texture development in deformed granodiorites from the santa rosa mylonite zone southern california abstract we report on texture development of granodiorite which has been progressively deformed to mylonite and phyllonite in the santa rosa mylonite zone in southern california since it is extremely difficult to measure pole figures of materials with complex diffraction patterns it was necessary to use sophisticated neutron diffraction techniques with high resolution and position sensitive detectors which enable the mathematical deconvolution of at least part of the spectrum we could construct separate pole figures for biotite quartz and plagioclase which are the major components in the granodiorite protolith the preferred orientations of all phases are weak in the mylonite biotite and quartz develop strong preferred orientations whereas feldspars remain randomly oriented quartz has c axes in the intermediate strain axis as is common in other mylonites interestingly biotite a axes show preferred orientation suggesting that slip may be involved rather than just rigid body rotation in phyllonite with a grain size of about 50 m the biotite fabric is very strong however the quartz fabric has largely vanished perhaps by superplastic processes which do not produce texture a weak plagioclase fabric has developed

sources of magnetic susceptibility in a slate abstract magnetic susceptibility and its anisotropy in the borrowdale volcanic slates at kentmere in the english lake district are attributed largely to preferred orientation of a paramagnetic chlorite of diabantite ripidolite composition in units of 10 6 cgs g the principal susceptibilities for the slates are 9 61 9 42 8 69 and for the chlorite grains the minimum anisotropy is represented by principal susceptibilities of 11 57 11 22 and 9 15 because the magnetic susceptibility is carried by a tightly packed matrix forming mineral that has recrystallised during the deformation it is not possible to imagine simple grain rotation as being responsible for the anisotropy of susceptibility

magnetic susceptibility fabrics in slates structural mineralogical and lithological influences abstract a single penetrative tectonic deformation of archean greywackes accompanied by low grade metamorphism generated low field magnetic susceptibility fabrics that are coaxial with the rock s schistosity although there are multiple sources of magnetic susceptibility pre metamorphic magnetite and the metamorphic minerals pyrrhotite biotite and chlorite variety thuringite daphnite aphrosiderite are responsible for the anisotropy of susceptibility these minerals grew with preferred crystallographic orientations influenced by the prevailing stress regime on its own rotation cannot be invoked to explain the alignment of the metamorphic minerals there are varying proportions of at least two of these minerals in most specimens for 153 specimens the degree of anisotropy increases as the mean susceptibility increases over the range from 100 to 700 in units of 10 6 s i theory shows that this is to be expected either due to increasing the traces of strongly oriented ferrimagnetics or due to increasing the proportion of chlorite in the latter case this simultaneously increased its preferred orientation by closer packing stress influenced crystallization of chlorite rapidly achieves saturation alignment and a limiting magnetic anisotropy although strain may continue to increase samples taken through individual graded beds show that the susceptibility ellipsoid varies from oblate in slaty rocks to a neutral shape in sandier rocks however its orientation is always structurally controlled with the minimum susceptibility axis perpendicular to schistosity

magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

the effect of terrain topography on commonly used resistivity arrays in this work a study of the effect of surface topographical variations on several dc resistivity arrays is presented a 2 5 d finite element modeling scheme is used to examine the performance of several arrays over buried features that exist below a range of different topographical terrain contexts such as valleys hills and steep slopes a mesh generating algorithm allows a realistic representation of terrain topography the results confirm that topographical variations can have a significant impact on the field resistivity data values for all resistivity arrays further topographical variations can be treated flexibly using a realistic resistivity forward modeling process thus topographic effects to the degree they can be modeled are predictable and should be taken into account when designing surveys and interpreting data

coercive forces and coercivity spectra of submicron magnetites abstract remanent coercivity spectra have been determined for four dispersions of magnetite with mean particle sizes of 0 04 0 22 m using four different methods 1 acquisition of isothermal remanent magnetization irm in continuous fields 2 stepwise reversal of saturation irm by opposite polarity continuous fields 3 stepwise alternating field af demagnetization of saturation irm and 4 af demagnetization of weak field thermoremanent magnetization trm alternating field methods gave narrower distributions with lower median coercivities than continuous field cf methods the two cf spectra are offset the remanent coercive force hr which is the median coercivity determined by cf reversal of saturation remanence is always less than the median coercivity h r determined from irm acquisition the median destructive field of saturation irm h 1 2 the median coercivity determined by af demagnetization is less than eitherhr orh r the relation reported by dankers 1 h r h 1 2 2hr is obeyed for these samples and for a companion suite of single domain magnetites and maghemites ordinary coercive force hc is substantially affected by reversible magnetization and is a poor indicator of the median coercivity of remanence hc is much smaller thanh 1 2 hr orh r and varies in a different way with particle size

the anisotropy of magnetic susceptibility in biotite muscovite and chlorite single crystals the anisotropy of magnetic susceptibility ams of single crystals of biotite muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock forming minerals the low field ams and the high field paramagnetic susceptibility are defined for the high field values it is necessary to combine the paramagnetic deviatoric tensor obtained from the high field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals this leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice the ellipsoid of paramagnetic susceptibility which was obtained for the three phyllosilicates is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c axes the anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements the degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite is smaller than previously published values the shape of the chlorite ams ellipsoid for all the samples is near perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks d 2003 elsevier science b v all rights reserved

