a good correlation with the calcite texture for calcite concentrations above 70 the paramagnetic sub fabric is shown to be a reliable texture parameter for muscovite even at a concentration of 5 the results demonstrate the capability of the ams method as a texture parameter in impure carbonate rocks

comment on interplay of deformation and magmatism in the pangong transpressional zone eastern ladakh india implications for remobilization of the trans himalayan magmatic arc and initiation of the karakoram fault by k sen b k mukherjee and a s collins journal of structural geology 62 2014 13 24

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

magnetic susceptibility as a tool to study deformed calcite with variable impurity content 1 a set of calcite mylonites were systematically sampled along the basal shear zone of the morcles nappe located in the southwestern swiss alps to investigate the relationship among crystallographic preferred orientation cpo chemistry and anisotropy of magnetic susceptibility ams second phase minerals are 10 vol of the rock the substitution of trace impurities for ca in the calcite crystal lattice in particular fe mn mg and sr varies among the samples with total values ranging from 1450 190 ppm in the most pure sample to 2450 320 ppm for the most impure sample fe shows the largest variation among the sample group and varies by a factor of nearly four from the purest 230 30 ppm to the most impure sample 840 110 ppm all samples have negative bulk susceptibility and the concentration of ferromagnetic in a broad sense phases is very low a correlation is identified between trace element chemistry cpo of calcite and the magnetic susceptibility the bulk susceptibility and the ams vary systematically with fe and mn elemental concentration in the matrix calcite and this variation is apparent for the entire length of the shear zone a simple model that takes into account the cpo and trace element chemistry is used to explain the resulting bulk susceptibility and ams calcite mylonites and marbles from other locations fit the model indicative of a general relationship between cpo chemistry and ams in calcites where second phase minerals are sparse for lithologies that are dominated magnetically by calcite with a deficiency of secondary phases magnetic susceptibility can be used to rapidly screen rock samples for the variation of paramagnetic trace elements fe and mn and cpo which can then be used for detailed microfabric studies

methods and applications of magnetic anisotropy a special issue in recognition of the career of graham j borradaile methods and applications of magnetic anisotropy a special issue in recognition of the career of graham j borradaile

the influence of deformation mechanisms on magnetic fabrics in weakly deformed rocks abstract magnetic fabrics of weakly deformed rocks are not simply correlated with the principal finite strain directions depositional magnetic fabrics persist at 30 shortening where the rocks are deformed by particulate flow grain boundary sliding rocks with a spaced stylolitic cleavage believed to be formed by pressure solution and shortened by 20 50 have maximum susceptibility parallel to the intersection of cleavage and bedding this type of magnetic fabric is attributed to the interference of a depositional magnetic fabric between cleavage surfaces with a tectonic magnetic fabric in the cleavage surfaces

evolution of soil properties on stabilized sands in the tengger desert china abstract the spatial and temporal patterns of pedogenesis on stabilized dunes at shapotou northwestern china were studied on the time sequences of 0 18 35 and 43 years the spatial pattern of soil formation was estimated by measuring the thickness of accumulated sand fractions on the stabilized dune surface and by analyzing the characteristics and properties of soil the results showed that the environment of soil formation and circulation of soil material were influenced in the processes of shifting sand fixation and the mean soil particle size changed from 0 2 to

biogenic magnetite as a primary remanence carrier in limestone deposits abstract studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at sugarloaf key fl u s a and calcareous laminated sediments from laguna figueroa baja california mexico have revealed the existence of magnetotactic bacteria and ultrafine grained single domain magnetite in both environments magnetotactic bacteria were identified by light and electron microscopy the single domain magnetite was detected by coercivity spectra analysis with a squid magnetometer and examined under the transmission electron microscope the similarity in terms of size and shape between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine grained magnetite in these two marine environments was biologically formed these results combined with the common occurrences of ultrafine grained magnetite in limestone deposits detected rock magnetically suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks several cambrian limestone samples separately collected from siberia china and kazakhstan were examined for the presence of bacterial magnetite samples from the lower cambrian sinskian formation at siberia platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component post cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them assessing the potential role of biogenic magnetite as a primary remanence carrier in other phanerozoic limestone deposits ought to be further pursued

