magnetic fabric determined from arm and irm anisotropies in paleozoic carbonates southern appalachian basin we have undertaken a comparative study of arm and irm anisotropies in paleozoic carbonate rocks from the nashville and jessamine domes in the southern appalachian basin the arma ellipsoids differ markedly from the irma ellipsoids arma appears to reflect a pre deformation magnetic fabric due to deposition and or compaction with minimum axes near vertical and a weak lineation conversely irma has minimum axes near horizontal and oriented ne sw which is compatible with a tectonic fabric due to alleghanian deformation percent arma is consistently greater than irma arm was imparted at low alternating fields 30 mt and thus arma may reflect the fabric residing in low and intermediate coercivity coarse grained magnetite however the irm was imparted in saturating fields 300 mt and appears to be dominated by the single domain fraction which is believed to carry the observed kiaman superchron remagnetization

deformation and magnetization of the hudson valley eastern new york results of a study of calcite twinning and anisotropy of magnetic remanence in the onondaga limestone abstract results of a study of calcite twin analysis and anisotropy of anhysteretic remanent magnetization aarm for the onondaga limestone in the hudson valley fold and thrust belt hvb raise questions about the deformation history of this area and the nature of aarm two bedding parallel shortening directions are inferred from calcite twinning one normal to hvb fold axes nw se to ene wsw and the other at high angles to this direction nne ssw to n s two hypotheses are put forward to explain the inconsistent shortening direction 1 even during the initial stages of folding and thrusting in the hvb deformation was three dimensional in character 2 the hvb developed synchronous with main phase alleghanian deformation aarm results are consistent with the magnetic anisotropy being in part caused by the deformation associated with folding and thrusting this is a surprising result because the magnetization is presumably of diagenetic origin and it was acquired after folding

erosion charts for selected geomaterials abstractknowledge of erodibility is essential to the analysis of problems related to soil erosion including bridge scour embankment and floodwall overtopping dam spillway erosion stream stabilit

subtle stretching lineation revealed by magnetic fabric of callovian oxfordian black shales french alps abstract the anisotropy of magnetic susceptibility ams of callovian oxfordian black shales from the subalpine massifs of the french alps has been measured the magnetic susceptibility of these formations originates from ferromagnetic titanomagnetite and paramagnetic phyllosilicates illite and chlorite although the magnetic fabric is complicated by the interaction of planar and linear structural elements and the presence of variable amounts of minerals two types of lineation are preserved a stretching lineation which exhibits counterclockwise rotation as a result of deformation from south to north and an intersection lineation parallel to the fold axis this set of new structural data together with semiquantitative analysis of ams is in agreement with the geodynamic evolution of the western alps

magnetic and petrofabric studies in the multiply deformed thomson formation east central minnesota abstract two contrasting zones of deformation have been previously recognized in the proterozoic thomson formation of east central minnesota the northern zone consists of open upright e w trending folds with a well developed steeply south dipping axial planar cleavage the southern zone is marked by a pervasive subhorizontal cleavage s 1 crenulated by a second cleavage that is vertical to steeply south dipping s 2 paleomagnetic results and anisotropy of anhysteretic remanent magnetization aarm in these rocks indicate that the ferrimagnetic particles preserve a record of the field during deposition and primary compaction neither nrm nor aarm were much affected by tectonic deformation by contrast the low field magnetic susceptibility which is dominated by paramagnetic minerals particularly chlorite reflects the tectonic fabric of the rock anisotropy of magnetic susceptibility ams is characterized by minima normal to the cleavage in the northern zone and to the s 1 cleavage in the southern zone thus the s 2 deformation did not significantly affect the ams fabric march strains were obtained by measuring the 002 lattice preferred orientation of chlorite using an x ray pole figure goniometer the principal orientations of march strain match well those of ams and ams and march strain ellipsoids for selected samples in both northern and southern zones plot in the flattening field of a flinn diagram based on ams signature it is possible that the deformation in the northern zone occurred at the same time as the first deformation in the southern zone this could have involved simple shear on subhorizontal planes giving way to pure shear with horizontal shortening towards the north in this scenario the second deformation the result of regional late stage shortening was only marked by a cleavage s 2 in the southern zone due to the favorable orientation there of s 1