anisotropy of magnetic susceptibility of heated rocks abstract heating produces changes which does not always correspond to simple enhancement of the magnetic fabric two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating the first diagonalizes the tensor from the difference between each tensor term before and after heating the second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment when the same magnetic fabric is obtained from several thermal steps it cannot be related to randomly oriented ferrimagnetic minerals instead the newly formed fabric must be related to characteristics of the pre existing rock by comparing this ferrimagnetic minerals fabric with the initial whole rock fabric we can distinguish cases where heating simply enhances pre existing fabric from those where thermal treatment induces a different fabric relative to the pre heating fabric this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction related to petrostructural elements other than those defining the initial fabric

granite emplacement in an extensional setting an ams study of the magmatic structures of monte capanne elba italy abstract monte capanne elba is an excellent example of a granite pluton 42 km 2 emplaced 6 m y ago in a well known extensional environment that of the tyrrhenian sea its internal fabric has been studied using the magnitude of magnetic susceptibility and anisotropy measurements at 74 regularly distributed stations susceptibility magnitudes reveal a normal and asymmetrical petrographical zoning in the pluton the magnetic lineations are remarkably well organized mostly of nw se azimuth with slight plunges and always normal to the walls of the late aplitic dykes these lineations are also parallel to the stretching lineations of the immediate country rocks of the pluton in the western and probably deepest part of the pluton a ne sw trending magnetic magmatic lineation is also imprinted the following scenario is proposed for the emplacement of monte capanne during the opening of the tyrrhenian sea an approximately e w trending sinistral transform fault is taken as being responsible for the creation of a pull apart void that permitted upwelling of the magma orientations of the magmatic structures at depth are parallel to the elongation of the feeding zone of this void i e ne sw the very top part of the pluton was sheared parallel to the granite country rock interface i e nw se parallel to the local stretching of the tyrrhenian sea at the time of emplacement

field impressed anisotropies of magnetic susceptibility and remanence in minerals the application of alternating af or direct df magnetic fields to samples containing dispersed particles of magnetite titanomagnetite or maghemite has been found to alter significantly the measured low field susceptibility anisotropy even though the particle orientation is unchanged the acquisition of isothermal remanent magnetization irm is also strongly dependent on the previous af treatment given to the samples a method of quantifying the changes brought about by application of the fields is described the samples studied were intrinsically isotropic or only weakly anisotropic and the computed impressed susceptibility or irm ellipsoids produced by the applied fields were ellipsoids of revolution with the unique axis aligned with the applied field axis the size and shape of the field impressed ellipsoids were strongly dependent upon particle size these effects might thus form the basis of a rapid nondestructive means of estimating particle size in rocks the results also have important implications for magnetic fabric analysis and the static af demagnetization of rocks in particular to ensure that the intrinsic magnetic fabric is measured without any field impressed component of anisotropy samples should not be subjected to static af demagnetization prior to the determination of either low field susceptibility anisotropy or irm anisotropy samples should also not carry a significant remanence such as an irm acquired in a field greater than about 5 mt prior to low field susceptibility anisotropy analysis

magnetic fabric and its significance in the sills and lava flows from taimyr fold belt arctic siberia abstract magnetic fabric research on the emplacement of the taimyr fold belt is still absent therefore we have performed magnetic fabric studies on samples collected from 23 sites from mafic igneous rocks 75 n 100 e in south taimyr this study indicates a correlation between the anisotropy of magnetic susceptibility ams magma flow and fold related compression in the rocks ams measurements on 183 unheated and 122 heated samples reveal the magma flow directions the magma flow direction is mainly parallel to the ese wnw and secondarily parallel to the ssw from the nne in the mafic sills the unheated and heated basaltic flows reveal a ne trending flow vector in some of the sills ams shows a tight clustering of the maximum k1 axes close to the bedding pole which is not thought to define the flow direction corresponding ams measurements on igneous rocks allow us to infer the existence of magnetic fabrics of tectonic origin linked to the main folding episode that occurred subsequent to the mid triassic magmatic event in latest triassic early jurassic times in the sills the distribution of most maximum k1 axes is close to ns that corresponds to the maximum compressive stress or folding directions contrarily the minimum k3 axes have an elongated distribution along the ew direction or parallel to the fold axis before exchanging k1 and k3 axes for inverse fabrics but close to the ns stress direction after exchanging k1 and k3 axes for inverse fabrics in the basaltic flows the minimum k3 axes almost parallel to the ns folding direction the structural interpretation of all ams data taken from the igneous bodies is in accordance with a nnw sse stress related folding taking place around 198 ma in the taimyr peninsula arctic siberia the relationship of susceptibility axes to bedding surfaces and magnetic foliation planes is a criterion that permits differentiation of normal magnetic fabrics from inverse fabrics in the igneous samples single domain sd and small pseudo single domain psd magnetite crystals in the sill samples are the main carriers of the ams fabrics implying the abundance of inverse magnetic fabrics psd and multidomain md titanomagnetite tm grains as well as minor hematite are identified in the basalt flows suggesting the occurrence of more normal magnetic fabrics in the basalts than in the sills