inverse magnetic fabric in carbonate bearing rocks abstract the observation of a magnetic susceptibility ellipsoid whose maximum axis corresponds to the minimum axis of petrofabric pole of bedding or schistosity is referred to as an inverse magnetic fabric the investigation of the magnetic properties of some ferroan carbonate monocrystals and paramagnetic limestones demonstrates that c axis preferred orientation of paramagnetic carbonates results in a maximum susceptibility parallel to the flattening direction inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones a mineralogical model based on the property of single domain grains to have a zero susceptibility parallel to their long axis is proposed however more complex cases are also encountered

magnetic fabrics of the morcles nappe complex abstract anisotropy of magnetic susceptibility ams measured with high fields at ambient and low temperatures 77 k has been used to separate ferromagnetic sensu lato paramagnetic and diamagnetic sub fabrics for deformed limestones located in a nappe structure from southwest switzerland the magnetic fabrics are dominantly controlled by paramagnetic and diamagnetic minerals and ferromagnetic minerals contribute significantly only at two of the eight study locations separation of the magnetic sub fabrics is possible in the root zone and overturned limb of the morcles nappe which contains a high ratio of diamagnetic matrix calcite to paramagnetic secondary phases each of the sub fabrics displays a relationship with structural cleavage and the crystallographic preferred orientation cpo of calcite in the root zone and overturned limb the maximum axis of the paramagnetic ams ellipsoid kmax lies close to the pole to cleavage with the intermediate kint and minimum kmin axes near the cleavage plane conversely the diamagnetic kmin lies close to the pole to cleavage with kmax and kint axes near the cleavage plane slight offsets in the poles to cleavage and in the principal ams directions are observed they are interpreted to be the consequence of a second phase deformation event affecting only the cpo but not the macroscopic structural elements the diamagnetic sub fabric reflects the cpo of calcite produced through recrystallization during nappe deformation iron rich calcite appears to control the paramagnetic sub fabric susceptibility ellipsoids approach oblate shapes for the diamagnetic sub fabric with the most extreme shapes reaching susceptibility differences k kmax kmin close to that of single crystal calcite whereas the paramagnetic sub fabric is represented mainly by prolate shape ellipsoids the low field ams ellipsoids show mixed oblate and prolate shapes without consistency within or between sites it is evident that when the magnetic sub fabrics of the root zone and overturned limb are separated their individual shapes and distribution of principal directions mirror the developed cpo

paleomagnetic dating of fracturing using breccia veins in durness group carbonates nw scotland abstract a paleomagnetic study of red fault related breccia veins in the cambro ordovician durness group in nw scotland was conducted to determine the time of brecciation the origin of the veins and the nature and timing of associated fluid related diagenetic alteration the veins contain brecciated fragments of the host durness group and strike either east west or north south clasts of breccia cemented by calcite suggest multiple brecciation events the host durness limestone is a gray dolomite and contains a devonian chemical remanent magnetization crm that resides in magnetite the veins contain magnetizations that reside in hematite and are interpreted as crms the breccias in north south veins contain a triassic crm whereas the veins with east west strikes contain a jurassic crm authigenic hematite is common in the breccias along growth planes in the calcite cements the two crms within the veins are interpreted as dating two separate brecciation and fluid flow events that precipitated authigenic hematite the brecciation and fluid flow events are interpreted to be related to extension in the mesozoic which is consistent with the extensional history of the northern atlantic margins