sem and magnetic fabric study of a compacting sediment scanning electron microscopy sem anisotropy of magnetic susceptibility ams and anisotropy of anhysteretic remanence aar studies were conducted on kaolinite magnetite samples compacted to pressures between o and 0 2 mpa to determine if clay and magnetic fabric development mimic a change in compaction behavior marked by a break in slope in the compaction curve this break in slope indicates that the rate of void ratio decrease with increasing pressure abruptly slows down at pressures of about 0 04 mpa about one half of a compaction caused inclination decrease occurs at pressures less than 0 04 mpa sem photomicrographs show that horizontal clay fabric develops inhomogeneousiy within a sample at pressures below the break in slope since pore spaces are larger than magnetite grains at low pressures below the break in slope the observed pattern of inclination decrease during compaction may be due to magnetite grains being attached to clay particles and constrained to follow clay fabric development this interpretation is supported by the observation that aar minimum axes become vertical at pressures above the break in slope this may indicate that magnetic fabric is developing at low pressures and that most of the magnetite particles are constrained to lie subhorizontally the ams maximum axes tend to lie 90 from a strong remanence in these samples suggesting single domain magnetic behavior measurement of ams after alternating field demagnetization yields maximum axes which cluster near to the aar maximum axes suggesting multidomain behavior this apparent contradiction aay be due to magnetic conditions prior to ams aeasurement controlling domain wall nucleation

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

late paleozoic remagnetization of the trenton limestone samples of the middle ordovician trenton limestone were collected from 8 sites in quebec ontario and new york state for paleomagnetic study stepwise thermal demagnetization reveals a very well defined characteristic magnetization that has a corresponding paleomagnetic pole at 53 0 n 126 9 e k 97 7 a 95 2 7 this pole is not at all close to other ordovician poles from cratonic north america but is near late carboniferous and early permian ones indicating that the trenton was remagnetized in the late paleozoic an earlier paleomagnetic investigation of the trenton gave a pole that did not appear to be late paleozoic in age however we demonstrate that this earlier study failed to isolate the ancient characteristic magnetization rock magnetic experiments and analyses of magnetic extracts show that the carrier of magnetization is fe3o4 that does not contain ti or other elements in solid solution that would indicate a detrital source the magnetite and its associated remanence appear to have a diagenetic origin

magnetization of the austell gneiss northwest georgia piedmont the austell gneiss is located in the piedmont 16 km west of atlanta georgia and 8 km northwest of the brevard fault zone this metamorphosed granite gneiss lies refolded in the nose of the austell frolona antiform a doubly plunging structure of regional extent subsequent to intrusion the unit has undergone four episodes of deformation the first and second generation folds f1 and f2 are coaxial but not coplanar the two later fold events were less intense and have little effect on local outcrop patterns in the study area the age of the unit has been estimated to be greater than 325 m y but less than 365 m y the induced and remanent magnetic properties for at least six cores drilled at each of 10 sites within the austell gneiss have been determined the magnetic foliation for the unit is approximately parallel with field estimates for s1 foliation planes exhibited by mineral orientations within the body indicating post emplacement magnetic grain mobility the stable remanent magnetization exhibited by the body was therefore probably acquired during or slightly after the f1 or f2 deformational events as a result of cooling from metamorphic temperatures 600 c or alternatively from secondary magnetite grain nucleation and growth a paleopole position calculated for these data without structural correction 126 2 34 4 dp 6 6 dm 13 2 can be explained by magnetic acquisition in at least three possible geologic settings from simple to complex these are the following first since a large oval of confidence is observed for the unit and minor tilting can not be resolved the data may be explained by magnetization of the austell at its present site during cooling from metamorphic temperatures second a northwestward thrust event may have resulted in bringing the austell to its present location third the data can be explained by two major translational events the austell may have initially been magnetized and then moved to the northeast by transcurrent strike slip faulting 1000 km into juxtaposition with the southern appalachians followed by thrusting of the unit to the northwest to its present location in support of this last interpretation is the similarity between the paleopole for the austell and paleopoles determined for the maritime provinces of north america which were suggested to have moved 1500 km to the northeast during the mid to late paleozoic against such an interpretation is the lack of geologic evidence in support of large scale left lateral strike slip faulting in the southern appalachians