magnetic fabric of araguainha complex impact structure central brazil implications for deformation mechanisms and central uplift formation article i nfo the weakening mechanisms involved in the collapse of complex impact craters are controversial the ara guainha impact crater in brazil exposes a complex structure of 40 km in diameter and is an excellent object to address this issue its core is dominated by granite in addition to microstructural observations magnetic studies reveal its internal fabric acquired during the collapse phase all granite samples exhibit impact related planar deformation features pdfs and planar fractures pfs which were overprinted by cataclasis cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure fracture planes are systematically decorated by tiny grains b10 m of magnetite and hematite and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility ams and anhysteretic remanence aar are perfectly coaxial in all studied sites therefore we could track the orientation of deformation features which are decorated by iron oxides using the ams and aar the magnetic fabrics show a regular pattern at the borders of the cen tral peak with orientations consistent with the fabric of sediments at the crater s inner collar and complex in the center of the structure both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of complex im pact structures the widespread occurrence of cataclasis in the central peak and its orientations revealed by magnetic studies indicate that acoustic fluidization likely operates at all scales including the mineral scales the cataclastic flow made possible by acoustic fluidization results in an apparent plastic deformation at the macroscopic scale in the core

thermally induced alterations of minerals during measurements of the temperature dependence of magnetic susceptibility a case study from the hydrothermally altered soultz sous forets granite france identification of magnetic minerals using the temperature dependence of magnetic susceptibility in low field t combined with optical microscopy microprobe x ray diffraction and chemical analysis provided new constraints on the alteration of fe bearing minerals of the magnetite bearing soultz sous forets granite from the eps 1 borehole upper rhine graben france while relatively fresh granite shows largely reversible t curves typical of magnetite the altered granite revealed a wide variety of irreversible heating and cooling curves which allowed an assignment to different alteration stages under specific geochemical conditions though paramagnetic minerals like fe bearing carbonates pyrite or antiferromagnetic hematite could not be detected according to their curie or neel temperature they were identified due to reactions to new ferrimagnetic phases during the heating cooling experiments at specific temperatures mineral reactions were proved by measurements of the single mineral phases hematite fe carbonates and illite our mineralogical results combined with the thermomagnetic measurements imply that first faulting of the granite occurred already during cooling of the magma which caused a first magnetite oxidation event during uplift of the granitic body and exposure to a paleo erosion surface strongly acidic fluids emerged from pyrite oxidation caused a decomposition of fe bearing minerals like martite hematite derived from magnetite oxidation and fe carbonates and an ongoing transformation of magnetite to martite subsequently precipitation of fine grained hematite was restricted only to the upper part of the pluton in the deeper part of the borehole pyrite was preserved from oxidation in an active fault zone martite was reduced back to magnetite which can be explained with the occurrence of organic matter transported by fluids

three dimensional imaging of subsurface structures using resistivity data we have developed a three dimensional inverse scheme for carrying out dc resistivity surveys incorporating complicated topography as well as arbitrary electrode arrays the algorithm is based on the finite element approximation to the forward problem so that the effect of topographic variation on the resistivity data is effectively evaluated and incorporated in the inversion furthermore we have enhanced the resolving power of the inversion using the active constraint balancing method numerical verifications show that a correct earth image can be derived even when complicated topographic variation exists by inverting the real field data acquired at a site for an underground sewage disposal plant we obtained a reasonable image of the subsurface structures which correlates well with the surface geology and drill log data

three dimensional terrain corrections in resistivity surveys a three dimensional finite element computer algorithm which can accommodate arbitrarily complex topography and subsurface structure has been developed to model the resistivity response of the earth the algorithm has undergone extensive evaluation and is believed to provide accurate results for realistic earth models testing included comparison to scale model measurements analytically calculated solutions and results calculated numerically by other independent means computer modeling experiments have demonstrated that it is possible to remove the effect of topography on resistivity data even under conditions where such effects are large with respect to the subsurface responses this can be done without resorting to lengthy and costly trial and error computer modeling after correction the data can be interpreted as if the anomalies are due only to subsurface structure the results of case studies on field data measured in high relief topography indicate the following 1 pre or postsurvey 3 d compu