long term effects of restoration on soil hydraulic properties in revegetation stabilized desert ecosystems 1 improving structure of soils found at 0 5 cm depth and increasing the thickness of biological soil crusts are both associated with sand dune revegetation stabilization in arid northwestern china since 1956 research on sand dune stabilization has included the use of straw chequer boards to facilitate development of soil structure one method to gauge the degree of stabilization is to compare undisturbed soil hydraulic properties including water retention h and hydraulic conductivity k h functions with properties from stabilized sites of different ages this study examined properties at five experimental sites of different ages since revegetation 51 42 34 20 and 0 years saturated hydraulic conductivity ks was determined in situ from all five sites and the h curve was determined from samples collected from three sites a significant negative correlation existed between ks and the clay silt and organic matter contents differences in most van genuchten parameters for h were observed between the revegetated plots and the migrating sand dune area results of this long term study show that changes in soil hydraulic properties and improvement in soil structure were associated with migrating dune stabilization

orthogonal switching of ams axes during type 2 fold interference insights from integrated x ray computed tomography ams and 3d petrography abstract we applied x ray computed microtomography ct in combination with anisotropy of magnetic susceptibility ams analysis to study metamorphic rock fabrics in an oriented drill core sample of pyrite pyrrhotite quartz mica schist the sample is extracted from the paleoproterozoic martimo metasedimentary belt of northern finland the ct resolves the spatial distribution shape and orientation of 25 920 pyrrhotite and 153 pyrite grains localized in mm thick metapelitic laminae together with microstructural analysis the ct allows us to interpret the prolate symmetry of the ams ellipsoid and its relationship to the deformation history ams of the sample is controlled by pyrrhotite porphyroblasts that grew syntectonically during d1 in subhorizontal microlithons the short and intermediate axes k3 and k2 of the ams ellipsoid interchanged positions during a subsequent deformation d2 that intensely crenulated s1 and deformed pyrrhotite while the long axes k1 maintained a constant position parallel to the maximum stretching direction however it is likely that all the three ams axes switched similar to the three principal axes of the shape ellipsoid of pyrite porphyroblasts from d1 to d2 the superposition of d1 and d2 produced a type 2 fold interference pattern

anisotropy of diamagnetic susceptibility in thassos marble a comparison between measured and modeled data abstract a study of shear zones within the calcite marble complex of the island of thassos greece shows that the low field anisotropy of magnetic susceptibility ams technique can be successfully applied to diamagnetic rocks for characterizing rock fabrics the strain path involves both an early pure shear stage and a simple shear overprint that is documented by a transition from triaxial neutral to uniaxial prolate shapes of ams ellipsoids the maximum susceptibility is oriented perpendicular to the rock foliation reflecting the preferred orientation of calcite c axes in the protolith as well as in the mylonites for three samples that represent different types of calcite fabrics the ams was recalculated from neutron and electron backscatter diffraction textural data a comparison of the measured and modeled data shows a good coincidence for the orientation of the principal ams axes and for the recalculated anisotropy data both measured and modeled data sets reflect the change from neutral to distinct prolate ellipsoids during progressive deformation

magnetic fabrics in the basal ice of a surge type glacier 1 anisotropy of magnetic susceptibility ams has been shown to provide specific useful information regarding the kinematics of deformation within subglacially deformed sediments here we present results from debris rich basal glacier ice to examine deformation associated with glacier motion basal ice samples were collected from tunabreen a polythermal surge type glacier in svalbard the magnetic fabrics recorded show strong correlation with structures within the ice such as sheath folds and macroscopic stretching lineations thermomagnetic low temperature susceptibility varying field susceptibility and isothermal remanent magnetism acquisition experiments reveal that the debris rich basal ice samples have a susceptibility and anisotropy dominated by paramagnetic phases within the detrital sediment sediment grains entrained within the basal ice are inferred to have rotated into a preferential alignment during deformation associated with flow of the glacier an up glacier directed plunge of magnetic lineations and subtle deviation from bulk glacier flow at the margins highlight the importance of noncoaxial strain during surge propagation the results suggest that ams can be used as an ice petrofabric indicator for investigations of glacier deformation and interactions with the bed