relationship between remagnetization magnetic fabric and deformation in paleozoic carbonates abstract we have studied the magnetic fabric of paleozoic carbonates from numerous sites in the north american midcontinent for comparison with paleomagnetic behavior and calcite twinning strains sites carrying a kiaman remagnetization have significantly lower anhysteretic remanent magnetization arm anisotropy less than 3 than other sites minimum axes of arm ellipsoids are distributed in a vertical plane trending approximately se nw this trend is very similar to the orientation of alleghenian layer parallel shortening as inferred by mechanical twinning in calcite sites lacking a kiaman overprint invariably have steeply oriented minimum arm axes and relatively strong anisotropy whereas the most strongly remagnetized sites have nearly horizontal minima and weak anisotropy these observations suggest that diagenetic authigenic magnetites formed independently of any pre existing fabric of precursor sulphides and host rock and that the magnetic fabric of the whole midcontinent was at least partially affected by alleghenian syn orogenic activity

strain anisotropy of anhysteretic remanence and anisotropy of magnetic susceptibility in a slaty tuff finite strain data for the borrowdale slaty tuffs compare variably with the anisotropy of magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm finite strain determined from lapilli rims shows that slaty cleavage was formed by coaxial flattening with x y z in the ratio 1 74 1 21 and 0 48 aarm was measured in different coercivity windows to isolate contributions from magnetite of different grain sizes a 0 3 mt for multi domain md b 3 15 mt for pseudo single domain psd and c 15 60 mt for single domain sd ams combines petrofabric contributions from silicates as well as magnetite magnetite grains may grow recrystalize or rearrange domains after or during metamorphism and postdate or overlap with the silicate s fabric evolution ams foliation defined by paramagnetic chlorites is parallel to slaty cleavage aarm foliation for sd magnetites is offset clockwise from ams foliation which may reflect late crystallization or domain rearrangement of magnetites in response to a latter noncoaxial increment ams fabric shape consistently corresponds to strain ellipsoids and indicates that the strain induced ams fabric is susceptible to the change of oblateness rather than strain intensity furthermore investigation of the different aarm subfabrics and finite strain shows that only sd magnetite s aarm correlates with finite strain and weakly at that 2001 elsevier science b v all rights reserved

the origin of tectonic lineation in extensional basins combined neutron texture and magnetic analyses on undeformed clays abstract in extensional sedimentary basins fine grained sediments that appear undeformed at the outcrop scale can carry a magnetic fabric consistent with the regional deformation pattern the origin of the magnetic lineation which is often found in extensional basins is not yet well understood in clays from extensional basins in southern italy the magnetic lineation is tectonically controlled and oriented perpendicular to the main normal faults a combined analysis of magnetic and mineral fabrics was made to gain insight into the processes that lead to a lineation in extensional settings low field high field and low temperature susceptibility measurements were used to distinguish the ferrimagnetic and paramagnetic contributions to the magnetic susceptibility and its anisotropy the magnetic anisotropy of the sediments is predominantly carried by paramagnetic phyllosilicates neutron texture analysis was used to evaluate the spatial distribution of chlorite basal planes results demonstrate that the orientation of the magnetic lineation is related to the spatial distribution of chlorite lying parallel to the common axis of differently oriented basal planes a quantitative correlation between the magnetic and rock fabric was made comparing the low and high field magnetic anisotropy ams hfa to the theoretical anisotropy calculated from the chlorite preferred orientation a good linear correlation is found between the degree of theoretical anisotropy and the ams and hfa results show that the integrated approach of magnetic and mineral fabric investigations represents a valid alternative tool for detecting grain scale and regional deformation patterns in weakly deformed extensional basins where macroscopic evidence of deformation is often not visible