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

petrogenetic modelling of in situ fractional crystallization in the zoned loch doon pluton scotland the late caledonian loch doon granitic intrusion ranges in composition from hypersthene diorite at the margin through quartz diorite granodiorite and granite to cordierite microgranite at its core petrogenetic modelling of trace element variations and least squares analysis of major elements indicate that two distinct magmas are involved each magma controlled by crystallization of plag opx cpx bio late stage rocks related to the second magma include the cordierite microgranites and aplites which are interpreted as the final residue which crystallized rapidly after a build up and loss of volatiles analyses of whole rocks and minerals for ree s and other elements of moderate high ionic potential indicate that these elements are strongly controlled by minor phase crystallization apatite zircon sphene and allanite are dominant at intermediate compositions but other accessory minerals such as monazite and xenotime may also become important at acid compositions it is probable that within each magma the mechanisms of crystal settling and filter pressing operated the former being initially dominant and the latter becoming more important with increasing degree of fractional crystallization

partial anhysteretic remanence and its anisotropy applications and grainsize dependence the distribution of remanence coercivities in a rock sample can be obtained by differentiating alternating field af demagnetization curves of anhysteretic remanent magnetization arm however significant errors can be introduced by grain interactions and by the noise inherent in differentiation we have found that 1 this error can be significantly reduced by measuring partial arm s parm s and 2 parm patterns show a strong empirical correlation with grain size parm s are imparted by exposing a sample to a small direct magnetic field in the presence of a large decreasing af the direct field is switched on over a specified range of af s thus magnetizing a fraction of the magnetite grains in the corresponding coercivity range we have further found in both synthetic and natural sediments that measurement of the anisotropy of parm enables us to detect the preferred orientations of distinct subpopulations of magnetite particles related to the size and shape of the grains

magnetic properties of the high himalayan leucogranites structural implications the magnetic properties of the high himalayan leucogranites have been investigated on 527 specimens in three plutons everest makalu 6 sites and manaslu 40 sites in nepal and gangotri 43 sites in india susceptibility varies between 2 and 100 10 6 si with an anisotropy ratio up to 1 16 high field and low temperature magnetic measurements together with comparison with weight percent iron demonstrate that anisotropy of magnetic susceptibility is carried by paramagnetic biotite and tourmaline the latter produces an inverse fabric i e with the minimum axis parallel to mineral lineation the magnetic fabric demonstrates complex patterns of stretching lineations during magmatic emplacement and its usefulness in semi quantitatively estimating petrofabric intensity is demonstrated for the biotite bearing facies natural remanent magnetization was measurable at only two sites in everest makalu where there are well defined reverse directions carried by titanomagnetite and pyrrhotite comparison of these preliminary results with predicted directions for stable india suggests northward tilting of about 10 and a small clockwise rotation of this massif

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

slaty cleavage development and magnetic anisotropy fabrics the shale to slate transition preserved in the ordovician martinsburg formation at the lehigh water gap pennsylvania provides an opportunity to study the relationship between magnetic anisotropy fabrics and the development of slaty cleavage our previous work has indicated that anisotropy of magnetic susceptibility ams does not record changes in finite strain associated with cleavage development in these rocks but instead measures the degree of dissolution and new growth of ehlorite additional ams data presented in this paper lend further support to this conclusion conversely anhysteretie remanent magnetization anisotropy arma which is not affected by paramagnetie ehlorite accurately reflects the strain induced rock fabrics associated with cleavage formation arma results show that magnetite dimensional orientations vary from bedding parallel in shale samples to cleavage parallel in samples with well developed slaty cleavage samples with weak and pencil cleavage display scattered arma orientations which lie in between bedding and cleavage these intermediate orientations may be due to either passive rotation of magnetite from bedding parallel to cleavage parallel or re crystallization of magnetite if rotation occurred grain rotation was highly heterogeneous in the samples with incipient cleavage the intermediate arma orientations may also reflect the varying contribution of two magnetite preferred orientations a depositional orientation parallel to bedding and a new growth orientation parallel to cleavage

anisotropy of magnetic remanence a brief review of mineralogical sources physical origins and geological applications and comparison with susceptibility anisotropy the magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low field magnetic susceptibility ams however alternative methods based on remanent magnetization measured in the absence of a magnetic field rather than induced magnetization measured in the applied field have distinct advantages for certain geological applications this is particularly true for 1 adjunct studies in paleomagnetism in order to assess the fidelity with which a natural remanence records the paleofield orientation 2 studies of weakly magnetic or weakly deformed rocks for which susceptibility anisotropy is very difficult to measure precisely and 3 quantitative applications such as strain estimation the fundamental differences between susceptibility and remanence and their respective anisotropies are due to several factors 1 susceptibility arises from all of the minerals present in a sample whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals 2 ferromagnetic minerals are generally more anisotropic than para and diamagnetic minerals 3 for ferromagnetic minerals remanence is inevitably more anisotropic than susceptibility and 4 a number of common minerals including single domain magnetites possess an inverse anisotropy of susceptibility i e they tend to have minimum susceptibility parallel to the long axis of an individual particle remanence is immune to this phenomenon as a consequence of all these factors remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample

paleomagnetism and geochronology of the early cretaceous dipilto batholith nw nicaragua chortis block large rotation with respect to sw north america