anisotropy of magnetic susceptibility measurement schemes the precision of ams determination is enhanced by measuring susceptibility in directions with a uniform orientation distribution that include the four body diagonals some standard 10 5 cm samples with mean susceptibility 100 si possess too few magnetic grains for reliable petrofabric interpretation whatever the measurement strategy we should only interpret their ams if they pass fabric homogeneity tests

inverse magnetic fabric in deformed limestones of the morcles nappe switzerland anisotropy of magnetic susceptibility ams was measured at 15 sites in deformed helvetic limestones of the morcles nappe in western switzerland inverse magnetic fabrics characterized by axes of maximum susceptibility subperpendicular to bedding or cleavage were observed at four sites in the root zone of the nappe acquisition and thermal demagnetization of isothermal remanent magnetization show that the ferromagnetic mineralogy is dominated by pyrrhotite with various proportions of magnetite hematite and goethite the matrix minerals were studied in thin section with a staining technique to distinguish different carbonates by x ray texture goniometry and by microprobe analysis observations of ams at room and liquid nitrogen temperatures and of the temperature dependence of bulk susceptibility lead to the conclusion that the inverse fabrics result from a paramagnetic component due to fe2 ions in the calcite matrix

anisotropy of magnetic susceptibility ams and diamagnetic fabrics in the durness limestone nw scotland anisotropy of magnetic susceptibility ams and diamagnetic fabrics in the durness limestone nw scotland

single domain particles in rocks and magnetic fabric analysis magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes we have recently demonstrated however in laboratory produced samples that as theoretically expected susceptibility anisotropy is dependent on particle size multidomain particles of magnetite or similar strongly magnetic particles with shape anisotropy exhibit a maximum susceptibility parallel to their easy remanent magnetization axes whereas uniaxial single domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state by demonstrating uniaxial single domain anisotropy characteristics for the first time in a rock specimen we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro or ferrimagnetic particles in rocks in particular apparently foliated rocks can actually be lineated and vice versa moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment

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

magnetic fabrics and petrofabrics their orientation distributions and anisotropies abstract magnetic fabric and other petrofabric anisotropies may be described by second rank tensors represented by ellipsoids for a homogeneous petrofabric that is adequately sampled a stereoplot of the orientation distribution of the tensors principal axes maximum intermediate and minimum should show three orthogonal concentrations the concentrations form some combination of shapes from circular clusters through partial girdles to full girdles the concentrations elliptical eccentricities are constrained by the symmetry of the sample orientation distribution i e l l s l s etc as well as the individual sample anisotropies the mean orientations of principal axes must be orthogonal just as with individual sample tensors this requires tensor statistics for their calculation jelinek 1978 furthermore elliptical confidence cones for the means should parallel principal planes preserving overall orthorhombic symmetry however in practice sub orthorhombic symmetry may arise from unrepresentative sampling but it may also be a useful indicator of multiple or heterogeneous petrofabrics in the case of magnetic fabrics the wide range in average susceptibility values and variation in magnetic mineralogy permit small numbers of high susceptibility samples to deflect the orientation of the tensor mean away from the majority of samples normalizing the samples by their bulk susceptibility overcomes this but the orientation of high susceptibility outliers may signify an event or subfabric of importance that we should not discard therefore stereoplots of both normalized and non normalized orientation distributions should be compared preferably also identifying the outliers it is important to distinguish the shape of the orientation distribution ellipsoid from the shape of the individual magnetic fabric ellipsoids the qualitative l s nomenclature is best replaced by tj where tj 1 oblate tj 1 prolate jelinek 1981 invariably the orientation distribution is described by an ellipsoid whose shape is more spherical than that of the individual sample anisotropy ellipsoids because the latter have scattered orientations furthermore the shape of the orientation distribution ellipsoid need bear no relation to the shape of individual sample ellipsoid anisotropies the concepts are illustrated with 1119 measurements of anisotropy of magnetic susceptibility ams from seven areas and with 188 measurements of anisotropy of anhysteretic remanence aarm from two areas