magnetic fabrics in the bjerkreim sokndal layered intrusion rogaland southern norway mineral sources and geological significance abstract magnetic anisotropy can provide important information about mineral fabrics and thus magmatic processes particularly when it is known how multiple mineral species contribute to the anisotropy it may also affect the direction of induced or remanent magnetization with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies here we aim at describing the magnetic fabrics in the bjerkreim sokndal layered intrusion and identifying their carriers anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the bjerkreim sokndal layered intrusion the intrusion is characterized by magmatic layering and has a synform structure with strong foliation on the limbs detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial but the minimum susceptibility and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering the minimum susceptibility and anhysteretic remanence are associated with pyroxene 100 axes and the maximum susceptibility and anhysteretic remanence are sub parallel to the pyroxene 001 axes in layers mcu ivc and mcu ive for which electron backscatter data are available however the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy we propose that the magnetic anisotropy of magnetite free specimens is carried by hemo ilmenite exsolutions within pyroxene in addition to pyroxene itself when present multi domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence due to shape preferred orientation and distribution anisotropy the orientation of the magnetic fabric appears independent of carrier due to their common deformation history but the degree of anisotropy is stronger for magnetite bearing specimens the results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection

the quantification of geology from abacus to pentium a chronicle of people places and phenomena abstract the geological profession has only recently become numerically literate but highly so in a relatively short time mathematics was applied to geological problems mostly in the fields of hydrology engineering geology and geophysics until the past few decades statistics were used by sedimentologists and paleontologists to describe populations with some univariate bivariate and multivariate statistics used by a few avant garde workers geocomputing really started in the 1960s but the slow start became an avalanche in the 1980s with the introduction of microcomputer personal computer pc the trend towards increasing quantification of the discipline is noticeable and in recent years this trend has been accelerating there is seemingly no limit to the information and communication revolution

a successful rock magnetic technique for correctng paleomagnetic inclination shallowing case study of the nacimiento formation new mexico the accuracy of an inclination shallowing correction technique was tested on the remanence of the paleocene nacimiento formation the correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy the remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains the nacimiento formation was selected for this test because its inclination is shallow by 7 to 8 and previous paleomagnetic studies indicate that it has a primary remanence with a well constrained age and that magnetic overprints are minimal tectonics are an unlikely explanation for its shallow inclination these conditions allow the inclination corrected remanence to be compared to a well defined paleomagnetic pole to determine the accuracy of the technique the characteristic remanence from 20 nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site individual particle anisotropy was determined by drying a magnetic separate epoxy mixture in a 35 50 mt magnetic field laboratory compaction of disaggregated nacimiento material was also used to yield an effective individual magnetic particle anisotropy the two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections both corrections were successful and the inclination corrected formation mean direction was indistinguishable from the direction predicted by north america s paleocene paleopole the inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes e g compaction

directional petrological characterisation of deep marine sandstones using grain fabric and permeability anisotropy methodologies theory application and suggestions for integration abstract different methods for quantifying preferred grain orientation or grain shape fabric in sandstones are reviewed and their value for the reconstruction of palaeocurrent direction and preferred direction of pore fluid flow is reassessed critically based on existing knowledge from literature and new grain fabric data it is shown that petrographic analysis has distinct merits because grain fabric can be determined with low equipment costs and population statistics are based on the orientation of individual grains several recommendations are given how to standardise procedures for acquisition of grain fabric through microscopy these recommendations include 1 the careful selection of cross section orientations to acquire apparent long grain axes that are representative of true long grain axes 2 the preferred measurement of the coarsest grains in cross section and 3 a new statistically founded method to select a subset of grains that is representative of the total population of grains in a sandstone sample magnetic techniques that use anisotropy of magnetic susceptibility ams as a proxy for grain orientation require specialised equipment and are more restricted in statistical evaluation but these methods benefit greatly from being able to measure a large number of grains in a three dimensional space in a short amount of time and with a lower sensitivity to user bias of the two magnetic techniques discussed in this paper the new enhanced ams method in which rock magnetic properties are enhanced by introducing magnetite into the pores and as precipitates on grain surfaces is regarded to be an improvement over the traditional natural ams method in which only the natural assemblage of magnetite particles present in the rock is analysed enhanced ams measures the preferred orientation of essentially all grains in a rock sample thus providing the shape fabric of a hydrodynamically more accurate representation of framework grains than natural ams through a small technical modification the enhanced ams method can be used to obtain a magnetic cast of the pore network of a rock sample so that the resulting ams is controlled by the shape anisotropy of the individual pores it is commonly assumed that long axes of sand grains in sedimentary deposits parallel the direction of the flows that formed the deposits this concept is challenged by a new systematic study of grain fabric in deep marine turbidites it indeed appears that grains in ripple cross laminated divisions of turbidites dominantly attain a preferred orientation parallel to the palaeoflow direction but grains in plane parallel laminated divisions have an additional flow perpendicular mode which suggests a contribution of rolling grain transport three different fabric modes occur in massive structureless sandstones and massive divisions in turbidites apart from flow aligned and flow perpendicular modes there is a statistically significant but hydrodynamically enigmatic flow oblique mode with an average azimuthal deviation of 40 this large variation of fabric types suggests that a similar variation exists in preferred paths of pore fluid flow within turbidites controlled to at least some degree by depositional process this is confirmed by independent laboratory measurements of permeability anisotropy on samples for which grain fabric data are available and thus defines grain fabric analysis as a viable alternative to traditional permeametry techniques