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

magnetite grain shape fabric and distribution anisotropy vs rock magnetic fabric a three dimensional case study abstract shape fabric and distribution anisotropy of magnetite grains in a highly ferromagnetic s l syenite are determined for three mutually perpendicular planes on about 600 magnetite grains the grains are lenticular with an average aspect ratio of about 2 0 their preferred orientation determined by the inertia tensor method is consistent with the principal directions of the magnetic susceptibility ellipsoid and the axial ratios of the magnetic fabric ellipses are very close to those of the shape fabric ellipses e g 1 41 and 1 39 in the section plane perpendicular to the magnetic foliation and parallel to the magnetic lineation the anisotropic wavelet transform can detect and quantify the anisotropic distribution of the magnetite grains which are located interstitially along the feldspar grain boundaries this anisotropic distribution has no noticeable effect on the rock s anisotropy of magnetic susceptibility ams since only a few percent of grains are close enough to interact magnetically therefore it is realistic to consider that the ams of ferromagnetic granitic rocks originates mainly from the shape fabric of anisotropic magnetite grains and a close correlation between the magnetic fabric and the magnetite grain shape fabric is expected

thermally enhanced mimetic fabric of magnetite in a biotite granite the thermally induced growth of mimetic magnetite after biotite is here demonstrated by the equivalence of anisotropies of magnetic susceptibility ams and anhysteretic remanence aar after heating of biotite single crystals and whole rock specimens from a magnetite free granite elba island italy it is concluded that between 500 and 725 c magnetite nucleates and grows within and parallel to the biotite cleavages yielding a magnetic fabric mimetic of the biotite fabric at higher temperatures magnetite undergoes partial haematization accounting for the decrease in remanence susceptibility and anisotropy degree this experiment may be used as a proxy for fabric study of granites having composite tourmaline biotite paramagnetic fabrics in which the fabric of biotite can be selectively thermally enhanced

two dimensional inversion of direct current resistivity data incorporating topography by using finite difference techniques with triangle cells investigation of kera fault zone in western crete in this study we suggest the use of a finite difference fd forward solution with triangular grid to incorporate topography into the inverse solution of direct current resistivity data a new inversion algorithm was developed that takes topography into account with finite difference and finite element forward solution by using triangular grids using the developed algorithm surface topography could also be incorporated by using triangular cells in a finite difference forward solution initially the inversion algorithm was tested for two synthetic data sets inversion of synthetic data with the finite difference forward solution gives accurate results as well as inversion with finite element forward solution and requires less cpu time the algorithm was also tested with a field data set acquired across the kera fault located in western crete greece the fault location and basement depth of sedimentary units were resolved by the developed algorithm these inversion results showed that if underground st

magnetic anisotropy of some phyllosilicates magnetic susceptibility anisotropy of susceptibility and hysteresis of single microcrystals of chlorite biotite phlogopite muscovite zinnwaldite and fuchsite were measured in low and high magnetic fields with an alternating gradient force magnetometer micromag their properties are sufficient to account for the low field susceptibility ams of most micaceous rocks nearly all samples show some ferromagnetic contribution at low fields due to inclusions of pseudosingle domain and multidomain magnetite the paramagnetic contribution isolated at high fields usually exceeds the ferromagnetic contribution the paramagnetic susceptibility is intrinsic to the silicate lattice and agrees with values predicted from chemical composition within the limits of error the minimum susceptibility is nearly parallel to c another axis is parallel to b and the third susceptibility usually the maximum is close to a the paramagnetic susceptibility has a disk shaped magnitude ellipsoid with strong anisotropy p 2 the ferromagnetic contributions at low fields have more variably shaped ellipsoids with greater eccentricity p 5 the silicate lattice does not constrain their orientation our technique cannot determine the principal axes of the ferromagnetic component however its principal values usually correspond with the paramagnetic principal susceptibilities in order of magnitude thus the combined paramagnetic ferromagnetic anisotropy recognised in routine studies of ams should faithfully represent the petrofabric of most micaceous rocks nevertheless nearly 10 of our samples have incompatible anisotropy ellipsoids for the silicate host and magnetite inclusions these yield a net inverse ams that does not correctly represent the orientation of the silicate lattice therefore some caution is necessary in petrofabric ams studies of micaceous rocks

chlorite control of correlations between strain and anisotropy of magnetic susceptibility abstract anisotropy of magnetic susceptibility ams has in the past been used as an indicator of finite strain mathematical relationships between the measured ams ellipsoid and the finite strain ellipsoid in slates are numerous the ordovician martinsburg formation at lehigh gap pennsylvania provides a unique opportunity to test this correlation as a complete shale to slate transition is preserved magnitudes of the principal susceptibility axes appear to reflect a strain gradient at this location apparent strain values derived from ams using equations from rathore and hrouda have x y z ratios from 2 79 1 87 0 19 shale to 2 92 0 82 0 42 slate the orientations of the susceptibility axes characterize two fabrics i k int k max planes at 20 to bedding and ii k int k max planes parallel to cleavage anisotropy of magnetic susceptibility fails to show any intermediate orientations of the principal susceptibility axes in the zone in which cleavage is developed comparison with phyllosilicate crystallographic orientations indicates that the ams intermediate and maximum susceptibility axes are parallel to chlorite basal planes because chlorite reoriented via dissolution and new growth as cleavage developed in this location and because models correlating ams and strain instead assume that strain is reflected by the rotation of the magnetic susceptibility carriers the apparent strain values calculated from ams reflect only the degree of dissolution and neocrystallization in these rocks and not the finite strain