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

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

magnetic anisotropy in the trenton limestone results of a new technique anisotropy of anhysteretic susceptibility a new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described the magnetic fabric of a collection of trenton limestone specimens has been determined using this method as well as by conventional anisotropy of magnetic susceptibility the results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the trenton in which the bulk magnetic susceptibilities are low a model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses the original fabric is inferred to have been isotropic the anisotropy resides in secondary magnetite of late paleozoic age it is argued that the observed magnetic fabric must therefore be alleghenian or younger in age our method has the potential to determine paleostress directions in carbonates elsewhere provided our assumptions are correct

paleomagnetism of ordovician alkalic intrusives and host rocks from the pedernal hills new mexico positive contact test in remagnetized rocks jackson m van der voo r and geissman j w 1988 paleoma etism of ordovician alkalic intrusives and host rocks from the pedemal hills new mexico positive contact test in remagnetized rocks tectonophysics 147 313 323 a set of thin dikes from central new mexico dated at 469 7 ma rb sr loring and armstrong 1980 have yielded a virtual geomagnetic pole which lies on the late paleozoic segment of the north american apparent polar wander path the remanence of the dikes appears to be a product of late paleozoic hydrothermal alteration paradoxically however the magnetization of the host rocks is most simply explained in terms of a positive contact test samples collected between 0 2 and 0 5 dike widths from the contact contain a component of remanence parallel to the magnetization in the dikes with unblocking temperatures which decrease with distance from the dikes host rocks from a distance of more than 1 dike width show no evidence of the characteristic dike magnetization there are two possible resolutions of this paradox 1 the magnetization of the host rocks is secondary despite the apparent positive contact test and is a product of hydrothe al fluid migration through the dikes or along the contact zones or 2 the magnetization of the dikes is primary but not representative of the ordovician paleofield for north america possible reasons for inaccurate representation include a incomplete averaging of secular variation b tectonic rotation with respect to the stable craton or c erroneous age determination for the rocks we argue that explanation 1 is the most likely

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

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

paleomagnetic results from the snake river plain contribution to the time averaged field global database 1 this study presents paleomagnetic results from the snake river plain srp in southern idaho as a contribution to the time averaged field global database paleomagnetic samples were measured from 26 sites 23 of which 13 normal 10 reverse yielded site mean directions meeting our criteria for acceptable paleomagnetic data flow ages on 21 sites range from 5 ka to 5 6 ma on the basis of 40ar 39ar dating methods the age and polarity for the 21 dated sites are consistent with the geomagnetic reversal time scale except for a single reversely magnetized site dated at 0 39 ma this is apparently the first documented excursion associated with a period of low paleointensity detected in both sedimentary and igneous records combining the new data from the srp with data published from the northwest united states between the latitudes of 40 and 50 n there are 183 sites in all that meet minimum acceptability criteria for legacy and new data the overall mean direction of 173 normally magnetized sites has a declination of 2 3 inclination of 61 4 a fisher concentration parameter of 58 and a radius of 95 confidence 95 of 1 4 reverse sites have a mean direction of 182 4 declination 58 6 inclination of 50 and 95 of 6 9 normal and reversed mean directions are antipodal and indistinguishable from a geocentric axial dipole field at the 95 confidence level virtual geomagnetic pole dispersion was found to be circularly symmetric while the directional data were elongate north south an updated and corrected database for the northwestern u s region has been contributed to the magnetics information consortium magic database at http earthref org