joint inversion of geophysical data summary by jointly inverting several different kinds of geophysical measurements at a site we avoid some of the ambiguity inherent in the individual methods we show how this can be done for the combination of dc resistivity and magnetotelluric measurements on a layered medium by considering a simple 3 layer model the combination resolves the resistivity of the thin resistive second layer even though neither of the two methods can do so alone the method is then applied to field data from a shallow sedimentary basin a blind zone occurs beneath a thick near surface conductive shale by a study of the eigenvalue structure of the model it can be seen that resolution in this zone would be slightly enhanced by higher frequency magnetotelluric data but additional dc data at larger spacing would yield no improvement

a systematic study of topography effect of ert based on 3 d modeling and inversion in this study a quick method using a digital elevation model dem to obtain real terrain points for generating tetrahedral mesh has been developed based on tetgen then three dimensional 3 d forward modeling and inversion based on the patching method of electrical resistivity tomography ert and which have been compared favorably with those obtained using other software were used to study topography effect asystematic research of apparent resistivity features of different topographies with pole pole array and wenner array has been conducted in this study based on that the solutions for removing topography effect are given to the two dimensional 2 d survey and the 3 d survey respectively comparing to the inversion result the solution for the 2 d survey can effectively remove topography influence a 3 d inversion algorithm incorporating topography is proposed at the same time two synthetic models incorporating real topography with fault and ellipse anomalies were created to test the 3 d inversion algorithm and the results show that the relative image error is less than 30 and the correlation coefficient is more than 90

the tourmaline bearing granite pluton of bodmin cornwall uk magnetic fabric study and regional inference the magnetic fabrics and microstructures of the variscan granite pluton of bodmin cornwall have been studied its low susceptibility magnitude consistent with its lack of magnetite comes from biotite and tourmaline as the magneto crystalline behaviour of tourmaline is inverse compared with that of the phyllosilicates the magnetic fabric was remeasured after heating the specimens to enhance the biotite magnetic signal by the growth of mimetic magnetite the roughly similar magnetic fabrics before and after heating demonstrate that the effect of tourmaline is not important as long as the tourmaline content does not exceed that of the phyllosilicates regionally this study reveals a well defined nnw trend of the magnetic lineations with opposite plunges on each side of a separation line crossing the pluton centre this pattern very similar to that documented in the nearby carnmenellis pluton is ascribed to unroofing during emplacement it agrees with the late to post magmatic nw stretch that characterizes the devonian to carboniferous formations located to the north of this region the thrust and fold structure of which is attributed to southward compression we conclude that syntectonic emplacement may have characterized the whole cornubian batholith at the very end of the variscan orogeny in cornwall

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

thermal enhancement of magnetic fabrics in high grade gneisses the orientation distributions of ams and aarm ellipsoids are non coaxial in the kapuskasing zone granulites aarm is due to the preferred orientation of magnetite which mostly postdates the silicate fabric the ams orientation distribution combines susceptibility anisotropies of magnetite and the silicates whereas heating does not increase the bulk susceptibility it does improve the definition of the orientation distributions of both ams and aarm however the orientation distribution of aarm is most enhanced and therefore post heated samples emphasize the magnetite sub fabric thus heating does not clarify the interpretation of the silicate fabric in this study however it does change the samples ams fabric in such a way as to reveal the magnetite subfabric which can only otherwise be detected by more tedious aarm measurements

cumulate like textures and chemical relationships in the bressanone brixen granodiorite eastern alps italy a new genetic approach abstract the shallow level pluton of bressanone is a late hercynian multiple intrusion into the south alpine basement of the eastern alps most of this complex is composed of anatectic granodiorites and granites intruded in separate stocks 282 14 ma ago gabbros and leucogranites occur in smaller quantities the chronological intrusion sequence is layered gabbro granodiorites and granites two mica cordierite leucogranite and fayalite leucogranites the granodiorites and granites may contain hornblende or garnet the hornblende and garnet rocks differ both in chemistry and 87 sr 86 sr i ratio and may be identified as i type and s type respectively according to the chappell white classification textural and chemical patterns show that the granites may be linked to the granodiorites by cumulate like processes the granodiorite granite transition attributed to filter pressing expresses an increase in the liquid xenolith ratio in a magma where the liquid fraction was a minimum melt and the solid fraction was restitic material

flow lobes in granite the determination of magma flow direction in the trawenagh bay granite northwestern ireland using anisotropy of magnetic susceptibility anisotropy of magnetic susceptibility ams has been determined from 152 sites over the 64 km2 trawenagh bay granite part of the 400 ma caledonian donegal batholith microscopic observations thermomagnetic analyses high field torque magnetometry and heating experiments demonstrate that the ams fabric is carried by biotite with varying amounts of mimetic magnetite in the biotite cleavage planes the ams fabric is interpreted as a flow fabric because microstructures indicate magmatic state deformation it defines large scale macroscopic flow features flow lobes in this weakly deformed granite at pluton scale the data set shows a gentle westward plunging lineation at a smaller scale many lobate magnetic foliation patterns can be discerned in which the magnetic lineation is usually symmetrically disposed about the long axes of the lobes in three dimensions the lobes are tongue like 1500 m wide subunit scale and elongated east west parallel with the general lineation they close consistently westward and younger lobes in the east appear to deflect older lobes in the west the trawenagh bay granite is west of a major synplutonic shear zone that hosts the main donegal granite pluton these structures are interpreted as frozen flow lobes and indicate that the magma flowed in a westward direction from within the active shear zone of the main donegal granite into the weakly deformed wall rock trawenagh bay granite intra unit pulses or surges have long been suspected in granite plutons however this study reports the most detailed description of such structures to date