strain decoupling across the decollement of the barbados accretionary prism the interrelation between deformation styles and behavior of fluids in accretionary prisms is under debate particularly the possibility that overpressuring within the basal decollement may enable mechanical decoupling of the prism from the subducting material anisotropy of magnetic susceptibility ams data from sediments spanning the basal decollement of the barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is markedly overpressured in the accretionary prism above the decollement the minimum ams axes are subhorizontal and oriented nearly east west whereas the maximum ams axes are oriented nearly north south and shallowly inclined at the top of the decollement the minimum ams axes orientations abruptly change to nearly vertical this orientation is maintained throughout the decollement and in the underthrust sediments below the ams orientations in the prism sediments above the decollement are consistent with lateral shortening due to regional tectonic stress as the minimum axes generally parallel the convergence vector of the subducting south american plate and the maximum axes are trench parallel because the orientations of the ams axes in deformed sediments usually parallel the orientations of the principal strains the ams results indicate that the incremental strain state in the barbados prism is one dominated by subhorizontal shortening in contrast the ams axes within and below the decollement are consistent with a strain state dominated by vertical shortening compaction this abrupt change in ams orientations at the top of the decollement at site 948 is a direct manifestation of mechanical decoupling of the off scraped prism sediments from the underthrust sediments the decoupling horizon occurs at the top of the decollement zone coinciding with the location of flowing high pressure fluids

differences between magnetic and mineral fabrics in low grade cleaved siliciclastic pelites a case study from the anglo brabant deformation belt belgium abstract theoretically within a given pelitic rock in which the main paramagnetic carriers are white mica and chlorite and in which judging from the ams parameters ams is controlled by the paramagnetic carriers the results of x ray pole figure goniometry should show a qualitative relationship with ams this idea is tested on single phase deformed low grade pelites of the lower palaeozoic brabant massif belgium representing the southeastern part of the anglo brabant deformation belt for the vast majority of the samples investigated the different lf ams parameters suggest a paramagnetic control on lf ams different parameters of lf ams measured at 77 k low t lf ams and a comparison of these with the lf ams parameters indicate that lf ams is controlled virtually entirely by paramagnetic carriers high field torque magnetometry hf ams indicates further that ams is controlled entirely by paramagnetic carriers for the vast majority of the samples two apparent discrepancies are observed however these are 1 a frequent mismatch between the lf ams fabric on the one hand and the paramagnetic fabric indicated by low t lf ams and hf ams on the other hand and 2 a rather common mismatch between the preferred orientation of chlorite and white mica suggested by x ray pole figure goniometry on the one hand and the lf ams fabric low t lf ams fabric and hf ams paramagnetic fabric on the other hand these apparent discrepancies may partly be attributed to the composite nature of both the lf ams fabric and the paramagnetic susceptibility fabric the presence of different populations of paramagnetic carriers possibly in combination with minor traces of ferromagnetic s l carriers which are not picked up by the methods applied may explain some of the mismatches observed and are compatible with the basin evolution history of the brabant massif in addition it is possible that the low temperatures low t lf ams and high fields hf ams are capable of enhancing one of the paramagnetic phases thus leading to discrepancies between methods that define the mineral and the magnetic fabrics the results of this study show that even when the lf ams data suggest that the lf ams of a given pelitic rock in which phyllosilicates form the main paramagnetic carriers is controlled entirely by paramagnetic carriers alternative methods such as phyllosilicate x ray pole figure goniometry will not necessarily reflect the paramagnetic carrier orientation although a number of cases have successfully correlated lf ams with mineral fabric and occasionally even strain this study serves as an example for how methods to define mineral and magnetic fabrics can be sensitive in detecting different paramagnetic phases

composite ferromagnetic fabrics magnetite greigite measured by ams and partial aarm in weakly strained sandstones from western makran iran summary we present examples of composite magnetic fabrics ams and partial aarm in five sites from the western makran accretionnary prism these rocks are weakly strained sandstones of mio pliocene age rock magnetic investigations show a large occurrence of magnetite and also some greigite at two sites partial anhysteretic remanent magnetization coercivity spectra reveal that grain size is not homogeneous suggesting several grain fractions ams foliations are close to bedding except at one greigite bearing site where ams foliation is almost perpendicular to bedding soft aarm 0 50 mt foliations are slightly oblique both to bedding and ams foliation surprisingly hard aarm 50 100 mt foliations are perpendicular to bedding comparison of two limbs of one fold suggests that ams foliations are pre tilting while hardaarm foliations are post tilting corrected degree of anisotropy parameters display similar trends for all sites p 1 04 p 1 11 p hardaarm 1 19 we suggest that ams soft aarm and hard aarm reflect distinct magnetic grain populations however a possible relationship between hard aarm and a component of natural remanent magnetization is possible our study shows that two orthogonal fabrics exist in these weakly strained rocks coarse grains are parallel to bedding while fine grains are aligned perpendicular to bedding

tectonics of the circum troodos sedimentary cover of cyprus from rock magnetic and structural observations the calcareous sedimentary cover of the troodos ophiolite complex dips gently away from the troodos complex it has rare usually open folds and locally stylolitic cleavage s1 stylolitic cleavage is 050 more eaective than primary beddingstylolites in removing matrix and the variation in the s1 vergence directions are compatible with gravitational sliding radially away from the troodos range and into sedimentary basins two diaerent types of magnetic fabric anisotropy of magnetic susceptibility ams and anisotropy anhysteretic remanence aarm reveal the sequence of preferred orientations of diaerent minerals and the vergence of their magnetic foliations with respect to bedding reveals the sense of over shearing of the stratigraphic sequence ams defines crystallographic alignments of clay minerals and dimensional alignments of magnetite in the limestones aarm isolates the preferred dimensional orientation of magnetite although traces of magnetite dominate the bulk susceptibility its anisotropy is low in comparison with phyllosilicates so that ams essentially records the clay fabric throughout the study area ams foliation indicates s vergence and ams lineation indicates n s extension ams foliationvergence indicates shearing of the entire sedimentary cover southward from troodos this motion is attributed to gravity sliding controlled by regional uplift to the north in contrast aarm foliation indicates a subsequent ese vergence due to later wnw ese extension the nne aarm lineation is a composite of bedding and tectonic fabric contributions that do not directly yield a faithful extension direction the shapes of the magnetic fabrics indicate that ams clay fabrics have a broad range of shapes about the neutral ellipsoid shape whereas aarm magnetite fabrics are bimodal with equally developed bedding oblate and tectonic linear planar ola su components hysteresis properties plotted on a new three dimensional diagram show clearly that the magnetite has a pseudosingle domain response so there is no reason to expect complications from inverse fabrics due to single domain magnetite moreover it shows that hysteresis properties vary systematically with depositional depth reflecting systematic changes in magnetite granulometry with sedimentary environment 2000 elsevier science ltd all rights reserved

crystallization in flow i palaeocirculation track by texture analysis and magnetic fabrics summary in order to better define the dependence of the mineral texture on flow anisotropy of magnetic susceptibility ams and microscopic observations have been performed on calcite rich precipitations occurring in a horizontal pipe where the flow direction is clearly defined a total of 65 cubes were cut from five slices magnetic studies identified pseudo single domain magnetite as the major ams carrier horizontal foliation characterizes the magnetic fabrics and the lineation is parallel to the pipe axis that is the flow direction the origin of this lineation is discussed and is interpreted to be mainly the consequence of elongated pseudo single domain magnetite to complete the magnetic fabric studies shape preferred orientation statistics were performed on 563 calcite sections in the 0001 calcite plane it shows elongated shapes with a general orientation parallel to the pipe axis the mean shape orientation is the average of two distinct subpopulations that deviated slightly from the pipe axis observation on calcite shapes and the direction of the magnetic lineation are coherent suggesting that it is possible to track hydrothermal palaeocirculation using magnetic lineation and petrographic fabrics