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

chilled margins and commingling of magmas in the bressanone brixen hercynian granodiorites eastern alps northern italy abstract the brixen hercynian pluton is the result of a multiple intrusion of crustal anatectic magma granodiorite granites and two mica cordierite bearing leucogranites and subcrustal magma gabbro diorites and fayalite bearing leucogranites some adjacent granodioritic stocks show chilled margins containing globular inclusions in the size of several centimetres of a coarse grained leucogranite the granodiorite margins also show narrow zoning with more acidic external borders this phenomenon is correlated to differentiated processes such as filter pressing and the soret effect which were active during the rise of these stocks the strong petrographic and chemical analogies between the globular inclusions of some chilled margins and the fayalite leucogranites suggest their common subcrustal origin the granodiorite containing these inclusions is of crustal origin the occurrence of globular inclusions inside it thus indicates only mechanical mingling between crustal anatectic and subcrustal magmas as shown by the absence of hybrids and chemical interactions

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

magnetic anisotropy of chloritoid 1 the magnetocrystalline anisotropy of monoclinic chloritoid a relatively common mineral in aluminum rich metapelitic rocks has been determined for the first time by measuring the high field anisotropy of magnetic susceptibility hf ams using two independent approaches i e a directional magnetic hysteresis measurements and b torque magnetometry on a collection of single crystals collected from different tectonometamorphic settings worldwide magnetic remanence experiments show that all specimens contain ferromagnetic s l impurities being mainly magnetite the determined hf ams ellipsoids have a highly oblate shape with the minimum susceptibility direction subparallel to the crystallographic c axis of chloritoid in the basal plane of chloritoid though the hf ams can be considered isotropic the degree of anisotropy is found to be 1 47 which is significantly higher than the anisotropy of most paramagnetic silicates and even well above the frequently used upper limit i e 1 35 for the paramagnetic contribution to the ams of siliciclastic rocks the obtained values for the paramagnetic curie temperature parallel and perpendicular to the basal plane indicate that this pronounced magnetocrystalline anisotropy is related to strong antiferromagnetic exchange interactions in the direction of the crystallographic c axis 0 as a consequence chloritoid bearing metapelites with a pronounced mineral alignment can have a high degree of anisotropy without the need of invoking a significant contribution of strongly anisotropic ferromagnetic s l minerals the newly discovered magnetocrystalline anisotropy of chloritoid thus calls for a revised approach of magnetic fabric interpretations in chloritoid bearing rocks

assessment of the reliability of 2d inversion of apparent resistivity data the reliability of inversion of apparent resistivity pseudosection data to determine accurately the true resistivity distribution over 2d structures has been investigated using a common inversion scheme based on a smoothness constrained non linear least squares optimization for the wenner array this involved calculation of synthetic apparent resistivity pseudosection data which were then inverted and the model estimated from the inversion was compared with the original 2d model the models examined include i horizontal layering ii a vertical fault iii a low resistivity fill within a high resistivity basement and iv an upfaulted basement block beneath a conductive overburden over vertical structures the resistivity models obtained from inversion are usually much sharper than the measured data however the inverted resistivities can be smaller than the lowest or greater than the highest true model resistivity the substantial reduction generally recorded in the data misfit during the least squares inversion of 2d apparent resistivity data is not always accompanied by any noticeable reduction in the model misfit conversely the model misfit may for all practical purposes remain invariant for successive iterations it can also increase with the iteration number especially where the resistivity contrast at the bedrock interface exceeds a factor of about 10 in such instances the optimum model estimated from inversion is attained at a very low iteration number the largest model misfit is encountered in the zone adjacent to a contact where there is a large change in the resistivity contrast it is concluded that smooth inversion can provide only an approximate guide to the true geometry and true formation resistivity

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

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

field impressed magnetic anisotropy in rocks the application of alternating af or direct df magnetic fields to two weakly anisotropic rock specimens which remain static significantly alters their measured low field anisotropy of magnetic susceptibility ams by superimposing an anisotropy which has the form of an ellipsoid of revolution with its unique axis aligned along the field axis previous af history is also shown to influence the acquisition of isothermal remanent magnetization irm these effects can be quite substantial and thus it is essential that rocks should not be used for low field ams analysis or for irm anisotropy studies if they have previously been subjected to static af demagnetization ams studies should also not be carried out on rocks which carry a significant remanence e g an irm acquired in 5 mt the field impressed effects were also strongly dependent upon whether the ferrimagnetic particles in the rocks were predominantly multidomain md or uniaxial single domain sd the results were consistent with our recent work on synthetic specimens containing magnetite titanomagnetite or magnemite and add support to our suggestion that these effects could provide a rapid nondestructive means of determining the predominant domain state md or uniaxial sd of ferrimagnetic particles in rocks

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

electrical resistivity ground imaging ergi a new tool for mapping the lithology and geometry of channel belts and valley fills efforts to map the lithology and geometry of sand and gravel channel belts and valley fills are limited by an inability to easily obtain information about the shallow subsurface until recently boreholes were the only method available to obtain this information however borehole programmes are costly time consuming and always leave in doubt the stratigraphic connection between and beyond the boreholes although standard shallow geophysical techniques such as ground penetrating radar gpr and shallow seismic can rapidly obtain subsurface data with high horizontal resolution they only function well under select conditions electrical resistivity ground imaging ergi is a recently developed shallow geophysical technique that rapidly produces highresolution profiles of the shallow subsurface under most field conditions ergi uses measurements of the ground s resistance to an electrical current to develop a two dimensional model of the shallow subsurface 200 m called an ergi profile ergi measurements work equally well in resistive sediments

devastation of aquifers from tsunami like storm surge by supertyphoon haiyan the northwest pacific ocean is a hot spot for sea level rise and increasing frequency of stronger storms it is where supertyphoon haiyan formed the strongest storm to hit land which provided a window into the hydrologic impacts of an extreme storm through detailed documentation of flood levels groundwater table elevations and salinity electrical resistivity and modeling we found that haiyan s storm surge reached 7 m above sea level along samar island philippines which led to contamination of crucial aquifers by infiltrating seawater a contaminated surficial aquifer will take years to recover groundwater in an underlying deeper aquifer saw widespread contamination immediately after the storm but here salinity has decreased significantly after 8 months however this deeper aquifer remains vulnerable to seawater slowly percolating through the surficial aquifer as warmer seas generate more powerful storms the vulnerability of aquifers to persistent contamination from intense storm surges is a growing concern for coastal communities

pluton shapes in the cornubian batholith new perspectives from gravity modelling gravity modelling of the dartmoor bodmin st austell and carnmenellis plutons of the cornubian batholith of sw england strongly supports a tabular form for the exposed granites modelled thicknesses for the individual bodies with the exception of dartmoor are consistent with those derived from empirical relationships for plutons and laccoliths in the case of the dartmoor bodmin and st austell plutons the deepest parts are located near their southern margins suggesting a steep conduit with northerly and outward direct flows modelling of the carnmenellis granite suggests a more centrally located feeder in the case of dartmoor at least the granite appears to have exploited the devonian carboniferous interface during its emplacement

silicate versus trace mineral susceptibility in metamorphic rocks rates of change of magnetic susceptibility during leaching can characterize the presence of certain common types of magnetic minerals in metamorphic rocks in this study the results of leaching are confirmed by mineral separation optical microscopy scanning electron microscopy sem and microchemical analysis under sem leaching curves provide a simple rapid way of determining the relative roles of oxides sulfides and silicates in carrying the susceptibility in metamorphic rocks

incorporation of topography into two dimensional resistivity inversion direct current resistivity data acquired on rough terrain can be interpreted by using an appropriate inversion technique after a topographic correction in order to avoid the influence of a possibly incomplete topographic correction an improved two dimensional resistivity inversion algorithm has been developed to estimate the subsurface resistivity distribution in the presence of topography in this paper fully discretized modeling is based on the finite element method and the iterative inversion scheme is derived from the second order marquardt damped least squares method the algorithm has been tested on both synthetic and field resistivity data with topography incorporated explicitly into the inversion model both theoretical and field studies indicate that the technique is computationally efficient and provides an improved geologic interpretation for complex subsurface structures a four electrode configuration is used in the algorithm so that the inversion can represent most field measurements

identification of multicomponent anisotropies in rocks using various field and temperature values in a cryogenic magnetometer abstract a new technique involving magnetization measurements on a rotating sample in a cryogenic magnetometer with variable field from 0 to 4 t and variable temperature is presented it can be used to separate and identify multi component anisotropy in rocks the results obtained for anisotropy due to magnetite haematite pyrrhotite or phyllosilicates are interpreted according to different magnetization models

thermal enhancement of magnetic fabric in cretaceous sandstones heating of samples of the san juan cretaceous sandstones ne venezuela results in large increases in their low field susceptibility mostly above 650 c which allows measurement of their low field susceptibility ellipsoids where measurements are available before and after heating the same fabric is almost always enhanced samples previously too weak for measurement showed thermally induced fabrics which are consistent within each site but vary between different localities the susceptibility changes are all largely attributed to magnetite formation probably from vermiculite at 450 c in the proximal sediments and from illite at 850 c in all areas the induced magnetic fabrics can be interpreted in terms of either sedimentary structures or low grade metamorphism related to cleavage parallel to the fold axes it is considered that this thermal enhancement technique offers a method for measuring low field magnetic fabrics in a wider range of geological conditions and lithologies than reviously although the processes involved need considerable study

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